Radio World Engineering Extra 265 - February 10, 2021

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TVSS for Broadcast Facilities The author is owner of Rural Florida Communications Cooperative. He is a retired AT&T employee who has a great deal of experience protecting communications facilities from lightning and surges. He saw Mark Persons’ recent account in Radio World of a lightning strike to the pole supporting the KRJM STL antenna and was inspired, first, to provide some tips for John Bisset’s Workbench column in the Dec. 9 issue, then

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an old myth. I have heard from a number of engineers that “If you take a direct hit, it’s all over; there is nothing you can do to prevent that.”

Such a statement is patently false.

The tier one carrier I retired from has tens of thousands of cell sites and switching offices all over the U.S., and they take thousands of direct hits annually without sustaining any damage at all.

Why? Because we engineer layered lightning protection into each site. Be it standalone, collocated on a broadcast tower or on a rooftop, proper protection equals greatly lowered losses and greatly increased uptime.

So how is this done? Well, let’s use the example of KRJM(FM), which Mark Persons wrote about in “What Happens When Lightning Hits? A Case Study,” which appeared in Radio World in October, and whi ch you can find at radioworld.com by searching for keyword KRJM. Beginning with the improperly protected and grounded pole and

It’s not impossible to protect your facility from damage from a direct lightning hit

BY WAYNE ECKERT

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offer the following in-depth article for readers of RW Engineering Extra. We in turn are inspired to share it with you.

A wise man who invented the air terminal, Benjamin Franklin, once stated, “An ounce of prevention is worth a pound of cure,” and the lightning damage at KRJM bears witness to that.

Now, let me start with dispelling

(continued on page 4) Getty Images/Dnes Mszros / 500px

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Xa Z in From Transmission Line

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OK Everybody It’s Tee Time!

Dave Dybas offers a primer for calculating 90-degree T networks.

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FROM THE TECH EDITOR

February 10, 2021

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Are You Prepared for Lightning?

It can strike anywhere, in any month of the year

BY CRIS ALEXANDER

The worst lightning damage I ever saw happened in the month of February; and it was, in more than one sense, a perfect storm.

We were in the process of upgrading our Detroit FM facility, replacing a 1985-vintage Continental 27.5 kW transmitter with a much newer version of the same transmitter. The aux had long been an RCA BTF20, and our chief engineer had pulled it out of service in preparation for installing the new rig.

The new transmitter was at the site and still on the skid and wrapped in plastic. In other words, we had no working auxiliary transmitter.

In February in Michigan, we’re usually much more concerned about snowstorms and antenna icing than lightning; but this particular year, a warm weather system moved through and fired off a line of severe thunderstorms.

VAPORIZED At that time I had been with the company for about a decade, and I was well acquainted with the propensity of that particular tower, a 500-foot Pirod free-standing Detroit landmark, to take “grand mal” lightning hits.

I had done everything I knew at the time to mitigate that. I had an array of ground rods at the tower base, and all transmission lines were bonded to that array where they departed the tower. It had been a good while since we’d taken a hit that produced damage.

When the storm system came through, BOOM! The tower took one of those grand mal hits, and the station was knocked off the air.

The air staff called the engineer, who took one look at the situation and called me. I caught the next flight from Dallas to Detroit.

Looking at the Continental transmitter, I couldn’t believe my eyes.

Those familiar with that design know that it has two rows of seven panel-mount fuse holders in the middle of the front panel of the rightmost (power supply) cabinet. Every one of those fuse holders was … gone!

Vaporized!

Looking behind the panel, the wires, some not in too good a shape, were still there, the ends turned into beads where the copper strands had melted.

It didn’t take me long to make an executive decision.

There was no telling what other damage there was inside that transmitter, so for the moment I forgot about it and focused on getting the new-on-the-skid transmitter connected and running.

Make it part of your routine to check ground connections like this UFER ground.

That took a few hours, and it wasn’t the prettiest installation, but it got the station back on the air.

Thankfully there was no other significant damage to studio equipment, although as I recall, a good bit of ancillary equipment in the transmitter room was blasted to smithereens and had to be replaced or bypassed.

MISSING CONNECTION With that done, I started doing a post-mortem of sorts to try to figure out what had happened. After all, if it happened once, it could happen again.

I found that the strap connecting the main 3-1/8(continued on page 4)

THIS ISSUE FEBRUARY 10, 2021 TVSS for Broadcast Facilities . . . . . . . . . . . . . . . . . . . . . . . . 1 Are You Prepared for Lightning? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Broadcast Frequency Measurement Methodology. . . . . . . . . . . . . . . . . . . . . 10 OK, Everybody, It’s Tee Time!. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 The Natural Progression of Virtualization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Using the SNMP Protocol in Broadcast Monitoring . . . . . . . . . . . . . . . . . . . . . . . . 18 Reader’s Forum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

I found pieces of those fuse holders on the floor several feet from the front of the transmitter.

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TVSS (continued from page 1)

ending at the last burned-out device, this is a perfect example of where an ounce of prevention could have prevented a lot of heartburn.

Let’s start with the pole. When I see a pole shelled out from lightning, the first thing I look for is a down conductor. I could be incorrect, but I didn’t see one on that pole.

A down conductor is a simple lightning protection device. Before the pole is placed, an AWG #6 hard-drawn copper conductor is placed on it using fencing staples, attaching it from the top of the pole to the butt. The installer leaves six or so inches of the conductor standing above the top of the pole and coils up a few feet of it on the butt, starting in the center, and where the conductor crosses over itself, places a staple diagonally to connect the two conductors to each other.

The goal is to produce a low-resistance, low-impedance grounding electrode on the butt of the pole. When the pole is placed in the mounting hole, it will make good contact with the earth, producing a good solid grounding electrode.

The down conductor provides a

PREPARED? (continued from page 3)

inch rigid transmission line to the station reference ground (that array of rods I mentioned earlier) had been severed.

Lightning evidently had hit the main antenna, which is on a pole on the top of the tower, and the current travelled down the rigid line right to the older Continental transmitter.

Some of the current no doubt flowed through the tower structure to ground, but evidently the path to ground through the transmitter cabinet had a much lower impedance.

The transmitter cabinet was connected to a safety ground that was eventually tied to the service entrance ground and whatever ground Edison provided. And evidently, enough potential was developed across the Bakelite (or whatever the material) panel-mount fuse holders that they vaporized. I found pieces of those fuse holders on the floor several feet from the front of the transmitter.

One side of most if not all those fuses was connected directly to the incoming AC power phases, and (again, evidently) those phase conductors, which were shunted to ground with an LEADynatech lightning suppressor, present-

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bypass for the lightning energy to earth, sparing the pole and any attachments from damage. See Fig. 1.

If the pole is already placed, a down conductor can be added by placing a ground rod with a minimum length of eight feet into the earth.

One important note: Always call for a utility locate before driving a ground rod or doing any digging. In most states, the number is 811. Failure to do so can get you killed, or if you survive hitting a buried utility line, the least you’ll get is a substantial bill from the utility for the underground damage you caused.

The installer then installs the down conductor in the same manner as a new pole, minus the below-grade work, connecting the down conductor to the grounding electrode.

All that protects the pole, but what about attachments?

February 10, 2021

The first step is to bond all attachments to the down ground lead with an AWG #6 copper conductor. That’s your first layer of protection.

The next step is surge protection, commonly referred to as Transient Voltage Surge Suppression.

A TVSS can be installed in one of two locations. The best location is where the cables coming off the tower or pole enter the building. However, if the building is metal, it is better to deploy a primary TVSS near the base of the pole or tower.

If the TVSS is rated for outdoor installation, a cabinet is not required.

However, most of the time you will want to keep the TVSS in a dry location and out of sight.

Either way, the TVSS should be installed on a common insulated buss (continued on page 6)

I have heard from a number of engineers that “If you take a direct hit, it’s all over; there is nothing you can do to prevent that.” Such a statement is patently false.

Fig. 1: A properly installed pole with down lead.

ed a lower impedance to ground than the safety ground on the transmitter cabinet.

All the grounds were at one time tied together, but in the process of uninstalling the old RCA aux, somehow one critical ground conductor was disconnected.

We wasted no time repairing the broken strap that gave lightning current a place to jump off transmission lines before entering the building, and we brought in an electrician and had him bond all the grounds together so that the tower ground, station reference ground, service entrance ground and lightning suppressor ground were all at the same potential.

I can’t remember what all was blasted in that Continental transmitter, but we did fix it and it saw many years in auxiliary service.

NOW IS THE TIME The point of this sordid account … well, there are a couple of points.

First, a lightning hit can happen at any time, during just about any month of the year.

In Colorado where I live, we sometimes experience a phenomenon called “thundersnow,” where we get lightning and thunder during snowstorms. I’ve never experienced that anywhere else, but my guess is that it’s not uncommon

anywhere it snows. Everywhere else, a passing weather system can fire off lightning just about anytime.

Next, the best way to prevent lightning damage is to give lightning a place to go other than into or through your equipment.

That means a jumping-off point where transmission lines, power conductors and control cables leave the tower.

It also means a low-impedance ground, which is more easily achieved in some locations than others. Some soil types may require chemical rods (which you have to maintain). And it means keeping all grounds at the same potential, with heavy-gauge wire used to connect rods and bond everything together.

There are a lot of good resources out there dealing with lightning protection, including NFPA780, which is the National Fire Protection Association’s “Standard for the Installation of Lightning Protection Systems.”

Radio World has an excellent ebook available titled “Is Your Transmitter Ready for Lightning Season?” Find it at radioworld.com/ebooks and scroll down to the 2019 issues.

Broadcast Engineering Conference papers have been presented on the topic.

And of course there’s an excellent chapter on the topic in the 11th Edition of the

NAB Engineering Handbook. A search of the internet will turn up numerous other resources.

Whether or not you get lightning in your area outside the usual thunderstorm season, now is the time to inspect your facilities and prepare. Forewarned is forearmed.

We still get the occasional lightning strike on that Detroit tower — we have experienced two in the past 12 months — but we seldom see any direct damage. Our excellent engineering staff in Detroit is well aware of the potential (no pun intended) for damage and makes a visual inspection of the entire ground system, including transmission line bonds, part of their routine.

That kind of thing should be part of every broadcast engineer’s routine, especially as we head into thunderstorm season.

In this issue, Wayne Eckert, who spent many years protecting AT&T sites from lightning damage, offers tips so that you can apply some of his techniques to broadcast studio and transmitter facilities. We would do well to pay attention to what he has to say.

Cris Alexander, CPBE, AMD, DRB, is director of engineering for Crawford Broadcasting. Email him at rweetech@ gmail.com.

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TVSS (continued from page 4)

bar. This bar should be bonded to the pole or tower ground and grounded to the main grounding bar (MGB) in the building. The minimum recommended grounding conductor is AWG #4 copper, unless the distance between the pole ground bar and the MGB exceeds eight feet. Then it should be stepped up to an AWG #2 copper conductor. See Fig. 2.

BRINGING IT INSIDE The common method when bringing cables into a studio or transmitter building is to use a bulkhead. This can be as simple as a sheet of non-flammable water-resistant material such as Hardie Panel sheeting or acrylic plastic.

In a metal building, this is mandatory. You do not want to penetrate a metal building where a cable can be in direct contact with the metal wall. That is begging for arc-over problems.

The point where the cables enter the building is where you will place your primary or secondary TVSSes. The MGB should be placed as close as practical to this penetration, but no more than a couple of feet away.

Each TVSS should be connected

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POWER AND UTILITIES More commonly than not, the power, telco and CATV are not going to enter the building at the same location as the cabling off of the tower or pole, but they must also be protected, so let’s start with power.

Your building will have a watt-hour meter, and it’s usually installed outside (Fig. 4). Even if it is a “smart meter,” it’s a simple device that allows your electric utility to vacuum funds out of your bank account and transfer them into the utilities bank account. However, that watt-hour meter can be used to your advantage.

There are devices known as meterbased TVSSes, which are sleeves that are installed by either a licensed electrician or the utility between the meter base and the meter. Some utilities will allow your electrician to do the work; others insist that you rent the TVSS from them for a small monthly fee.

Either way, they are good protection for the cost. They are crude first-line devices that can take the brunt of a hit.

But don’t stop there. Always have a hard-wired TVSS installed as close as possible to the service entrance. Some will require a circuit breaker to be installed ahead of the TVSS to prevent a catastrophic burn-down of the TVSS

Do yourself a favor and don’t go cheap.

Real TVSSes for power run anywhere from $400 to $2,500+.

to the MGB with its own grounding conductor. Never double up grounding conductors. The grounding conductors should be placed either right or left of the center of the MGB. The conductors should be swept in from above and not reversed below and upward where they connect to the MGB. The primary grounding conductor for the MGB should be connected at the center of the MGB. The reason for connecting the main grounding conductor to the center of the MGB is to isolate incoming conductors that introduce surges on a regular basis (dirty) from the sensitive equipment (clean), which must also be connected to the MGB to be protected.

The center MGB ground serves to provide this grounding isolation or PANI, an acronym describing a way of bonding conductors to the MGB in a specific order, depending on their origin: surge energy Producers, Absorbers, Non-isolated equipment and Isolated equipment (Fig. 3).

February 10, 2021

Fig. 2: TVSS devices installed on a ground bar.

Fig. 3: This installation follows the PANI order, with surge Producers, Absorbers, Non-isolated and Isolated from the center MGB out.

should it go into a total failure after doing its job.

That breaker should be installed as close as possible to the main breaker in the service panel. If that requires moving breakers around to clear a double space, so be it. If the service panel is choked up with no spare spaces, drop in a sub-panel and unload other circuits to the sub-panel to make space for the TVSS breaker.

Do yourself a favor and don’t go cheap. Real TVSSes for power run anywhere from $400 to $2,500+, depending on the rating of your service entrance and if it is single phase or multi-phase power (Fig. 5).

ON THE INSIDE If you have a load center within your building, this is a great place to add a secondary panel buss TVSS. While not breaking the bank, most manufacturers of load centers offer TVSSes that simply slot in like a multi-pole breaker. Like a (continued on page 8)

Fig. 4: The watt-hour meter is a good place for additional TVSS protection.

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February 10, 2021

TVSS

GROUNDING & BONDING

(continued from page 6)

primary TVSS, they should be installed next to the feed to the panel. If needed, have your electrician relocate one or two breakers to clear up slots and you are good to go. See Fig. 6.

Moving down in voltage, we have telco, cable and CATV. These incoming services need to be protected as well.

Normally your telco provider will provide a Network Interface Device (NID). Within it will be protectors, commonly gas-based protectors with a 400 VDC breakdown. As long as the NID is properly grounded and bonded to the building’s ground system, that provides good primary protection.

Internally, you want to back that up with TVSSes rated for no more than 200 VDC for POTS phone lines. Special circuits, however, are a horse of a different color. Digital or audio circuits behind the telco’s mounting need to be protected by very low voltage TVSSes, 50 VDC or less.

Commonly, cable and CATV will utilize a spark-gap protector, and like telco, it must be grounded and bonded to the buildings grounding system. However, a spark-gap protector is unsuitable in this application. Back it up with a reputable coaxial TVSS mounted to the dirty side of the MGB ahead of the distribution of the cable or CATV signal within the building.

GROUNDING AND BONDING Now, for a somewhat more complicated subject, grounding and bonding. It is extremely important that all connections to earth be bonded to each other.

A few tips: lways bond devices, racks, A attachments, etc., swept toward the MGB. ire-brush paint from both W sides of equipment mounting tabs where grounding connectors will be placed and the rack faces they will be mounted to, exposing bare metal. ost modern racks and rackM mounted equipment are aluminum-framed, therefore always use bi-metallic connectors when bonding such equipment to copper grounding conductors. lways apply a thin coat of antiA oxidant such as NO-OX to the bare metal before assembly to ensure a long-lasting corrosion free connection.

Fig. 5: Get the best TVSS device you can find.

Lacking that bonding, surges entering your facility over various services connected to disparate earth connections will wreak havoc within your facility.

An example would be if your facility was not built from the ground up as a broadcast facility. While the structure is wired to code, you later add something simple such as a DTV or other satellite dish.

That dish must have a clear view of the southern sky, but your electrical

service and other utilities may enter a different side of the building, say the north side. Not a problem.

The contractor for the DTV provider or the dish installer installs the dish on the south side of the building’s roof, then runs the coax from the dish down and through a spark-gap protector, which is earthed to a ground rod he placed. That ground rod, however, is commonly not bonded to the building’s grounding system by the contractor.

Later, during a storm, lightning

lways use compression connecA tions and the proper crimping tools and dies or use exothermic welds. If a connection must be soldered, silver solder is required. No exceptions.

Always tag the grounding connections at the MGB and subsystem ground bars. You may be the next person who has to work on the system, long after it was built, and memory may not serve to remind you what all these are.

Incoming services like telco, cable and CATV need to be protected as well.

Fig. 6: Panel-mounted TVSS devices are a great additional line of protection.

tickles that dish. The resulting energy saturates the un-bonded ground rod and the remaining energy seeks out all other forms of an earth ground it can find. In the process, it passes through the connected receiver and then enters the building’s electrical system. From there it passes through and into any source of a ground to earth. This would include neutral, any electronic device with a three-wire cord, or grounded rack-mounted equipment. Sometimes lightning energy will find a ground just by arcing within the building’s wiring system and device’s, destroying them in the process.

The solution to preventing such dam-

age is to bond all connections to earth to each other with a minimum of an AWG #4 direct-buried copper conductor. This provides a low-resistance path from all grounding connections to earth, eliminating any differences in voltage potential between those connections.

Yes, all those connections may become saturated for a moment, and yes, there may be substantial rise in potential on all of them, but the bonds prevent any flow of energy through the protected equipment within the structure.

See the above box for some additional tips.

Comment on this or any article.

Email rweetech@gmail.com.

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February 10, 2021

Broadcast Frequency Measurement Methodology A unique way of measuring AM carrier frequencies off-air

AM Station to measure

BY ROBERT W. MEISTER

The author is a broadcast contract engineer who has a unique way of measuring the carrier frequency of the AM stations in his care.

Making off-air frequency measurements of AM broadcast stations can be a bit of a challenge.

Unless you are at the transmitter site and have a high-level RF sample of the transmitter output available, it’s unlikely that you can use a frequency counter to make the measurement. Another method has to be used to measure the low-level (millivolt-range) off-air signal. I have found an easy, “zero-beat” method that works reliably.

I use the following complement of equipment: • Field intensity meter (such as PI FIM21/41, RCA WX-2, or Nems-Clarke 120).

• R F signal generator with 0.01 Hz adjustability (such as Agilent E4430B).

• GPS-disciplined 10 MHz reference oscillator (such as HP Z3801A) and antenna.

• Loop antenna (such as Chris Scott LP-3).

The physical setup is shown in Fig. 1, and the measurement procedure is as follows: 1. Connect the equipment as shown.

The loop antenna can be oriented in any way and should be placed about a foot away from the FIM. 2. Tune the RF signal generator to the frequency of the station to be measured, then tune the FIM to that signal. It is not necessary to calibrate the FIM; it will only be used to receive the station and the actual field intensity reading is unimportant. You don’t even need to listen to the signal on the speaker or headphones. Set the meter to LIN mode, not LOG mode.

3. Disconnect the loop antenna from the RF signal generator or disable its RF output. Orient the FIM to maximize the signal coming from the station. Adjust the FIM’s RANGE switch and GAIN controls for a mid-scale indication (3–6) on its meter.

4. Reconnect the loop antenna or enable the RF output of the signal gen-

Antenna In Loop Antenna in Cover

HP GPS Receiver 10 MHz Oscillator Ext. Ref. In Agilent E4430B RF Signal Generator

RF Out

C. Scott LP-3 Antenna

Field Intensity Meter

Fig. 1: Zero-beat frequency measurement equipment setup

erator and adjust its output level so the meter swing remains within the limits of the scale. Set the RF output level based on the position of the FIM’s RANGE switch: for the 1 V/m range, start with –20 dBm; set it lower by 20 dB for each lower position of the RANGE switch.

position where changing the frequency up or down by 0.01 Hz causes the meter indication to reverse direction, indicating you’re as close as you can get.

With practice you can dial in the exact zero-beat frequency in less than 30 seconds. Read the station’s exact carrier

I’ve been using this method for more than eight years with results that match or exceed the commercial frequency measuring company’s reports.

On my setup, I need around –10 dBm feeding the loop antenna for a usable indication on the FIM’s 1 V/m range.

If necessary, change the RF signal generator’s frequency up or down by a few Hertz to see the meter swing back and forth due to the beat frequency.

5. Adjust the RF signal generator’s frequency to zero-beat the station so the meter swing is minimized and eventually stands still. Go right down to 0.01 Hz steps. Take your time as you get near the exact frequency, as the meter will be moving up or down very slowly. Make sure you’re not at a maximum or minimum of the zero-beat cycle. You want a

frequency on the RF signal generator.

Stations running IBOC, most of which are locked to a GPS reference frequency, are usually very close to their assigned frequency, within 0.1 Hz. Most modern analog transmitters will show some seasonal drift with temperature.

I am currently checking the carrier frequency of four local stations. The IBOC station (that is not using an external GPS antenna) has drifted up 0.04 Hz over five years. The others tend to move up or down by as much as 3 Hertz as the equipment temperature changes. The FCC rules require the carrier frequency to be within +/- 20 Hertz, so a few Hertz

won’t matter.

I’ve been using this method for more than eight years with results that match or exceed the commercial frequency measuring company’s reports.

I have the equipment listed above, but you can make substitutions if necessary. For example, in place of the Chris Scott loop antenna, a couple of clip leads and a series 30-50 ohm resistor can be used to form a loop that can be loosely draped on top of the FIM’s loop antenna. Even a short whip antenna can be used on the signal generator if the FIM is close enough to it.

An RF signal generator that lets you specify a frequency within 1 Hz or better can be used as long as it can utilize a 10 MHz reference signal. The carrier frequency you measure will only be as accurate as the equipment you have available to measure it with.

The 10 MHz reference signal could come from a rubidium oscillator, which has been adjusted to zero-beat a GPSdisciplined oscillator (GPSDO). These can often achieve accuracies of 0.0001 Hz on the 10 MHz signal.

An AM radio with a VTVM or DMM on its AVC line can also be used as an indicator if you don’t have an FIM. The author is an amateur radio operator (WA1MIK) and FCC licensed contract radio engineer in Southern Connecticut. Email him at wa1mik@ comcast.net.

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February 10, 2021

Okay, Everybody, It’s Tee Time!

A primer for calculating 90-degree T networks

BY DAVE DYBAS

The author is owner-engineer of AM Detuning Service.

Currently there are more than 4,500 AM broadcast stations on the air in the U.S. These stations still rely, to a limited degree at least, on a technology that, at its heart, hasn’t changed much in over 80 years.

One area that uses well-established and core electronic techniques is the AM antenna system.

AM broadcast antenna systems rely heavily on the use of L/C (that is Inductor/Capacitor) networks to accomplish things like impedance matching, phase shifting, broadbanding, frequency

trapping, harmonic filtering and numerous other applications.

The most common configuration of L/C components in an AM antenna system is the ubiquitous “T” network, so named because of the circuit configuration.

Nearly every AM antenna system uses one of these networks to match the complex impedance of the antenna to the impedance of the transmission line feeding it. We remember from early electronics training that when the source impedance (the impedance of the transmission line) equals the load impedance (the impedance of the antenna), we get the most efficient power transference to the load. Getting every last watt of power to the antenna is usually good thing!

Antenna Z out

Xa Z in From Transmission Line

Xb R Xc jX

Fig. 1: Schematically, the components form the letter “T,” thus the descriptor “T network.”

AM antenna systems have a resistance value (R) and a reactive value (X). Most often, this is expressed as a complex number R+jX. This complex number is the “impedance” of the antenna system and is measured at the operating frequency of the antenna.

When an engineer designs an AM antenna, its complex impedance can be estimated with a good degree of accuracy using empirical data gathered decades ago, or it can be even more accurately estimated using momentmethod modeling. However, ground conductivity at the antenna, variations in the ground radials configuration and nearby structures may have some effect on that value.

The best way to determine the exact AM antenna base impedance is to use an impedance bridge or network analyzer and measure the impedance on the frequency of operation.

Fig. 2: The physical layout for an AM antenna tuning unit “T” network. Note that the layout doesn’t exactly resemble a letter “T,” but it could if desired. Also note that coils are physically mounted at right angles to one another to minimize mutual coupling.

COMPONENT ARRANGEMENT As noted, the “T” network gets its name from the schematic arrangement of the components in its makeup. As Fig. 1 shows, the components Xa, Xb, Xc are electrically connected in a way to form the letter “T.” For those just getting into radio engineering, I’d like to mention that the components do not have to physically form the letter “T” when they are mounted in place, though they can if you desire. It is wise, though, to mount inductors perpendicular to each other, so they don’t inductively couple energy and do unpredictable things.

Fig. 2 shows a layout for a commer(continued on page 16)

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TEE TIME (continued from page 14)

cially-built ATU. You will notice the input component Xa is a capacitor-coil series combination. This coil in series with a capacitor “subtracts” from the capacitive reactance to allow a wide range of adjustment. Xc is the shunt coil of this “T” network, and one end is grounded through a paralleled pair of capacitors. Again, the series coil provides for adjustment of this leg. At the top right is Xb, which is the output inductor. Notice this inductor is mounted at a 90-degree angle to Xc, and in a different plane, to Xa. This is done to minimize mutual coupling between the coils. The component layout may not form the letter “T” mechanically, but electrically it does.

IMPEDANCE MATCHING One function of the “T” network is impedance matching, that is, taking a high or low value of antenna impedance and transforming it to the same value as its transmission feedline.

Along with the impedance transformation, the “T” network also has some inherent shift of the current from the input to the output of the network. It’s possible to design either a phase-lagging or phase-leading network.

In many, but certainly not all, AM broadcast antenna tuning units, the “T” network is designed for a 90-degree phase shift. The focus of this article will be limited to 90-degree “T” networks.

One of the reasons for choosing a 90-degree phase shift is that this value simplifies the component calculation.

The reactance values for Xa, and Xb are the same and Xc is the same numerical value but of opposite sign. Typically, in a phase-lagging network, Xa and Xb are equal value inductors and Xc is a capacitor of the same, but negative, reactance value. The opposite is true of phase-leading networks. But as we shall

February 10, 2021

90 Degree Phase Shift “T” Network

Example

Xa = Xb = Xc = Zin x Zout

Antenna =100 +j250 ohms

Where Za, Zb, Zc is the reactance in

Tx Line = 50 ohms

ohms of the “T” network

Xa, Xb, Xc =

Zin is the transmission line impedance Z out is the load/antenna resistance.

100x50

Xa, Xb, Xc = 70.7 ohms

For Phase Lagging Networks

L = 70.7 / (6.28 x 1,000,000) = 11.3 uhy

Xa, Xb = Inductive Xc = Capacitive

C = 1/(6.28 x 70.7 x 1,000,000) = .0022 ufd

For Phase Leading Networks

the antenna reactive component.

Note: See text regarding the treatment of

Xa, Xb = Capacitive Xc = Inductive Fig. 3: 90-degree “T” network equation and example calculation.

see, that’s not always the case.

To calculate the values for an example “T” network, the follow the equations in Fig. 3. For the moment, we will ignore the reactive part of the antenna’s impedance and use only the antenna resistance and the characteristic impedance of the transmission line.

In the example shown in Fig. 3, all three component reactances are calculated to be 70.7 ohms, but that doesn’t take into account the reactive component of the antenna impedance. What we need to do is to make that reactive part go away by adding an opposite sign reactance in the output leg of the “T” network.

To do this, we must add a negative 250-ohm reactance to the output leg of the “T” network. This gives us a

+j 70.7 ohms

+j 70.7 ohms

Xa Z in =50 ohms From Transmission Line

RadioWorldMagazine

Xb

The most common configuration of L/C components in an AM antenna system is the ubiquitous “T” network, so named because of the circuit configuration. network that looks like the one shown in Fig. 4.

Now we can combine the two reactive components in the output arm of this network into one component by adding Xb + (– jX ) together to get –j 179.3 ohms, which changes this arm of the network from inductive to capaci-

Antenna Z out

-j 250 ohms

-jX +j 250 ohms

jX

100 ohms

R

Xc -j 70.7 ohms

Fig. 4: Block diagram of a complete “T” network, including a reactance to cancel antenna reactance.

tive. We calculated the capacitor value to replace both of these components to be 888 pF. This isn’t a standard value of transmitting capacitor, so we can either use a vacuum variable capacitor adjusted to that value or employ a standard-value capacitor with a slightly lower capacitance value and put an inductor in series with it. The value of the inductor is then adjusted to achieve the exact net let reactance that we need.

For example, using a 750 pF capacitor and a 10 uH coil, we would adjust the coil for a net combined reactance of –j 179.3 ohms in the output arm of the “T” network.

“T” networks with phase shift values different than 90 degrees require a bit more math. We’ll explore that in a future article.

The author started his radio career as an amateur radio operator in his teens and worked for various AM and FM stations in the Chicago region. After 30 years as a chief engineer he retired from radio, got bored shortly thereafter and started AM Detuning Service to mitigate wireless tower effects on AM antennas.

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RADIO TECHNOLOGY

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February 10, 2021

The Natural Progression of Virtualization A recent Radio World ebook explores the concept of “virtualizing the air chain.” The following is an excerpt from a roundtable discussion and interview.

Roz Clark is senior director, radio engineering for Cox Media and chair of the Next Generation Architecture working group of the NAB Radio Technology Committee; he chaired of the HD Time Alignment Working Group that published the document that the National Radio Systems Committee adopted related to HD/FM time alignment.

Shane Toven is senior broadcast engineer, Educational Media Foundation and a national director of the Society of Broadcast Engineers.

Philipp Schmid is chief technology officer of Nautel.

Alan Jurison is senior operations engineer, iHeartMedia Centralized Technical Operations and chair of the Metadata Usage Working Group of the National Radio Systems Committee.

Greg Shay is chief technology officer of Telos Alliance.

Their conversation was moderated

by Michael LeClair, chief engineer of WBUR in Boston.

Michael LeClair: What does virtualization of the air chain mean to you?

Roz Clark: I think it’s a natural progression of our business, as we look at the technologies that surround it and how we can use those technologies.

Once we can get a process into the world of software and it can be networked and connected together, it lends itself to, “Okay, where is this software?

Where does it live? Does it live in a purpose-built box or does it live in a server amongst other instances of the software?” And once it’s into that “virtual environment,” the location of it becomes optional, depending on available networking.

There’s this building block approach.

Things like dependencies, networking and business criticality, how much do you really want to go and touch that box that it lives in, all become considerations.

We’re doing a tremendous amount of it on the business side but less on the broadcast side at the moment, mainly because of the networking and perceived reliability requirements.

Shane Toven: We’ve been living in a software-based world now for quite a number of years. Every box that you have, at some level or another, likely has

some software driving it in the back end.

So it really doesn’t matter whether that software lives in a dedicated purpose-built box or elsewhere in your environment.

In our particular case, again, we have a lot of virtualization going on in the business side of the house. We’re just starting to dip our toes into it in the broadcasting side of the house. I’ve been doing some projects to investigate that, and look at what it would take to basically create a virtual air chain, so to speak.

In our case, a lot of the challenge is around connectivity. We have a number of remote mountaintop sites with limited bandwidth to them; so I think the real limiting factor would be being able to get reliable connectivity to those sites.

But, if we could get a couple of forms of connectivity, say something like the combination of a Starlink [microwave link] and LTE [4G wireless data] to the site, that would be a very robust solution for connecting a lot of these sites.

Philipp Schmid: At Nautel, whether it’s virtual or whether it’s a software-only system running on physical hardware, it doesn’t make that much of a difference.

Our goal is to come up with a transmitter design where essentially you just have an Ethernet jack and it takes care of everything. Really simplify the transmitter site. That’s where we want to get to.

We’re definitely not there today, in today’s architecture. Our hope is that this virtualization approach will reduce complexity. And perhaps, it may not necessarily eliminate all the complexity, but by moving it into a virtual site, Nautel can come along and help with customized solutions.

Now that it’s no longer at a remote transmitter site, if it’s not a purpose-built box with a very rigid set of firmware that has a very specific purpose, and it’s a more flexible virtual system, Nautel can come along and help the broadcaster tailor the solution to best integrate with the rest of the broadcast plant.

I’m not going to lie; particularly with HD Radio, there will still be a lot of complexity. But maybe we can shoulder some of that.

In particular, for broadcasters that don’t necessarily have all of the technical know-how to deal with the finer details of HD, perhaps we can provide the technical support in a virtual environment. So, I believe virtualization will lead more into customized solutions.

We’re moving away from products that have very rigid specifications to a more

Getty Images/BlackJack3D

We asked five experts to discuss what the word virtualization means for them

agile type of environment where a set of standard building blocks can be used.

The way they’re put together as a system will be customized.

HD is here to stay, but it has a lot of complexity, and we’ve noticed that even though there is a significant build-up in side channels, the overall station conversion has somewhat plateaued over the last five to eight years. Everybody is planning for HD, but there are a lot of broadcasters that have not pulled the trigger because there is significant complexity, cost and a requirement for technical specialists.

We’re hoping to make it easier for people to get into HD. Greg Shay: Of course, at Telos Alliance we make studio broadcast equipment as well as interconnectivity over long distances.

Virtualization, to us, is really about intentionally taking advantage of the latest technologies from the IT world. Full stop. We believe that by intentionally following the best practices of the IT industry, you end up in a place where you actually have the best possible performance.

In the same way that 20 years ago when we first figured out how to do audio over IP, it was an intentional choice to leverage standard networking equipment, here we are again: an intentional choice to take advantage of the best server technology, which is virtualization.

tions into either a software environment, or single-purpose hardware and facility functions at the transmitter site. That’s our focus on it.

There are big challenges with virtualizing something as critical as broadcast.

Putting parts of your air chain right now into the cloud, you are going to need that “last mile” connectivity to the site, and at this point, there’s just not a universal solution for getting stations connected at the level of reliability to run air chains completely virtually.

We’re able to do parts of it now. As virtualization increases in other businesses, the infrastructure becomes more available.

As broadcast companies — manufacturers, software providers and broadcasting companies — come together and work through some of those things, I think within a matter of a few years we’ll see increased options for connectivity. We have been seeing the overall rollout of 4G LTE, and now soon we’ll be on 5G wireless connectivity. With new and different satellite networks that will have much more robust connectivity in the next few years, we can start planning now to harness that.

This isn’t something that everyone is jumping into today, but it’s a topic the industry is starting to delve into. It’s good to see companies look at the challenges of how do we make something live and robust for broadcast stations and systems.

Alan Jurison: [Virtualization] is really consolidating all the broadcast func-

Read “Virtualizing the Air Chain” at radioworld.com/ebooks.

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WHITE PAPER

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February 10, 2021

Using the SNMP Protocol in Broadcast Monitoring IT industry software tool unlocks equipment diagnostics

Simple Network Management Protocol.

The SNMP standard is a set of rules that defines certain data points, or objects, in a piece of equipment that is connected to an IP network.

The protocol establishes a standard syntax that allows users to query that equipment via the network and receive the real-time values of those data objects.

And in certain cases, the user can even change those values using SNMP, which can allow them to control the functions or settings of that equipment.

This article will attempt to explain at least the basics of the SNMP structure and capabilities, and how broadcast technical personnel could benefit by integrating the SNMP protocol into their overall facility control and management systems.

BY TONY PETERLE

The author is manager of Worldcast Systems Inc. in Miami.

Without question, the introduction of IP network systems into broadcast operations has had a generational impact on our industry.

Audio distribution (both in-house and across the Internet), IBOC/DTV broadcasts and Internet streaming are some of the ways IP networks have become essential in broadcast operations, and ATSC 3.0 is taking networked data on to the actual broadcast signal.

Along with new ways to reach our customers, IP networks offer benefits for engineers, like remote equipment management; reduced time and complexity of studio builds using AoIP; and TeamViewer connections with manufacturer support personnel.

And along with the more familiar interfaces with networked equipment like embedded websites or Telnet links, many networked devices support the

Fig. 1: SNMP Manager and Agent Devices

HISTORY AND DEFINITIONS The Simple Network Management Protocol was developed during the late 1980s as a way to facilitate communications with and control of distant network devices as the internet was burgeoning.

Management Station Management Applications

MIB SNMP Manager

SNMP

IP Network

SNMP Agent

SNMP Agent

SNMP Agent

SNMP Agent

SNMP Agent

SNMP Agent

MIB

MIB

MIB

At the time, there were many protocols used to communicate with and control the routers and other network infrastructure. Around 1987, the protocol we know as SNMP was chosen by the market as a unified and efficient way to communicate with the hardware.

The earliest version of SNMP was called V1.

Currently, the most commonly used versions are V2c, which added some new commands, and V3, which applies a somewhat more secure process to send the SNMP data back and forth across the network.

SNMP DEVICES To clear a semantic point, the terms “Manager” and “Agent” refer specifically to the software modules running in each device that enable and support the transfer of data using SNMP. But for the purposes of this paper, the terms are applied equally to the hardware and software on each end of an SNMP message.

Also keep in mind that the device running the Manager application can also itself be an Agent, managed either by its own internal NMS software or perhaps by another Manager system elsewhere on the network. A Manager will normally send SNMP requests to the Agent on IP port 161.

OBJECT IDENTIFIERS (OIDS) SNMP is simply a way to retrieve and set data points in a remote system. These data points, called Objects, are defined by the maker of that system, usually at the time of manufacture. The type of data presented by an SNMP object is very flexible — it could be an integer, a floating-point number, even a text/numeric string.

Each Object has a unique identifying number, which is used by the Manager and Agent systems to locate and deliver the requested data to the user. These numbers are called Object Identifiers, or OIDs. A Manager will use the OID to request data from an Agent, and the Agent, in its response to the request, will include that same number along with its current value.

The OID is also used when the Manager device sends a SET command to change the value of a data point in the Agent, and also on those rare occasions when the Agent device is the one to initiate communications.

Object Identifiers are written with a specific (and rather old) syntax called Abstract Syntax Notation One, or ASN-1. A typical OID might look like this: 1.3.6.1.4.1.186.1.19.2.1.2.4.0 MANAGEMENT INFORMATION BASES (MIBS) When a device with an SNMP Agent is manufactured, the designers will usually organize all of the OIDs for that device into a structured text file, called the Management Information Base, or MIB (see Fig. 2).

The MIB can be viewed as a “menu” used

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in the NMS software to identify the OIDs associated with the desired data points in that Agent device. There are many software applications, called MIB Browsers, that will allow a user to examine these files, and most browsers display the MIB as a directory-tree type structure.

There are some exceptions. As we will see, some OIDs cannot be completely defined at the time of manufacture, and some makers of Agent equipment publish only “truncated” MIBs that refer to industry standard files (other MIBs or RFC files) to define their own structure and OIDs. More in a moment.

SNMP COMMANDS The commands and messages that pass between the Manager and Agent devices can be divided into three categories: GET commands, SET commands and TRAP messages.

SET commands are perhaps the simplest. They are messages sent from the Manager device to the Agent, containing the OID for a particular data point, and the desired value to be set for that point.

This is the way a Manager can command the remote device to enable or disable functions or change a value, such as the frequency setting of a transmitter.

No matter if the value of that particular object is an integer, a floating-point number, even a text/numeric string; the SET command is used in the same way each time.

The TRAP messages also are simple. They are unique in that they are generated autonomously by the Agent and sent to the Manager. As you might guess from this characteristic, they generally are used to alert the Manager (and its users) to abnormal conditions in the Agent device that could negatively impact the performance or stability of the system.

Put simply, they are alarms. High CPU temperature, RF output failure, a hard drive approaching full status — these are examples of some of the conditions for which SNMP Agents might send a TRAP message to a Manager.

Fig. 2: SNMP MIB Structure

is important to know that this acknowledgement handshake must also be supported by the NMS software.

The standard port on which a Manager will “listen” for traps is IP port 162.

The GET command is certainly the most-used in SNMP systems. A Manager will send a GET command containing a specific OID to an Agent, and the Agent will “look up” the current value for that data point. It will then send that value in a message called a

A Manager will use the OID to request data from an Agent, and the Agent, in its response to the request, will include that same number along with its current value.

Since SNMP was designed in the early days of connectivity, minimizing bandwidth usage was a high priority, which is why there is no default automatic acknowledgement for SNMP messages.

However, the critical nature of TRAP messages has led many designers of SNMP Agent software to include optional settings that will cause the Agent to resend the TRAP message until the Manager replies that the message has been received and understood. It

SNMP Version SNMP V1 SNMP V2e SNMP V3 Table 1

GETRESPONSE that also repeats the OID. All versions of SNMP support these message types, as well as the GETNEXT command. GETNEXT is a command that, when repeated, will cause the Agent to methodically research and deliver data on each and every object that has been defined within its configuration (see Table 1).

In some Agent systems, data is defined in tables — for example, the connection status and speed of

each port on a router. For these types of systems, the GETNEXT command is an easy way to retrieve a large number of data points without having to write multiple GET commands, each with the specific OID of a single object in the Agent. When the Agent returns a response to the Manager, the Manager can issue another GETNEXT, which causes the Agent to look up and deliver data on the next OID in its MIB. An additional GET command, GETBULK, was introduced in SNMP V2c and is also supported by version 3. The GETBULK command allows an NMS to request from the agent a large portion of a data table with one command. Again, this is done to minimize the need to send GET commands for each data point and so minimize the demands on programmers and network bandwidth.

The GETNEXT and GETBULK commands are used primarily for SNMP analysis and design by human operators, they have little to no use in a 24/7 monitoring system.

For each GET command received from a Manager system, the Agent device will return a GETRESPONSE. Or in cases where the structure of the GET command is improper, the Agent might return an error message.

There are several other types of SNMP messages that were defined in the V2c and V3 standards, including INFORM, NOTIFICATION and REPORT. INFORM is the mechanism by which an NMS can acknowledge SNMP Traps sent by an Agent. A NOTIFICATION is a variation on the SNMP TRAP message, created to share the same data structure as the GET and SET commands (the original TRAP messages as defined in V1 had a different structure). And the REPORT message is designed to allow different NMS systems on a network to communicate easily with one another about the status of the devices and network segments each is monitoring. INFORM V2c messages are the most common alarm messages used today, and although they don’t have the same data structure of the original V1 alarm messages, they are still commonly called TRAPS. SNMP TRANSPORT AND PACKET STRUCTURE (OR “DOWN IN THE WEEDS”) SNMP communications can be carried out via a number of different transport protocols, but typically employ the User Datagram Protocol, or UDP, a simplex protocol with no built-in confirmation of messages traveling in either direction.

UDP is a Transport protocol, operating in Layer 4 of the OSI model. SNMP itself operates in the Application layer 7. UDP allows systems to communicate with each other without having to first establish communications ports and protocols.

Since an SNMP message (particularly the GETRESPONSE) can contain different amounts of data, they cannot be a fixed length. Instead, the data are defined as variable bindings.

An SNMP message packet follows the basic encoding rules, and the data contained in the variable bindings can be divided into three parts: Type, Length and Data.

Commands supported GET, GETNEXT, SET, GETRESPONSE, TRAP GET, GETNEXT, GETBULK, GETRESPONSE, TRAP, INFORM, NOTIFICATION, REPORT All of the above commands, adds encryption and password protection

(continued on page 20)

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SNMP Message Type = Sequence, Length = 44 SNMP Version

Type = Integer Length = 1 Value = 0

30 2C

02 01 00

SNMP Community String Type = Octet String Length = 7 Value = private

04 07 70 72 69 76 61 74 65 A0 1E

Request ID

Type = Integer Length = 1 Value = 1

Error

Error Index

SNMP PDU Type = GetRequest, Length = 30 Varbind List Type = Sequence, Length = 19 Varbind Type = Sequence, Length = 17

Type = Integer Type = Integer Length = 1 Length = 1 Value = 0 Value = 0

02 01 01 02 01 00 02 01 00 30 13

Object Identifier

30 11

Value

Type = Object Identifier Length = 13 Value = 1.3.6.1.4.1.2680.1.2.7.3.2.0

Type = Null Length = 0

06 0D 2B 06 01 04 01 94 78 01 02 07 03 02 00 05 00

Fig. 3: An illustration of SNMP Packet Structure

SNMP PROTOCOL (continued from page 19)

Type is a single byte that specifies the type of data being transported — Octet, Integer, String, etc. Length delineates the size or amount of data, in bytes; and Data is of course the data itself. It is noteworthy that the data can span multiple bytes (see Fig. 3). In such cases, the highest order bit is used as a flag to indicate that the data is being spread into two or more bytes, and only the lower 7 bits are actual data.

A message in SNMP will contain three elements — the SNMP version being used (V1, V2c or V3), the Community string, (or username and password for V3 messages) and finally the SNMP function itself — be it a SET request, a GET Response, TRAP, etc. This central part of the message is called the Protocol Data Unit, or PDU. The Community string is like a password that must be used to retrieve or set data points using SNMP (from V2c on).

This is absolutely no guarantee of security, as the strings are sent using open text on the network, so it would be comically easy for someone to intercept and read that information. SNMP version 3 offers some more robust security, with data encryption and stronger passwords.

PRACTICAL ASPECTS OF SNMP MANAGEMENT Understanding an OID — When we look at an OID, we’re really seeing an address — a way to navigate through a MIB tree until we arrive at the specific data point in which we are interested. But the OID starts from a much broader viewpoint. To understand this perspective, we should examine the meaning of each of the numbers in a typical OID, for example:

TRAP messages are generated autonomously by the Agent and sent to the Manager, generally to alert the Manager (and its users) to abnormal conditions in the Agent device that could affect the system.

Toshiba UPS power system (see Fig. 4).

After the manufacturer number, the remaining numbers delineate a path down the manufacturer’s MIB tree for that device. Each branch of that tree has a number assigned to it and by following the numbers down the MIB we come to the object specified by this OID. After the manufacturer number 186, the 1 says it belongs to a piece of Toshiba equipment, the 19 indicates a Toshiba UPS system, the 2 specifies the Toshiba Industrial line of said systems, and the 1 identifies the particular software. This same software (and thus the same MIB) could be in use on any number of different models.

The SNMP name of the data object is upsEstimatedChargeRemaining, and from the description in the MIB browser we can see that it is an Integer type data point, from 0 to 100. In raw text form, the OID definition in the MIB would look like this: upsEstimatedChargeRemaining OBJECT-TYPE SYNTAX INTEGER(0..100) UNITS “percentage”

Note that the full OID is not present in the MIB file, only upsBattery 4. This is an example of a “Truncated” MIB. Truncating is used by manufacturers to avoid having to write complete OID numbers for each data point in the equipment with the associated costs in man hours and possible errors. So the OID is defined by naming the “branch” of the MIB that contains this data point, plus an integer to define the specific data point in that branch.

Imports — By examining the beginning of the Toshiba MIB, we can see that a number of “imports” are described: DisplayString, TimeStamp, TestAndIncr, AutonomousType FROM SNMPv2-TC enterprises FROM RFC1155-SMI OBJECT-TYPE FROM RFC-1212 TRAP-TYPE FROM RFC-1215;

1.3.6.1.4.1.186.1.19.2.1.2.4.0 The first six numbers in the OID will be the same for most OIDs in nearly every commonly encountered object. They are intended to allow traceability of the OID, as follows: 1 – Identifies the International Standards Organization, or ISO 3 – Specifies that this OID belongs to an ISO recognized organization 6 – Indicates the Department of Defense, creator of the Internet 1 – Confirms that this is an Internet OID 4 – Shows that this OID belongs to a private organization 1 – Tells us that this organization is a business enterprise The next number identifies the manufacturer of the equipment to which this OID refers. In this instance, the 186 refers to Toshiba; this OID comes from a

ACCESS read-only STATUS mandatory DESCRIPTION “An estimate of the charge to battery charge depletion under the present load conditions if the utility power is off and remains off, or if it were to be lost and remain off” ::= { upsBattery 4 }

Fig. 4: MIB Tree for Toshiba UPS

TimeInterval,

SNMPv2-TC, RFC1155-SMI, RFC-1212 and RFC-1215 are “standard” of “community” MIBs published by the IETP and ISO that organize basic structural elements and methods such as data types, table structures, definitions, etc. By incorporating these standard files into their own MIB structures, manufacturers help ensure compliance with the standards while also saving themselves from having to write the full syntax of each OID in the MIB files for their equipment. As long as your browser or NMS software can import and integrate these standard MIBs, it should remain relatively easy to locate and use the OIDs you need.

An example of an OID from a different MIB (this one from an APT Horizon audio codec), looks like this:

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+ Index value can identify a specific object in the Agent, even one that was created only moments ago.

As an example, here is the “base” definition of a user created button in Worldcast Systems’ Scripteasy software:

(continued from page 20)

--1.3.6.1.4.1.22425.2.1.4 alarmTrap NOTIFICATION-TYPE OBJECTS { sequenceNo, name, status } STATUS current DESCRIPTION “This trap is sent when an alarm is either set or cleared on the unit. The variable bindings which are sent are:Sequence No Alarm Name Status of alarm. “ ::= { horizonalarm 4 }

1.3.6.1.4.1.5299.15.42.1.10.3.4.1.3 : scripteasyActionsState scripteasyActionsState OBJECT-TYPE SYNTAX INTEGER ACCESS read-write STATUS mandatory DESCRIPTION “No description” ::= { scripteasyActionsEntry 3 }

Note that while this MIB uses a truncated OID, the designers have included the full OID as a comment line before the object description, preceded by the double dash --. This convention helps make the MIB more legible to the human readers.

Regardless of structure, the MIB file serves only to indicate the correct OIDs for each data point. An address book, or map, if you like. If there is another way to obtain accurate OIDs, the MIB file itself may be unnecessary.

Special Cases — In many instances, the OIDs for a particular system can be defined and the MIB file published at the time of manufacture.

There are notable exceptions to this rule, such as systems where data objects can be created by the user after the equipment is delivered. One example group are facility control systems, where inputs, controls and analog information can all be integrated, polled and controlled by remote equipment using SNMP. Since the inputs and controls are sometimes software based, they can be created and altered as needed by the user, so their OIDs cannot be completely defined when the equipment is built.

For these types of data objects, two things are defined, the first being a “base” OID, which identifies a class of object (say a reading from an analog voltage input) in the software. Second, a table is built into that definition in the MIB that will allow something called an Index value to be appended to the base OID. Together, the base

February 10, 2021

INTEGRATING SNMP MANAGEMENT WITH OTHER INPUTS When we begin to formulate a plan to incorporate SNMP monitoring and control within a broadcast facility, we must remember that this protocol comes from the world of computers and IT, and thus we must be able to correctly interpret the information before we can use it.

In most cases, the object description in the MIB will give some indication. In other instances, a user might have to perform multiple GET commands and look at the raw values in order to understand the results. And depending on the data returned, it is sometimes necessary to perform some mathematical operation in order

We must remember that this protocol comes from the world of computers and IT, and thus we must be able to correctly interpret the information before we can use it.

Just above that definition of the base OID, the MIB file defines the Index table: 1.3.6.1.4.1.5299.15.42.1.10.3.4.1.1 : scripteasyActionsidScriptObject scripteasyActionsidScriptObject OBJECT-TYPE SYNTAX INTEGER (0..2147483647) ACCESS read-only STATUS mandatory DESCRIPTION “Scripteasy action button identifier” ::= { scripteasyActionsEntry 1 } As each button is created in the software, a specific Index number is assigned to it and made visible to the user. So while the base OID for ‘a’ button in the software would look like this: 1.3.6.1.4.1.5299.15.42.1.10.3.4.1.3 The OID for a specific button with an index value of 361 would append that index value at the end: 1.3.6.1.4.1.5299.15.42.1.10.3.4.1.3.361

The FCC has given the green light for U.S. AM radio stations to turn off their analog and switch on all-digital HD Radio if they wish.

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to provide data that can be easily interpreted by the users of the system.

There are also hurdles on the path to integrating SNMP monitoring alongside more traditional broadcast facility control connections (GPIO, etc.). NMS systems do not normally have any capabilities for connecting with physical analog and status inputs, nor do they generally have any kind of relay outputs or control functions outside the SET command. For broadcast operations, users expect these functions plus robust monitoring and alerting, and even the capability to preform automatic actions based on the information received. Fortunately, there are several solutions on the market that can “bridge” the functions of an SNMP Manager with the more traditional capabilities of a facility control system so the users can monitor and control the entire length and breadth of their operations.

SUMMARY AND CONCLUSION Just as IBOC, ATSC, AES and basic IP structure and languages were once little understood by broadcast engineers, there is growing recognition of SNMP as a useful tool. More and more manufacturers of transmitters and other broadcast equipment are including support for SNMP operations within their systems, and that is a trend that is not likely to change soon.

It is a powerful option, since it expands the user’s capabilities in two important ways. First, SNMP provides the ability to control and monitor more different types of equipment. Since SNMP is a standardized system, not only broadcast gear, but IT devices such as routers, servers, printers, etc. can be incorporated into the overall facility control plan. And second, the concept that the equipment being monitored can be remotely located from the facility control device, whatever it may be. This allows great possibilities for inter-site communications and being able to monitor a large network of sites and equipment that may be spread over great distances. With SNMP, anything you can reach over a network connection can be controlled.

With a proven record of easing engineering workloads and reducing downtime for broadcasters, anyone with responsibilities that involve IP networks should consider the Simple Network Management Protocol as a part of their facility monitoring and control toolkit.

23

READER’S FORUM

RADIOWORLD ENGINEERING EXTRA

A LIFT FROM A DIP Just a comment on Buc Fitch’s great design for the unbalanced-to-balanced adaptor in the Oct. 21, 2020 issue: Since there are a couple “extra” positions on the DIP switch, I had one modification that can help for those cases where ground loops might exist.

Using the spare two positions on the DIP switch, wire them in series between the XLR Pin 1 and ground. When PROJECT JOURNAL the switch is closed it permits the XLR to be Build an Unbalanced-to-Bala nced Adaptor Buc Fitch provides an easy solutio n grounded. But when to an everyday engineering problem it’s opened it makes the output configured as “ground lift.” In some cases, this can reduce ground loops, especially when using the adapter in less-thanoptimum environments!

Of course, you should use the shortest possible wire length for the connection, as the DIP switch itself This project is will introduce a slight divided into two parts, one for the amount of resistance signal flow area and the other for into the ground circuit. power supply, 20

RADIOWORLD ENGINEERING

BY CHARLES S. FITCH, P.E.

Our recent contribution to the Workbench column in Radio World about our home-brew constructi on an XLR-3 cable tester (https://tin of yurl. com/rw-xlr) produced a notable, positive reader reaction.

It seems the “maker instinct” exists in almost all of us in this profession .

Although I’d promised in our next article to jump cold into a complex follow-up project of a step oscillator, our distinguished editors thought something as useful as the cable tester but slightly more complicated might be a better next step … sort of an organic progressio n.

Advancing somewhat by inculcatin g some simple active circuitry would also be apropos. Staying in the universe of audio, our project in these pages is elegantly straightforward unbalancean dto-balanced audio converter.

As is often said, the beauty and joy of technical standards is that there are so many to choose from!

This dilemma holds true for the world of audio as well. We often need to make a marriage in our stations between the near-universal broadcast impedances and signal levels with the ubiquitous

Ed Dulaney CPBE, AMD, CBNT Regional engineer Wichita Falls, Texas

EXTRA

October 21, 2020

“consumer” output levels.

Audio from handheld digital recorders, air tuners, computer sound cards needs to be taken into the broadcast etc. plant.

The situation is complicated by differences in impedances between devices, to mention the unique characteris not tics of odd sources such as “pro” audio signals with their slightly higher levels.

ON WITH THE SHOW We’ve attempted to accomplis h several goals here. In our example unit, we’ll use a similar enclosure to the one from our cable tester article so that, at least on our workbench, our new tools Breadboarding the circuit. will have that matching look.

We’ll provide some flexibility in uses and setup. We’ll keep complexaudio from something like a ity and fabrication to a CD player minimum is expected to peak at 0.447 and consequently, minimize volts with the cost, an RMS (Root Mean Square) especially as we may be making value of many 0.316 volts, which is well of these. within the handling capability of an We’ll provide enough design operationa l descripamplifier (op-amp). tion and suggestions for changes and If our audio is headed to customization so you can a typimake the cal 600-ohm standard broadcast project truly your own and as mixer you need input of +4 dBm, the level at the device to work in your plant that point will need to be a peak of 1.736 Let’s get oriented. Take a look volts or at the 1.228 volts RMS. As such, schematic. the RMS voltage gain will need to be Addressing just levels, consumer about 3.9, or 11.8 dBV.

connectors and enclosure.

CIRCUIT DETAILS Like most electronic circuits, one can generally divide the beast into main activity and support sections.

The main in this case is the signal flow area; the support is essentially the power supply, connectors and enclosure.

Start with main. In our circuit, the unbalanced signal enters the converter and encounters a 10k ohm shunt resistor to ground, which is included as industry-suggested load impedanc the e that provides the flattest signal response transfer. The 220 uF series capacitor isolates the audio, stabilizing the gain of Fig. 1: Project Schematic the first active stage to follow.

The op-amp sections in this device are configured in the inverting format, providing great flexibility and stability.

Many readers will remember that most of my previous audio processing work in these pages used the classic 1458 op-amp, as I had been given and used over 100 of them. Now that been given an even larger number I’ve of the similar but higher performin g 5532 opamps, we’ll start using this new group to a similar depletion.

The 5532 is a self-comp ensated, dual-section unit. The first op-amp section provides two functions for us. The first is gain, and with that in mind, required gain might be that voltage our gain of 4 noted above to achieve line level output, but could also be a negative gain to help us get down to mic level.

AUDIO “ENHANCEMENT”

10

AUDIO ESSENTIALS

ERING EXTRA

RADIOWORLD ENGINE

Tips and truisms I greatly appreciated the thoughtful article by Robert Orban and Greg Ogonowski [“Maintaining Quality in Digital Audio Chains, RWEE Dec. 16, 2020], and couldn’t agree more with the last paragraph: to sit a PD down in front of a high-quality monitor system to hear the damage that an audio “enhancement” system may cause to a station’s air sound. “Less is more” is often the best route to a compelling, attractive signal.

One thing that should be addressed is cascading and transcoding of lossy coders in the air and stream audio chains.

Such coders are part of remote links, transmitter links and of course HD Radio and online streams at stations. It only takes one extra step of lossy coding to drop the quality significantly to listeners. There are few tools to identify this effect, short of our ears and a good monitor system. I hope someone will address this in a future article!

AND

BY ROBERT ORBAN GREG OGONOWSKI

) 810 This is at KTBI(AM one in Ephrata, Wash., of three AM stations

n excerpted from owned by America The following is .

Quality in the Christian Network “Maintaining Audio Facility.” In Running 50 kW dayBroadcast and Netcast deal with the time, it covers Spokane this excerpt, the authors often misunderover 100 miles away. many-faceted and in digital audio stood subject of quality chains. g devices, In digital signal processin per word of bits the lowest number nal qualprofessio achieve necessar y to are several reasons ity is 24 bits. There for this. need ions workstat Digital audio odate gain headroom to accomm of several adjustments and mixing are a numthere sources. Moreover, operations (like DSP common of ber e filtering) infinite-impulse -respons the digital that substantially increase bits allows enough noise floor, and 24 odate this without headroom to accomm (This assumes audibly losing quality. ated enough sophistic is that the designer s to control to use appropriate measure rly difficult filters noise when particula point arithmetic is are used.) If floatingle word length acceptab lowest the used, is 32 bits (24-bit for professional quality t; sometimes mantissa and 8-bit exponen ). called “single-precision” systems, 20-bit In digital distribution range) are usuwords (120 dB dynamic t the signal represen to ally adequate bits can retain the accurately. Twenty source even after full quality of a 16-bit on by a mixer. as much as 24 dB attenuati very elevated levels. converters that been at least Moreover, there has There are almost no A/D ng 48 kHz 20 bits of real can achieve more than one rigorous test compari This test rates.

“24-bit” converters accuracy, and many and 96 kHz sample below the ably is no audible difconsider have accuracy concluded that there rates ing bits” in A/D between these two sample 20-bit level. “Market ference to sing filter is usly abused converters are outrageo if these A/D if the 48 kHz rate’s anti-alia and, s, ately. deceive customer designed appropri r products, a controversial However, in 2016, converters were consumebe actionable tests comthese bogus claims would “meta-analysis” of existing CD-quality sion. and Commis by the Federal Trade paring high-resolution in the AES Journal. audio was published “Eighteen VERSY According to the author, which sufSAMPLE RATE CONTRO disagreement nts for There is considerable (if any) of published experime be obtained were about the audible benefits 44.1 kHz. ficient data could lysis that above , providing a meta-ana raising the sample rate included using nts in more lind test An extensive double-b combined over 400 participa inserting a that showed trials.

554 trials than 12,500 but statistiinto the output “Results showed a small CD-quality A/D/A loop of test subjects (SACD) player was of a high-resolution cally significant ability listening content, n o-loud resolutio undetectable at normal-t to discriminate high d dramatically subjects, on any of levels by any of the and this effect increase The noise of the received extensive four playback systems. when test subjects audible only at CD-quality loop was

was verified by a training. This result exploring different sensitivity analysis studies and difchoices for the chosen es. ferent analysis approach studies, effect of “Potential biases in ntal design, experime logy, test methodo were also investiand choice of stimuli n is that the gated. The overall conclusio recording audio an of perceived fidelity can be affected by and playback chain onal resoluoperating beyond conventi tion.” , it can Assuming perfect hardware down comes be shown that this debate a given antiy of entirely to the audibilit as is discussed aliasing filter design, below. on of the 2016 Far before the publicati marketing-driven meta-analysis, in a attempted to push the record industry r standard from change the consume sampling fre44.1 kHz to a higher and SACD, neither quency via DVD-A

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the mass marketof which succeeded in trying again with place. The industry is remains to be seen Blu-ray audio, and it ul than they if they will be more successf SACD. were with DVD-A or FM STEREO scientifically Regardless of whether ly proves that accurate testing eventual l, sampling rates this is audibly beneficia have no benefit in higher than 44.1 kHz the effective samFM stereo because 38 kHz, so is pling rate of FM stereo be lowpassly the signal must eventual or less to prevent filtered to 17 kHz l in DAB, which aliasing. It is beneficia audio bandwidth, kHz 20 has typically at all in AM, whose but offers no benefit than 10 kHz in bandwidth is no greater 4.5 kHz. any country and is often have built-in rs Some A/D converte to act when the soft clippers that start

14) (continued on page

John Kean Cavell Mertz & Associates Inc. Manassas, Va.

(continued on page 22)

ñol Espa a l b a Se H

December 16, 2020

s in Digital Audio Chain ing Qualiisty Maintainabou misunderstood t a subject that often

TO INVERT OR NOT TO INVERT A fundamental facet of a non-invert ing op-amp (the alternative arrangeme of the inverting circuit configurat nt ion used herein) is that one cannot achieve

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February 10, 2021

February 10, 2021

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