TECHNOLOGY: HOW TO
www.audiomediainternational.com
INCREASING WIRELESS SIGNAL-TO-NOISE RATIO
RF Venue’s Alex Milne provides some of his best advice on improving SNR using ‘physical layer tools.’
W
hen it comes to wireless audio systems, there is never a situation where a lower signal-to-noise ratio provides better performance over a higher signal-to-noise ratio, all other factors being equal. Never. So, you should use every opportunity and every tool at your disposal to improve that ratio. Signal-to-noise ratio (SNR) is the ratio of the amplitude of the signal of interest to the amplitude of the surrounding noise floor or competing signals. SNR measurements are used across many systems and components within an audio signal chain, like microphones and amplifiers, but here we’re talking about the signal-to-noise ratio of the amplitude of a radio signal (coming from a wireless microphone or IEM transmitter) in comparison to the amplitude of surrounding radio noise floor when measured at the receiver front-end (antenna input). Because the majority of the ‘noise’ part of wireless signal-to-noise ratios usually exists outside of the electrical audio and RF system, manipulating SNR is something that must – and in fact can only – be done by manipulating what network engineers refer to as the ‘physical layer’. The physical layer in wireless audio includes the hardware components that move electrons and electromagnetic waves that make up audio and RF signal, as well as the processing of digital bits. There are very few electrical components, software processing, or DSP chips that can improve SNR downstream in the signal chain from the antenna input (inside the device), but many simple and accessible techniques available using physical layer tools – like transmitting and receiving antennas, coaxial cables, and filters – that profoundly improve SNR upstream from the antenna input (outside the device). 26
The simplest, and perhaps most powerful, method is to shorten distances between transmitters and receivers. By closing the distance gap between receiver and transmitter, signal strength increases dramatically because of the relationship of distance to received power and the inverse square law. For example, if you double the distance, the strength of your received signal will be four times less powerful. This works the other way around, too: halve distance, and received power increases four times over. Of course, moving an entire rack of equipment closer to a performance area is not always practical, but for some one- or two-channel systems, it certainly can be.
Tricks of the Trade Another extremely effective way to improve SNR is to use a high gain directional antenna. When used as a receiving antenna, a high-gain antenna can increase the received strength of a signal by focusing the RF energy in a given area, thereby increasing the apparent strength of your signal in relation to noise and other unwanted signals in the area from the point of view of the receiver. When used as a transmitting antenna (commonly for
IEMs), high gain antennas can project a narrower and more intense beam of RF energy, to similar effect. When external directional antennas are used, it’s very important to deploy them correctly. Using an external antenna lofted on a lightstand above the audience is a common method to obtain proper lineof-sight, but even better SNR is possible by remotely deploying antennas closer to the stage, backstage, or stage left/right using long runs of low-loss coaxial or fibre-optic cable. Long runs of coaxial cable should be treated with similar care. Use only high-quality, low-loss, undamaged coaxial cable for placing remote antennas, such as RG8X or better. Coaxial cable is fragile. Small defects can cause a dramatic reduction in signal quality. So it should always be inspected for damage before use. And cable runs over 100ft/30m often require in-line amplification to maintain sufficient signal strength lost due to in-line attenuation, which occurs in all coaxial cable to some degree. An additional technique for improving SNR is to increase transmitter power – though this should be used with caution, as unscrupulous indulgence in increased power to multiple transmitters may cause
more noise than the noise you are trying to avoid. In the US certain end-users are eligible for a licence that allows them to use transmitters as powerful as 250mW. In the UK, power exemptions are given to operators of PMSE equipment under the control of Ofcom. Finally, it’s also possible to block or reduce noise and competing signals coming from outside the performance venue by shielding the interior of the space with electromagnetic barriers. Broadcast studios and some secure facilities will hire consultants to do this for them at significant cost, but most of you are doing this already to a lesser degree: all buildings attenuate (weaken) signals and noise from outside. By operating a wireless microphone inside a physical structure (especially metal ones) you are keeping a significant amount of the ambient radio noise from intruding into your venue, which lowers the noise floor and improves signal-to-noise ratio at the receiver. Alex Milne is marketing manager at Bostonbased RF Venue, a manufacturer of wireless audio antennas and hardware. www.rfvenue.com
November 2015
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