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No PiCture

PerFeCt PiCture




Q. I am receiving signals from Ridge hill transmitter but can only receive 4 of the digital mux. Analogue A. Replace the Group A aerial with a Wideband aerial to receive Digital all channels.


Optimum gOOd medium

Security margin

SuFFic. marginal bad

fig 2

~1dB 6db


such as Ghosting and Snow are very effectively removed by the COFDM and FEC system, meaning the viewer should see an excellent picture even when moderate levels of noise and interference are present.

The Digital Cliff Once the signal strength falls too far or if the local noise and interference levels become too great, then too many bit errors will cause the system to become unstable and the picture could suffer from blocking, freezing or complete loss. The threshold for this happening is fairly sharp and is known as the Digital Cliff – see fig 2. As the signal strength becomes weaker with an analogue signal,



rF signal quality

the picture quality gradually becomes worse (grainy and snowy). As the signal strength becomes weaker with a DTT signal, the picture quality will remain good until the signal strength drops below the digital cliff, at which point the picture quality will be very poor or disappear – see fig 3. COFDM/FEC is therefore an excellent combination but not invincible. A signal level at the DTT receiver is recommended to be 45 to 65dBµV. When looking at digital signals a number of measurements need to be taken and collectively considered in order to determine the quality of the signal, and these will be looked at next time.

Decoding Margin

picture quality

fig 3


rd rd

Matthew Boulton

Digital Signal

SNr picture quality


technicalsolutions Ask

Signal Strength


Q. I am receiving signals from the Storeton relay with a C/N of 4dB on Channel 30. A. Moel-y-Parc is in almost the same direction and transmitting on CH30, so look at aerial alignment and check the exact heading of both transmitters from your location and see if Moel-y-Parc can be placed in a null on the aerial. A good site to use is www. Q. Will the old Group B aerials work for Sutton Coldfield after switchover? A. No, I am told Sutton is to have two extra multiplexes after switchover, the lowest being Channel 29. Q. I have lost one of the digital multiplexes completely at the outlet plate. A. In this particular case, the first thing to do was measure the signal at the aerial, where all multiplexes were present. The outlet socket was then removed from the wall where it was found that two cables had been connected to the back, as such acting as an open circuit stub. Removing the second cable cleared the fault.

Matthew Boulton College of Further & Higher Education

spring09 49

Rigger's Digest Spring 2009  
Rigger's Digest Spring 2009  

Hello, and welcome to Rigger’s Digest. Published quarterly in the UK, Rigger’s Digest is a magazine crammed full of Trade News for the Insta...