Digital TV can sometimes throw up nasty little image break-ups. Barry Street explains.
The advent of digital TV has eliminated snow, grain, sparklies and ghosting on TV pictures. But the glitches, image streaking and pixelation effects which occasionally crop up now are difficult to deal with, as are the viewers who experience them.
Data corruption
All digital TV transmission systems impress the picture and sound data onto a carrier, UHF for terrestrial broadcasts, SHF for satellite transmissions. In its passage to the receiver’s antenna socket the carrier can become weak, distorted, irregular or laden with impulse interference. The broadcast datastream carries quite robust electronic armour to protect it, but there comes a point in a deteriorating signal at which the receiver’s data correction and repair section can no longer cope, and the picture (and sound) starts to break up as shown in the photo above. More often than not this occurs on an intermittent or spasmodic basis, making diagnosis difficult.
Satellite pictures
Satellite transmission systems are relatively free of impairment thanks to a clear direct transmission path and QPSK modulation. When Sky or Freesat pictures fall to pieces it can be due to a really heavy downpour weakening the received signal, a rare and short-lived problem. If it happens often in rain the likelihood is that the signal is weak for some other reason – see below – and that rain attenuation tips the marginal signal over the edge, so to speak.
Incorrect positioning/pointing of the dish is a common cause of weak signals; a loose dish mounting gives rise to troubles when the wind gets up. It sometimes happens that the cable, especially if it’s many years old, becomes ‘lossy’ due to gradual ingress of moisture, for which the only cure is replacement. This cable fault may cause mysterious frequency-dependent effects in which individual channels/multiplexes or blocks of them go down. LNB failure is becoming rarer as reliability improves, but still it can happen: the symptoms range from intermittent picture and sound glitches to complete loss of all signals.
Terrestrial pictures
Terrestrial signals are very vulnerable to deterioration (in spite of being ‘ruggedised’ by their excellent COFDM and QAM modulation system) because of their much more perilous path from the transmitter to the receiving aerial, which may itself be inadequate, unsuitable or misaligned. Terrestrial digital broadcasts (Freeview) are subject to large variations in signal strength and to multipath reception, interference, co-channel reception and fading. If the received signal is unreliable, then, the first check should be on the strength and quality of reception, measured at the point where the flylead plugs into the TV’s aerial socket, with a digital checker. This keeps distribution amplifiers etc. in the loop; too strong a signal can cause data corruption by overloading the TV’s tuner, while an aerial whose bandwidth (‘grouping’) does not embrace all the digital multiplexes’ frequencies causes trouble, in the form of image glitches or complete loss of channels falling beyond the aerial’s response.
Especially where reception is weak or impaired, impulse interference can play havoc with Freeview reception, and the stronger the signal can be made, e.g. by a larger/higher aerial array or a masthead booster, the less the effect of interference. Careful positioning, maybe at the rear of the premises, can reduce the severity of road traffic interference, while household troublemakers like fridge and boiler thermostats can be tamed by fitting them with suppressors. It is good to use satellite (low-loss double screened) cable with terrestrial signals for its low loss and interference-screening properties.
A special case is interference from TETRA transmitters, used by mobile emergency services for speech and data exchange. Near ground-based TETRA relay transmitters interference to Freeview reception can be rife, especially with older receivers and masthead signal boosters. A solution can often be found with purpose-designed bandstop filters, now fitted internally to masthead and distribution amplifiers.
Diagnosis
As with most equipment failings in this complex world the easiest method of diagnosis is substitution: of the receiver, the downlead, UHF amplifier, LNB or whatever, once the obvious causes have been eliminated. If the TV set or receiver box is responsible – or contributing to the problem – it’s worth checking with the manufacturer or importer to see if a software update is available to address the issue. As analogue services are shut down, the strength of Freeview signals is being bumped up, reducing the incidence of image glitching.
