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ABC guide to... Step-by-step guides to understanding digital TV
May 2011
Top 3 things to know about digital TV Want to see more?
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Digital TV
Before you resort to calling in specialist help, try our ‘first-aid’ checklist to diagnose the more common digital TV reception problems You could spend a lifetime learning about digital TV broadcasting and reception and still not know it all, but here are our top three subjects to hone up on overnight to hold your own down the pub, at the dealer’s and in the living room. Why is digital TV compressed?
01 The simple answer is to bring you more channels to watch. Whichever medium your TV signals use to get to you – by satellite or from a terrestrial transmitter, or even via the internet – it takes up a lot of the capacity of that
medium to carry a TV signal. By condensing the signal to take up less ‘room’, capacity is released to carry more signals. For terrestrial TV, compression enables the switchover to change from about five TV channels available to more than 40. When analogue TV was used satellite viewers could watch hundreds of channels; now with digital they can receive thousands. TV via the internet would simply not be possible without compression – your broadband connection would work flat-out and still not be able to stream a single channel.
Intraframe compression: Parts of a TV frame that repeat the same pixels can be stored and transmitted in a shortened form to take up less space
TV frame
Compression is possible because the TV signal is digital. Each frame of the TV picture is made up of thousands of picture elements, or pixels, which are described in brightness and colour by numbers in the digital TV data stream. Amazingly, not all of those numbers are really needed. By eliminating the repetition of the numbers describing an area of pixels of the same brightness and colour within a frame, and an area of pixels that repeats across several frames, the amount of data can be reduced considerably. With more sophisticated techniques, such as following a moving area of the image, huge reductions in data are possible. There is some reduction in the quality of the TV picture when it is compressed, and the greatest levels of compression are reserved for ‘simple’ images such as news while complex and detailed natural history or sports programmes are given less compression to maintain the perceived quality onscreen. Why your antenna is crucial
02 Satellite TV signals come from a
A block of identical pixels can be coded as one
A block of different pixels must be coded separately
2 What Satellite & Digital TV May 2011
transmitter with the power of a light bulb in orbit over 36,000km away, and to say that these signals are weak is a huge understatement (but they have to be or we’d all cook like food in a microwave oven!). TV signals from terrestrial transmitters are a lot stronger but they still need sensitive pick-ups and careful handling. That’s why the antenna that first plucks the signals out of the air before they embark on the electronic path to your TV screen is arguably the most important component in the whole system. Picking up any kind of radio signal (and both terrestrial and satellite TV signals are radio waves) is basically a matter of sticking a metal probe in the path of the radio waves. This induces a tiny current in the metal that can be amplified and processed electronically. A TV aerial requires little more than
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Frame 1
Areas between the image that change between frames must be coded in each individual frame Frame 2
Frame 3
A part that remains the same but moves in the image can be coded once and tracked between frames
The same area of the image containing the same data can be coded just once
that. On a normal TV aerial the metal probe is held at one end of a boom with a reflector behind it and other pieces of metal across the boom in front that direct the waves onto the antenna. The size of the probe and the reflector and directors determine the frequency range of the aerial; different shapes and arrangements of directors and reflectors – and even more than one boom – are used to adjust the sensitivity and the directional reception pattern of the aerial. To make the most of all the signals from the transmitter but reject the unwanted signals from transmitters in other directions and noise from the surroundings, the choice of aerial is very important. Satellite dishes are even more critical. Because the incoming signals are so much weaker, a lot of signal is gathered together by the parabolic reflector and focused on a tiny aerial probe inside the LNB, where the pathetically weak electrical signal is amplified and processed. It is very difficult to amplify these tiny high-frequency electrical signals without introducing noise (which kills the usefulness of the signal) so the initial strength of the signal received is all-important. It is the reflector that determines a dish antenna’s sensitivity (and makes it very directional so satellite dishes must be
carefully aligned) and there is no substitute for a large dish to pick up weak satellite signals. It is vitally important that the right size of dish is used for any satellite TV reception if reliable reception, in all weathers, is to be achieved. What’s gone wrong?
01 When your TV screen goes blank, or shows only ‘snow’, before you reach for the phone to bring in the professionals, see what you can find out yourself. Assuming that your TV and receiver power up OK, failure to display a satellite channel can be down to the dish, the LNB, the receiver, the TV or the connections between them. The trick to diagnosis is to eliminate each possible cause until you come to the actual problem. Only if you get absolutely nothing onscreen can it be the TV. Then, try another AV source such as a DVD player, feeding the TV or another TV connected to the receiver – but first check the connections between the TV and receiver carefully, substituting another cable if possible. If you can see the receiver’s menus onscreen, then the TV is working and so (probably) is the receiver (but check the LNB cable is still connected). However, a TV image that comes and goes can be a fault with the receiver – try to borrow another to substitute; if that works, then you can surely blame the box.
Interframe compression: When parts of a TV image are the same between successive frames, they only have to be stored and transmitted once, saving on the space taken up by the frame sequence
If you can get some channels but not others then the dish has not been knocked off alignment, or off the wall. Consult Channel Check at the back of this issue and see if the missing channels are all from one transponder, or very close in frequency. It may just be down to the broadcaster moving the channels (re-scan to fix that), or it may be that local interference is swamping this frequency. Only better shielding and moving the dish, plus a fair amount of luck will sort that out. If you’re getting only horizontal or only vertical channels, or miss all those below 11.700GHz or the channels above 11.700GHz, this probably means that your LNB has failed. Because LNBs are relatively cheap you could even try substituting a new one (if the dish is reasonably accessible) to test the theory. All this leaves the co-ax cable from the dish to the receiver. It is surprising what faults this can cause while apparently in perfect condition. If certain channels disappear for a while after a period of rain but reappear after a spell of hot, dry weather it’s likely that water in the cable is the problem. Although not too expensive, new cable is a devil to fit properly but, if you have correctly narrowed down the suspects, that should restore your system to operation n Geoff Bains May 2011 What Satellite & Digital TV 3