Digital versus analogue television

Television signals used to be recorded in an analogue format. That simply means the signal could be of any value from 0 to 1 volt.

Whenever you pass an electronic signal down a wire, through a connector, into another machine, recorded onto tape, etc., it loses some quality – the signal becomes slightly distorted, and It is impossible for the receiving machine to work out exactly what the original signal looked like.

Once a tape had been copied a few times, that quality loss (generation loss) became apparent, so we tried not to copy tapes too often.

Ones and zeros

The other way of sending a signal is to take thousands of measurements, and send a series of binary numbers down a cable. So, if at a particular time the picture signal was 0.5 V, it would be converted to a series of ‘1s’ and ‘Os’ that, in binary terms, represents 0.5.

Now we send this new signal down a slightly grubby cable, through a seriously dodgy couple of connectors and onto another tape. By this stage our signal has again picked up a whole bunch of distortion and noise.

However, even covered in this mess it’s obvious which bits were meant to be ‘1s’, and which ‘0s’. So the receiving machine reconstructs the signal as a perfect copy of the original. Now, theoretically at least, we can copy from machine to machine, without ever losing any quality.

Compression

The problem everyone faces when dealing with digital signals is that they are huge. You need a massive computer memory to remember even a small video clip, so engineers have ‘compressed’ images.

For example, the difference between two successive television frames is actually very small. So if, instead of storing a complete new picture for every frame, you simply store the difference between two successive frames, that information would take up much less space in a computer memory.

There is a limit to how much you can compress pictures, after which you really notice the quality loss on screen, and we need to be particularly careful of compressing pictures that have already been compressed – they can end up a real mess.

Transmission

It’s possible to transmit many digital television channels in the space that was used by a few analogue channels, so broadcasters are keen to move over to digital transmission systems. There are additional benefits to the viewers – they should receive better quality pictures, as long as the digital signal has not been compressed too much by the broadcaster. Viewers need either digital decoders attached to their old televisions, or new television sets.

The same part of the signal once it has been copied a few times

A digital signal. This can only consist of either a high voltage (represented by ‘1’) or zero volts (represented by ‘0’)

Even if the digital signal gets badly distorted it is still clear which bits are meant to be ‘1s’ and which bits are ‘0s’. So a perfect copy of the original signal can be reproduced and recorded no matter how many times it is copied.