What is Digital TV?
Digital TV is a modern thing - or is it? While it has only been popular in the
UK for a couple of years, in the United States it has enjoyed nearly a decade
of being on air. Indeed the principles that allow Digital TV to exist today were being
put in place over 15 years ago.
In the old days (!) analogue television was transmitted as a modulated video and audio signal.
This meant that if anything happened to the signal between where it left the transmitter and
where it entered your antenna, it would either affect the picture, or sound, or both. Many viewers
over the age of twenty will remember the ghosting seen on pictures when atmospheric conditions were
poor (the technical name is multi-path interference).
Error Correction
In the digital world, the continuous variation of the video and audio signal is done away with, being
replaced instead by an instantaneous digital representation of the signal at each moment in time.
This means that each signal is only transmitted as either a string of '0's or '1's - nothing in between.
If at the receiving end, a zero is received as 0.01 or 0.1, then it is a simple matter to tell that it
started off as a '0'. Likewise if a '1' is received as a 0.89, then we can assume a '1' was transmitted.
All of this is a gross simplification but it serves to show how errors in the received signal can be detected
if we know we only ever transmit '0's and '1's. If you're technically minded, advanced mathematic techniques
called Coded Orthogonal Frequency Division Multiplexing (COFDM), Viterbi Decoding and Reed-Solomon decoding are
used to make sure that the signal received is as close to the original as we can get.
Multiple Channels
Having decided that the TV programmes needed to be digitised before transmission, the next logical step was to use
compression techniques to squeeze as much into a single UHF channel as is possible. With analogue TV, only one
programme could be loaded into a single channel, but with digital compression, as many as 8 digital programmes can
be levered in. All of this means that many, many more programmes can be transmitted using the same frequency ranges as
for analogue - very important considering how crowded the airwaves are getting these days.