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Information supplied by Joe (Bored_bean) British expats Forum
A TV signal consists of 6 separate signals ( 7 if you have stereo sound ). These are
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To transmit these signals over the air we have to combine (multiplex) them into one signal. The way we combine these signals together is called a television standard.
In a receiver the first signal we deal with is the off-air signal. This arrives from your aerial via a co-axial cable and usually a Belling-Lee plug.
We need to pick the signal we want from the hundreds of other radio signals out there, and we do this by selecting the frequency (channel) that we wish to receive. There are 2 groups of channels used in Australia VHF and UHF. UHF is used for high-density urabn areas, whereas VHF is used for rural areas (it covers larger areas more efficiently). In the UK we stopped using VHF in the 1980s and consequently most modern UK TV sets don't cover those channels.
Once we've tuned in to the correct channel we get what is called a baseband signal. This contains all of our 6 signals jumbled up together and the bad news is that the UK puts it's audio(sound) signal in a different place to Australia. The picture is encoded in the same way in both the UK and Australia(PAL), so if we've got this far we'll have a picture but no sound.
The final bit of decoding we have to do is to turn the baseband video into our 5 separate signals. This is done using a decoder chip. There are 3 common ways of doing this worldwide, PAL, SECAM and NTSC. Both Australia and the UK use PAL. These 5 separate signals are used to control the picture tube and make the picture that you see.
As I've explained, whilst the signal is a separate audio and baseband video signal, there is no difference between the UK and Australian system. This usually uses a SCART lead in Europe and RCA connectors elsewhere in the world. SCART can also be used to carry the 5 separate video signals as well, it's just not so common.