Post
by Albert H » Sun Apr 14, 2024 3:14 am
All the BBC content - on FM as well as DAB has been digitally distributed since the late 70s! They used a custom-made PCM system with 16-bit words, but with just 13-bit audio (the other bits were used for control functions). The PCM system was developed to distribute stereo programme to the transmitter sites, and eliminate the problems that Post Office "Music Lines" could cause when trying to send stereo. The Post Office - in their infinite wisdom - could never seem to find identically routed pairs of lines from London to the transmitter sites, so the phasing of each channel could be somewhat arbitrary, leading to some weird stereo effects.
Their plan was to effectively combine Left and Right, and send it down a single line - they tried (and rejected) simple time division multiplexing, because this was wasteful of bandwidth and was still susceptible to noise pick-up. They decided to digitise, and as long as the bitstream arrived at the TX sites, it's condition didn't really matter - it could be squared up again, and demodulated back to a pair of audio channels to feed the transmitter.....
When the Post Office microwave network was commissioned in the late 70s, the BBC could use the full potential of the PCM system they'd developed, and the bitstream would carry all the national services, and would be separated out at the TX sites by the demodulators. I don't know if the system is still in use, but at the time of its development, it was world-leading. This was in the days when the BBC employed engineers and had a Designs Department.....
"Why is my rig humming?"
"Because it doesn't know the words!" 