This week sees the end of a digital overlay service, which carried stereo sound to televisions in the UK. NICAM (Near Instantaneous Companded Audio Multiplex) will rest in peace this week.
The BBC started using NICAM in 1986, but it took many years for the service to reach all of the main transmitters, and even longer for all television sets to have the ability to decode it.
NICAM was an "out of band" 728 kilobits per second digital data signal which formed part of the broadcast signal and was added in using the QPSK digital coding system.
It started with 32,000 samples per second at a resolution of 14 bits (compared with the 44,000 samples/s, 16 bits of a CD) and used a system called companding to compress the data down to 10 bits.
Companding works by removing the less significant parts of the data. For samples where the values are large, the top "bits" are sent, for those where the values are small, the lower "bits" are sent instead, with two intermediate steps. As the data is carrying an audio signal, this was sufficient to keep the sound the same to the human ear.
By the end of the analogue era, most television sets could decode these NICAM signals, but most equipment was unable to record them (so they did not get recorded onto domestic video tape) or produce them. This means that the "RF out" of Sky and Freeview boxes was always mono.
However, given that the service was created in the era of the BBC Micro and ZX Spectrum, and implemented in "pure logic" it was quite an achievement.
I'm a little insulted that you reckon I can't tell the difference between mono and stereo! :-) As a retired BBC engineer and amateur sound recordist, I don't think I have cloth ears to that extent! As other correspondents have confirmed, NICAMvideo recorders right from the start of the service uesd to output stereo sound on the left and right legs of the Scart - this was how I uesd to get it, as my fist NICAM VCRs didn't have any other audio outputs - and they could also record it onto the video cassette Hi-Fi audio tracks, and indeed later onto DVD. If you don't believe me, you're welcome to come round to hear some of my old recordings made from analogue TV in that way - I think you, like me, live in Brighton!
Briantist: Possible error on your site? (Or am I going nuts!). Whilst trying to manually tune a rogue set that won't allocate the channel numbers properly I notice that on Mendip Xmitter all the programs you attribute to C52 are really on C56 and vice versa! If so, are the MUX names reversed as well?
I can confirm that ArqB is on C52 and ArqA is on C56 at Mendip whatever Ofcom's document says!
I did point this out to you back in April on the Full Mendip page but I think you must have missed my post.
I did repeat the information a couple of times for a number of confused posters.
Funnily enough, I was about to post about this on the feedback page but happened to read this first.
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"By the end of the analogue era, most television sets could decode these NICAM signals, but most equipment was unable to record them (so they did not get recorded onto domestic video tape) or produce them. This means that the "RF out" of Sky and Freeview boxes was always mono."
Many later VCRs could record the decoded NICAM onto the FM helical soundtrack, what they couldn't do is re-encode the NICAM onto the RF output on playback.
NICAM encoder chipsets just weren't an economic option for domestic equipment.
SKY, Freeview , and cable boxes would have no need of a NICAM decoder of course, but, in principle they could have encoded the RF modulator output with NICAM. They didn't do that for the reason given.