Weirdness: Round three

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Weirdness: Round three

Postby Té Rowan » Mon Dec 17, 2018 3:25 pm

So what is it this time? OK, I was googling up the FM stereo system used in Russia and a few other OIRT countries on their low FM band (66–74 MHz).

In short, it is not a pilot tone system like the US system, but uses the (14dB reduced) subcarrier to signal stereo. And, man, is the subcarrier modulation odd. The system is called polar modulation. One channel is modulated onto the positive half-waves, the other, presumably inverted, goes onto the negative half-waves.

In any case, decoding stereo is dead simple: Isolate the subcarrier, readjust the carrier/sideband relation (with a damped bandpass filter), use two opposing half-wave rectifiers to extract the stereo information and then de-emphasise and filter as needed before putting it on headphones or speakers.

Yes, the system sounds fiddly as fourecks. But if it works well enough, why worry?
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Re: Weirdness: Round three

Postby Spica75 » Mon Dec 17, 2018 4:20 pm

As someone with good basic knowledge, it's when you read more specific explanations like this, that it becomes clear just how much more there is to systems like those, despite how "simple" they are...

At least i know enough to vaguely understand your detail description, even without having any special interest in the subject. :P
Still, it's interesting to hear that different places have chosen completely different implementations.

Any clear advantage/disadvantage with the various tech use?
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Re: Weirdness: Round three

Postby Té Rowan » Tue Dec 18, 2018 11:49 am

If there is one ‘best’ system… that I do not know. I have read a few details on FM stereo history, though. The US system chosen allowed them to continue distributing muzak over subcarriers, which was a source of much-needed income for the FM stations there. In the UK, BBC has experimented with muzak subcarriers, but IIRC they were not overly impressed. Can’t remember which report I read about that in.

Insert: It’s here: https://www.bbc.co.uk/rd/publications/rdreport_1976_14

Anyway, I first found a diagram of a very simple polar stereo receiver on the web.

http://zpostbox.ru/index_e.htmlhttp://zpostbox.ru/fm_receivers_with_pll.html (Fig.3)

Later I found a few stereo coders here:

http://vrtp.ru/index.php?showtopic=27758&st=0

A few FM stereo threads:
https://www.vintage-radio.net/forum/showthread.php?p=349456
https://www.vintage-radio.net/forum/showthread.php?t=149892

Incidentally, I think Cheb is in the best position of us to compare stereo systems.
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