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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Re: Weirdness: Round three

Postby Té Rowan » Thu Jan 24, 2019 1:41 pm

And now for something completely different…

Plaid403.gif
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sjokul.gif
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A couple of plaids and a chocolate piece, all carved from raw bits. Really, I made them by filling out XPM (Xwindows PixMap; actually C code fragments) files in a text editor and converting them to GIFs. Copyright? Dont care; can’t be hedgehogged; do whatever-the-fuck you want to with them.
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Re: Weirdness: Round three

Postby Té Rowan » Wed Feb 06, 2019 3:38 pm

Felt the need to hack. Ended up extending a CRC-8 calculator I wrote and last hacked on close to two years ago from eight to sixteen variants with a choice of using pre-calculated tables or calculating CRCs on the fly. Still have checks to run to be sure nothing is wrong or broken.

http://reveng.sourceforge.net/crc-catalogue/ is the Big Catalogue of things that go CRC in the night, btw.
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Re: Weirdness: Round three

Postby Té Rowan » Tue Feb 26, 2019 7:28 am

Back to FM stereo… Had an idea simple enough that it must have occurred to multiple others, but here goes:

Isolating the 19 kHz pilot tone, doubling it to 38 kHz, combining it with the difference signal and then rectifying as per the simplest Russian stereo decoders… crazy enough to work? And if it does, is three-channel stereo really that far out for either OIRT or multiplex stereo?
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Re: Weirdness: Round three

Postby Cheb » Wed Feb 27, 2019 10:27 am

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

Eh, I never had interest in radio receivers and I have firmly forgotten what I learned in the university - despite signal degradation and modulation methods being a notable part of the course.

The most complex circuit I ever constructed was a RF modulator to allow connecting VHS recorder to a TV that lacked AV input (the vast majority of them, at the time). But even there I cut corners making the device a big speaker with built-in amplifier instead of trying to modulate the sound into the radio signal - which was beyond my skills.
Yeah, I know, normal video recorders do have alternate RF output - except most soviet TVs weren't equipped with the optional decimeter block, limited to channels 1..12 while VCRs output at channels 21+

We also had to call a TV master to install an additional PAL decoder into our first TV so it could show those bootleg films in color (before Dad bought a Japanese TV). By the way SECAM's true superiority to PAL really shines when the signal becomes noisy, the tape is worn (or re-copied N times) or you try recording in LP (aka double density crap) mode. PAL shifts, breaks and flickers in horizontal bands of distorted saturation while SECAM gains multi-colored static that grows in intensity with signal degradation but the underlying colors stay true, never shifting.
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Re: Weirdness: Round three

Postby Té Rowan » Wed Feb 27, 2019 1:27 pm

Heh, our first VCR was a JVC machine Dad bought in West Germany, and that one had the mark SECAM-Ost, meaning it could record East German TV in colour.

Anyway, I was thinking more like there was a chance you could still receive broadcasts in the 66-MHz band using OIRT stereo as well as in the 88-MHz band using FCC stereo, and hence had the best chances of anyone on here to tell how they compare in daily life.
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