The Crosby FM stereo system, which was runner-up to the Zenith-GE system in the US FCC evaluation, used an FM subcarrier at 50 kHz, with a maximum deviation of ±25 kHz, meaning a deviation ratio, referred to 15 kHz, of just 1.67. Nominally the subcarrier occupied the band 25 to 75 kHz. It was allowed 50% of the main carrier deviation. Thus because there was no interleaving with an FM subcarrier, the main channel had to be restricted to 50% deviation, meaning a non-negligible reduction in the mono service area for a given S/N. At the time, the FCC 75 kHz upper limit on modulating frequencies (later removed) meant that the Crosby system could not accommodate an SCA (subsidiary communications authorization) subcarrier. This did go against it as compared with the Zenith-GE system, which allowed for an SCA subcarrier at 67 kHz, but that aside, the Crosby system was judged to be not as good overall.
Interleaving considerations aside, a bigger deviation ratio for the subcarrier would help with its own S/N. But accommodation of a wider bandwidth subcarrier would require that it be move upward in frequency, say to 75 kHz for a ±50 kHz deviation. Then the subcarrier as a whole would have a lower deviation ratio referred to the main carrier, which could adversely affect the net S/N. So there would be a trade-off in that respect.
Nonetheless, FM subcarriers are required where a completely separate programme is to be carried, in order to minimize crosstalk from the main channel. Thus the SCA subcarriers are FM. So was the SAP (second audio programme) subcarrier of the American Zenith-DBX TV stereo sound system. It used DBX companding in order to have an acceptable signal-to-noise ratio. The Japanese (EIAJ) TV stereo sound system used an FM subcarrier, with companding. In that case the same subcarrier was used either for the stereo difference signal or the SAP signal. Its modulation of the main carrier was additional to that of the baseband, since there was plenty of bandwidth available around the sound carrier in a TV channel. A derivative of the EIAJ FM-FM system, with an additional SAP subcarrier, was proposed for use in the USA, but the Zenith-DBX system, with AM subcarrier, was judged by the FCC to be better.
There was some discussion on SCA subcarriers towards the end of this thread:
https://www.vintage-radio.net/forum/...ad.php?t=56858.
Quote:
Originally Posted by G6Tanuki
I spent 6 months working with a bunch of Honeywell techies in Dallas/Fort-Worth back in the early-80s and one of them had built a subcarrier-receiver using one of the LM56x PLL decoder chips to extract this.
|
I think that the LM565 was fairly standard for this job. Interestingly, the original Zenith decoder circuit for the Zenith-DBX TV stereo sound system used an LM565 for the SAP channel.
Quote:
Originally Posted by G6Tanuki
Practically, I've always thought that FM is really just a variant of SSB with a 90-degree-shift; in times-past there were FM-detectors using strange valves [EQ90 Nonode?] that used this approach.
|
That was the EQ80 enneode/nonode. It was one of several valve-era quadrature-type FM demodulators, some quasi-synchronous and some fully synchronous. There was some mention of these in this post:
https://www.vintage-radio.net/forum/...6&postcount=15.
Some of the valves used for quadrature FM demodulation, such as the EH90 (6CS6) were also proposed for Zenith-GE stereo decoding, e.g. by Mullard.
I suppose that one could say that FM and SSB are both forms of angle modulation, the latter involving amplitude modulation as well. Quadrature AM (QAM) is also a combination of angle and amplitude modulation. Demodulation of a QAM signal along the I-axis recovers the I signal, along the Q-axis recovers both the Q signal and any FM. And phase matrixing of the demodulated I and Q signals allows individual recovery of the LSB and USB components. The various AM stereo systems were combinations of amplitude and angular modulation. An interesting IC of the 1970s was the Plessey SL624C, which could demodulate both AM and FM, and also act as a self-oscillating SSB demodulator. It was essentially the same as a TV intercarrier FM sound IC, the kind with an electronic volume control and an audio driver (e.g. TBA120S).
Returning to the FM stereo case, I’d say that choice of system was well-researched on both sides of the Atlantic, with the Zenith-GE system receiving almost universal approval. The USSR was the only dissenter. Apparently, it found the Zenith-GE system less suitable for its ±50 kHz maximum deviation, and instead opted for its polar modulation system, which used an AM subcarrier at 32 kHz with partially suppressed carrier. I don’t know for sure, but it looks as if the objective was to get the subcarrier to as low a frequency as reasonably possible, thus minimizing the bandwidth of the composite signal. In turn this meant that there was no room for a pilot carrier, hence the use of a partially suppressed rather than a fully suppressed subcarrier. It is conceivable that an FM subcarrier (which would not require a pilot tone) at around 32 kHz was considered, but rejected in favour of the AM subcarrier. The 32 kHz subcarrier would have had sidebands covering 17 to 47 kHz, the 17 kHz number being about as close to the 0 to 15 kHz baseband as one might want to go. I understand that the thinking was that with the subcarrier adjusted to 31.25 kHz (twice the 625-line TV line frequency), the same system might also have been suitable for TV stereo sound.
In contrast to the near-unity in worldwide FM stereo system choice, there was much diversity in TV stereo sound systems. But there in any case certain parameters had to be tied to the basic TV characteristics, such as line frequency, and also to the channel characteristics. So the diversity was effectively built-in.
I still haven’t managed to find any information on the Japanese FM tuner that used on the subcarrier LSB to decode stereo. I am fairly sure that it was in one of the British magazines in the 1980s, I think not HFN. The report might have given some information as to the claimed S/N benefit.
Cheers,