I too had pondered the original question about the use of stereo subcarriers with VHF-AM. On the face of it, it should have been possible. Given that AM noise has a square shape, not triangular as in FM, then the significant reduction of signal-to-noise ratio that occurs with FM at subcarrier frequency does not occur with AM. On the other hand, VHF-AM has a higher noise floor than wideband FM. Impulse noise reduction might be more difficult, in that noise limiters would need to respond only to noise above 53 kHz, which might make them less effective. I suppose though that with frequency division decoding, a lower frequency noise limiter could be used in the baseband L+R channel.
Anyway, I recently stumbled across a form of proof-of-concept insofar as that in the mid-1950s, the French had dual-language TV sound in Algeria with System E, which used AM sound. I included the scant information – plus some deductions – on a recent posting in another forum, but will repeat here if wanted.
Of course the whole VHF-AM saga smacked more of industrial politics than investigation of valid technical options. The BBC had decided in favour of FM by 1945, and had wanted to start the Third Programme as a VHF-FM only service. As Michael W. has said, Pye was the chief antagonist when it came to FM, and used BREMA as its voice, thus more-or-less forcing the powers-that-be to take notice. In fact it was something of a three-system debate, involving FM, AM wide (allowing the use of impulse noise limiters) and AM narrow options. Apparently Stanley of Pye had a second preference for AM narrow, his first preference having been no VHF sound broadcasting at all. In the end technical sanity prevailed, and the TAC recommended FM and the Government/PMG accepted this. One of the reasons was that with AM, the capital and running costs of the transmitter network would have been very much higher. At equal radiated powers, VHF-AM, with its higher noise floor, had a shorter range than VHF-FM, so more transmitters were needed. But even when the usable signal ran out, the carrier still had the ability to cause co-channel interference at what might be called FM-like distances. Add that to the lack of capture effect with AM, and it limited channel re-use possibilities as compared with FM.
The BBC was in a bit of a no-win situation in all of this. It was being beaten up – questions in the House and all that - because many were suffering from poor reception MF AM conditions in some parts of the country, particularly at night (I think East Anglia was one such area), but going to VHF was the only workable option to solve that problem. Thus the VHF-FM network was conceived on the basis of providing interference-free reception across virtually the whole country. The high quality audio aspect was downplayed, in part because the BBC would need to use the existing landline network, which for the most part did not get past 10 kHz, sometimes less. The BBC was then criticized for not aiming higher in audio quality terms. Then it would seem that many of those who had complained about the poor MF AM service were not in a hurry to move to FM. (Although I suspect that the uptake of FM was higher amongst Third Programme listeners than the average, given that it had content that was more demanding in terms of audio quality.) In part though the slow uptake may have been because by the late 1950s a large part of the evening audience had moved from radio to TV, and their daytime MF AM reception was deemed to be adequate.
The use of AM sound for System A television did not really create a precedent for VHF sound broadcasting. The attached excerpt from BBC Monograph #39 more-or-less why AM, although not as good as FM, was acceptable for television.
Some VHF-AM receiving equipment was made available on a normal commercial basis. At least three makers included VHF-AM in the initial FM tuners, namely Lowther, Chapman and Sound Sales. There may have been others. The initial design brief for the Quad FM included VHF-AM capability, but this was not needed by the time that it was in production.
How the above-mentioned tuners handled AM demodulation is unknown. But quite possibly they used the limiter grid, as was done by Jason with its JTV/2 FM-TV Sound Tuner. The latter had a three-stage IF strip, consisting of an EF89 with agc, an EF89 with fixed gain and an EF80 limiter, which fed the FM discriminator. Both the agc and the demodulated AM were taken from the EF80 limiter grid, and the AM output circuit included a diode noise limiter. Thus the IF strip differed from the customary FM-only three-stage type, which usually consisted of a gain stage (perhaps with agc), a combined amplifier/high-level limiter (from whose grid the agc feed was usually taken) and a regular limiter.
The Quad FM (whose core design was done by Geoffrey Horn, evidently in part from a Brimar original circuit) IF strip shows what might be evidence of its original VHF-AM option intent. It originally comprised a pair of 6BH6 gain stages, non-limiting, followed by a 6AU6 limiter, whose grid provided agc bias for the 6BJ6 RF stage. With the second iteration of the Series B version, the RF stage was changed to a 6BH6 without agc, and the 1st IF stage was changed to a 6BJ6 with agc of the “fast” type. That is, effectively, the agc bias included any AM that was present, fed back in antiphase to improve overall AM rejection. Thus the 6AU6 limiter grid was acting as an AM demodulator.
Whilst grid leak AM demodulation has by the conventional wisdom higher distortion than the diode variety, one assumes that Jason et al were able to pick valve operating conditions and time constants, etc., to obtain adequately low-distortion. Maybe the use of sharp cutoff valves helped.
MF AM stereo systems were first proposed in the USA in the late 1950s, at about the same time that FM stereo was under consideration. C-QUAM was a much later development, though. With MF one constraint was that any system chosen must fit into a standard MF channel, ±15 kHz. All proposed systems were variants on theme of an AM envelope carrying the sum signal on a carrier that was somehow phase-modulated with the difference signal. With VHF-AM, the bandwidth constraint would not have been there, thus allowing the use of a subcarrier. Nonetheless, one imagines that C-QUAM would work well.
Perhaps indicative though is that when the time came to add stereo/bilingual sound to TV system L, the French chose NICAM on the same carrier frequency as was used for system B/G/H, thus chopping into the vision bandwidth, which was reduced from 6 to 5.1 MHz as a consequence. I wonder if C-QUAM was considered, given that it was established technology with decoding ICs readily available. But whilst it would have worked for stereo, I doubt that it would have been satisfactory for bilingual sound.