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Old 5th Nov 2016, 3:32 am   #21
Synchrodyne
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Default Re: Multiplex stereo Zenith/GE what was the difference?

That would have referred to the SCA subcarrier services, which were used from the mid-1950s. SCA was “Subsidiary Communications Authority”. These used an FM subcarrier that accounted for a small portion (often 10%) of the main carrier deviation. In the mono days, 41 kHz was one of the frequencies used. When stereo was in the offing, FM stereo broadcast proposals essentially needed to make continued provision for an SCA subcarrier, and as noted above, both GE and Zenith allowed for this at 67 kHz. So from 1961 67 kHz became the standard SCA subcarrier frequency. At some later time provision was made for a second SCA subcarrier at 92 kHz.

SCA, or “storecasting” as it was sometimes called, given that its main, but not its only use was delivering background music to stores, was an important revenue stream for many American FM stations. Finding room for an SCA subcarrier was an issue when proposals for quadraphonic FM broadcasting were being developed.

For some background, See “Radio-Electronics” for 1957 October (http://www.americanradiohistory.com/...cs-1957-10.pdf) page 89ff.

Construction articles for home-built SCA decoders did appear from time-to-time in the American consumer magazines. I think that there may have been some prohibition on the sale of commercially-built adaptors to users who were not subscribed to and thus paying for an SCA service.


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Old 5th Nov 2016, 12:04 pm   #22
emeritus
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Thanks for the explanation. I had wondered what the SCA subcarrier was.
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Old 6th Nov 2016, 9:02 pm   #23
Synchrodyne
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Default Re: Multiplex stereo Zenith/GE what was the difference?

There is a more detailed treatment of FM subcarriers and SCA in a two-part article “FM Multiplex - Its Present and Future”, carried in Radio & TV News Magazine for 1959 January and February.

See: http://www.americanradiohistory.com/...ws-1959-01.pdf, page 57ff; and:

http://www.americanradiohistory.com/...1959-02-R.pdf; page 59ff.


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Old 11th Dec 2016, 8:13 pm   #24
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Default Re: Multiplex stereo Zenith/GE what was the difference?

As noted previously, in their respective FM stereo system proposals, both GE and Zenith included the use of a 67 kHz subcarrier for SCA purposes. It would appear that the choice of 67 kHz was not so much a coincidence, but the result of each simply following the precedent set by the earlier Halstead FM stereo proposal. Essentially this used a 41 kHz FM subcarrier, with ±8 kHz deviation, for the right channel, with the main carrier used for the left channel. Since 41 kHz was then a commonly used SCA subcarrier frequency, the latter was moved up to 67 kHz. I suspect that 67 kHz may have been derived as the highest frequency that would accommodate ±8 kHz deviation (with an 8 kHz audio bandwidth) without its significant sidebands going beyond 75 kHz. At the time that the various FM stereo proposals were being developed, the FCC required that SCA subcarriers and their sidebands be within the range 20 to 75 kHz. (The 75 kHz upper limit must have been relaxed at a later date, post stereo implementation, as evidenced by the later use of 92 kHz SCA subcarriers.)

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Old 14th Feb 2017, 3:50 am   #25
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Returning to the “merging” of the GE and Zenith FM stereo proposals, a pointer to this event is provided by a brief item in Radio-Electronics magazine for 1959 October (pages 8 & 10), in which it was said that the National Stereo Radio Systems Committee (NSRC) was endeavouring to reduce the competing systems to three, and then choose from amongst that group. To that end the entrants were divided into like groups, namely FM-AM and FM-FM, with the EMI-Percival system standing alone. The FM-AM group included the GE, Philco, and Zenith proposals. Philco had proposed an SSB subcarrier (as did Siemens in Europe), and I imagine that this would have been outruled on the receiver complexity score. Then given that FM-AM was superior to FM-FM when it came to preserving the monophonic service areas of FM transmitters, it would not have been such a big step to choosing a combined Zenith-GE system from what remained in the FM-AM basket.

That the chosen FM stereo system was a combination of the GE and Zenith proposals was probably happenstance, but the two had previously worked together on FM matters, notably in respect of the 6BN6 gated beam valve FM limiter-demodulator in the late 1940s. Zenith did the basic development work, whose objective was to include adequate limiting in the single-valve quadrature demodulator previously developed by Zakarius and improved by Kalmus. GE did the design and development of the gated beam valve itself.

On SCA subcarrier frequencies, 67 kHz does seem to have predated even the Halstead stereo proposal, going back to the monophonic days. It was used a second SCA subcarrier frequency by stations which were using a lower frequency (typically 41 kHz) subcarrier for their first SCA service. Whilst a plausible rationale for 67 kHz is easily derived, the reason for the choice of 41 kHz is not obvious. It might be noted that both 41 and 67 are prime numbers, but this line of thinking does not seem to go anywhere. Perhaps it was found desirable that the SCA subcarrier frequency be high enough that its post-demodulator level in domestic receivers would be substantially reduced by the de-emphasis, simpler receivers having little other defence against the entry of potentially troublesome ultrasonic signals into their audio amplifiers. With 75 microsecond de-emphasis, a 41 kHz subcarrier would be around 28 dB down.


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