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Old 27th Jul 2010, 6:35 pm   #1
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Default Multiplex stereo Zenith/GE what was the difference?

For years I have wondered what the difference between the Zenith and GE multiplex stereo systems was. It is said 'they where so close in spec...' the question is how close.
 
Old 28th Jul 2010, 11:24 am   #2
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Default Re: Multiplex stereo Zenith/GE what was the difference?

As best as I can determine, the GE and Zenith proposals were effectively identical. Any differences would have been very minor and easily reconciled.

The best source I have found for information on this topic is the book “Audio Anthology, Volume Six”, published by Audio Amateur Publications, Inc., in 1994, ISBN 1-882580-06-0. It is a compilation of articles originally published in Audio Magazine in the period January 1960 to December 1961, under the editorship of C.G.McProud.

Therein are several articles on the matter of FM Stereo, including one by Csicsatka and Linz of GE, and another by Eilers of Zenith.

GE and Zenith seemed to have differed only in their starting points. GE approached the problem as one of frequency division multiplex (FDM) in which the use of (L+R) and (L-R) signals allowed interleaving. On the other hand Zenith arrived at the same point by starting with an analysis of time division multiplex (TDM). Both seem to have selected the same 19 kHz pilot carrier frequency and its associated 10% modulation. But whether those numbered differed at all in their respective original proposals is hard to say, as both the articles seem to have been written from the perspective of the final system. Possibly there were adjustments along the way.

That said, there was probably not a lot of choice of subcarrier and so pilot carrier frequencies. There was a need to allow room for an SCA subcarrier. Such had mostly landed on 41 and 67 Hz by the late 1950s. Clearly the 41 kHz subcarrier would need to make way for stereo, but I would guess that there was a preference to retain the 67 KHz frequency. (Quite how these numbers were chosen I have never been able to ascertain. Both are primes, which might or might not be significant; but the 92 kHz SCA subcarrier adopted later was definitely not a prime.) Anyway, centring the stereo subcarrier between the audio baseband (assumed to end at 15 kHz) and the lower edge of the SCA carrier and sidebands (assumed to be around 60 to 61 kHz) kHz pretty much takes one to 38 kHz.

I suspect that the AM subcarrier systems from GE and Zenith may have been late entrants into the US FM Stereo “competition”. Audio Anthology, Volume Five mentions several times FM-FM systems, including the Crosby system that was eventually “runner-up” in the FCC evaluation, but has no references to AM subcarrier systems.

Possibly the pertinent FCC papers are available somewhere, wherein all might be revealed. Also, there could well be some AES and/or IEEE papers that address the topic. One may search respectively at:

http://www.aes.org/e-lib/; and:

http://ieeexplore.ieee.org/xpl/periodicals.jsp

As a sidebar, the US AM Stereo and TV Stereo systems seem to have fared quite well in what might be called the non-professional literature. A good summary of the lead up to, details of and implementation of each is provided in the book “AM Stereo & TV Stereo – New Sound Dimensions”, by Stan Prentiss; published by TAB Books Inc, 1985; ISBN 0-8306-0932-6 & 0-8306-1932-1 (paperback).

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Old 28th Jul 2010, 6:47 pm   #3
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Could it be that the GE TDM method used, in essence, a square wave carrier to switch between the L and R signals, while the Zenith FDM method used a sine wave carrier and DSB of L-R. Many stereo decoders use a switching method, but then have to use some cross-mixing to get proper separation. This is because the two methods of encoding/decoding are not the same, but similar. I once saw (in Wireless World?) a sample and hold decoder, which is yet another way to do it - this would suffer from a sinc frequency response but I'm not certain the author realised this.
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Old 28th Jul 2010, 7:30 pm   #4
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Thank you both for the replies, it makes sense now, the result from FDM and TDM (with the FDM phase adjusted if needed) would be the same, bandwidth limiting of a square wave TDM would be the same as doing it with multipliers and a sine wave rather than a switch.

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Old 29th Jul 2010, 7:01 am   #5
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Default Re: Multiplex stereo Zenith/GE what was the difference?

The Zenith analysis was based upon square-wave switching at supersonic frequency between L and R, followed by low-pass filtering of all but the first set of switching harmonics. The need for cross-mixing after square wave switching during the decoding process was noted.

The GE analysis was sine-wave based in that the L-R signal obtained by matrixing was used to modulate a subcarrier to produce a DSBSC signal. Imputing some history (always risky business), it seems that GE approached the issue with a subcarrier system in mind, and saw that a DBSC subcarrier for the L-R signal overcame one of the main objections to FM subcarriers, namely that the available modulation depth had to be shared between the main and subcarrier signals, thus reducing that available for the main signal and so in turn reducing transmitter mono service areas as measured at any given combination of power output and received S/N ratio. GE also claimed that the 19 kHz pilot carrier, being removed from the audio modulation spectrum, was more readily accessible and so simplified receiver design. (I suppose an alternative might have been DSBRC, with a 38 kHz pilot carrier at 20 dB or so down. This might have been satisfactory once IC-based PLL decoders became available, but not in the early days when the pilot needed to be extracted by an LC tuned circuit.) FDM decoding was proposed, and the suggested circuit included a double-diode balanced demodulator fed with L-R and a 38Hz carrier (sine wave) obtained by doubling the amplified 19 kHz pilot, followed by matrixing with L+R.

TDM decoding seems to have been preferred in practice, and the MC1310 PLL decoder IC and all of its derivatives have been TDM-oriented. But Philips had a decoder (TDA1005A) that could be used in both TDM and FDM modes. Some tuner builders seem to have ventured into more complex decoders (usually semi-discrete or semi-integrated) that use Walsh functions for switching, Meridian and Sansui amongst them.

Please send me a PM if you want scans of the GE and Zenith articles.

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Old 29th Jul 2010, 11:13 am   #6
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Default Re: Multiplex stereo Zenith/GE what was the difference?

The problem with FDM encoding and decoding is that you have to ensure that the levels are exactly right, with L+R and L-R channels having exactly the same gain and phase throughout the modulation/demodulation process. This is quite hard to achieve with sine wave carrier, as modulator gain depends on the carrier level. Not too hard for the broadcasters as they only have to buy one good encoder for each transmitter, but expensive and liable to drift at the receiver.

TDM keeps the two channels together, so they are more or less guaranteed to have the same level. I think this is why most decoders use TDM. The snag is that the L-R is then out by a factor of 4/pi (?) so a bit of adjustment is needed, but at least this is a fixed known adjustment. Incidentally, this difference means that those sources which say that Zenith and GE were essentially the same (such as Wikipedia?) are actually wrong, but in a subtle kind of way.

This is a very educational forum!
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Old 31st Jul 2010, 3:20 am   #7
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Dave, I think you must be omniscient! Regarding the use of FDM at the transmission end, the editorial article in AA V6 notes that although Zenith’s analysis and system development was based upon TDM, it actual proposal to the FCC used an FDM system at the transmission end, with, as expected, TDM at the receiving end. (This aspect is not mentioned in Zenith’s own article, though.) And one of the decoder designs described in the book (Pilot) is stated to use a sample-and-hold system.

I suspect that the oft-stated essential similarity between the GE and Zenith systems is something of an urban legend. It could be that some who have used this statement have not checked with original source literature.

This is definitely a very educational forum. Questions such as this cause one to look back over materials on-hand – and probably not looked at for many a year – with a much more critical eye, seeing detail previously not taken in.

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Old 31st Jul 2010, 6:22 am   #8
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Curiosity got the better of me, and I downloaded a copy of the 1964 paper “Stereophonic Broadcasting and Reception”, by Spencer and Phillips of the BBC. Most interesting!

As well as discussing the pilot tone system from both the FDM and TDM viewpoints, it tabulates the various FM subcarrier and AM subcarrier systems that had been proposed up until that point.

The AM sub-carrier systems used subcarriers that ranged from 30 to 38 kHz. The various proposals included one from Siemens to use the lower sideband only of a 30 kHz subcarrier, with unmodulated subcarrier at 6% of maximum deviation, and one from the USSR, using a double sideband reduced carrier system, with unmodulated subcarrier (31.25 kHz) at 20% of the maximum deviation. Hardly surpisingly, the Siemens system was viewed as requiring relatively complex receiver circuitry. (Even in the PLL IC era, when it would have been somewhat easier, use of the lower sideband only for decoding the pilot tone system seems to have been rare – it was briefly touched upon in the thread “FM Deviation”, at: https://www.vintage-radio.net/forum/...349#post317349.

In the light of this article, one might infer that there was reasonable latitude in subcarrier frequency selection, unlike the case for TV sound subcarrier systems where multiples of line frequency were desirable. Given that 38 kHz was at the high end of the proposals, it may have been chosen to allow adequate “clear space” around the pilot tone (for recovery using LC tuned circuits) whilst not running afoul of the 67 kHz SCA channel. I suppose that given the triangular noise spectrum of FM, keeping the subcarrier frequency as low as possible was desirable, but even more so, having the pilot carrier on the low side of the subcarrier, but well clear of any AF modulation was important. By the time stereo TV sound arrived, PLL recovery techniques could be assumed, which I would think meant that having a line frequency pilot tone only fractionally above the edge of the audio baseband, was less of a concern.

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Old 31st Jul 2010, 11:53 am   #9
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Default Re: Multiplex stereo Zenith/GE what was the difference?

I think the difficulty in distinguishing between GE and Zenith systems boils down to trying to explain to someone that a square wave contains a fundamental sine wave component with a greater amplitude than the square wave. This may go over the head of the typical Wikipedia author. Of course, you first have to explain that TDM is just FDM with a square wave carrier.

There is are advantages in having the pilot tone at half the subcarrier frequency, rather than some other (sub)multiple. Frequency doubling and halving are fairly easy to do, even with valve circuits. A PLL naturally aligns itself in quadrature to the signal, but doubling then puts you back in phase. A PLL at the subcarrier frequency would need an accurate 90 degree phase shift - not easy for valves. A pilot tone at twice the subcarrier would be ambiguous so some other means would be needed to distinguish right from left.

Once you have decided that the pilot is at half the subcarrier, and know you need to keep clear of the SCA at 67kHz, then the rest of the system more or less designs itself.
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Old 3rd Aug 2010, 3:16 am   #10
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Something that has since occurred to me in connection with GE's proposal for an FDM system - I wonder if its choice of DBSC for the subcarrier had any connection with the late 1950s work by its employee Costas on DBSC communications systems. That might at least have disposed it to consider DBSC. However, I imagine that the use of a Costas loop for decoding would have been too complex and costly to execute in the valves era, at least for domestic receivers, hence the need for a pilot carrier.

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Old 10th Aug 2010, 9:19 am   #11
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Default Re: Multiplex stereo Zenith/GE what was the difference?

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Originally Posted by G8HQP Dave View Post
I once saw (in Wireless World?) a sample and hold decoder, which is yet another way to do it - this would suffer from a sinc frequency response but I'm not certain the author realised this.
Perhaps it was the article by D.E.O’N. Waddington, “Stereo Decoder using Sampling”, in Wireless World, February 1971?

The author posits that sampling at the correct instants allows L and R recovery without crosstalk, but that incorrect phasing and the need for a finite, not infinitely small sampling time introduce some crosstalk but not necessarily too much.

Possibly though the Waddington design was already outmoded. The Portus & Hayward PLL decoder had already appeared in WW September 1970 I think ( I didn’t keep a copy of this article, so I can’t be sure) and by early the CA3090 and MC1310 PLL-based decoder ICs could not have been too far away.

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Old 10th Aug 2010, 12:09 pm   #12
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Default Re: Multiplex stereo Zenith/GE what was the difference?

That was probably the one. I think he is right in what he says about crosstalk. As I recall he also claimed a reduction in noise, but I find that less convincing - I would expect an increase in noise. The main problem is that a sampled system needs the correct filtering both before and after sampling to avoid both aliases and images, and I don't think he did this. However, it was a long time ago so my memory may be defective.

I considered using the Portus & Hayward decoder in a tuner I was building, but eventually decided to take the easy way and used a Hitachi PLL chip instead.
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Old 2nd Dec 2015, 9:47 am   #13
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Default Re: Multiplex stereo Zenith/GE what was the difference?

A little more on this topic....

Radio-Electronics magazine for 1960 September reported that the FCC was testing six FM stereo systems.

In particular, it was said: “The Crosby Telectronics, Calbest, Halstead (Multiplex Development Corp.) and British Percival (EMI) sys¬tems were described in RADIO-ELECTRONICS, July, 1959, page 73. General Electric and Zenith are sponsoring the other two systems.”

So at that stage, the GE and Zenith systems were separate candidates. That they were not covered in earlier issues of radio-Electronics suggests that they were quite late entrants, and that at the time of writing, not much information about them was in the public domain.

Electronics World for 1961 July reported on the FCC’s decision in its editorial section, and also in a detailed article.

In the editorial; section, it noted: “The FCC approval, which became effective as of June 1st of this year, gave the nod to a system similar to the one recommended by General Electric and Zenith”.

This was essentially repeated in the main article, as: “The system adopted by the Federal Communications Commission is along the lines of the methods proposed by General Electric Company and Zenith Radio Cor¬poration, with certain aspects of the system developed by the FCC.”

One is left wondering though as to whether GE and Zenith combined forcers or whether the “combination” was as FCC idea.

A little later in the article it was stated:

“The FCC system of stereophonic broadcasting had its inception in a time-division multiplex switching system. Here, the input of a transmitter is switched rapidly between the left and the right stereophonic program channels, switching at a 38-kc. rate. The system was analyzed mathematically and with test equipment. It was discovered that this system was basically a "sum and difference" system. The sum of the left and right stereophonic channels appeared as audio modulation of the main carrier, whereas the difference between the left and right stereophonic channels appeared as suppressed-carrier amplitude modulation of a series of odd harmonics of the switching rate. Since FCC regulations do not permit the radiation of any signals in excess of 75,000 cycles, filtering of the higher harmonics of the switching rate resulted in the adopted system.”

So it looks as if the starting point was time division multiplex (TDM), not frequency division multiplex (FDM) with a DSBSC subcarrier, but that the two were found to be essentially equivalent. With the previously developed assumption that the TDM idea came from Zenith, then it could be seen as the primary developer. Perhaps that is why it is often referred to as the “Zenith-GE” system, rather than the “GE-Zenith” system.

A very useful source is BBC Monograph #29 of 1960 April, “A Summary of the Present Position of Stereophonic Broadcasting”. (http://downloads.bbc.co.uk/rd/pubs/a...nograph_29.pdf) It was quite comprehensive, and included information on the various FM stereo (and AM stereo) system proposal extant at the time. It noted the effective equivalence of TDM and DSBSC subcarrier systems. At the time, it noted that a subcarrier frequency in the vicinity of 40 kHz was required, with either a residual carrier or a 20 kHz reference tone for receiver synchronization. It was recorded that systems of this kind had been proposed by Zenith in the USA and by Grundig in Germany.

Also recorded was that GE, Philips and RTF (France) had proposed systems with AM subcarriers without any carrier suppression, apparently in the interests of receiver simplicity, although at the expense of performance.

From that one may deduce that GE, once it had seen the Zenith proposal and that it was not a world apart from its own, decided to back the Zenith system.

Conceivably the FCC contribution may have been the choice of 38 kHz for the subcarrier frequency. I’d say that that number was not “in play” when the BBC Monograph was written, otherwise it would surely have been mentioned. A observation made in the Monograph was that a 2 dB improvement in the difference channel signal-to-noise ratio was obtainable if the subcarrier frequency could be moved from 40 to 32 kHz. Presumably then 38 kHz was chosen as the lowest reasonably possible with anticipated receiver technology whilst allowing adequate space for the pilot tone.

The overall inference is that:

The Zenith-GE FM stereo system was primarily derived from the Zenith proposal to the FCC.

The original GE proposal was somewhat different, although with basic similarities, and GE moved to align with and support the Zenith proposal.

The fine details, most likely including choice of subcarrier frequency, and possibly modulation levels for the various components, were worked out amongst Zenith, GE and the FCC, with some FCC original input.


I suppose one could debate as to just how similar were the original Zenith and GE proposals, but it is worth noting that the 1964 BBC paper (mentioned in post #8 upthread) said this about the then-new Russian FM stereo system proposal (later known as Polar modulation):

“The U.S.S.R. system, as put forward at the C.C.I.R. Plenary Assembly in Geneva in 1963, is in many ways similar to the pilot-tone system. A normal amplitude-modulated sub-carrier is passed through a notch filter centred on the subcarrier frequency, and a compensating peaking filter is used in the receiver. In comparison, the pilot-tone system transmits the sidebands fully, but completely suppresses the sub-carrier, a tone of one-half of the sub-carrier frequency being transmitted as a reference signal from which the sub-carrier can be regenerated in the receiver.”

And the Russian system was very similar to the original GE proposal, differing only in that it had a partially reduced subcarrier. Not requiring a pilot tone, it used a 31.25 kHz subcarrier, which number I understand was chosen because it was twice the line frequency for 625-line television, thus making the system potentially suitable for use with TV sound as well as with FM. (I don’t know if it was ever thus used; the American work leading up to the Zenith-DBX MTS system indicated that AM subcarriers needed quite serious companding, amongst other measures, if vision-on-sound buzz was to be avoided.)

The American NRSC (National Stereo Radio Committee) of the late 1950s had as its remit the development of compatible stereo systems for AM and TV sound as well as for FM. (This was noted in Electronics World for 1959 October, page 6.) Only the FM case was progressed at the time, but AM and TV sound proposals were made by various parties. Radio-Electronics for 1966 November ran an article on the early TV stereo sound proposals, and that from Zenith was a PWM system that I think was essentially a TDM variant, more evidence of this company’s interest in TDM techniques.

Anyway, I think that we now have a reasonable fix on the original question, which was how close were the original Zenith and GE proposals. Admittedly the case is somewhat circumstantial and deductive, but is probably as close as we can get without seeing the original Zenith and GE documents.

By the way this late posting came about because when recently looking again at the list of BBC Monographs, which I had first seen many moons ago, I realized that I had not read #29. Quite why I do not know, given that it was an obvious choice to put on the reading list. Anyway, once I started reading it, I was reminded of this older thread.

Perhaps redundant to say by now, but the above-mentioned issues of “Electronics World” and “Radio-Electronics” are available at: http://www.americanradiohistory.com/.


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Old 2nd Nov 2016, 7:22 am   #14
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Thread re-opened at Synchrodyne's request.
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Old 2nd Nov 2016, 8:25 am   #15
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Thanks for reopening!

Quote:
Originally Posted by Synchrodyne View Post
Anyway, I think that we now have a reasonable fix on the original question, which was how close were the original Zenith and GE proposals. Admittedly the case is somewhat circumstantial and deductive, but is probably as close as we can get without seeing the original Zenith and GE documents.
The difference in the Zenith and GE proposals may be seen in the respective US patent documents, available on-line, at Google Patents, for example.

In the GE case, the patent was US 3122610, filed 1960 July 22 by A. Csicsatka, and granted 1964 February 25. It was entitled “Circuitry for Multiplex Transmission of FM Stereo Signals with Pilot Signals”.

The basic premise was a subcarrier system in which the subcarrier was of the suppressed carrier AM type rather than the FM type as proposed in earlier systems. The low amplitude pilot carrier was a subharmonic of the AM subcarrier and transmitted in the gap between the baseband and the subcarrier sidebands. The numbers 19 kHz for the pilot carrier and 38 kHz for the subcarrier were mentioned in the worked detail, but were apparently not fundamental to the proposition. Decoding in the receiver was by frequency division multiplex, with subcarrier recovery either by doubling the pilot carrier or by using the pilot carrier to synchronize an oscillator.

It was claimed that by keeping the pilot carrier frequency relatively low, it was recovered with less noise than if it had been higher in frequency. And also that it was easier to recover than say a reduced carrier at subcarrier frequency. Improved overall signal-to-nose ratio (presumably as compared with previously proposed FM subcarrier systems) was an objective.

GE suggested 67 kHz, with ±8 kHz deviation, for the SCA subcarrier.

The Zenith patent was US 3257511, "Stereo FM Transmission System", filed 1960 April 18, by R. Adler, A.J. De Vries and C.G. Eilers, and granted 1966 June 21.

The basic premise was also an AM subcarrier system, but one in which the subcarrier was derived by time-division multiplexing of the left and right audio signals, with a switching frequency in the range 30 to 50 kHz for high-fidelity systems. This generated the subcarrier fundamental and odd harmonics thereof, but only the fundamental was transmitted, the harmonics being filtered out. A pilot signal was added, and for this Zenith specified a modulation level of 10%, leaving 90% for the interleaved baseband and subcarrier signals. Use of a pilot carrier of half the subcarrier frequency was recommended to ease its recovery, and to avoid the need to specify its polarity. Zenith suggested a subcarrier frequency of 39 kHz. Zenith also saw maintaining capability of carrying an SCA channel as important, and like GE, suggested a frequency of 67 kHz for this. Receiver decoding could be by time-division multiplex, with appropriate compensation for the form of the switching wave.

So the GE and Zenith systems were in fact very close in net form, and so quite easily combined, with each contributing specific details. The 19 and 38 kHz numbers came from GE, the specific modulation levels came from Zenith, and the 67 kHz SCA frequency came from both GE and Zenith.

Neglecting the SCA component, one could describe the Zenith system as having a 19.5 kHz pilot carrier at 10% modulation, with baseband and the subcarrier at 39 kHz sharing 90% of the modulation. The GE had a 19 kHz pilot carrier at an apparently unspecified but low level of modulation, with baseband and the 38 kHz subcarrier sharing the larger part of the modulation. Synchronizing the two, as it were, was but a minor operation. So the simple answer to the original question as to how close they were is “within 1 kHz of each other”.

It has taken over six years, but now I think we have the definitive answer. The odd aspect is that nowhere in the literature have I found an accounting of this aspect of FM stereo history. One wonders if the magazine article writers in the early 1960s just accepted that the GE and Zenith proposals were very close, and did not dig any deeper. Or perhaps they did, but their editors thought it not worthwhile to publish such detail.


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Old 2nd Nov 2016, 9:24 am   #16
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Default Re: Multiplex stereo Zenith/GE what was the difference?

I wouldn't be too quick to call one approach 'FDM' and the other 'TDM'. With only two channels they are rather convergent. DSB demodulation is performed, or is that demodulator a mixer? or is it a synchronous rectifier? There is really only one process going on, but multiple ways of viewing it.

In terms of circuitry, the difference comes down to whether the demod/mixer core is a full-blown analogue multiplier producing the product of the signal with a 38kHz sine, or whether it is a switching mixer producing the product of the signal with a 38kHz squarewave.

The former needs more care to get it linear, the latter needs filtering upstream and downstream to remove products involving LO harmonics.

The same signal components are being mangled to get the same results, so the differences of method come out in the wash in general terms, but allow some choice of detail which allows room for circuit design preferences.

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Old 2nd Nov 2016, 8:48 pm   #17
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Thanks, David.

I guess that the convergence is illustrated by the fact that the GE and Zenith systems were combined with but minor detail adjustments.

Still, FDM and TDM respectively is how their developers characterized their systems. Zenith placed particular emphasis on the TDM nature of its approach.

This mathematically “loose” use of FDM and TDM to describe different physical implementations of what is essentially the same process seems somewhat analogous to the use of the terms “additive” and “multiplicative” to describe different physical implementations of heterodyne mixing. It is useful in a practical sense but does tend to hide the fact that the underlying process is the same in both cases, that is mixing depends on the fact that the net transfer curves include second order terms, easy to see in the "additive" case but harder to visualize in the "multiplicative" case.

Another illustration of both convergence and the use of somewhat imprecise established terminology may be seen in the Philips/Mullard TDA1005A multiplex decoder IC. This could be configured to operate in two modes, which in its application note Philips referred to as TMPX (time multiplex) and FMPX (frequency multiplex). In the former case, the whole signal (baseband and subcarrier) was switched at 38 kHz. In the latter case only the subcarrier was switched at 38 kHz, with sum-and-difference matrixing following. Most other multiplex decoder ICs operated only in the TMPX mode, to use the Philips terminology.


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Old 3rd Nov 2016, 12:27 pm   #18
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Default Re: Multiplex stereo Zenith/GE what was the difference?

The difference between FDM and TDM is the difference between 1 and 4/pi. It seems that Zenith understood this. However, what is actually transmitted in either case is FDM - yet almost all decoders use TDM methods with correction for the difference.
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Old 5th Nov 2016, 1:20 am   #19
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Default Re: Multiplex stereo Zenith/GE what was the difference?

Thinking along that vector, even if one used TDM techniques to generate the 38 kHz subcarrier, the 19 kHz pilot carrier still needed to be added, as well as optionally the 67 kHz SCA subcarrier, and that turned the system as a whole into FDM.

Perhaps a fully TDM case was the French system once used for bilingual TV sound in North Africa. This did not require the addition of a pilot carrier, as switching was done at vision line frequency, and line sync was used to do the switching at the receiver.

Interesting is that in the quadraphonic era, of the proposed Dorren Quadraplex system for four-discrete channel FM broadcasting it was said that it whilst either FDM or TDM techniques could be used to generate the two subcarriers (38 and 76 kHz), TDM had a lower first cost and was easier to maintain. I imagine that generating the QAM 38 kHz subcarrier with the correct relative levels would have been the difficult part with an FDM approach.

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Old 5th Nov 2016, 1:51 am   #20
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Default Re: Multiplex stereo Zenith/GE what was the difference?

In the late 1960's I read an article in an American electronics hobbyist magazine that was then being sold in the UK, about how some American FM stations were piggy-backing advert-free "muzac" for shops onto a conventional FM broadcast signal using an additional subcarrier. There was a description of how to modify a conventional FM receiver to decode them. I don't recall the name of the journal or any technical details. Possibly this was a fore-runner of the quadraphonic system.
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