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Old 7th Oct 2018, 10:11 am   #81
percival007
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Default Re: FM stereo

This is a great thread folks, what an interesting read.

In the article linked to in an earlier post;
( https://www.bbceng.info/Technical%20...digits-fm.html )
the mention of those PCM DAC Cards fascinated me.
Does anyone know how they worked? Were they 'Ladder Dac's' or some other 'architecture'?
Do any of those Cards still exist? Any circuit diagrams?

Again, all fascinating ! Cheers all,

James.
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Old 7th Oct 2018, 10:38 am   #82
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It would have been quite practicable to use ladders at the time, as these convertors were made in small numbers and regularly maintained. It is interesting to note that the huge quality lift was noted at the time, and enjoyed, yet when CD arrived, some chose retrospectively to condemn the PCM system. Some nit on the net asserts that FM quality "plummeted" when it was introduced...
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Old 7th Oct 2018, 9:10 pm   #83
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Ted corrected me on my [mistaken] perception of a lack of interest in stereo Broadcasts at post 50*. Frank was talking about a distinct lack of interest from customers in the Japanese units at post 63* but unless I'm wrong there is a bit of "crossover" going on between Broadcast experiments and the likes of Sony trying to interest punters in Quadrophenic sound from vinyl. I'm not sure if Frank means Tuners or Record Deck equipment Certainly the latter seemed to be very expensive kit to the average person. Most people were already adjusting to the possibility of having [relatively] good quality stereo record playing equipment and spending a lot of money to decode the relatively few costly 4 Channel LP's didn't seem attractive. As it happens I do have one of the Decoders from Sony but I've not had it running as I have never owned any records or the associated deck.

Wireless World did refer to various encoding systems over the years as Synchrodyne says [post 74*] whether Broadcast or on Vinyl but I couldn't really understand the theory and maths or see the point [except for Pink Floyd performances using their Azimuth Coordinator Joystick projecting sounds around the room]. Although designed for the very circular Royal Albert Hall, it was deployed to great effect at the somewhat oblong Free Trade Hall in Manchester.

The Vinyl Quad sound turned out to have a simpler process-a not so sophisticated but very cheap alternative. The Wireless World cutting I have [undated but probaly 1972] refers to David Hafler of Dynaco and his technique for recovering out of phase content from the original recording. That in turn seemed to be simplified down to variations of running two speakers at the back of the room connected IN SERIES between the + side of each channel. The results varied and were dependent on the original recording technique employed but it was cheap and simple. As the recovered info tended to be in the mid range any sort of speakers would do for the "back channels". I used two redundant car spkrs [uncased] and hidden behind the couch in 72. Two of the recommended records Abbey Road by the Beatles and Surfs Up, by the Beach Boys, sounded great. With Surfs Up you could actually hear the water lapping at the back of the room

Dave W.

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Old 7th Oct 2018, 9:19 pm   #84
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Hi Dave,
It was Vinyl Quad record decks, tuner amplifier with stereo FM decoder and speakers, sorry don’t remember the make.
Yes expensive, few records, and little customer interest, don’t blame them, if we had two in stock it was two too many.
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Old 7th Oct 2018, 9:27 pm   #85
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Thanks for that Frank. Your info from being in the trade is invaluable. I didn't expect such a quick response. My personal impression, at the time, was that some people thought it was an attempt to sell the same thing twice. Much like the attitude others expressed [rightly or wrongly] when CD's appeared

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Old 7th Oct 2018, 9:49 pm   #86
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The main problem with quadraphonics as promoted in the 70s was that it simply didn't work - either the two additional channels were carried in some kind of matrix arrangement, where the separation and definition were lamentable (despite the use of "logic" decoders) or a carrier system which stretched vinyl technology to its limits and beyond. Either way, stereo performance was noticeably degraded, with little benefit.

The mathematically more rigorous Ambisonics system came into its own as a way of making steerable microphones and the like, but this was almost something different in kind, not degree.

The change in perception with quad was not nearly as great as that going from mono to stereo, and in any case much of the subjective benefit could be derived from the Hafler arrangemnet , as has already been noted.

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Old 7th Oct 2018, 10:45 pm   #87
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Sony were trying to flog their 'SQ' matrix system and I used to get invited to an annual doo where people from Sony demonstrated their full range for that year.

The quadrophonic demos were underwhelming. The bunch of us who went together discussed each afterwards and no-one perceived anything good at all. The stereo presentation was less clear if anything. As Ted said, it simply didn't work.

There was one system, CD4, that did produce real, separate channels, using difference channels on high frequency subcarriers - just like the Zenith stereo radio format. However this was priced at an even higher level AND the difference info on the records wore off. Not a very good selling point.

I remember reading a very favourable review of the SQ version of 'Several species of small furry animals gathered together in a cave and grooving with a pict' I think this probably attributes to a reviewer who must have been smoking something in close accord with whatever the group was on.

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Old 7th Oct 2018, 11:26 pm   #88
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Interesting comments on the Sony equiment. A few years ago I was given a Sony "Compact Hi-Fi Stereo System" [LBT -V302CD] which has an FM tuner, 45/33 record player, and CD player. While it came with 2 speakers, (connected via bare wires to their speaker terminals) it also has two phono sockets on for connection to a "Sony SS-V5 surround speaker system". No details of this are given. However, as the terminals are marked as suitable for 8 to 16 Ohm speakers, I assume the additional system is simply a pair of additional speakers. The main control panel has a knob to adjust the surround sound. As I don't have a spare pair of speakers, I haven't tried connecting anything to the surround sound phono sockets. The user manual is dated 1990, somewhat later than the experimental FM broadcasts, so I wonder if surround sound would be somehow synthesized from conventional L and R stereo audio channels rather than source material that had actually been encoded with surround sound information, possibly as per #83? No details of the surround sound feature, such as what sort of source material it is intended for use with, are given.

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Old 8th Oct 2018, 7:25 am   #89
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There was a fashion for having a pair of rear speakers connected in series differentially across the live sides of the feeds to the two normally-connected speakers at the front. An elaboration was to have a suitably beefy variable resistor from the otherwise floating junction of the two rear speakers.

It was supposed to produce quad-like effects from SQ or QS matrix encoded records, and to a lesser extent from plain old stereo records. It smeared the apparent locations of sounds around somewhat, it's difficult to say whether it was much worse than a full matrix quad system. Neither did what it said on the can. At least this way reduced the amount of money wasted.

The pseudo-quad trick was usually attributed to David Hafler.

All this stuff just goes to show how effective the Zenith pilot-tone stereo radio system was at stuffing two channels into one and then separating them again!

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Old 8th Oct 2018, 7:44 am   #90
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Originally Posted by Radio Wrangler View Post
All this stuff just goes to show how effective the Zenith pilot-tone stereo radio system was at stuffing two channels into one and then separating them again!
Absolutely - it was a big deal at the time, wasn't it? Potentially, and frequently, the best stereo source domestically available for over a decade. I still think it's pretty neat, even if dulled by familiarity and about by over-processing.
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Old 8th Oct 2018, 9:25 am   #91
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Interesting that GE and Zenith independently came up with similar systems, which the FCC then combined into one with one or two minor changes. GE had approached the question from an FDM viewpoint, Zenith from the TDM side. Both recognized the interleaving advantage that came from using a suppressed carrier DSB subcarrier, as compared with say an FM subcarrier, which in turn meant minimum reduction of the mono service area of existing FM transmitters. Both also recognized the noise advantage of using a subharmonic pilot tone of as low a frequency as reasonably possible, placed in the spectrum in such a position that its recovery was not too difficult to do in domestic receivers. In the US context both also understood that any FM stereo system should still leave room for an SCA subcarrier.


Cheers,

Edit: The Zenith and GE contributions were discussed in this earlier thread: https://www.vintage-radio.net/forum/...d.php?p=349456.
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Old 8th Oct 2018, 11:02 am   #92
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At least we were spared the Percival steering system. The BBC didn't try this on air but did closed-circuit tests.

The death blow was dealt by a BBC recording of the Nine Lessons and Carols service from King's, Cambridge. The reverberation confused the steering detector and the boy soprano singing Once in Royal David's City oscillated from full left to full right at rates up to 16Hz, with effects on the listening panel which were comic and distressing by turns. Several people ran out of the room and vomited, whilst others just fell flat on their backs.
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Old 8th Oct 2018, 3:12 pm   #93
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I may be alone in my contrary view, but I've always felt that advent of the GE/Zenith stereo did rather spoil the performance of FM radio. I have fond memories of upgrading from MW/LW (when it had a decent 10kHz bandwidth) to the new-fangled FM (mono initially of course). The silent background was a revelation and tape recordings were comparable with the quality of vinyl records. So having got a near-perfect radio system, what did the industry then do? Ruin it by going stereo!

I'm probably something of a quiet background audio junkie, but the theoretical 6dB degradation in signal-noise ratio on switching to stereo often seems a great deal worse in practice, no matter how good the aerial and tuner. I've just looked up a few FM receiver specs, and it's quite difficult to find quoted figures for optimum S/N ratio with a strong aerial signal. More often we're quoted required input for 30dB quieting, which is hardly Hi-Fi. I did find some figures which do ring true in the spec of my trusty vintage Technics ST-3500, which quote (at +- 40kHz deviation), mono S/N is 75dB and stereo S/N 60dB, a 15 dB stereo degradation which is pretty audible. Judging on noise audibility, other more modern tuners, whether Quad, Technics, Denon etc also seem to give a similar degraded S/N ratio in stereo.

In addition to circuit/system random noise, we are also nowadays afflicted by sundry 'birdie' whistles and burbles from time to time. Before the advent of stereo, I don't remember ever using the term birdie in connection with FM - a good tuner and aerial gave effectively silent reception. It may be that here in the East of England, we're particularly afflicted with co-channel interference from the Continent - for example BBC Radio 2 today has a persistent background birdie whistle. Mercifully, BBC Radio 3 is usually clean, but a critical A/B comparison with the DAB version normally comes out in favour of DAB (both FM and DAB have good external aerials).

After half a century of FM stereo, with today's crowded FM band and with today's listeners accustomed to the relatively silent background of digital media, is it time to consign the GE/Zenith FM stereo system to history?

Martin
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Old 8th Oct 2018, 4:26 pm   #94
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No, I don't think so. I'd rather have a little hiss, which to be frank, has never troubled me with a decent aerial, than the deleterious effects of DAB data compression or, worse, mp3 at silly bit rates. Given that all engineering is a compromise, the multiplex stereo system is still pretty much in the sweet spot.
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Old 8th Oct 2018, 4:42 pm   #95
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Quote:
Originally Posted by Hartley118 View Post
After half a century of FM stereo, with today's crowded FM band and with today's listeners accustomed to the relatively silent background of digital media, is it time to consign the GE/Zenith FM stereo system to history?
That's an intriguing question.

'Relatively silent background' is key, I think. I have a Sony FM & DAB set; also an FM & CD stereo which will accept memory cards and sticks. Both contemporary, average items. Both exhibit low-level but distinct background noise.

In contrast, I recall hearing Sony's first CD machine for the consumer market, the CDP-101, in a radio studio. There was no background noise I could discern.

A later model CD machine, from Aiwa, was much slimmer and lighter! It also produced low-level, distinct background noise.

I imagine that modern digital devices could be designed so as to generate no audible noise, as was the CDP-101. I'm not sure if the average consumer would really notice the difference. Personally speaking, I am sure that I was made more critical of hi-fi performance by my exposure to broadcast-standard studios.

I suspect that an extra-special effort was put into the Sony CDP-101, to assist in launching the CD format. Later models generally didn't need to sound quite so good.
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Old 8th Oct 2018, 7:07 pm   #96
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I first heard CD via a demo at my local hi-fi dealer.
Background noise similar to line whistle was very evident to me, although the demonstrator claimed not to be able to hear it. I'd rather have the slight hiss of FM with a good aerial than that.
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Old 8th Oct 2018, 7:34 pm   #97
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Since continuity-to-receiver output s/n ratio can routinely exceed 70dB, what's the fuss? OK, you need a good aerial, but you need that to avoid other nasty effects anyway.

The first products in any given digital format have generally been over-engineered and hence are still sought after. The Sony F1 is a work of art. The CDP 101 is massively built, as are the early Philips players. So concerned were Sony that DAT should be a success that they put Philips D-A convertors in the DTC 1000, another copper-chassised tank.
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Old 8th Oct 2018, 10:06 pm   #98
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Quote:
Originally Posted by Ted Kendall View Post
At least we were spared the Percival steering system. The BBC didn't try this on air but did closed-circuit tests.

The death blow was dealt by a BBC recording of the Nine Lessons and Carols service from King's, Cambridge. The reverberation confused the steering detector and the boy soprano singing Once in Royal David's City oscillated from full left to full right at rates up to 16Hz, with effects on the listening panel which were comic and distressing by turns. Several people ran out of the room and vomited, whilst others just fell flat on their backs.
On that basis, I’d say that the Percival system was definitely an “F” fail, perhaps even suitable for inclusion in the list of “heroic failures”.

Still, to be fair, it was probably seen as an avenue worth exploring back in the late 1950s, when quite a few FM multiplex systems of diverse types were proposed. Given the triangular noise spectrum of FM, it was well-known that extending the modulation bandwidth beyond the 15 to 20 kHz range would incur a noise penalty that steadily increased with the bandwidth. So methods that might reduce that bandwidth were worth investigating, even if only empirically to prove that they were dead ends.

That said, in a qualitative sense, deriving a control signal from the audio signal itself does not sound like a formula for highly accurate decoding. Something similar was done, admittedly at the receiving rather than sending end, with some surround sound decoders, such as QS Variomatrix and SQ Logic, with, as already noted, categorically underwhelming results.

I can’t help wondering what would have happened if a King’s Cambridge performance had been recorded in four channels, then encoded into one or other of the matrix surround systems, and played back through a gain-riding decoder (which I think was the type recommended for use with Matrix H)….

A good overview of the broadcast stereo situation as it was in 1960 was provided in this BBC monograph:

BBC Monograph 29 196004 Stereophonic Broadcasting.pdf

The Percival system was discussed in reasonable detail from page 20. Note that there were also systems that used SSB (LSB) subcarriers in order to reduce the bandwidth extension. These would have been difficult from a receiver viewpoint back then, although feasible in the IC era.

The BBC monograph also referred to proposed AM stereo systems. In the USA, whilst the FCC remit at the time was only in respect of FM stereo, the industry body NSRC (National Stereo Radio Committee), set up to review and test the proposals and to assist the FCC accordingly invited (and received) proposals for AM stereo and TV sound stereo as well as FM stereo. In some ways AM stereo was an easier case, in that it allowed for two channels by using some form of quadrature modulation (to use that term very broadly) without the need to extend the bandwidth. The problem though was making any quadrature system reasonably compatible with the relatively crude envelope demodulators used in the vast majority of AM receivers.

A slightly later view of the FM multiplex stereo situation was provided in this 1962 BBC Research Department Report:

BBC RDR 1962-49 Multiplex systems for stereophonic broadcasting.pdf

In the USA, the quest for FM quadraphonic (other than by matrixing, which did not require any changes to the existing system) also led to a multiplicity of multiplex proposals being presented to the NQRC during the 1970s. NQRC recommended the Dorren Quadracast system, perhaps most easily visualized as a four-input TDM system with only the 1st and 2nd resultant sidebands transmitted. The FCC might have actually approved this circa 1983, but right now I cannot find corroboration. But anyway, by that time quadraphonics was analogous to the “Norwegian Blue”.


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Old 8th Oct 2018, 11:15 pm   #99
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Originally Posted by Hartley118 View Post
So having got a near-perfect radio system, what did the industry then do? Ruin it by going stereo!
Well, you could always switch to mono. One reason for the Zenith-GE system was that for mono reception, the S/N degradation was minimal. With a mono broadcast peaking at ±75 kHz deviation, and the sum channel of a stereo broadcast peaking at 90% of that, i.e. ±67.5 kHz deviation, then the S/N degradation when receiving a stereo broadcast in mono would be around 1 dB.

Preferably I think that you’d want a tuner that took a mono feed direct from the FM demodulator – with de-emphasis of course – and bypassed the stereo decoding circuitry. That might have been less common than simply using the “forced mono” input of the decoder IC. But the Quad FM3 did it that way (direct from demodulator mono) from its second iteration (when the MC1310 PLL decoder IC superseded the MC1305 non-PLL type originally used.)

Here are the S/N curves for the Revox A76, which was, back in the late 1960s and early 1970s, regarded as something of a “supertuner”, evidently designed for the difficult reception conditions existing in mountainous Switzerland:

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It gets to better than 70 dB in stereo, but requires at least 500 mV of signal to do that. Mono is 2 or 3 dB better, but it gets there with only 50 mV of signal.

And here are the curves for the Quad FM3:

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Not a lot different in ultimate terms, but around 6 dB less sensitive. The FM3, at least from its 3rd iteration, has what one might call a 1970s industry basic standard circuit, that is two dual-gate mosfet front end with 3 or 4 gangs, ceramic IF filter(s), CA3089 (or derivative) based IF strip and MC1310 (or similar or derivative) decoder. (The 1st and 2nd iterations of the FM3 could be described as “late transitional”.)

I guess a pertinent question is, what S/N (relative to full-scale) is needed to ensure that the noise level does not intrude. That probably varies by listener, but a number adequate for a good proportion of listeners might be available.

Of course, those noise measurements are probably made with clean single channel input signals, and whilst they would include any self-generated birdies, they would not take account of the birdies caused by less favourable reception conditions. At a given level relative to programme, such might be more annoying than broadband noise. (We have a NAD 412 that has avicultural leanings – one energetic day I’ll find where the Carver TX-11a and Quad FM66 have been packed away and re-install one of those. Meanwhile, RNZ Concert comes via the digital TV UHF multiplex.)

Not that the Zenith-GE stereo noise issue has not been the subject of improvement efforts in the past. At the transmission end, the abortive FMX system of the 1980s, based upon the CBS CX noise reduction system, was one example. At the receiving end, there was at least one Japanese FM tuner whose decoder used the lower sideband of the subcarrier signal in order to reduce noise. (I saw that in one of the hi-fi magazines long time ago, back in the 1980s or 1990s, but cannot retrace it.)


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Old 8th Oct 2018, 11:40 pm   #100
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This extremely technical debate is fascinating but like most types of audio reproduction it has a psychological aspect at the end of the day. It seems to me that one encoding system was clearly superior but from a recording perspective, I've sometimes found myself with exciting crystal clear reproduction that turns out to be an accidental doubled up mono signal [due to some fault or other] or maybe one channel has just failed altogether but needs to be doubled. The result rarely seems inadequate as it matches the previous mono technology in much better quality. Not everyone will agree of course. Maybe nobody.

When the early Dylan recordings were brought out in the original Whitmark mono masters it was a big seller. I'm not a "Back To Mono" enthusiast myself. I think Ringo Starr was just advocating a simpler approach with his campaign for that in the eighties and not necessarily one technology or another. Sometimes it's great though, just to be able to hear the material at all but I'll happily go with Mono or Stereo [a great leap forward] or Binaural sound as in Derek Jarman's great film BLUE [a technique pioneered by Alan Blumlien along with so much more] that just shows a blue screen while he's losing his sight. A cyclist nearly runs into him and shouts "Are You Blind?" He is! Wikki says the film has a stereo soundtrack but when it was Broadcast on Ch4 and Radio 3, in headphones you flinched as Derek crossed the road and the cyclist approached shouting in your ear! Binaural sound was a big thing for a short while with the Beeb. They generally binge on something trendy or an Anniversary and then move on!

Dave

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