"David Looser" wrote in message
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"Fleetie" wrote in message
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Title lifted from an Ebay auction with Pink Floyd
search terms.

I've read a little about quad LPs recently. If my understanding
is correct, the encoding is a little like FM stereo in that there
is information encoded in HF above the audible range, and it's
something to do with sum-and-difference, that when decoded, yields the
rear channel information. How close am I?

I was surprised when I read that, since I assume a lot of that
information is above 20kHz, and quad was en vogue back in the 70s.
Were cartridges and preamps of the time capable of picking up signals
way in excess of 20kHz accurately?

Let's suppose I were to buy a Floyd DSOTM quad LP from Ebay:
Is there any equipment now that could take the output from
my cartridge and decode it? (Kinda irrelevant in fact because
I only have 2 speakers and a stereo amp, but... In the bottom of
my heart...) I would imagine that you can't easily get that kind
of gear any more.

Also, since the groove modulation extends to much higher frequencies
than it does with normal stereo, would playing it with a normal
cartridge/stylus be likely to cause it great (brain) damage (I use a
Sumiko Blue Point Special Evo III)?

How different was the back-channel information from the main
front signal? Could you, in practice, have 4 completely
different signals (vocals, for example) coming from the 4
speakers with very little crosstalk? Would the strident
tintinnabulation starting "Time" be different from each corner?


There were several different "Quadraphonic" systems, and I don't know
which Dark Side of the Moon used. The original quadraphonic system was the
CBS "SQ" system. This, like most of the others, (such as Sansui's "QS" and
the BBC's "Matrix H") was a "matrix" system in which four original audio
tracks were combined into two by a process of phase-shifting and
combining, and then separated again using phase-sensitive circuitry and
gain-controlled amplifiers. The most commercially successful of these
systems is the Dolby pro-logic system. Though this has been marketed as a
surround-sound system, not a quadraphonic one, it is essentially the same
idea.

The BBC's Matrix H was yet another matrix system, but one that came out of
Michael Gerzon et. al's work on ambisonics, and which was evaluated for
broadcast. The BBC did some test transmissions in Matrix H, but it never
went into full service as it didn't have full mono compatibility (something
the BBC was somewhat paranoid about at the time) and anyway, the whole quad
thing had pretty much gone away by then.

There was also the RCA "FM4" system, which seems to be the one you are
thinking of. This used a frequency-modulated carrier around 20kHz to carry
the band-limited rear channels.

You're thinking of CD4, which was the system used by RCA, JVC, Denon and
others. It was developed by JVC.

With the matrix systems channel separation is very poor, hence the use of
gain controlled amplifiers which "enhance" the separation by reducing the
gain of the speaker channels into which the wanted signal has bled.
Clearly here it is NOT possible to carry 4 completely different signals.

With the RCA system in theory crosstalk should be very good (at least no
worse than normal disc stereo), but as you realised there were serious
problems with the sub-carrier at the top of the audio band. I had no
practical experience of this system, but I do remember that it didn't do
well commercially. If a record of this type is anything other than
pristine I would guess that the sub-carrier would be seriously reduced in
amplitude from it's new condition.

David.


As one who lived and worked through the Quadraphonic era, it was a very
interesting time, and might have succeeded if there hadn't been three
competing and incompatible systems (UD4 was the fourth system, but it never
came to Europe). SQ and QS worked acceptably well for classical music and
jazz if ambiance only was recorded in the rear channels, giving a much
better impression of the venue. For rock music, I thought it worked less
well, given that producers wanted to use the surround for effect, swinging
instruments not only side to side, but also front-back and diagonally. With
the earlier non-logic decoders, crosstalk was so bad as to prevent accurate
localisation, and with the later logic-steered decoders, pumping effects
could be unpleasant. It's a great pity that Ambisonics never caught on, it
was being sponsored by the state-run NRDC at the time, with the predictable
result when marketing is handled by civil-servants. Of the competing
systems, SQ was probably the most successful commercially, but CBS, EMI and
others lost interest and never fully developed the potential.

Dolby later realised that what didn't work for music could well work for
movies, and launched the analogue matrixed Dolby Surround, which then led to
the discrete Dolby Digital we know and love.

S.

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