"Fleetie" wrote in message
...
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?

Yours, not-old-enoughly-but-very-nearly,


Martin

There were three basic encoding methods for quad records in the 70s. SQ and
QS were matrixed systems which means that the rear channels were encoded
essentially by phase differences in with the front channels. This would make
it reasonably compatible with normal stereo and almost compatible with mono,
important for broadcasting which was mostly on AM in those days.

The third encoding method, which you allude to in your post was CD4, which
was a carrier system similar to FM in that the front plus rear channels were
mixed in the base-band, and the front-rear encoded on a carrier and then
decoded on playback in a similar way to FM stereo. The great advantage of
CD4 was that separation between front and back was much greater than a
matrix system, and stereo and mono compatibility was excellent. The great
disadvantage of CD4 was that it didn't work! Under laboratory conditions,
with clean, unworn records it would work fine, but in the real world, with
records of varying cleanliness and wear, it would just collapse. Cartridges
had to track up to 45 kHz to recover the carrier and sidebands, and even one
playing by the blunderbuss cartridges fitted to most record players those
days would render the LP unplayable as a quad LP.

Different labels adopted different encoding standards. DSOTM was on EMI and
they used the SQ system as did CBS and, I think, did Decca, so a quad DSOTM
on EBay would be SQ encoded and you wouldn't need to worry about recovering
an HF carrier. If it's in good condition for stereo, the quad encoding
should also be in good condition.

Decoding SQ these days is pretty much impossible unless you can find a good
decoder dating back to the 70s. Sony made a few good ones the best probably
being the Tate decoder. As standard, the SQ system has only something like
7dB of crosstalk between L and R and only 3 dB between front and back if I
remember my matrix co-ordinates correctly. The later decoders had logic
steering to improve the subjective separation at the expense of wavering
positions.

The combination of three incompatible systems that never really worked
properly (there was a fourth, Dr. Duane Cooper's UD4, which however never
came to Europe in significant numbers)and the need for four loudspeakers
arranged in a square around the listener meant that the quadraphonic craze
only lasted a few years in the mid '70s.

If you want to find out more about quadraphonics in general, there's
http://www.quadraphonicquad.com/ which has information on the systems, a
forum etc.

S.

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