"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?


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.

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.

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.