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