[OT] Re: [vorbis] Format comparison

[Craig Dickson]

Merijn Vogel wrote:

> Eh, just a (seemingly offtopic) question, but how do you create a 4 channel
> lp ? With just one needle ? 

Back in the '70s, there was a brief vogue for "quadraphonic", or
4-channel music. There were indeed 4-channel LPs that were played with a
single needle. There were a couple of ways of doing this -- mutually
incompatible, of course.

(1) The sums (left_front + left_rear) and (right_front + right_rear)
were used as the standard stereo left and right channels, but with the
differences (left_front - left_rear) and (right_front - right_rear)
modulated onto a supersonic carrier (such that all difference
information is above 20 kHz) and then added to the left and right
channels respectively. This meant that a standard stereo player would
produce a normal stereo output (since it would barely respond to the
supersonic carrier, and certainly would not demodulate it), while a quad
system could decode the left and right channels to create quadraphonic
sound.

(2) The four channels were matrix-encoded as follows, where A, B, C, and
D are signal-level multiplier constants and E, and F are phase-angle
constants:

 left = left_front * A + right_front + B + left_rear * CE + right_rear * DF

 right = left_front * B + right_front * A + left_rear * DF + right_rear * CE

This, on the one hand, made less demands on the cartridge (no supersonic
carrier to worry about), but channel separation was not as good as with
(1), and backward-compatibility with conventional stereo systems was
also inferior.

For the most popular system of this type, constants A and B were 1 and 0
respectively, providing excellent separation between the two front
channels. Rear separation was not as good. 

Craig

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