Re: Stero Card for the GS

[philip@labtam.oz.au (Philip Stephens)]

Richard J Wifall writes:

>This sounds neat and all, but what stereo equipment can handle more then
>2 channels?  I think all the surround sound equipment that I am aware of
>use some encoding technique instead of actually having greater then 2
>inputs/outputs.

  This is true enough, although I'd imagine that a card that performs
Dolby Surround encoding would be rather complex and/or expensive.  If
that isn't the case, I'd certainly agree it would be a better way to
provide 4 channels of sound.

  I've done a little bit of experimentation in generating Dolby
Surround output in software via a standard stereo sound card, and
it seems like it _may_ be plausible; the basic trick in generating
sound for the rear channel is to have one oscillator playing a
waveform 180 degrees out of phase with respect to a second oscillator.
  
  I wrote a simple program to do this, and it basically worked (the
sound came exclusively out of the rear speakers of my surround setup),
but there were beats in the sound, probably due to the fact that the
phase difference wasn't exactly 180 degrees.  (It's rather hard to
achieve this, since the Ensoniq DOC chip services each oscillator
sequentially in a round-robin affair.)  I haven't investigated the
issue further, but it should be possible to create two waveforms in
DOC RAM that when played back by two specific oscillators will be
exactly 180 degrees out of phase.

  There is a related problem that when generating normal stereo by
having two oscillators playing the same waveform, because they are
also not in phase the sound leaks into the rear channel and hence
generates a "false" surround signal!  So in order to create true
Dolby Surround output on a GS via software, it would be necessary for
each instrument to have two waveforms in DOC RAM that are properly
adjusted to sound in phase or 180 degrees out of phase when played
back.

  If anyone thinks that a Dolby Surround music player would be a
nice thing to have, I might try experimenting a little further
along the lines described above.  Of course, I'm sure this post
is probably lost on some people who aren't familiar with the
principles behind Dolby Surround (hell, I'm not really all that
familiar with it myself in all it's gory detail :-D)

--
=========== Philip Stephens, Systems Programmer, Labtam Australia ============
====== Owned by Motor (Mo for short), the power-purrer from Down Under =======
=== DS (B+S)t Y 2 X L  W C+++ I T+++ A+ E H+++ S+ V+ F Q+ P++ B+ PA+ PL+++ ===
"Many views yield the truth.  Therefore, be not alone." -- Viggies' Prime Song