
 EH> Is the surround sound-thing which the GUS can do with a Dolby
 EH> Pro-logic amplifier special for the ultrasound?

No, it's not.. Here is description of Dolby Surround that I picked up in
comp.sys.ibm.pc.soundcard a few months ago:

This discussion started in csipg.action, so it's crossposted there, but I
thought the soundcard and demo newsgroups would get more bang out of this. <g>

For those who are interested:

(fairly long)

How Dolby Stereo (aka Dolby Surround) works:

Dolby stereo is a system designed by Dolby Labs in the '70s for creating
better movie sound, including surround-sound. The other benefits are
increased directionality of front sounds, and especially improved
localization of on-screen sounds like dialog for the people sitting on the
edges. The system is totally separate from Dolby noise-reduction systems
like Dolby B and C.

There are four output channels: Left front, Right front, Center front, and
Surround. These are encoded on two channels in such a way that it can be
played back on a standard stereo device, a mono device, or a Dolby
decoder. (Or a THX system, which is a Dolby Pro-Logic decoder with
specifications licensed by Lucasfilm).

A word is in order about THX. It is NOT different from Dolby Pro-Logic, at
least not precisely. It is a licensing and quality-control wing of
Lucasarts with the stated aim of improving movie sound. It licenses the
name THX for movie equipment, movie theatres, home equipment, and
laserdiscs. The name means that the THX engineers certify it as good
quality. There are a few processing steps in home THX that are not
typically present in a plain Pro-Logic system, but they are merely
enhancements to Dolby Pro-Logic decoding, not a different system.

The Center channel is the most used one in a film. It contains all of the
dialog, and most of the on-screen sound effects. It is important to have a
center channel so that people on the edges of the theatre (or your living
room) still hear the on-screen sound from the direction of the screen, not
panned to one side or another. Without the center channel, people hear
most of the sound coming from the nearest speaker.

The Surround channel is used for "ambient" effects: sounds that should
envelop the listener. It is not a rear channel, and should not be used for
directional sounds. In a properly setup system, the listener should not be
able to tell where the surround speakers are located. In a movie theatre,
there are generally many surround speakers around the back of the hall. In
the home, this is impractical, so two speakers facing in such a way as to
maximize reflected sound are used.

Any device capable of playing back two distinct channels can produce Dolby
Stereo-compatible signals. This includes any stereo soundcard, stereo vcr,
or even CD player or record (there are Dolby Stereo encoded CDs).

Here's how:

I'll refer to the four output channels as LO, RO, CO, and SO, for left,
right, center, and surround, respectively. The input channels (the ones
coming from the source and into the decoder) I'll call LI and RI. (If you
don't like acronyms, feel free to use search-and-replace to make this more
readable.) :-)

The LO channel and RO channel are recorded normally on the LI and RI tape
channels (we'll assume tape recording for example purposes). The CO
channel is recorded on BOTH the LI and RI channel at exactly the same
volume, in-phase (i.e., no special processing). The SO channel is recorded
on LI and RI at the same volume, with inverted phase (i.e. every peak on
one channel is a trough on the other).

There are a few interesting nuances here. Because the CO channel is
recorded normally on LI and RI, it will still image in the center on any
stereo playback system, as long as the listener is basically in the
center. As the listener moves further left or right, the image moves in
the same direction, which is a little distracting, but acceptable. This is
what one gets with a simple "surround" decoder. Without the Center
channel, the imaging is fine for one or two people, but not so great for
groups. Dolby Pro-Logic adds (among other things) the center channel.

Note that because of the inverted encoding scheme for the surround
channel, it will disappear when played back on a mono system. Each peak is
precisely canceled by a trough on the other channel, and all surround
information is lost. This is another reason to put only ambient,
non-essential sounds in the surround channel. Many systems are still mono
(most VCRs, for example) and will not reproduce them.

The decoder takes the signals and reproduces a fairly good semblance of
the original four channels, though there will always be leakage and
crosstalk. Dolby Pro-Logic and THX circuitry have special processing that
minimizes perceived crosstalk.

When the Dolby Stereo is mixed, the engineers listen to it running through
a Pro-Logic decoder exactly like the one in the theatre or your home.
Thus, they design the four channels specifically so they will decode
properly. It is not a good idea to mix four distinct channels, then do the
Pro-Logic encoding as a post-process, because the results will not be
exactly the same as the original four distinct channels.

Doing it on a soundcard (kinky as it sounds)

On a soundcard, sending signals to the center channel is a simple as
playing the sound in dead-center (equal volume on both channels). Surround
is a bit tougher. The sound must be played back inverted on one channel.
One way would be to have two samples. Another would be to invert one side
on the fly. I have been told that DMP, a MOD music player by Otto Chrons, does
just this. I haven't heard it myself.

Inverting a sample would, I believe, consist of finding a "pivot" value,
preferably the median sample value, to map each sample across, then
"mirroring" each sample across that pivot value. In this way the dynamic
range and total volume would remain constant, but all the peaks would
become troughs. Anyone who has actually done this would be welcome to
email me with any corrections/modifications.

There are a few other issues to keep in mind:

It is difficult to get a sound to play on all four channels at once.
Generally, on a film soundtrack, a "big" sound, like an explosion, is sent
mainly to the surround channel. Since this will be lost in mono, a similar
sound is sent to the left and right channels as well.

A technique often used with thunder effects is to put the main sound in
the surround channel, followed by an echo in the front channels. The Dolby
stereo listeners hear the two-part thunder, and the mono listeners hear
just the second bit (or a scaled-down version of the first).

One technique is to slightly pitch-shift or delay sounds going to the
different speakers, but results can be iffy. YMMV. The important thing is
to do testing on a real Pro-Logic setup, and experiment until a good
balance is reached.

One can also send frequency band-limited chunks of the sound to each channel,
which will decode rather well. It requires that the bulk of each channel
be using a different band of the frequency spectrum, which is not
practical in some cases.

Keep in mind that there are many tricks of the trade that are used in film
mixing that only Dolby really knows. They don't tell how they do it,
because they want film companies to buy their technology, not the
competition (e.g. UltraStereo).

Fortunately, in computer gaming, you don't have to make the soundtrack
mono-compatible. You can ask the user if the sound system is Dolby
Surround, and place sounds in the surround channel as needed. On a mono
system, the sounds can all be sent to the single channel.

All of this information is from me, and any errors are my own darn fault.
None of this information has been endorsed by Dolby Labs. Dolby is a
trademark of Dolby labs, and should not be used on a product without
getting their permission. (Although I think if you called it "surround"
people would get the idea.)

Please email suggestions/criticisms/additions/subtractions/
multiplications/corrections to dmunsil@netcom.com or don@elseware.com.

This text is copyright 1993 by Don Munsil. It may be distributed freely,
as long as modifications are attributed and marked clearly.

===============

Extracted from CDN.MUSIC by Chris C.
