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study/sabre/os/files/Sound/dolby.v2.01.txt

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+How Dolby Stereo (aka Dolby Surround) works: 
+(version 2.01, 4/22/94)
+
+Don Munsil (don@munsil.com)
+
+2.01 changes:
+
+- Better explanation of inverting a sample.
+
+2.0 changes:
+
+- Added responses from Otto Chrons and Adrian Kwong.
+
+- Cleaned up text considerably.
+
+- Added more tips on practical aspects.
+
+** Introduction:
+
+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 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. Remember that there is only one encoding process: Dolby Stereo, 
+which can be decoded by a Dolby Surround, Dolby Pro-Logic, or THX 
+decoder, for varying degrees of home-theatre "quality."
+
+There are four output channels in Dolby Stereo: Left front, Right front,
+Center front, and Surround. These are encoded on two channels in such a
+way that the soundtrack 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. THX is a licensing and quality-control wing of
+LucasArts with the stated aim of improving movie sound and picture. 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. In addition, dialog 
+tends to be clearer when it is localized in the center channel.
+
+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 (in addition to the
+surround channel, of course). 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 Dolby decoder reproduces a fairly good semblance of the original four
+channels from the two input 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 "blind" 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, with one pre-inverted. 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 simply consists of negating it. If the sample values 
+are unsigned, the negation will still give the desired result because of 
+the nature of two's complement representation (though you should make 
+sure your compiler is actually doing the appropriate negation -- in C, 
+cast the value to signed, then negate it, then cast to unsigned to be 
+absolutely sure.)
+
+>>New addition: Otto Chrons (c142092@cc.tut.fi), author of DMP, emailed 
+me the following: 
+
+You wrote an excellent article about Dolby Surround sound at csip.soundcard
+but I wanted to comment on few things. DMP does work with surround channel
+and the technique I use is extremely simple. Dolby standard state that
+you should do +90 and -90 phase shifting on left & right channels, but I've
+found out that doing a 0 and +180 shift works as well. So basically I
+put the original sound data out of the left speaker and negate the data
+on the right speaker. In my mixing routines this is accomplished by
+using ADD and SUB instructions respectively. So doing surround sound is
+as easy as doing mono!
+
+>>End Chrons
+
+** A few other issues to keep in mind:
+
+>>Adrian Kwong (akwong@alfred.carleton.ca) had this to add:
+
+You might want to add that the surround channel in a normal Dolby
+Surround setup is placed through Dolby A type noise decoding.  It has
+something to do with making it 3dB larger following a specific
+frequency envelope.  To make the surround appear at the correct
+volume, you'll need to "double" the signal to the surround channel.
+(+3dB is about 2x the signal)
+
+>>End Kwong
+
+One thing to keep in mind is that the Surround channel is typically not 
+designed to handle a lot of bass. The speakers are generally small, and 
+the amplifiers are lower wattage. On a film soundtrack, the low bass is 
+generally filtered out of the soundtrack to avoid clipping.
+
+In addition, the high frequencies are generally filtered out of the 
+surround channel because higher frequencies are easier to localize, and 
+the surround channel is not supposed to be directional. The end result is 
+that the surround channel is only "supposed" to carry about 200-8000Hz, 
+which is a fairly narrow spectrum.
+
+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 (or
+sometimes vice-versa). 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). 
+
+Another 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, Chase Surround, StereoSurround). If anyone 
+reading this has useful techniques (that are not trade secrets of Dolby) 
+they would like to share, please email the author.
+
+Fortunately, in computer sound, 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.
+
+** Disclaimers:
+
+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 don@munsil.com.
+
+This text is copyright 1993 by Don Munsil. It may be distributed freely,
+as long as modifications are attributed and marked clearly.
+
+-- 
+------------------------------------------------------
+Don Munsil          | I respect faith, but doubt is
+don@munsil.com      | what gets you an education.
+                    |                 -- Wilson Mizner
+