How Dolby Stereo (aka Dolby Surround) works

@DrWatson

Published in

atari

· 1 year ago

(version 2.01, 4/22/94)

Don Munsil (dmunsil@netcom.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 theaters, 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 dmunsil@netcom.com or don@elseware.com.