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An Experiment into Pair-Wise Mixing and Channel Separation

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Equipment required: A stereo CD and something to play it on.

This evolved from an article I posted to the rec.audio.high-end Usenet newsgroup. It has been modified for clarity and to make me look good.


Subject: Re: Can surround modes be made to work?
Date: 31 Oct 1995 15:36:39 GMT
From: Martin J Leese

On 28 Oct 1995 18:08:37 -0400 Andre wrote:

In my opinion, the Ambisonic FAQ has an axe to grind, and I would take what it says with a grain of salt. Since no one has heard pure music applications of AC-3, I would tend to doubt anyone making absolute statements about its inherent musicality.

Yes, the Ambisonic FAQ has an axe to grind, but it is not Dolby Surround AC-3. The culprit is the pair-wise mixing style. Dolby Surround AC-3, like DTS Digital Surround, is just a delivery mechanism and is not tied to pair-wise mixing. It is true, however, that to date all Dolby Surround AC-3 sound tracks have been mixed using the pair-wise mixing style.

To understand why pair-wise mixing is a bad idea in surround sound, please perform the following very simple experiment. Play, in stereo, a pair-wise mixed CD that has good phantom images (almost all CDs use this mixing style). Turn your chair through 90 degrees. If you still hear stable phantom images when both speakers are to one side then you are a space alien because humans cannot do this. Pair-wise mixing did not work in the quadraphonic era and it will not work now with Dolby Surround AC-3 or DTS Digital Surround. Such an absolute statement can be made because the way that humans localise sound has not changed.

Note that you performed this experiment with a CD. CDs have complete separation between the two channels, so clearly good separation is not sufficient for accurate imaging in surround sound.

If you perform the above experiment, I am confident that you will then understand why the Ambisonic Surround Sound FAQ has an axe to grind about the pair-wise mixing style, and that you will join us at the grinding wheel. We will even lend you an axe.

To hear why the Ambisonic mixing style is better for surround sound requires an Ambisonic decoder and a CD that is UHJ encoded.

Regards,
Martin


The lesson here, I would suggest, is that surround in not just stereo with more speakers. What is needed is an entirely different approach.


References on Pair-wise Mixing

The stability of side phantom images generated using pair-wise mixing has been measured in a number of studies including:

  1. K. de Boer, 1947, A Remarkable Phenomenon with Stereophonic Sound Reproduction, Philips Technical Review, Volume 9, pages 9 to 38.
  2. P. A. Ratliff, 1974, Properties of Hearing Related to Quadraphonic Reproduction, BBC Research Department, Report BBC RD 1974/38 (November 1974).
  3. R. C. Cabot, 1977, Sound Localization in Two- and Four-Channel Systems: A Comparison of Phantom-Image Prediction Equations and Experimental Data, presented at the 58th Convention of the Audio Engineering Society. Preprint 1295.
  4. K. Nakabayashi, 1974, (in Japanese), in the Journal of the Acoustical Society of Japan, Volume 30, (March issue), page 151.
  5. G. Thiele and G. Plenge, 1977, "Localization of Lateral Phantom Sources,", in the Journal of the Audio Engineering Society, Volume 25 (April issue), pages 196 to 200.
  6. O. Kohsaka, E. Satoh, and T. Nakayama, 1972, "Sound-Image Localization in Multichannel Matrix Reproduction", in the Journal of the Audio Engineering Society, Volume 20 (September issue), pages 542 to 548.
  7. W. Zieglmeier and G. Theile, 1996, "Imaging of lateral sources using the 3/2-stereo format" (in German), in Proceedings of the 19th Tonmeistertagung, pages 159 to 169.

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