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AES

Audio Engineering Society

24th International Conference on Multichannel Audio
June 26-28, 2003, Banff, Canada

 

SURROUND AMBIOPHONIC RECORDING AND REPRODUCTION

by Ralph Glasgal

REVIEW OF BASIC AMBIOPHONICS (Fig. 1)

The simplest form of Ambiophonics is meant for the playback of ordinary stereo CDs, LPs, SACDs, MP3s, cassettes, stereo TV, etc. In stereo, the front stage is created between the speakers, in Ambiophonics the stage is created from the speakers outward and so can be much wider. The Ambiopole speaker pair form an angle to the listener of from twenty to thirty degrees. One can sit anywhere along the line between the speakers and can stand or recline, turns one head, lean etc. Some omnidirectional speakers produce larger sweet areas but in general the best Ambiopoles can only accommodate one or two listeners, perhaps best one behind the other for golden eared results.

Figure 1

The scattered surround speakers are fed hall ambience signals calculated for both the left and right channels by a computer which we call an Ambiovolver. The Ambiovolver has stored within it the impulse responses of some of the great halls, churchs, and auditoriums of the world and more such hall signatures are being accumulated all the time. One simply selects the hall best suited to the recording or the actual hall where the recording was made. The Ambiovolver is told the location of all the surround speakers in the room and it then generates the appropriate reflections and feeds them to a surround speaker that can then mimic a concert hall wall. In this way the levels, frequency responses, and the directionalities of the reverberant field are maintained. I have driven up to 24 surround speakers this way and, while clearly overkill, the results are gratifying. This is in contrast to normal 5.1 practice where recorded hall ambience whether from front, rear, overhead or the side is lumped together and launched from just two surround speakers. The attached references describe Ambiovolver design, hall impulse response measuring procedures and hall acoustic properties. It is always desirable to keep the listening room early reflection characteristics under control.  Absorptive panels are quite effective.  However, since the direct sound speakers are so close together and aimed forward, they are easier to position than for stereo or 5.1.  Bad room acoustics are actually less of an issue in Ambiophonics than in stereo.  Except that when room reflections interfere in stereo most listeners can hardly notice or care enough.  In Ambiophonics the average listener will notice bogus early reflections and feel deprived.  It is possible to adjust the Ambiovolver to compensate for listening room late reverb tails.

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