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For professional audio engineers who practice "surround without accompanying picture" such as music, 5.1 is an intentional compromise ñ but some measure of it would suggest how acceptable the compromise is. A subjective comparison with a technology that is superior in one or more ways would be useful in that determination, leading to techniques that work within its compromises. One such approach is termed Panor-ambiophonics (Figure 3). Using two closely-spaced speaker pairs in front and back and requiring four monaural transmission channels (two stereo), "PanAmbio" - essentially two Ambiophonic systems - is in the authorís experience superior to 5.1 in accurate 360ƒ localization, spatial impression, and envelopment with uncompromised frontal tone color (no comb filtering or pinna confusion). Bass management to redirect low frequencies from main channels, plus a ".1" LFE channel if used, are applicable to both ITU and PanAmbio, hence the 5.1 and 4.1 designations here ñ although these refer more precisely to the number of transmission channels, not speakers. PanAmbioís disadvantages, aside from limited popular acceptance and not being recognized by a standard, are its need for crosstalk cancellation and that it works for only one or at most two listeners, not a group. Still, regarding 5.1ís qualities, PanorAmbiophonics is at least a benchmark of excellence, if not an alternative for high-quality music listening.

This paper describes experimental recordings to evaluate subjectively each of these reproduction systems in the light of the other. The objective will be to put each to its highest use. In addition, we will explore compatibilities for producing recordings that play well on both systems and a method of critical multi-channel recording level calibration. Applications are not limited to music only, but include music and natural sounds for film and broadcast.

5.1 has been described elsewhere and is relatively well understood [2], so this paper will dwell more on Panorambiophonics. "PanAmbio" is two Ambiophonic systems, one for the front 180ƒ and a second for the rear 180ƒ as in Figure 3. (Note that Ambiophonics is not Ambisonics, a surround approach that uses coincident omni and figure-8s after Gerzon.) Each Ambiophonic system is two closely spaced speakers - an Ambiopole or stereo dipole - with crosstalk-cancellation provided by digital processing. Each Ambiopole more precisely reproduces recording angles up to 150ƒ with reduced "angular distortion" [3, 4], which is characteristic of phantom images in stereo and 5.1, where instruments "relocate" toward one speaker or another when the listener is off the central plane. With one Ambiophonic dipole, instruments are localized more precisely, within ±5ƒ where listening acoustics permit, failing due to pinna confusion (in this instance sounds intended for the sides coming from the front) only as they approach the extremes of a 180ƒ wide stage. Contrast this single Ambiophonic system with conventional stereo, where all sounds are heard within 60ƒ. Why and how Ambiophonics works ñ even for many existing stereo recordings - is the subject of Glasgal's papers available in AES publications [5] and at www.ambiophonics.org. Discussed here are its uses, limitations, recording techniques, comparison to, and compatibility with 5.1.

For surround reproduction, a second speaker dipole is added in back, and full 360ƒ PanAmbiophonic reproduction has been demonstrated by Ralph Glasgal and the author at the 111th AES Convention, December 2001. The experimental result is precise (±5ƒ) localization of sources around 360ƒ (Figure 4), virtually duplicating the recording layout, although with some coloration and soft focus of voices near ±90ƒ directly left and right. While anomalies in these two side regions are within the cone of confusion of human hearing and might be considered negligible, a PanAmbio listener is able to turn his/her head to confirm direction and tone color, just as in normal living, so the author considers these anomalies near ±90ƒ disadvantageous. PanAmbiophony works best when reproduced in a symmetrical, "dry" (cf. recording) acoustic and with speakers at less than the critical distance (room radius) of the listening environment. With four well-matched speakers and calibrated levels, the degree of precision possible can reveal subtle errors in recording ñ so PanAmbio is useful also for monitoring.

Fig. 4. Panor-Ambiophonic 4.1 (2/2) reproduction localizes sources accurately within ±5ƒ, virtually duplicating the recording session directions above. A guitar quintet and fans are placed as shown for experiments that contrasts two 360ƒ reproduction methods. Multichannel surround sound is more "realistic" by localizing both front stage instruments and sounds from around and behind, including antiphonal voices, audience participation, and ambience. [back]

In contrast, the ITU 5.1 (3/2) studio and home theater speaker layout "redirects" sounds as in Figure 5, but accommodates audiences of more than two, as the "sweet area" for "important" sounds from the center speaker is increased to 1.5m2 [6]. Both ITU 3/2 and PanAmbio 2/2 remove the 60ƒ confines of two-speaker stereo (Figure 6). However, PanAmbio offers a discriminating listener, or at most two, superior realism for critical music appreciation at home or automobile.

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