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The Ambiophone
Derivation of a Recording Methodology Optimized For Ambiophonic Reproduction -- 3

By Ralph Glasgal

Presented at the AES 19th International Conference on Surround Sound Techniques, Technology, and Perception
June 21-24, 2001, Schloss Elmau, Germany

For a copy of this article in its entirety, click here
(PDF, must have Acrobat Reader)

Contrasts with 5.1 Recording Setups

Figure 5 shows the Optimized Cardioid Triangle, OCT, method of 5.1 recording proposed by G¸nther Theile [13]. The left and right main microphones are supercardioids; the center microphone is a cardioid. Very directional supercardioids are used, despite their poor low frequency response and relative complexity, in order to reduce crosstalk between the sides and the center. The spacing between the left and right microphones needs to be just under one meter for capturing a full orchestra stage because of the presence of the center speaker which divides the stereo triangle into two halves thus requiring additional amplitude and time differences between the microphones to produce roughly the same phantom localization as two speaker stereo. Of course, the largest perceived stage angle in reproduction is still limited to plus and minus 30 degrees but the sweet listening area is enlarged.

Figure 6 shows a surround sound recording configuration for an orchestra proposed by Dr. David Griesinger of Lexicon. [12] It uses twelve microphones altogether and mandates that electronic reverberation generators be used to add early reflections to the stage spot and main microphones. The mixing and the formulation of the ultimate front and rear signal mix for engraving on the disc is left as a subjective process.

Compare these with the Ambiophone recording setup of Figure 3. Note that the impulse response of the hall will also have been measured at this point

In general, the fundamental conundrum in using microphones to record ambience, is that the main mic's are yielding a close up perspective while the ambience mic's are sampling part of say a 20th row sound perspective. If you move the ambience mic's forward to avoid this timing discrepancy, the direct sound pickup is likely to be too great. If the main mic's are moved back they may pick up too much reverb and the stage angle becomes too small. Thus, capturing ambience with microphones during the recording session will always involve compromises and subjective opinion. This is not the audiophile way to go.

Conclusion

Just as there are those who prefer LPs to CDs there are those who will likely continue to prefer stereo to any form of surround sound including 5.1, 7.1 Ambiophonics or Ambisonics. Stereo (2.0) is an art form. It is like oil painting compared to photography. Photography did not make artists obsolete just as stereo did not replace mono (Think of telephones, public address systems, Caruso records, short wave or AM radio, etc.) so surround or Ambiophonics will not replace stereo. (Think of clock radios, small TVs, laptops, walkmans, waiting rooms, FM radio, cassettes, LPs, MP3, stereophiles, etc.) Another factor to remember is that most recorded serious music listening is a solitary experience; which is why so many earphones are still sold. (I ignore parties and dances where psychoacoustics is about as relevant as the St Matthew Passion.) Thus compromises in the quality of music reproduction systems made merely to enlarge the ideal listening area, do not seem reasonable to a majority of audiophiles.

However, the development of digital signal processors and algorithms able to process digital audio in real time, without audible harmonic distortion or noise, has made it feasible and practical for music lovers to enjoy and recording engineers to deliver greater realism in music recording. Recordings made with the Ambiophone or the Schoeps KFM-6 have been shown to provide binaural realism and a normal perspective when coupled with an Ambiopole and Ambioconvolvers. Such Ambiophonic recordings require no spot microphone support, panning algorithms, artificial reflections, HRTF manipulation and not only need just two media channels but work better if there are just two.

Since even existing LP and CD discs reproduce well Ambiophonically, the future of Ambiophonics rests as much or more with the reproduction side than the recording side. It is hoped that the audio industry will begin to offer dedicated processors for the home market that include effective crosstalk cancellers tailored to work with touching Ambiopole room correction, and Ambiovolvers supplied with the hall impulse response libraries needed to create great sounding domestic concert halls.

References

[1] Ralph Glasgal. Ambiophonics, 2nd Edition. www.ambiophonics.org

[2] William B. Snow. Basic Principles of Stereophonic Sound. Stereophonic Techniques, An AES Anthology 1986.

[3] James Moir. Stereophonic Reproduction. Stereophonic Techniques, An AES Anthology 1986.

[4] Jens Blauert. Spatial Hearing, 1997 Edition, MIT Press. www.mitpress.mit.edu

[5] Y. Ando. Concert Hall Acoustics. Springer Verlag, 1984.

[6] Damaske & Ando. Interaural Crosscorrelations for Multichannel Loudspeaker Reproduction. Acoustica, Volume 27.

[7] Furuya, Fujimoto, Choi Young Ji, Higa. Arrival Direction of Late Sound and Listener Envelopment. Applied Acoustics. www.elsevier.com/locate/apacoust

[8] Henrik M¯ller, et al. Head-Related Transfer Functions of Human Subjects, J. Audio Eng. Soc., May 1995.

[9] Ronald M. Aarts. Phantom Sources Applied to Stereo-Base Widening. J. Audio Eng. Society, March 2000.

[10] Kirkeby, Nelson, Hamada. The Stereo Dipole. J. Audio Engineering Society, May 1998.

[11] Angelo Farina, Enrico Armelloni. Ambiophonic Principles for the Recording and Reproduction of Surround Sound for Music. Proceedings of the AES 19th International Conference.

[12] David Griesinger. The Theory and Practice of Perceptual Modeling. http://world.std.com/~griesngr/threedpm.pdf

[13] G. Theile. Multichannel Natural Music Recording Based on Psychoacoustic Principles. www.irt.de/irt/indexpubli.htm

[14] T. M. Bock, D. B. Keele. The Effects of Interaural Crosstalk on Stereo Reproduction. 1986 AES Preprints 2420A & B.

[15] G. Theile. On the Naturalness of Two-Channel Stereo Sound. J. Audio Eng. Society, Oct. 1991.

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