The Ambiophone
Derivation of a Recording Methodology
Optimized For Ambiophonic Reproduction (continued)
By Ralph
Glasgal
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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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