Audio Engineering Society
Chicago
Meeting Review,
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The November 16th meeting of the Chicago
Section featured a presentation on surround sound systems and associated
recording techniques by distinguished guest John Eargle.
Well-known as an author of several audio engineering books, recording
engineer and/or producer of over 280 compact disks, and for his decades of
product development experience, Mr. Eargle drew a
large and enthusiastic crowd of 68 attendees. This was our first
dinner/presentation meeting of the season. Recent developments in microphone
technologies and practice for surround sound fall broadly into four
categories. In a stereo-derived approach such as 5.1, the loudspeakers create
both real and phantom images as well as de-correlated ambience. Phantom
images are created in the same method as in stereo, and it is important to
avoid positioning an image both as phantom between front left and right and directly
in the center. Regarding ambience reproduction, it was shown via inter-aural
correlation measurements that with proper recording technique, four speakers
in a living room can replicate a very live, large space to a fairly close
degree. Several near-coincidence microphones have been used in practice, as
well as the Trinnov Azimuthal
Array and the Gerzon 2-3 Martrix.
� A different approach is the Mapping Technique. An array of
directional microphones is used to pickup sound from all three directions,
with each mic covering an equal sector. For
playback, a loudspeaker for each microphone/channel is placed in the location
of each mic's maximum pickup axis. The minimum
number of microphones required is four of the first-order type, and the Soundfield � Microphone is an example. More channels requires higher order microphones, which can be created
virtually from an array of omnidectional mics as in the Eigenmike � system. The mapping technique is probably most
suited for special venues and in conjunction with visual presentations. In the transaural technique,
binaural reproduction is achieved for a carefully placed listener with just
two loudspeakers. This method requires cross-talk signals to be canceled,
which is complex but the technique can be expanded to enable panning in post
processing. The final subject, systems with parallax, generated the
most audience discussion. A large array of microphones and a "wall of
speakers" are used to reproduce the original three-dimensional acoustic
field of a source, and thus the image location is independent of listener
location. This is the cutting edge of directional sound reproduction, with |
