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How Good Can Stereo Be? – Try Ambiophonics and See

Appendix

Microphones for Stereo, Ambio, & Surround

by Robin Miller ©2008

Suspended over the tenth row of a moderately live hall, the “dummy head” microphone was well outside the “critical radius.” Live, the concert sounded glorious. Back home, the binaural recording would recreate the event with compellingly like-like reproduction over headphones.

However a binaural recording (one involving that part of HRTF due to the pinnae) would be unusable over speakers conventionally positioned in the stereo “triangle.” For one thing, the recording would be confusingly “encoded” with filtration by pinnae other than your own. Moreover, too much ambience would be captured in the recording than would make sense to our ear-brain system coming entirely from only 60° in front, instead of all around. Reproducing a natural level of ambience using surround channels makes sense to the ear-brain. Natural hearing includes not only the entire horizontal circle, but a sphere of sound (including height cues) – anything less isn’t totally real. Thirdly, while binaural recordings work over headphones, they may not for more comfortable speaker listening, where the soundstage is external to the listener’s head, and stays put with head rotation (We unconsciously perform tiny head rotations to confirm localization).

No wonder that recording acceptable stereo over speakers has “devolved” from use of a main microphone, acting as surrogate ears for the listener, to many close-in microphones panned channel to channel, as though interaural level differences alone (ILD) made for the spatial sound we experience in real life. An entire class of coincident main microphones, from Gerzon’s ingenious Ambisonics and its mathematical subsets, including Blumlein’s XY figure eights and Mid-Sides (MS), also lack the interaural time differences (ITD) of natural hearing. At the other extreme, widely spaced pairs and moderately spaced Decca-Trees and variants capture time differences many multiples of the 640µs between our ears, and so deliver highly spacious but ambiguous, phasey sound that diffuses the image so totally as to make any “sweet spot” undetectable (i.e. the result sounds imperfect everywhere; no better in any one spot). Popularly employed, these microphones are usually placed well within the critical radius of acoustic spaces, where no one in the audience would want to be (often over the conductor’s head). Here, the ambience impression is purposely weakened for normal stereo release. But by the relative inverse squares of their distances, close instruments are too close and distant ones too distant. Fixing these resulting imbalances has led to a cure often worse than the illness – namely many spot microphones that increase costs for producers and artifacts for consumers.

 

Truthful microphone techniques for stereo reproduction lie somewhere in between. “Pinnaless dummy head” recordings played over stereo speakers are still too ambient: the auditory scene seems receded. The ORTF technique is an outstanding compromise, as the cardioid microphones it employs attenuate offstage ambience, although cardioids also lack bass response. Bruck’s pinnaless sphere microphone with bi-directional outriggers (Schoeps KFM-360) separates soundstages front and back, compatible with 5.1 surround. The baffled ellipsoidal Ambiophone developed by the writer separates the front 120° from the back 240° and ceiling for precise stereo and surround localizing, spatiality, and tone color, especially in PanAmbio (4-channel) as well as 5.1 surround. Sonic reality is completed by full-sphere 3D (with height) recordings, such as the author’s High Sonic Definition 3D.


An internationally recognized engineering consultant and Peabody award-winning producer, Robin Miller has presented advanced 2D and 3D audio solutions worldwide to the Audio Engineering Society, Society of Motion Picture & Television Engineers, Acoustical Society of America, Canadian Acoustical Association, and German Tonmeisters. As an invited panelist at the AES 2007 Italia conference in Parma, he demonstrated Ambiophonics, 5.1-compatible PanAmbio 2D surround, and full-sphere 3D using ten speakers using his original recordings. His company, Filmaker Technology, engages in applied research, systems design & integration, and has a patent for a system of full-sphere 3D recording & reproduction.

Angelo Farina (left) introduces invited panelist Robin Miller (third from left) at AES2007 Italia, Parma.