Thirty-nine people attended AES San Francisco's September meeting held at Dolby Laboratories in San Francisco.

Dr. Richard Duda gave an overview and a demonstration of a Motion-Tracking Binaural (MTB) recording technique developed at the CIPIC Interface Laboratory at UC Davis. Dr. Duda and his colleagues have devised a method of making audio recordings that allow a listener wearing stereo headphones equipped with a motion-tracking device to turn her head to face individual sounds. That is, a 360-degree sound field remains fixed and emulates what a listener would hear at the recording event as she turns her head.

MTB is a form of binaural recording – it requires 8 to 16 microphones mounted around a stationary spherical or cylindrical object. Many of the advantages of using MTB are similar to the binaural recording process: it is simple, it creates a realistic recording, it picks up a room’s natural reverb characteristics, it requires only 2 channels for playback, there are no problems with sweet spots, and it requires no special recording skills. On the downside, binaural playback requires headphones, there are elevation problems due to mismatched pinnae cues, the sound field moves if the microphone array shifted during recording, and there is front/back confusion as well as frontal collapse.

MTB recording handles the latter three issues especially well. If the listener’s ears are aligned with a pair of microphones, then the system will feed those signals to the headphones. If the listener’s ears are positioned between pairs of microphones, then the signals are interpolated.

The MTB system takes advantage of the fact that human hearing is sensitive to interaural time delays at low frequencies. That eliminates the phase interference that would otherwise occur if the interpolation were done on the full-bandwidth signals from the nearest and next-nearest microphones. The combined signal is run through a low pass filter and high frequencies are then restored using only the signal from the nearest microphone.

When listening to music recorded using eight microphones, some people may notice timbre changes when rotating their heads. Very few people will notice this artifact when listening to music recorded with sixteen microphones. For speech, eight microphones produce satisfactory results.

Anticipated applications for the MTB recording technology include training, remote listening, and entertainment.