Al Bregman's Website

Audio demonstrations of auditory scene analysis

Creating the sounds

Shaping onsets and offsets.

When a sound is turned on instantly, the listener hears a click. To prevent this, we turn sounds on and off gradually. Whenever the description of a signal mentions a rise (onset) time or a decay (offset) time, the amplitude of the signal is shaped, over time, by a quarter-sine-wave function, the first quarter of the sine wave for onsets and the second quarter for offsets. These functions seem to minimize the perceived onset and offset clicks, as compared, in our laboratory, with other functions typically used for this purpose.

Laboratory facilities.

The demonstrations were created in the Auditory Research Laboratory of the Department of Psychology at McGill University. The computers were IBM-compatible PC's using Data Translation 16-bit converters (DT-2823) for acquisition and playback. The sampling rate was 22255 samples/sec for all synthesized and sampled signals, except for the sine-wave speech which was synthesized and played back at 20,000 samples/sec. Signals were recorded and played back using 8-kHz low-pass filters with a passive Tchebychev design having 60 dB attenuation at 11.2 kHz, THD less than 0.1%. As a result of this filtering, the "white noise" referred to in various examples is actually 0-8 kHz flat-spectrum noise.

Software.

The signal processing software was version 8.1 of the MITSYN system of William Henke (1990). The playback program that controlled the sequencing of the sounds to form patterns, demonstrations and the overall program of demonstrations was written in MAPLE, a language specified by Albert Bregman and designed and implemented by André Achim and Pierre A. Ahad as a superset of the ASYST (1982) programming language.

Recording of announcements.

The spoken announcements were taped at the Recording Studio of the Faculty of Music of McGill University, using a Neumann U87 cardioid microphone placed about a foot from the announcer, and a Sony model DT-90 digital tape recorder, set to 48 kHz. The announcer was Albert Bregman.