Tone detection for automatic control of audio tape drives

This paper describes digital hardware for automatically stopping a cassette recorder upon detection of a prerecorded tone. This hardware is used in conjunction with experiments on computer assisted voice wiring experiments being performed at Western Electric locations [1]. For these experiments a sequence of instructions is automatically recorded on a cassette tape by a computer voice response system. At the end of each instruction, a tone is recorded. The hardware detects this tone and stops the cassette recorder. The operator, after performing the prescribed wiring instruction, manually restarts the cassette recorder for the next instruction. The technique used to detect the tone is a simple digital method comparing the axis crossings of the signal to a fixed threshold. This threshold is determined based on knowledge of the tone frequency, duration, and amplitude. When the signal axis crossings exceeds this threshold during two consecutive 40 ms nonoverlapping intervals the tone is detected and the tape recorder is stopped. The method described is a robust one which is rather insensitive to normal tape recorder problems, e.g., wow and loss of signal level due to battery drainage. The tone detection hardware requires nominal power and is portable.