Over-All Characteristics of a TASI System

TASI (Time Assignment Speech Interpolation) has been in service on transatlantic submarine cable channels since mid-1960. Measurement of service quality on one TASI system (White Plains-London) indicates that system performance equals or exceeds the original engineering objectives in all but a few cases. Field modifications now being made should bring these exceptions into closer agreement with objectives. A companion paper4 discusses in detail the design considerations for TASI speech detectors and describes subjective tests made to determine the maximum permissible loading of TASI circuits without impairment of service. TASI, an abbreviation of 7'ime Assignment Speech interpolation, is a high-speed switching and transmission system which uses the idle time in telephone calls to interpolate additional talkers. 1 2 In a normal telephone conversation each subscriber speaks less than half of the time. The remainder of the time is composed of listening, gaps between words and syllables, and pauses while the operator or subscriber leaves the line. Measurements on working transatlantic channels, Fig. 1, show that a TASI speech detector with a sensitivity of --40 dbm is operated by speech from one talker on the average about 40 per cent of the time the circuit is busy at the switchboard. Since long distance circuits use separate facilities for the two directions of transmission, each one-way channel is, on the average, free about 60 per cent of the time. In order to take advantage of this free time to interpolate additional conversations, a considerable group of channels must be available.