Measurement of Digital Channel Capacity for a Speech Compression System

Abstract

It has long been speculated that an information handling rate of 1000 binits/sec or lower might be adequate for transmission of speech, if first the redundancy of its original form could be reduced. The Formoder, a speech-band-compression system, accomplishes the reduction by extracting the main information-bearing elements from speech, presenting them in a seven channel output which requires a total band width of about 175 cps. Latest articulation tests using Harvard P.B. word lists processed by the Formoder and given to a trained crew have yielded scores of 75% and higher for some male speakers. In order to measure the digital channel capacity necessary to intelligibly transmit the Formoder output, the seven channels were digitized using a technique known as “delta modulation.” This scheme achieves time and amplitude quantization by means of a clipper, sampler, and a linear feedback network. Compared with standard pcm techniques, “delta modulation” represents considerable economy in instrumentation. Articulation tests were made for several rates in the range 500–2500 binits/sec. Results of these tests will be reported and the important features of the digitizing technique will be given. [This research is sponsored by the Air Force Cambridge Research Center under Contract No. AF 19(604)-3465.]