Speech Processing by the Selective Amplitude Sampling System
L.R. Spogen, H. N. Shaver, Daniel Baker, Bert Blom · The Journal of the Acoustical Society of America · 1960
Selective amplitude sampling is a speech processing technique in which the input speech waveform is sampled at the times of its maxima and minima. The original wave is reconstructed from its samples by means of a box-car circuit followed by suitable filtering. The maxima and minima are determined from the zero crossings of the differentiated speech wave. Center clipping is employed to provide amplitude discrimination and a reduction in the average sampling rate. The intelligibility of speech processed by the system is compared with that for clipped differentiated speech and the original speech. “Quality” tests attempted to measure the ability of listeners to distinguish between words processed by the selective amplitude sampling system and a system in which the original words were filtered to a pass band of 300 to 3000 cps. Distributions were obtained of the sampling role averaged over 1- and 10-sec intervals. Word intelligibility and sampling rate are also related to the degree of center clipping in the sampler. The degree of center clipping is measured in terms of an rms voice to-threshold ratio. The word intelligibility scores on the selective amplitude sampling system had a mean of 93.8% and a standard deviation of 4.3%. In addition, 97% of all the test scores were equivalent to 100% sentence intelligibility. The results of the “quality” tests indicate that at least 36% of the time, the listeners were unable to distinguish between the selective amplitude sampling system and the filtered system. The average sampling rate necessary for 100% sentence intelligibility is approximately 1600 samples/sec. This rate corresponds to an rms voice-to-threshold ratio of 7 db.