부산대학교 도서관

In aperture coding, one refrains from encoding waveform samples until the waveform crosses an appropriately wide aperture centered around the last encoded value. If the waveform is slowly varying in some sense, the above procedure can be a basis for bit rate reduction. The identification of aperture-crossing samples can be either explicit or implicit, and it is the latter case that this paper mainly addresses. We follow a finite length, converging-aperture procedure proposed recently for picture waveforms, and show that it can be used for speech coding as well if the aperture width is designed to be syllabically adaptive. We also describe, for Nyquist-sampled speech, desirable designs for aperture shape and aperture length L. The special case of L − 1 corresponds to ternary delta modulation with a constant encoding rate of log 2 3 ∼ 1.6 bits/sample. Using longer apertures (e.g., L = 2, 3), we show that it is possible to obtain average encoding rates as low as 1.2 bits/sample without significantly changing output speech quality. With 8- to 12 kHz sampling, the average bit rate would then be 9.6 to 14.4 kb/s. At these transmission rates, adaptive aperture coding, used in conjunction with a simple (first-order) adaptive predictor, can provide communications quality speech.