Perceptual Evaluation of the Effects of Dither on Low Bit Rate PCM Systems
L. R. Rabiner, J. A. Johnson · Bell System Technical Journal · 1972
It has previously been shown that by adding a pseudo-random “dither” noise to a signal to be quantized, and by subtracting an identical noise sequence from the quantizer output, it is possible to break up undesirable signal-dependent patterns in the quantization error sequence without increasing the variance of the error. The effect of the dither noise becomes significant when the number of bits per sample is less than about six. An experimental evaluation of the perceptual effects of dither on speech has shown: (i) strong preferences for dithered speech over straight PCM encoding at identical bit rates, (ii) for low bit rates (2–4 bits/sample), a preference for dithered speech over PCM encoded speech even when the PCM speech had one more bit per sample than the dithered speech, (iii) an increase in word intelligibility for dithered speech over PCM speech when 4 to 6 bits/sample were used, (iv) a decrease in word intelligibility for dithered speech over PCM speech when 2 to 3 bits/sample were used.