Subjective Evaluation of Pre- and Postfiltering in PAM, PCM, and DPCM Voice Communication Systems

In a recent paper [1] the authors obtained the pre- and postfilters that minimize the mean-square error between the message
x(t)
and its received replica
\hat{x}(t)
under very general conditions on both
x(t)
and the communication system under consideration. The present work compares the subjective effects of systems using optimum mean-square filters (OF) with those using conventional ideal low-pass filters (LF). The systems considered are pulse-amplitude (PAM), pulse-code (PCM), and differential pulsecode (DPCM) modulation systems for voice communication. For reasons presented in this paper, the OF and LF have double-sided bandwidth equal to the sampling frequency. In PAM systems, a white Gaussian channel model is assumed, while negligible channel transmission errors are assumed for PCM and DPCM systems. The subjective evaluation, based on preference tests, shows that subjective differences between PAM systems with OF and systems with LF are insignificant and are independent of sampling frequency and channel signal-to-noise ratio. Also, for those values of sampling frequency and number of bits of quantization considered, no significant differences exist between DPCM systems with OF and systems with LF. For PCM systems, however, significant improvement in subjective performance can be achieved when OFs are used instead of LFs.