Improvement in design of noise shaping filters

Addition of dither before the quantization (or requantization) linearizes the quantization characteristic for signals of low levels. However, the dither addition increases overall noise, which is uniformly distributed within the whole frequency range. By transposing a part of that energy into frequency regions where a human ear is less sensitive, the present noise can become less audible. Therefore, noise shaping is incorporated in the quantization (requantization). A noise shaping filter can be designed by applying suitable weighting functions that represent audibility to low level noise. The filter’s power spectral density should approximate the inverse of the audibility curve. In that way, perceived signal to noise ratio becomes higher. In this paper, some aspects of the noise shaping filter design are analyzed, and an improvement of the design procedure is proposed. The improvement is related to better matching of the filter’s power spectral density to the corresponding target curve. The results obtained by the proposed procedure are verified by listening tests.