5.3 Perceptual weighting

26.1903GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecSpeech codec speech processing functionsTranscoding functionsTS

The traditional perceptual weighting filter has inherent limitations in modelling the formant structure and the required spectral tilt concurrently. The spectral tilt is more pronounced in wideband signals due to the wide dynamic range between low and high frequencies. A solution to this problem is to introduce the preemphasis filter at the input, compute the LP filter A(z) based on the preemphasized speech s(n), and use a modified filter W(z) by fixing its denominator. This structure substantially decouples the formant weighting from the tilt.

A weighting filter of the form is used, where and ₁=0.68.

Because A(z) is computed based on the preemphasized speech signal s(n), the tilt of the filter 1/A(z/₁) is less pronounced compared to the case when A(z) is computed based on the original speech. Since deemphasis is performed at the decoder end, it can be shown that the quantization error spectrum is shaped by a filter having a transfer function W ^-1(z)H_de-emph(z)=1/A(z/₁). Thus, the spectrum of the quantization error is shaped by a filter whose transfer function is 1/A(z/₁), with A(z) computed based on the preemphasized speech signal.