Evaluation of lifters by frequency response Free

This paper describes the evaluation of lifters with respect to their frequency spectra. A lifter is the quefrency window for the cepstrum and is used for eliminating the pitch components of the speech signal to obtain the spectral envelope. We have already suggested that comb lifters are useful for separating the close formants even if the pitch frequency is high [G. Oyama et al., IEEE‐ICASSP 78, 19–22 (April 1978)]. It is desirable that the comb lifters should have a sharp main lobe and small side lobes in the frequency spectrum. To make the main lobe sharp, it is necessary that the quefrency of the first stopband of the comb lifter should be high, though the quefrency depends upon the pitch frequency. The lifter should have gentle slope along the quefrency axis to reduce the side lobes in the frequency spectrum. Therefore the envelope of the comb lifter should decrease gradually with an increase in the quefrency. To suppress the peaks due to the pitch, the stop bands in the comb lifter should have certain width, because the pitch frequency of natural speech usually fluctuates. Frequency spectra of various types of comb lifters are investigated on the basis of these considerations.