A Fixed Rate Speech Coder Based on the Filter Bank Method with Non-Uniform Bandwidths

Abstract

A fixed rate speech coder exploiting the filter bank with non-uniform bandwidth is proposed. The speech coder is based on the inflection point (IP) detection. The speech signal is filtered by a group of bandpass filters with non-uniform bandwidths. Each bandpass filtered signal is processed by the IP detector, and the obtained IP pattern is compared with the entries of the IP pattern database. To facilitate the database search, the IP pattern entries are grouped into two kinds: IP patterns for voiced and the unvoiced speech. Before the search, the obtained IP patterns are checked if they belong to the voiced or the unvoiced. For that purpose, the shorttime energy of IP pattern from the first band is calculated and compared to the predetermined threshold value. In addition to the unvoiced/voiced decision, the non-uniform bandwidth bandpass filters are used for the filter bank to speed up the database search. By using the non-uniformly spaced bandpass filters, the number of filters in the filter bank becomes smaller. Transmitted information for each subband includes the IP pattern address of the database and amplitude of each IP pattern. In the receiver, the decoder recovers the subband signals using the received addresses and the energy information, and reconstructs the speech via the filter bank summation. Through computer simulation, the usefulness of the proposed technique is confirmed. The SNR performance of the proposed method is approximately 9 dB with relatively low bitrate of 12.8 kbps.