A 4.8 KBPS voice excited DFT vocoder with time encoded baseband

The aim of this study is to achieve a simple speech codec at low bit rate for mobile radio applications. The vocoder part is obtained by using a low order Discrete Fourier Transform instead of a filter bank. Different baseband waveform coding schemes combined with different baseband bandwidths have been investigated. In particular we have studied a simple scheme for time encoded baseband in which the time intervals between zero crossings are quantized and transmitted. Amplitude information is transmitted with one bit for every zero crossing. The results concerning speech quality and implementation indicate that a simple 4.8 kb/s codec is feasible. It can be implemented with a few low-speed, low-power CMOS LSI circuits since no multiplications are required.