Residual excited linear predictive coder

Abstract

We describe a linear predictive vocoder excited by the speech residual with the transmission bit rate below 8k bits/sec. The features of the residual excited linear predictive (RELP) vocoder system are as follows: (1) no pitch extraction is needed; (2) the bit rate is relatively low; and (3) the system is simple for hardware implementation. The RELP vocoder combines all the attractive concepts of linear predictive coding (LPC), voice excited vocoder (VEV), and adaptive delta modulation (ADM); that is, the system employs LPC for a better spectral representation of the vocal tract, the concept of VEV for bandwidth compression of excitation signal, and ADM for its simplicity of implementation and ability of accurate coding of a low-frequency signal. The LPC residual is generated by a feed-forward LPC analyzer, is low-pass filtered at cutoff frequency 800 Hz, and then is coded by ADM. At the synthesizer the decoded residual is passed through a spectral flattener and its output is finally fed as an excitation signal to the LPC synthesizer to obtain synthesized speech. The quality of synthesized speech is, in our opinion, reasonably good compared with those of other vocoders with the same bit rate. [The study was supported by the Defense Communications Agency.]