An embedded-code multirate speech transform coder

In embedded multirate speech coding, it is desired to have a transmitter/receiver system that operates efficiently over a wide range of channel transmission rates. We are currently investigating a system based on adaptive transform coding of speech, in which we code and transmit the system parameters and the discrete cosine transform (DCT) coefficients of the fullband linear prediction residual waveform. The multirate property of the system is achieved by allowing the channel to discard some of the bits generated by the high data rate transmitter. Stripping off bits results in an absence of DCT components, which the receiver regenerates by a spectral duplication method. An inverse DCT at the receiver yields the time domain residual waveform to be used as input to the linear prediction synthesis filter. The lowest data rate achievable by the system is about 2.5 kb/s, in which case the system reduces to a narrowband LPC pitch-excited vocoder.