Real-time simulation of adaptive transform coding

Adaptive transform coding (ATC) has recently been proposed as a technique for speech coding at bit rates in the range of 9.6- 16 kbits/s. In this paper we report on two new developments: 1) the use of a homomorphic vocoder model for the "side-information" channel in ATC and 2) a real-time simulation of ATC on an array processing computer. It is shown that the choice of the homomorphic "side-information" model leads to a convenient form of the ATC algorithm for real-time block processing using array processing techniques. It is also shown that the log spectrum output of the homomorphic model is in a convenient form for input to both the bit assignment algorithm in ATC (which becomes a straightforward quantization operation) and the quantization of the transform coefficients (which may be done in the log domain). An array processor simulation of this form of the algorithm has been implemented and it serves as a highly useful and convenient tool for studying the ATC algorithm in real time in a Fortran programming environment. It has allowed us, for the first time, to perform actual telephone conversations over a transform coder. The quality of this ATC algorithm was found to be essentially equivalent to that of a previous version using an LPC vocoder model for the side information.