Multi-rate speech coding for wireless and Internet applications

Fixed-rate speech codecs are unable to provide synthesized speech with fixed delay when the channel capacity changes, and can not dedicate additional forward error correction bits for protection against noisy channels. We propose a multi-rate method for variable bandwidth applications, such as the Internet, and severely degraded wireless channels, such as mobile cellular. The technique uses a multi-rate version of the sinusoidal transform coder (MRSTC), operates at 9.6/4.8/2.4/1.2 kilobits/sec (kb/s), and is switchable “on-the-fly”. The algorithm produces high quality speech, even when transitioning between rates. We compare two switching techniques, one method uses a “frame-deletion” (FD) technique, and a second method which utilizes “parameter-history” (PH) information. PH produces the best speech quality. FD is attractive because it requires no additional speech memory. Experimental results show greater than a 9 dB gain in receiver C/N_o operating range using the MRSTC over a fixed-rate system with STC operating at 9.6 kb/s