A 32kbps ADPCM with improvement in coding characteristics for 9600bps modem signal

This paper describes a new 32 kbps ADPCM CODEC which shows high-quality speech coding characteristics and data transmission capability up to 9600 bps. The proposed ADPCM CODEC consists of two coding modes. The first coding mode can code the speech signal and the data transmission signal up to 4800 bps by using the coding algorithm of CCITT recommendation G.721. The second coding algorithm is tuned to handle the 9600 bps modem signal. These two coding modes are automatically switched depending on the input signal to the CODEC. The switching algorithm is based on detection of modem training signal and is found to be highly reliable in practical environments. This algorithm thus provides a means to be compatible with the standard CCITT algorithm for speech and data transmission up to 4800 bps and at the same time be transparent to data transmission at 9600 bps.