In-band tone signal coding in the low-bit-rate speech vocoder

In-band tone signals include the single tone signals from 200/spl sim/3800 Hz and DTMF signaling. Sometimes we hope the speech vocoder would not only transmit speech signals, but it can properly transmit the in-band tone signals as well. Unfortunately most vocoders can not fulfil these needs, especially when dealing with the DTMF signals, because the DTMF signaling needs more exact representation. A general tone signal coding scheme is presented to solve this dilemma. First, a reliable tone signals detection and analyzing algorithm is studied. Based on this a coding model is presented. Since this coding model is a separable part in the vocoders, it can be easily used in many different environments such as LPC and MELP. Since the FFT is used in the spectrum analysis, the computation load of this algorithm is also trivial for most vocoders.