Speech digitalization with channel vocoders

Three vocoders based on channel vocoder techniques are presented: - 2400 or 4800 bit/s channel vocoder which has 14 channels and pitch detector, -1200 or 1800 bit/s vocoder : spectral data are reduced by projection on a plane (out of eight possible) constitued with two factorial axis, - 9600 bit/s voice excited vocoder with acceptable quality up to five per cent bit error rate for the transmission. For such vocoders, digital processing will be a very attractive solution on account of using shared time processor which will provide large reduction of hard-ware. Each vocoder will be described and particularized. The second mentionned vocoder permits reduction of the binary transmission rate, in this field it can be considered as an improvement of first one. The last one uses delta modulation to transmit low frequency band of the speech and channels for upper band. It keeps acceptable quality even if bit error rate of the transmission reaches 5 per cent. For those vocoders, quality, complexity, transmission performances are given and compared.