Source coders operate using a model of how the source was generated, and attempt to extract, from the signal being coded, the parameters of the model. It is these model parameters which are transmitted to the decoder. Source coders for speech are called vocoders, and work as follows. The vocal tract is represented as a time-varying filter and is excited with either a white noise source, for unvoiced speech segments, or a train of pulses separated by the pitch period for voiced speech. Therefore the information which must be sent to the decoder is the filter specification, a voiced/unvoiced flag, the necessary variance of the excitation signal, and the pitch period for voiced speech. This is updated every 10-20 ms to follow the non-stationary nature of speech.
The model parameters can be determined by the encoder in a number of different ways, using either time or frequency domain techniques. Also the information can be coded for transmission in various different ways. Vocoders tend to operate at around 2.4 kbits/s or below, and produce speech which although intelligible is far from natural sounding. Increasing the bit rate much beyond 2.4 kbits/s is not worthwhile because of the inbuilt limitation in the coder's performance due to the simplified model of speech production used. The main use of vocoders has been in military applications where natural sounding speech is not as important as a very low bit rate to allow heavy protection and encryption.