Hybrid Codecs

Hybrid coding is an intermediate between waveform and source coding. Analysis-by-Synthesis (AbS) coding, which is performed in time domain, is the most famous type of hybrid coding. AbS codecs operate exactly as vocoders (source coding), but instead of using only two-state voiced-unvoiced model to determine the parameters of the linear prediction filter, the excitation signal is chosen by attempting to match the reconstructed speech waveform approximately the same as the original speech waveform. The speech signal is split into 20ms frames. For each frame, parameters are determined for a synthesis filter, and then the excitation to this filter is determined. This is done by finding the excitation signal which when passed into the given synthesis filter minimizes the error between the input speech and the reconstructed speech. Finally, for each frame, the encoder transmits information representing the synthesis filter parameters and the excitation to the decoder, and at the decoder the given excitation is passed through the synthesis filter to give the reconstructed speech. There are several types of AbS codecs. We can cite Multi-Pulse Excited (MPE), Regular-Pulse Excited (RPE), and the Code-Excited Linear Predictive (CELP) codecs. MPE and RPE codecs give good quality at rates of about 10 Kbps. CELP codecs can give the same quality at rates in the range 2.4 and 10 Kbps.