4.4 Principles of the adaptive multi-rate speech decoder

26.0903GPPAdaptive Multi-Rate (AMR) speech codecMandatory speech CODEC speech processing functionsRelease 17Transcoding functionsTS

The signal flow at the decoder is shown in figure 4. At the decoder, based on the chosen mode, the transmitted indices are extracted from the received bitstream. The indices are decoded to obtain the coder parameters at each transmission frame. These parameters are the LSP vectors, the fractional pitch lags, the innovative codevectors, and the pitch and innovative gains. The LSP vectors are converted to the LP filter coefficients and interpolated to obtain LP filters at each subframe. Then, at each 40-sample subframe:

‑ the excitation is constructed by adding the adaptive and innovative codevectors scaled by their respective gains;

‑ the speech is reconstructed by filtering the excitation through the LP synthesis filter.

Finally, the reconstructed speech signal is passed through an adaptive postfilter.