26.1903GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecSpeech codec speech processing functionsTranscoding functionsTS
The target signal for adaptive codebook search is usually computed by subtracting the zero-input response of the weighted synthesis filter from the weighted speech signal . This is performed on a subframe basis.
An equivalent procedure for computing the target signal, which is used in this codec, is the filtering of the LP residual signal r(n) through the combination of synthesis filter and the weighting filter . After determining the excitation for the subframe, the initial states of these filters are updated by filtering the difference between the LP residual and excitation. The memory update of these filters is explained in Section 5.10.
The residual signal r(n) which is needed for finding the target vector is also used in the adaptive codebook search to extend the past excitation buffer. This simplifies the adaptive codebook search procedure for delays less than the subframe size of 64 as will be explained in the next section. The LP residual is given by
( 36 )