Question: In the EVS (Enhanced Voice Services) codec, how is the target signal for adaptive codebook search typically computed on a per-subframe basis?
Option 1: By adding the zero-input response of the weighted synthesis filter to the unweighted input signal.
Option 2: Through direct analysis of the LP (Linear Prediction) residual signal without filtering.
Option 3: By subtracting the excitation signal from the output of the weighting filter only.
Option 4: By subtracting a zero-input response of the weighted synthesis filter from the weighted pre-emphasized input signal, or equivalently, by filtering the residual signal through the combined synthesis and weighting filters.
Explanation: The correct answer accurately describes the two equivalent methods used in the EVS codec for computing the target signal for adaptive codebook search on a subframe basis. This involves either subtracting the zero-input response of the weighted synthesis filter from the weighted pre-emphasized input signal or filtering the residual signal through the combined synthesis and weighting filters, crucial for efficient adaptive codebook search, especially for delays less than the subframe size, highlighting its importance in telecom networks for enhanced voice service quality.