Question: In the EVS (Enhanced Voice Services) codec, as specified by 3GPP, what is the primary purpose of filtering the pre-emphasized FD-CNG time-domain signal through the LP analysis filter in the decoder's memory update process?
Option 1: To enhance the overall voice quality by amplifying high-frequency components.
Option 2: To generate a noise reference for comfort noise generation.
Option 3: To analyze the speech signal's spectral characteristics for encoding.
Option 4: To obtain a residual signal for updating excitation domain memories.
Explanation: The correct answer is Option 4 because, in the context of EVS codec's FD-CNG decoder memory update, filtering the pre-emphasized time-domain signal through the LP analysis filter is specifically done to extract the residual signal. This residual signal is crucial for updating the excitation domain memories, which are essential for accurately modeling and synthesizing the voice signal's excitation source in speech coding algorithms.