Question: In the EVS (Enhanced Voice Services) codec, how are the weighted signal domain memories updated in the FD-CNG (Fullband Continuous Noise Generator) encoder?
Option 1: By directly copying the decoder memory update values
Option 2: Through a simple average of past and current LPC coefficients
Option 3: By filtering the FD-CNG time-domain signal through an LP analysis filter with weighted LPC coefficients
Option 4: Using only the unweighted LPC coefficients in a feedback loop
Explanation: The correct approach for updating the weighted signal domain memories in the FD-CNG encoder involves processing the FD-CNG time-domain signal. Specifically, this is achieved by passing the signal through a Linear Predictive (LP) analysis filter that utilizes a weighted version of the Line Spectral Pairs (LPC) coefficients. This method ensures that the memories are updated based on both the current signal characteristics and the predictive model adapted to the voice signal’s spectral envelope, thus maintaining high-quality voice synthesis in Enhanced Voice Services.