Question: What is the primary purpose of applying a 4th order linear prediction filter in the Adaptive Spectral Whitening process of the Enhanced Voice Services (EVS) codec?
Option 1: To amplify the excitation signal for louder audio output
Option 2: To compress the bandwidth of the excitation signal for lower bitrates
Option 3: To introduce non-linearity for improved voice quality perception
Option 4: To flatten the spectrum of the excitation signal after nonlinear processing
Explanation: The Adaptive Spectral Whitening process aims to counteract the effects of nonlinear processing on the excitation signal's spectrum. By estimating and applying 4th order linear prediction coefficients, the process effectively flattens the spectrum, ensuring a more uniform energy distribution across frequencies, crucial for maintaining voice quality in Enhanced Voice Services (EVS).