Question: In the EVS (Enhanced Voice Services) codec, why does the second stage of noise energy update not utilize the SAD (Signal-to-Noise Ratio per critical band) decision for determining frame inactivity and subsequent noise energy updates?
Option 1: To prioritize voice quality over noise reduction in low SNR conditions.
Option 2: Because SAD decisions are only relevant for stationary signals, not dynamic ones.
Option 3: To ensure the noise estimation remains robust against sudden changes in noise levels, preventing incorrect SNR assessments.
Option 4: Since SAD is more computationally intensive, an alternative method optimizes processing efficiency.
Explanation: The correct answer highlights the primary reason for not using the SAD decision in the second stage of noise energy update. By avoiding reliance on SAD, which could be misled by sudden increases in noise level (even in inactive speech frames), the noise estimation process remains resilient to rapid noise fluctuations, ensuring accurate SNR assessment and maintaining overall voice service quality.