Question: In EVS (Enhanced Voice Services) codec, as per 3GPP standards, under what condition is background noise energy updated in the first stage of the two-stage estimation process?
Option 1: When the current frame's signal energy exceeds the previously estimated background noise energy.
Option 2: Regardless of the current frame's signal energy, based solely on statistical closeness to the background noise model.
Option 3: Only for non-critical bands where voice activity is detected.
Option 4: When the current frame's signal energy is less than the previously estimated background noise energy, specifically for critical bands.
Explanation: The correct answer, Option 4, aligns with the described process in the EVS codec for estimating background noise energy. The first stage, known as the downward noise energy update, specifically updates noise energy for critical bands only when the current frame's signal energy is lower than the previously estimated background noise energy. This approach helps in accurately capturing and adapting to changes in the background noise level during periods of low signal energy in critical audio frequency bands.