Question: Under what conditions is the FD-CNG reset flag set to 1 in the EVS codec, triggering a reinitialization of the minimum statistics routine?
Option 1: When four consecutive frames show an increase in total noise energy with a sum of differences > 5, and the encoder is out of its initialization phase, OR when the current frame's input bandwidth exceeds the previous frame's.
Option 2: Only when the encoder detects a decrease in total noise energy over three consecutive frames, and the device is in the initialization phase.
Option 3: If the sum of the differences in total noise energy over the last two frames is less than 5, but the current frame's bandwidth is narrower than the previous one.
Option 4: When the FD-CNG mechanism has been inactive for exactly nine frames since its last reset.
Explanation: This option correctly identifies the conditions under which the FD-CNG reset flag is set to 1, as per the detailed algorithmic description. These conditions are crucial for maintaining optimal voice service quality by triggering a reinitialization of the minimum statistics routine when significant changes in noise energy or input bandwidth are detected, ensuring the encoder adapts to new audio signal characteristics effectively.