Question: In the Enhanced Voice Services (EVS) codec, what triggers the replacement of the UC (Unvoiced Coding) mode with the GC (Generalized Complexity) mode at a bitrate of 9.6 kbps, aside from exceeding the bitrate threshold?
Option 1: The occurrence of a voice activity detection (VAD) event.
Option 2: The counter of previous AC (Active Codec) frames exceeds a predefined value greater than 0.
Option 3: A mode switching event from NB (Narrowband) to WB (Wideband) operation.
Option 4: The gain quantizer in the NB VC (Variable Complexity) mode reaching its dynamic range limit at bitrates above 8.0 kbps.
Explanation: According to the EVS codec's detailed algorithmic description, at 9.6 kbps, the UC mode is replaced by the GC mode if the counter of previous AC frames is bigger than 0. This counter mechanism helps in adapting the coding mode based on recent frame types, ensuring optimal performance under varying signal conditions.