Question: In EVS (Enhanced Voice Services) codec, when a decoder operates in CNG (Comfort Noise Generation) mode and receives a SID (Silence Insertion Description) frame, how does the decoder differentiate between the two CNG modes for comfort noise synthesis?
Option 1: Based on the energy level of the received audio signal.
Option 2: By decoding the first bit in the SID frame, which points to the specific CNG mode.
Option 3: Through analysis of spectral envelopes generated during active voice frames.
Option 4: Automatically switching between modes after a predefined time interval.
Explanation: The correct answer is Option 2 because, according to the EVS codec specifications for CNG operation, the first bit in the SID frame explicitly indicates whether the decoder should operate in LP-CNG or FD-CNG mode, dictating how comfort noise will be synthesized.