Question: In EVS (Enhanced Voice Services) codec, after excitation post-processing, what happens to the decoded synthesis signal if the determined output mode is Narrowband (NB)?
Option 1: The entire frequency range is amplified to enhance voice clarity.
Option 2: Frequencies below 4 kHz are attenuated to reduce noise.
Option 3: The signal undergoes additional high-frequency enhancement for richer sound.
Option 4: Content above 4 kHz is attenuated using CLDFB synthesis, and high-frequency synthesis is skipped.
Explanation: In the context of EVS codec's synthesis filtering, when the output mode is determined to be Narrowband (NB), the focus shifts to optimizing the signal within the narrower frequency range. Attenuating content above 4 kHz using CLDFB synthesis ensures that the signal remains bandlimited, suitable for NB transmission. Skipping high-frequency synthesis on this bandlimited content is logical since it's outside the NB focus area, thus preserving bandwidth and processing resources. This approach aligns with efficient telecom practices for managing different bandwidth modes (e.g., NB vs. WB) in voice services.