Question: In the EVS (Enhanced Voice Services) codec, for bit rates at or above 13.2 kbps, how is the global gain quantization refined during residual coding?
Option 1: By allocating up to two additional bits to refine the quantized spectrum lines only.
Option 2: By sequentially refining the global gain quantization with up to three additional bits, starting from a base index.
Option 3: Exclusively through re-quantizing the zeroed spectrum lines with a multi-level quantizer without any additional bits for global gain refinement.
Option 4: Not refined; the initial 7-bit index is used for all bit rates above 13.2 kbps.
Explanation: According to the EVS codec's residual coding process for bit rates at or above 13.2 kbps, the global gain quantization can be refined using up to three additional bits. This refinement is done sequentially, indicating a stepwise approach to achieve more precise global gain representation when sufficient bits are available. This method enhances the overall coding efficiency and voice quality by allowing for finer adjustments to the global gain at higher bit rates.