Question: When switching from AMR-WB IO mode to a primary mode in EVS (Enhanced Voice Services) codec, what needs to be updated or converted for the primary mode encoded frame if it immediately follows a CELP-based AMR-WB IO encoded frame?
Option 1: Only the previous quantized end-frame LSFs and ISPs need conversion; all other memories are retained as is.
Option 2: The first three ACELP frames are processed using a safety-net LP quantizer, and conversions include end-frame ISFs to LSFs, end-frame ISPs to LSPs, and resetting BWE past buffers among others.
Option 3: No updates or conversions are necessary for the primary mode's memories when switching from AMR-WB IO mode.
Option 4: The entire audio signal improvement memory is overwritten with default values, and all previous frame quantizations are discarded.
Explanation: When transitioning from AMR-WB IO mode to a primary mode in EVS codec, to ensure seamless encoding, specific updates are mandatory. These include processing the first three ACELP frames with a safety-net LP quantizer, converting previous frame's ISFs to LSFs, ISPs to LSPs, and resetting buffers for BandWidth Expansion (BWE) among other adjustments. This ensures compatibility and maintains audio quality during mode switching.