Question: In an Adaptive Multi-Rate (AMR) speech codec, operating in Source Controlled Rate (SCR) mode on the Receiver (RX) side, what happens when a frame classified as SID_FIRST is received while the RX SCR handler is in SPEECH mode?
Option 1: The RX SCR handler remains in SPEECH mode and passes the frame to the speech decoder.
Option 2: The RX SCR handler substitutes the frame with comfort noise and stays in SPEECH mode.
Option 3: The frame is ignored, and the RX SCR handler continues in SPEECH mode.
Option 4: The RX SCR handler transitions into COMFO_NOISE mode and initiates comfort noise generation.
Explanation: Receiving a SID_FIRST frame triggers a mode switch from SPEECH to COMFO_NOISE. In COMFO_NOISE mode, the primary action is comfort noise generation to maintain a comfortable auditory environment during periods without usable speech frames, ensuring a seamless listening experience despite potential packet losses or errors.