Question: In a transition from MDCT (Modulated Discrete Cosine Transform) to CELP (Code-Excited Linear Prediction) coding in Enhanced Voice Services (EVS), what happens to the past excitation (adaptive codebook memory) in the CELP encoder when transitioning with MC1?
Option 1: It is updated with interpolated values from the previous MDCT frame.
Option 2: It is reset to 0, forcing the CELP coder to operate in Transition Coding (TC) without an adaptive codebook.
Option 3: Its contents are directly reused for the initial CELP subframe.
Option 4: It undergoes a predefined warm-up sequence before being used.
Explanation: When transitioning from MDCT to CELP using MC1, because the past excitation (adaptive codebook memory) is not available due to the coding scheme change, it must be reset. This reset forces the CELP coder into Transition Coding mode for that frame, meaning it cannot utilize an adaptive codebook, which relies on past excitation data. This ensures a clean start in the new coding context, preventing potential artifacts from mismatched coding schemes.