Question: In EVS (Enhanced Voice Services) codec, when transitioning from a frame where adaptive post-filtering was applied to a frame where it is not, what happens to the memories of the post-filter in the first frame after the transition?
Option 1: The memories of the post-filter are immediately cleared for the new frame.
Option 2: Only the memories of the post-filter are populated and updated for the next frame, without applying post-filtering in the current frame.
Option 3: A smoothing mechanism is applied to avoid discontinuities by computing the zero impulse response of the post-filter states.
Option 4: The entire decoded frame is filtered using the previous frame's LPC analysis filter before updating the memories.
Explanation: This is correct because, according to the EVS codec specifications for adaptive post-filtering, if post-filtering was not employed in the previous frame (or vice versa), in the first frame after the transition, the adaptive post-filter is not applied. Instead, only the memories of the post-filter are prepared for potential use in the next frame, ensuring continuity without unnecessary processing in the transitional frame.