Question: In the EVS codec, how is the counter (Δt_corr) updated when a correlation or harmonic event is detected in a frame, and what happens to its value under specific conditions related to short-term variance of input frame energy?
Option 1: The counter is incremented by one, and its value is doubled if the short-term variance estimate exceeds 8.0.
Option 2: The counter is reset, and if the short-term variance estimate is larger than 8.0, its current value can be reduced to one quarter of its value (or 1 if it was less than 4).
Option 3: The counter remains unchanged until two consecutive correlation events are detected, at which point it resets to zero.
Option 4: The counter is reset when a correlation or harmonic event occurs; otherwise, it increments by one. Additionally, if the short-term variance estimate exceeds 8.0 and specific conditions (C_ton & Δt_corr > 1) are met, its value may be limited to 1.
Explanation: This option accurately reflects the described behavior of the counter (Δt_corr). It correctly states that the counter resets upon detection of a correlation or harmonic event and increments otherwise. Furthermore, it outlines the specific condition under which the counter's value may be adjusted (limited to 1) based on the short-term variance estimate and additional logical conditions (C_ton & Δt_corr > 1). This demonstrates a nuanced understanding of the EVS codec's algorithmic handling of correlation and harmonic events in relation to input frame energy variability.