Question: In EVS (Enhanced Voice Services) codec, how are the MDCT coefficients of the upper band reconstructed for a low bit rate core?
Option 1: By predicting envelopes from all decoded frequency envelopes and using them directly as coefficients.
Option 2: Through linear interpolation of the two lowest frequency envelopes.
Option 3: By predicting envelopes from the 2 decoded highest frequency envelopes and the average envelope of all decoded envelopes, combined with reconstructed normalized coefficients via random noise.
Option 4: Solely based on the weighted average of all previously transmitted frames' MDCT coefficients.
Explanation: For low bit rate cores in EVS codec, the reconstruction of upper band MDCT coefficients involves a two-step process. First, the envelopes are predicted using both the 2 highest frequency decoded envelopes and the average of all decoded envelopes. Then, these predicted envelopes are combined with normalized coefficients that are reconstructed using random noise. This approach balances bit rate efficiency with perceptual quality, which is crucial for enhanced voice services over lower bandwidths.