Question: In EVS (Enhanced Voice Services) codec, when decoding the upper band at 64 kbps, what is the primary purpose of multiplying each spectral coefficient in the overlap region (7.6 kHz – 8 kHz) by a factor lower than 1.0?
Option 1: To equalize the spectral gap due to the attenuation of resampling filters where ACELP lower band synthesis is suppressed.
Option 2: To enhance the global gain of the decoded spectrum for louder output.
Option 3: To normalize the spectral envelope before denormalizing with the quantized spectral envelope.
Option 4: To adjust the bit-rate to match lower bandwidth conditions.
Explanation: This process ensures a seamless transition between the lower and upper bands of the decoded spectrum. By applying a factor less than 1.0 to coefficients in the overlap region, the EVS codec mitigates the spectral gap caused by resampling filter attenuation suppressing ACELP lower band synthesis, thus maintaining audio quality.