Question: In EVS (Enhanced Voice Services) codec at 64 kbps, how is the upper band (extending to 16 kHz or 20 kHz) coded differently in terms of frequency division between normal and transient modes?
Option 1: Normal mode divides the spectrum into two bands of 3.4 kHz each, while transient mode uses four bands of 1.7 kHz width each.
Option 2: Both normal and transient modes divide the spectrum (between 7.6 kHz and 14.4 kHz) into four bands of equal width, but with different encoding schemes.
Option 3: Normal mode divides the spectrum into four bands of 1.7 kHz width each, while transient mode uses two bands of 3.4 kHz each.
Option 4: The frequency division is identical in both modes, with the only difference being the number of sub-frames used.
Explanation: In EVS codec at 64 kbps, the upper band coding employs different strategies for normal and transient modes to optimize quality. Specifically, normal mode splits the relevant spectrum into four narrower bands of 1.7 kHz each for detailed representation of steady-state signals. In contrast, transient mode divides this spectrum into two broader bands of 3.4 kHz each, focusing on capturing changing signals with less granular but more robust encoding, balancing quality and artifact minimization in different signal contexts.