Question: When switching the internal sampling rate from 12.8 kHz to 16 kHz in an EVS (Enhanced Voice Services) codec, how is the power spectrum of the Linear Prediction (LP) filter originally estimated at 12.8 kHz handled for autocorrelation computation?
Option 1: The entire original power spectrum is directly mapped onto the new frequency range without modification.
Option 2: The power spectrum unknown at higher frequencies (above 6.4 kHz) is approximated by extending the value at 6.4 kHz over this range for autocorrelation computation.
Option 3: Only the lower frequency components of the original power spectrum are used, ignoring the need for higher frequency components.
Option 4: The LP filter is entirely re-estimated from scratch at the new sampling rate without using any information from the original estimate.
Explanation: When up-sampling from 12.8 kHz to 16 kHz, the EVS codec needs to handle the power spectrum of the LP filter for frequencies beyond what was originally estimated (up to 6.4 kHz at 12.8 kHz). To do this efficiently without losing information, the codec approximates the unknown higher frequency range by extending the last known value (at 6.4 kHz) across this new range for computing autocorrelation and solving LP filter parameters at the new sampling rate, ensuring continuity and a practical adaptation to increased bandwidth.