Improving 16 kb/s G.728 LD-CELP speech coder for frame erasure channels
C.R. Watkins, Juin-Hwey Chen · 2002
We have improved G.728 output speech quality for frame erasure channels. Three cases are considered: (1) no change to G.728, (2) change only the G.728 decoder, and (3) change both the encoder and decoder. In case 1, we synthesize a bit-stream during erased frames so that the decoder decodes an excitation with low energy or with characteristics similar to the excitation of previous good frames. In case 2, the gain-scaled excitation and LPC coefficients are extrapolated, and vital operations of backward LPC and gain adaptations are continued. Case 3 adds spectral smoothing and increases bandwidth expansion for the LPC and gain predictors. These techniques are quite effective, as the speech quality degradation due to 1% frame erasures ranges from just slightly noticeable in case 1 to almost unnoticeable in case 3. For case 3, the output speech is still intelligible for frame erasure rates up to 10% or even 20%.