CELP Speech Coders

1
CELP Speech Coders

• Code Excited Linear Prediction.
• Code refers to the fact that the excitation
  signal is created using VQ codebooks.
• CELP coders typically incorporate an
  analysis-by-synthesis (AbS) procedure.

2
Not Analysis by Synthesis
ADPCM encoder
Framing
Excitation Q
LP Analysis
Spectrum Q
Gain Q
3
Not AbS Cont.
ADPCM decoder
Spectrum
Excitation
Gain
4
Analysis by Synthesis
AbS ADPCM encoder
Spectrum
Excitation
-
Gain
Try all combinations of indices and pick the one
that minimizes
5
Analysis by Synthesis Conclusions

• Optimal search of encoding alternatives.
• Easy to include error weighting (noise shaping)
• Increased complexity.
• The encoder structure is still suboptimal. An
  unconstrained VQ with dimension frame length
  would perform better.
• In principle, any structured VQ (split, MS) could
  be used, but obviously the structure should fit
  the source.

6
Code Excited Linear Prediction with AbS
LP analysis
Spectrum VQ
Subframing
Fixed CB
-
AdaptiveCB
7
The EM algorithm
Given the current parameters for component
k, we obtain a new set of parameters by
where