CELP coders

Code Excited Linear Prediction (CELP) coders are state-of-the-art in telephony coding:

• Start from basic LP vocoder
• Apply a long term predictor to remove redundancies through periodicity
• Vector quantise the LP parameters - as before
• Vector quantise the LP residual - typically 40 samples (5ms) using 1024 codebook entries
• Coding is much more computationally expensive than decoding (need codebook searches)
• Resultant bit rate is about 4 kbps

The theory says that after all the linear correlations have been removed, the prediction residual is uncorrelated values with a Gaussian distribution. Therefore generate the excitation codebook as a set of random numbers.

Complete system looks like figure 48:

  
Figure 48: Typical CELP coder

Where all parameters of the system are obtained by minimising the final mean squared error. Sequential parameter estimation is non-optimal.