(Sorry, this messsage can appear twice in this newsgroup because of Yahoo). CELP is the name for the family of the speech coders based on analysis-by-synthesis (examples - G723 5.3 Kbps and 6.3 Kbps, FS 1016, G729 etc). ACELP - is a coder from CELP-family with algebraic codebook (example - G723 5.3 Kbps). MELP - belongs to the family of parametric coders (STC, WI, MELP, FS 1015), and it is significantly different from CELP although it also uses LPC. About zero excitation. Filter is linear operator WITH HISTORY. The history is what makes the output of filtering zero excitation non-zero. Symbolically it can be presented as follows. Suppose A is the filter with the history, A0 is the same filter with zero history, x is the signal to be filtered and 0 is zero signal. Then you can easily check that the following relation holds: A*x = A*(x+0) = A*0+A0*x Both elements on the right side are generally non-zero! If you understand this formula you have got the key to understanding the analysis-by-synthesis method. Ilya Druker |