abt VAD algorithm 1 of AMR codec | Speech Coding

Technical discussions related to Speech Coding (all itu and other vocoders, ACELP, CELP, AMR, etc)

Hi all! i have a query regarding the VAD algorithm 1 used in GSM AMR speech codec.one of the steps in this algo is the use of a filter bank. this bank consists of 5th and 3rd order filter blocks. for the 3rd order filter block the equations are as follows : x_lp(n) = 0.5 * (x(n) + A3(x(n-1))) eq (3.2a) x_hp(n) = 0.5 * (x(n) - A3(x(n-1))) eq (3.2b) where x(n) is the input signal x_lp(n) = low pass component x_hp(n) = high pass component A3() is said to be a first order direct form all-pass filter, whose transfer function is given by : A3(z) = [C3+z^-1]/[1+Cz^-1] where C3 = filter coefft now, the term A3 in eq 3.2a and 3.2b are confusing since all the other terms are in time domain. in fact, going by their C code, i found that the term is C3*x(n-1) and not A3(x(n-1)). even for 5th order filter, same results are obtained.(A1 to be replaced by C1 etc). is this observation correct?if it is then there is a printing mistake in the standard which is generally unlikely. if anyone has worked on this earlier please let me know the solution. Thank you Sameer Kibey

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abt VAD algorithm 1 of AMR codec - Sameer Kibey - Jul 25 8:14:00 2002