Problem Description: In codec2.c we have a function called codec2_encode_700b() and an instruction:
inverse_filter(&c2->bpf_buf[BPF_N], bpfb, 4*N, bpf_out, BPF_N); BPF_N is 101, 4 * N = 320 (which could be M instead) The number of coeficients of the BPF filter is 101. In lpc.c we have the location of the inverse_filter function. Since there are 101 coeficients, you can address these as 0..100 in an array. This function however references 0..101 as you can see the "j" loop goes from 0 to "order" which is 101. I believe this function is falling out of the coeficient array when j = 101. Solution: Change the "<=" in the "j" loop to "<" on line #209 http://sourceforge.net/p/freetel/code/HEAD/tree/codec2-dev/src/lpc.c#l209 Reference Code: void inverse_filter( float Sn[], /* Nsam input samples */ float a[], /* LPCs for this frame of samples */ int Nsam, /* number of samples */ float res[], /* Nsam residual samples */ int order /* order of LPC */ ) { int i,j; /* loop variables */ for(i=0; i<Nsam; i++) { res[i] = 0.0; for(j=0; j<=order; j++) res[i] += Sn[i-j]*a[j]; <---Note a[j] can be a[0]..a[101] } }
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