ref: 9d742e2840dacfffe8ecda3ef25ed6aa7974ac6b
dir: /src/lowp.c/
/* * Sound Tools Low-Pass effect file. * * (C) 2000 Chris Bagwell <cbagwell@sprynet.com> * See License file for further copyright information. * * Algorithm: Recursive single pole lowpass filter * * Reference: The Scientist and Engineer's Guide to Digital Signal Processing * * output[N] = input[N] * A + output[N-1] * B * * X = exp(-2.0 * pi * Fc) * A = 1 - X * B = X * Fc = cutoff freq / sample rate * * Mimics an RC low-pass filter: * * ---/\/\/\/\-----------> * | * --- C * --- * | * | * V * */ #include <math.h> #include "st.h" /* Private data for Lowpass effect */ typedef struct lowpstuff { float cutoff; double A, B; double outm1; } *lowp_t; /* * Process options */ int st_lowp_getopts(effp, n, argv) eff_t effp; int n; char **argv; { lowp_t lowp = (lowp_t) effp->priv; if ((n < 1) || !sscanf(argv[0], "%f", &lowp->cutoff)) { st_fail("Usage: lowp cutoff"); return (ST_EOF); } return (ST_SUCCESS); } /* * Prepare processing. */ int st_lowp_start(effp) eff_t effp; { lowp_t lowp = (lowp_t) effp->priv; if (lowp->cutoff > effp->ininfo.rate / 2) { st_fail("Lowpass: cutoff must be < sample rate / 2 (Nyquest rate)\n"); return (ST_EOF); } lowp->B = exp((-2.0 * M_PI * (lowp->cutoff / effp->ininfo.rate))); lowp->A = 1 - lowp->B; lowp->outm1 = 0.0; return (ST_SUCCESS); } /* * Processed signed long samples from ibuf to obuf. * Return number of samples processed. */ int st_lowp_flow(effp, ibuf, obuf, isamp, osamp) eff_t effp; LONG *ibuf, *obuf; LONG *isamp, *osamp; { lowp_t lowp = (lowp_t) effp->priv; int len, done; double d; LONG l; len = ((*isamp > *osamp) ? *osamp : *isamp); for(done = 0; done < len; done++) { l = *ibuf++; d = lowp->A * l + lowp->B * lowp->outm1; if (d < -2147483647L) d = -2147483647L; else if (d > 2147483647L) d = 2147483647L; lowp->outm1 = d; *obuf++ = d; } *isamp = len; *osamp = len; return (ST_SUCCESS); } /* * Do anything required when you stop reading samples. * Don't close input file! */ int st_lowp_stop(effp) eff_t effp; { /* nothing to do */ return (ST_SUCCESS); }