ref: c61a5fec6258b484fc7e5fbd2a088eff1c4571be
dir: /src/lowp.c/
/* * July 5, 1991 * Copyright 1991 Lance Norskog And Sundry Contributors * This source code is freely redistributable and may be used for * any purpose. This copyright notice must be maintained. * Lance Norskog And Sundry Contributors are not responsible for * the consequences of using this software. */ /* * Sound Tools Low-Pass effect file. * * Algorithm: 1nd order filter. * From Fugue source code: * * output[N] = input[N] * A + input[N-1] * B * * A = 2.0 * pi * center * B = exp(-A / frequency) */ #include <math.h> #include "st.h" /* Private data for Lowpass effect */ typedef struct lowpstuff { float center; double A, B; double in1; } *lowp_t; /* * Process options */ void 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->center)) fail("Usage: lowp center"); } /* * Prepare processing. */ void lowp_start(effp) eff_t effp; { lowp_t lowp = (lowp_t) effp->priv; if (lowp->center > effp->ininfo.rate*2) fail("Lowpass: center must be < minimum data rate*2\n"); lowp->A = (M_PI * 2.0 * lowp->center) / effp->ininfo.rate; lowp->B = exp(-lowp->A / effp->ininfo.rate); lowp->in1 = 0.0; } /* * Processed signed long samples from ibuf to obuf. * Return number of samples processed. */ void lowp_flow(effp, ibuf, obuf, isamp, osamp) eff_t effp; LONG *ibuf, *obuf; int *isamp, *osamp; { lowp_t lowp = (lowp_t) effp->priv; int len, done; double d; LONG l; len = ((*isamp > *osamp) ? *osamp : *isamp); /* yeah yeah yeah registers & integer arithmetic yeah yeah yeah */ for(done = 0; done < len; done++) { l = *ibuf++; d = lowp->A * (l / 65536L) + lowp->B * (lowp->in1 / 65536L); d *= 0.8; if (d > 32767) d = 32767; if (d < - 32767) d = - 32767; lowp->in1 = l; *obuf++ = d * 65536L; } } /* * Do anything required when you stop reading samples. * Don't close input file! */ void lowp_stop(effp) eff_t effp; { /* nothing to do */ }