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avfilter/af_biquads: implement 1st order allpass

This commit is contained in:
Paul B Mahol 2020-05-23 17:54:54 +02:00
parent f3068be15b
commit 1206a10d9c
2 changed files with 25 additions and 6 deletions
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3
doc/filters.texi
View File

@ -1568,6 +1568,9 @@ Specify which channels to filter, by default all available are filtered.
@item normalize, n
Normalize biquad coefficients, by default is disabled.
Enabling it will normalize magnitude response at DC to 0dB.
@item order, o
Set the filter order, can be 1 or 2. Default is 2.
@end table
@subsection Commands

28
libavfilter/af_biquads.c
View File

@ -113,6 +113,7 @@ typedef struct BiquadsContext {
double mix;
uint64_t channels;
int normalize;
int order;
double a0, a1, a2;
double b0, b1, b2;
@ -264,6 +265,7 @@ static int config_filter(AVFilterLink *outlink, int reset)
AVFilterLink *inlink = ctx->inputs[0];
double A = ff_exp10(s->gain / 40);
double w0 = 2 * M_PI * s->frequency / inlink->sample_rate;
double K = tan(w0 / 2.);
double alpha, beta;
if (w0 > M_PI) {
@ -389,12 +391,24 @@ static int config_filter(AVFilterLink *outlink, int reset)
}
break;
case allpass:
s->a0 = 1 + alpha;
s->a1 = -2 * cos(w0);
s->a2 = 1 - alpha;
s->b0 = 1 - alpha;
s->b1 = -2 * cos(w0);
s->b2 = 1 + alpha;
switch (s->order) {
case 1:
s->a0 = 1.;
s->a1 = -(1. - K) / (1. + K);
s->a2 = 0.;
s->b0 = s->a1;
s->b1 = s->a0;
s->b2 = 0.;
break;
case 2:
s->a0 = 1 + alpha;
s->a1 = -2 * cos(w0);
s->a2 = 1 - alpha;
s->b0 = 1 - alpha;
s->b1 = -2 * cos(w0);
s->b2 = 1 + alpha;
break;
}
break;
default:
av_assert0(0);
@ -773,6 +787,8 @@ static const AVOption allpass_options[] = {
{"c", "set channels to filter", OFFSET(channels), AV_OPT_TYPE_CHANNEL_LAYOUT, {.i64=-1}, INT64_MIN, INT64_MAX, FLAGS},
{"normalize", "normalize coefficients", OFFSET(normalize), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS},
{"n", "normalize coefficients", OFFSET(normalize), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS},
{"order", "set filter order", OFFSET(order), AV_OPT_TYPE_INT, {.i64=2}, 1, 2, FLAGS},
{"o", "set filter order", OFFSET(order), AV_OPT_TYPE_INT, {.i64=2}, 1, 2, FLAGS},
{NULL}
};