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avfilter/asrc_sine: increase frequency accuracy

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Previously the delta phase was fixed point fractional with 2^32 fractions,
which caused inaccuracies in the output frequency, unless the input
frequency*2^32 was divisable by the sample rate.

This patch improves frequency accuracy by tracking subfractions of the delta
phase fractions. For this we are using a denominator which is a multiple of the
sample rate, making sure that integer frequencies are always accurately
represented.

Signed-off-by: Marton Balint <cus@passwd.hu>
This commit is contained in:
Marton Balint 2024-11-10 01:14:06 +01:00
parent 06247ae746
commit 8d6f3bcb96
1 changed files with 23 additions and 1 deletions
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24
libavfilter/asrc_sine.c
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@ -33,6 +33,9 @@
typedef struct SamplingContext {
uint32_t phi; ///< current phase of the sine (2pi = 1<<32)
uint32_t dphi; ///< phase increment between two samples
int phi_rem; ///< current fractional phase in 1/dphi_den subfractions
int dphi_rem;
int dphi_den;
} SamplingContext;
typedef struct SineContext {
@ -148,12 +151,31 @@ enum {
static void sampling_init(SamplingContext *c, double frequency, int sample_rate)
{
c->dphi = ldexp(frequency, 32) / sample_rate + 0.5;
AVRational r;
int r_den, max_r_den;
max_r_den = INT_MAX / sample_rate;
frequency = fmod(frequency, sample_rate);
r = av_d2q(fmod(frequency, 1.0), max_r_den);
r_den = FFMIN(r.den, max_r_den);
c->dphi = ldexp(frequency, 32) / sample_rate;
c->dphi_den = r_den * sample_rate;
c->dphi_rem = round((ldexp(frequency, 32) / sample_rate - c->dphi) * c->dphi_den);
if (c->dphi_rem >= c->dphi_den) {
c->dphi++;
c->dphi_rem = 0;
}
c->phi_rem = (-c->dphi_den - 1) / 2;
}
static av_always_inline void sampling_advance(SamplingContext *c)
{
c->phi += c->dphi;
c->phi_rem += c->dphi_rem;
if (c->phi_rem >= 0) {
c->phi_rem -= c->dphi_den;
c->phi++;
}
}
static av_cold int init(AVFilterContext *ctx)