[FFmpeg-devel] [PATCH 3/4] avfilter/asrc_sine: increase frequency accuracy

[Marton Balint]

On Sun, 10 Nov 2024, Marton Balint wrote:

> 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.

Will apply the series soon.

Regards,
Marton

>
> Signed-off-by: Marton Balint <cus at passwd.hu>
> ---
> libavfilter/asrc_sine.c | 24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/libavfilter/asrc_sine.c b/libavfilter/asrc_sine.c
> index 854fba6e37..6a591b551b 100644
> --- a/libavfilter/asrc_sine.c
> +++ b/libavfilter/asrc_sine.c
> @@ -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)
> -- 
> 2.43.0
>
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