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audioconvert: align low-/highpass calculations

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The current biquad calculations are based on RBJ's cookbook [1],
except for low-/highpass. Since the filter configuration is also
based on using the definition of Q, it makes sense to also align
the remaining calculations to use the same filter cookbook instead
of using resonance which doesn't result in the same coefficients
as when using Q.

[1] = https://www.w3.org/TR/audio-eq-cookbook/
This commit is contained in:
Niklas Carlsson 2024-12-06 12:57:34 +01:00
parent e4ff13a75d
commit 7785f61677
1 changed files with 36 additions and 36 deletions
Download patch file Download diff file Expand all files Collapse all files

72
spa/plugins/audioconvert/biquad.c
View file

@ -15,8 +15,8 @@
#define M_PI 3.14159265358979323846
#endif
/* S = 1 in Q */
#define BIQUAD_SHELVING_DEFAULT_Q 0.707106781186548
/* Q = 1 / sqrt(2), also resulting Q value when S = 1 */
#define BIQUAD_DEFAULT_Q 0.707106781186548
static void set_coefficient(struct biquad *bq, double b0, double b1, double b2,
double a0, double a1, double a2)
@ -29,7 +29,7 @@ static void set_coefficient(struct biquad *bq, double b0, double b1, double b2,
bq->a2 = (float)(a2 * a0_inv);
}
static void biquad_lowpass(struct biquad *bq, double cutoff, double resonance)
static void biquad_lowpass(struct biquad *bq, double cutoff, double Q)
{
/* Limit cutoff to 0 to 1. */
cutoff = fmax(0.0, fmin(cutoff, 1.0));
@ -43,27 +43,27 @@ static void biquad_lowpass(struct biquad *bq, double cutoff, double resonance)
return;
}
/* Set Q to a sane default value if not set */
if (Q <= 0)
Q = BIQUAD_DEFAULT_Q;
/* Compute biquad coefficients for lowpass filter */
resonance = fmax(0.0, resonance); /* can't go negative */
double g = pow(10.0, 0.05 * resonance);
double d = sqrt((4 - sqrt(16 - 16 / (g * g))) / 2);
/* H(s) = 1 / (s^2 + s/Q + 1) */
double w0 = M_PI * cutoff;
double alpha = sin(w0) / (2 * Q);
double k = cos(w0);
double theta = M_PI * cutoff;
double sn = 0.5 * d * sin(theta);
double beta = 0.5 * (1 - sn) / (1 + sn);
double gamma = (0.5 + beta) * cos(theta);
double alpha = 0.25 * (0.5 + beta - gamma);
double b0 = (1 - k) / 2;
double b1 = 1 - k;
double b2 = (1 - k) / 2;
double a0 = 1 + alpha;
double a1 = -2 * k;
double a2 = 1 - alpha;
double b0 = 2 * alpha;
double b1 = 2 * 2 * alpha;
double b2 = 2 * alpha;
double a1 = 2 * -gamma;
double a2 = 2 * beta;
set_coefficient(bq, b0, b1, b2, 1, a1, a2);
set_coefficient(bq, b0, b1, b2, a0, a1, a2);
}
static void biquad_highpass(struct biquad *bq, double cutoff, double resonance)
static void biquad_highpass(struct biquad *bq, double cutoff, double Q)
{
/* Limit cutoff to 0 to 1. */
cutoff = fmax(0.0, fmin(cutoff, 1.0));
@ -79,24 +79,24 @@ static void biquad_highpass(struct biquad *bq, double cutoff, double resonance)
return;
}
/* Set Q to a sane default value if not set */
if (Q <= 0)
Q = BIQUAD_DEFAULT_Q;
/* Compute biquad coefficients for highpass filter */
resonance = fmax(0.0, resonance); /* can't go negative */
double g = pow(10.0, 0.05 * resonance);
double d = sqrt((4 - sqrt(16 - 16 / (g * g))) / 2);
/* H(s) = s^2 / (s^2 + s/Q + 1) */
double w0 = M_PI * cutoff;
double alpha = sin(w0) / (2 * Q);
double k = cos(w0);
double theta = M_PI * cutoff;
double sn = 0.5 * d * sin(theta);
double beta = 0.5 * (1 - sn) / (1 + sn);
double gamma = (0.5 + beta) * cos(theta);
double alpha = 0.25 * (0.5 + beta + gamma);
double b0 = (1 + k) / 2;
double b1 = -(1 + k);
double b2 = (1 + k) / 2;
double a0 = 1 + alpha;
double a1 = -2 * k;
double a2 = 1 - alpha;
double b0 = 2 * alpha;
double b1 = 2 * -2 * alpha;
double b2 = 2 * alpha;
double a1 = 2 * -gamma;
double a2 = 2 * beta;
set_coefficient(bq, b0, b1, b2, 1, a1, a2);
set_coefficient(bq, b0, b1, b2, a0, a1, a2);
}
static void biquad_bandpass(struct biquad *bq, double frequency, double Q)
@ -161,7 +161,7 @@ static void biquad_lowshelf(struct biquad *bq, double frequency, double Q,
/* Set Q to an equivalent value to S = 1 if not specified */
if (Q <= 0)
Q = BIQUAD_SHELVING_DEFAULT_Q;
Q = BIQUAD_DEFAULT_Q;
double w0 = M_PI * frequency;
double alpha = sin(w0) / (2 * Q);
@ -201,7 +201,7 @@ static void biquad_highshelf(struct biquad *bq, double frequency, double Q,
/* Set Q to an equivalent value to S = 1 if not specified */
if (Q <= 0)
Q = BIQUAD_SHELVING_DEFAULT_Q;
Q = BIQUAD_DEFAULT_Q;
double w0 = M_PI * frequency;
double alpha = sin(w0) / (2 * Q);