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Move Spectral Compressor knee coefs to functions

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This commit is contained in:
Robbert van der Helm 2023-03-27 17:22:53 +02:00
parent 20d165052a
commit 732cfdfa3a
1 changed files with 43 additions and 16 deletions
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59
plugins/spectral_compressor/src/compressor_bank.rs
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@ -1139,13 +1139,12 @@ impl CompressorBank {
// This is the formula from the Digital Dynamic Range Compressor Design paper by
// Dimitrios Giannoulis et. al. These are `a` and `b` from the `x + a * (x + b)^2`
// respectively used to compute the soft knee respectively.
*knee_parabola_scale = if downwards_knee_width_db != 0.0 {
(2.0 * downwards_knee_width_db * *ratio).recip()
- (2.0 * downwards_knee_width_db).recip()
} else {
1.0
};
*knee_parambola_intercept = -threshold_db + (downwards_knee_width_db / 2.0);
(*knee_parabola_scale, *knee_parambola_intercept) =
downwards_soft_knee_coefficients(
*threshold_db,
downwards_knee_width_db,
*ratio,
);
}
}
@ -1165,15 +1164,9 @@ impl CompressorBank {
.zip(self.upwards_knee_parabola_intercept.iter_mut()),
)
{
// For the upwards version the scale becomes negated
*knee_parabola_scale = if upwards_knee_width_db != 0.0 {
-((2.0 * upwards_knee_width_db * *ratio).recip()
- (2.0 * upwards_knee_width_db).recip())
} else {
1.0
};
// And the `+ (knee/2)` becomes `- (knee/2)` in the intercept
*knee_parambola_intercept = -threshold_db - (upwards_knee_width_db / 2.0);
// The upwards version is slightly different
(*knee_parabola_scale, *knee_parambola_intercept) =
upwards_soft_knee_coefficients(*threshold_db, upwards_knee_width_db, *ratio);
}
}
}
@ -1230,3 +1223,37 @@ fn compress_upwards(
threshold_db + ((input_db - threshold_db) / ratio)
}
}
/// Compute the `(scale, intercept)`/`(a, b)` coefficients for the parabolic formula `x + a * (x +
/// b)^2`. The formula is taken from the Digital Dynamic Range Compressor Design paper by Dimitrios
/// Giannoulis et. al. This version applies to downwards compression. It can be precalculated for
/// the regular modes, since it's dependent on the threshold it has to be recomputed for every
/// sample with the sidechain matching mode.
fn downwards_soft_knee_coefficients(
threshold_db: f32,
knee_width_db: f32,
ratio: f32,
) -> (f32, f32) {
let scale = if knee_width_db != 0.0 {
(2.0 * knee_width_db * ratio).recip() - (2.0 * knee_width_db).recip()
} else {
1.0
};
let intercept = -threshold_db + (knee_width_db / 2.0);
(scale, intercept)
}
/// [`downwards_soft_knee_coefficients()`], but for upwards compression.
fn upwards_soft_knee_coefficients(threshold_db: f32, knee_width_db: f32, ratio: f32) -> (f32, f32) {
// For the upwards version the scale becomes negated
let scale = if knee_width_db != 0.0 {
-((2.0 * knee_width_db * ratio).recip() - (2.0 * knee_width_db).recip())
} else {
1.0
};
// And the `+ (knee/2)` becomes `- (knee/2)` in the intercept
let intercept = -threshold_db - (knee_width_db / 2.0);
(scale, intercept)
}