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Implement upwards compression

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This commit is contained in:
Robbert van der Helm 2022-07-24 16:56:01 +02:00
parent 8ca66cf9bb
commit 4a2373b664
1 changed files with 38 additions and 11 deletions
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49
plugins/spectral_compressor/src/compressor_bank.rs
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@ -504,11 +504,13 @@ impl CompressorBank {
) {
// Well I'm not sure at all why this scaling works, but it does. With higher knee
// bandwidths, the middle values needs to be pushed more towards the post-knee threshold
// than with lower knee values.
// than with lower knee values. These scaling factors are used as exponents.
let downwards_knee_scaling_factor =
((params.compressors.downwards.knee_width_db.value * 2.0) + 2.0).log2() - 1.0;
// Note the square root here, since the curve needs to go the other way for the upwards
// version.
let upwards_knee_scaling_factor =
((params.compressors.upwards.knee_width_db.value * 2.0) + 2.0).log2() - 1.0;
(((params.compressors.upwards.knee_width_db.value * 2.0) + 2.0).log2() - 1.0).sqrt();
// Is this what they mean by zip and and ship it?
let downwards_knees = self
@ -536,17 +538,14 @@ impl CompressorBank {
.zip(upwards_values)
.skip(skip_bins_below)
{
let (
(downwards_threshold, downwards_ratio_recip),
(downwards_knee_start, downwards_knee_end),
) = downwards_values;
let ((upwards_threshold, upwards_ratio_recip), (upwards_knee_start, upwards_knee_end)) =
upwards_values;
// This works by computing a scaling factor, and then scaling the bin magnitudes by that.
let mut scale = 1.0;
// All compression happens in the linear domain to save a logarithm
let (
(downwards_threshold, downwards_ratio_recip),
(downwards_knee_start, downwards_knee_end),
) = downwards_values;
if *downwards_ratio_recip != 1.0 {
// The soft-knee option will fade in the compression curve when reaching the knee
// start until it mtaches the hard-knee curve at the knee-end
@ -565,7 +564,7 @@ impl CompressorBank {
nih_debug_assert!((0.0..=1.0).contains(&raw_knee_t));
// TODO: Apart from a small discontinuety in the derivative/slope at the start
// of the knee this equation does exaclty what you'd expect it to, but it
// of the knee this equation does exactly what you'd expect it to, but it
// feels a bit weird. Should probably look for a cleaner way to calculate
// this soft knee in linear-space at some point.
let knee_t = (1.0 - raw_knee_t).powf(downwards_knee_scaling_factor);
@ -581,7 +580,35 @@ impl CompressorBank {
}
}
// TODO: Upwards compression
// Upwards compression should not happen when the signal is _too_ quiet as we'd only be
// amplifying noise
let ((upwards_threshold, upwards_ratio_recip), (upwards_knee_start, upwards_knee_end)) =
upwards_values;
if *upwards_ratio_recip != 1.0 && *envelope > 1e-6 {
// This goes the other way around compared to the downwards compression
if envelope <= upwards_knee_start {
// Notice how these ratios are reversed here
let threshold_ratio = upwards_threshold / envelope;
scale /= threshold_ratio.powf(*upwards_ratio_recip) / threshold_ratio;
} else if envelope <= upwards_knee_end {
// When the knee width is set to 0 dB, `upwards_knee_start == upwards_knee_end`
// and this branch is never hit
let linear_knee_width = upwards_knee_end - upwards_knee_start;
let raw_knee_t = (envelope - upwards_knee_start) / linear_knee_width;
nih_debug_assert!((0.0..=1.0).contains(&raw_knee_t));
// TODO: Some note the downwards version
let knee_t = (1.0 - raw_knee_t).powf(upwards_knee_scaling_factor);
nih_debug_assert!((0.0..=1.0).contains(&knee_t));
// The ratios are again inverted here compared to the downwards version
let knee_ratio = upwards_knee_start / envelope;
let threshold_ratio = upwards_threshold / envelope;
scale /= (knee_t * (knee_ratio.powf(*upwards_ratio_recip) / knee_ratio))
+ ((1.0 - knee_t)
* (threshold_ratio.powf(*upwards_ratio_recip) / threshold_ratio));
}
}
*bin *= scale;
}