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Fix gain compensation for the Moog filter #11177

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merged 5 commits into from
Jan 9, 2023
Merged

Fix gain compensation for the Moog filter #11177

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99f2781
Fix gain compensation for the Moog filter
robbert-vdh Jan 2, 2023
ac5f6cf
Link the PR containing the Jupyter notebook
robbert-vdh Jan 2, 2023
8d76038
Also fix Moog filter gain compensation for HPF
robbert-vdh Jan 3, 2023
ee17b31
Revert Moog HPF filter gain compensation to 1.0
robbert-vdh Jan 3, 2023
1b98c4e
Elaborate on Moog LPF gain compensation formula
robbert-vdh Jan 8, 2023
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18 changes: 15 additions & 3 deletions src/engine/filters/enginefiltermoogladder4.h
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Original file line number Diff line number Diff line change
Expand Up @@ -93,11 +93,23 @@ class EngineFilterMoogLadderBase : public EngineObjectConstIn {
// Resonance correction for self oscillation ~4
m_kacrNew = resonance * (-3.9364f * (kfc * kfc) + 1.8409f * kfc + 0.9968f);

// See https://github.com/mixxxdj/mixxx/pull/11177 for the Jupyter
// notebook used to derive this
if (MODE == MoogMode::HighPassOversampling || MODE == MoogMode::HighPass) {
m_postGainNew = 1;
// This for all intents and purposes is just 1.0, so let's just stick with that
// m_postGainNew = 0.9999999983339118f + (4.58745459575558e-13f * resonance);
m_postGainNew = 1.0;
} else {
m_postGainNew = (1 + resonance / 4 * (1.1f + cutoff / sampleRate * 3.5f))
* (2 - (1.0f - resonance / 4) * (1.0f - resonance / 4));
// Analyzing this filter as a linear system will show a small dip in
// passband/DC gain when the cutoff frequency is around 10 kHz:
// https://github.com/mixxxdj/mixxx/pull/11177#issuecomment-1374833386
//
// In practice this is either not a noticeable issue when running
// music through the filter, or it might even be desirable as it
// keeps the overall gain increase a bit lower when the resonance is
// turned up and the cutoff frequency is around 10 kHz. This may be
// worth more experimentation in the future.
m_postGainNew = 1.0001784074555027f + (0.9331585678097162f * resonance);
}

m_doRamping = true;
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