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Fix: don't export loudness info to Engine #11979

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Sep 15, 2023
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Fix: don't export loudness info to Engine #11979

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12 changes: 5 additions & 7 deletions src/library/export/engineprimeexportjob.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -210,13 +210,11 @@ void exportMetadata(
snapshot.sample_count = frameCount;
snapshot.sample_rate = pTrack->getSampleRate();

// Set track loudness.
// Note that the djinterop API method for setting loudness may be revised
// in future, as more is discovered about the exact meaning of the loudness
// field in the Engine Library format. Make the assumption for now that
// ReplayGain ratio is an appropriate value to set, which has been validated
// by basic experimental testing.
snapshot.average_loudness = pTrack->getReplayGain().getRatio();
// Track loudness controls how the waveforms are scaled on Engine players.
// However, getting it wrong and accidentally scaling a waveform beyond a sensible maximum
// can result in no waveform being shown at all. In order to be safe, no loudness information
// is exported, resulting in waveforms being displayed as-is.
snapshot.average_loudness = 0;
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Judging form t he variable name it needs to be_

Suggested change
snapshot.average_loudness = 0;
snapshot.average_loudness = 1/pTrack->getReplayGain().getRatio();

Because the ReplayGain value is used as extra gain to have all tracks with the same loudness.
This means for instance normalized track has an average loudness of -18 LUFS (on a dB scale)
The replay Gain will be 0 for this track.
If the same track has a doubled sample voltage it has an average loudness of -12 LUFS, and the replay gain will be 0.5.
Applied this, we are back to the desired -18 LUFS.

Then the question is: What is the scale of average_loudness. Is it a ratio or a dB value?
But does it mean compared to the ReplayGain2 LUFS?

Do you have a set of tracks where the average_loudness has been set before we have messed it up with Mixxx?
Maybe we can spot a rule how it compares the the Mixxx Replay Gain column.

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@daschuer This is indeed the purpose of upstream issue xsco/libdjinterop#103 - the exact meaning of average_loudness isn't currently clear.

In the past, I had made an assumption that ReplayGain ratio was a good fit, but Denon players use the value to scale the waveforms, and the bug being fixed here is that some bad values cause the waveform to be scaled out of range, at which point it is no longer rendered.

I've been working through more methodically with an analysis suite of tracks to try and determine the exact meaning of the fields in Engine format. When we have confidence in the true meaning of the fields, we can come back to Mixxx and set the value properly.


// Set main cue-point.
mixxx::audio::FramePos cuePlayPos = pTrack->getMainCuePosition();
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