Flac has -6dB gain in mixxx 2.1.0

[mixxxbot]

Reported by: jamie.gifford
Date: 2018-04-22T01:52:11Z
Status: Fix Released
Importance: High
Launchpad Issue: lp1766042

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In Mixxx 2.1.0, FLAC files are decoded with a -6dB gain. Tested on Linux and Mac.

The problem is reproducible always. Eg, create a 440Hz full-scale sine in both WAV and FLAC formats with "sox -r 44100 -n a440.wav synth 20 sine 440" and "sox -r 44100 -n a440.flac synth 20 sine 440". The WAV file plays correctly; the FLAC file will play at -6dB. 

Looks to me that the problem is in soundsourceflac.cpp, in void SoundSourceFLAC::flacMetadata(const FLAC__StreamMetadata* metadata), in the code:

m_sampleScaleFactor = CSAMPLE_PEAK
                    / CSAMPLE(FLAC__int32(1) << bitsPerSample);

Since FLAC samples are signed, a FLAC full-scale sample will only reach bitsPerSample-1, not bitsPerSample (eg, in a 24 bit stream the maximum sample magnitude would be 1<<23). This means that m_sampleScaleFactor is out by a factor of two, hence -6dB. 

I think the code should read

m_sampleScaleFactor = CSAMPLE_PEAK
                    / CSAMPLE(FLAC__int32(1) << (bitsPerSample - 1));

[mixxxbot]

Commented by: Pegasus-RPG
Date: 2018-04-22T06:02:37Z

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I /thought/ my FLAC files sounded quieter! Thank you very much for the detailed analysis! (Do you use GitHub and want to make a pull request so you get credit in the commit history?)

[mixxxbot]

Commented by: jamie.gifford
Date: 2018-04-22T06:27:44Z

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Pull request sent...

[mixxxbot]

Commented by: uklotzde
Date: 2018-04-22T13:47:50Z

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#1636

[mixxxbot]

Issue closed with status Fix Released.