Change `Use Replay Gain metadata if it is available` checkbox
into a radio button and add button to disable any loudness normalization.
Add second group(+radio button), for EBU R 128 loudness normalization.
There is only one tunable: Target Level,
defaulting to EBU R 128-recommended `-23 LUFS`.
Care should be taken when changing Target Level!
You probably don't want to go outside of `-30..-16` range!
At least as implemented, there is only support for per-song normalization,
i.e. no per-album normalization.
We do not do anything with loudness range,
although i have some further thoughts about compression.
We do not do anything about clipping / peak level.
NOTE: we do not need `libebur128` to *perform* the audio normalization,
only for the initial analysis.
Co-authored-by: Jonas Kvinge <jonas@jkvinge.net>
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