demux: boost read EBU R128 gain values to ReplayGain's reference level

Without this change the same track encoded as Opus - which requires R128
tagging - and e.g. Vorbis with ReplayGain tagging have different volumes.
This is caused by ReplayGain 2 having a higher reference level of -18 dB
LUFS, while EBU R128 has a lower reference level of -23 dB LUFS.

For the results of gain application to match, the read EBU R128
values need to be boosted according to the difference in reference
levels.

Patch inspired by mpd's source code.

1 files changed, 5 insertions, 0 deletions

diff --git a/demux/demux.c b/demux/demux.c
index e1ef65a733..b6e85330ed 100644
--- a/demux/demux.c
+++ b/demux/demux.c
@@ -2963,6 +2963,11 @@ static struct replaygain_data *decode_rgain(struct mp_log *log,
         }
         rg.track_gain /= 256.;
         rg.album_gain /= 256.;
+
+        // Add 5dB to compensate for the different reference levels between
+        // our reference of ReplayGain 2 (-18 LUFS) and EBU R128 (-23 LUFS).
+        rg.track_gain += 5.;
+        rg.album_gain += 5.;
         return talloc_dup(NULL, &rg);
     }