Loudness normalization¶

Musium can normalize the perceptual playback loudness based on loudness information in flac tags.

The volume slider in Musium treats 0 dB volume as 0 dB gain for the output device, but it does compensate for track or album loudness relative to a target loudness of -23 LUFS. When the Musium volume slider is set to 0 dB, this means that a track with a loudness of -23 LUFS will play back without volume adjustment (at maximum volume of the device), and a track with a loudness of -10 LUFS will play back with device volume at -13 dB. When the Musium volume slider is set to -5 dB, a track with a loudness of -10 LUFS will play back with device volume at -18 dB.

It is possible to set volumes beyond 0 dB in Musium, but this is only effective for inherently loud tracks: a track with a loudness of -23 LUFS is already playing at the maximum device volume, but a track with a loudness of -13 LUFS has enough headroom to allow the volume to be set to 10 dB.

Tags¶

Two tags affect loudness normalization:

• BS17704_TRACK_LOUDNESS
• BS17704_ALBUM_LOUDNESS

The tags must store the integrated loudness as defined in ITU-R BS.1770-4, for the track, and the concatenation of all tracks respectively. The value must be a decimal number, followed by the suffix “LUFS” for Loudness Units Full Scale. For example, -9.317 LUFS. The value for album loudness must be consistent across all tracks in the album.

ReplayGain¶

ReplayGain has been very influential for loudness normalization, and it is widely supported by both taggers and players, but unfortunately it has become ambiguous over time. ReplayGain does not store the loudness of the track directly, instead it stores the gain that is needed to bring the track to target loudness. The target loudness was initially well-defined, but tools started offering different options to accomodate to the reference loudness of different standards such as EBU R-128, ATSC A/85, and ReplayGain 2.0. This means that ReplayGain tags from different sources do not necessarily have the same meaning, which defeats the purpose of normalization. In practice this means that ReplayGain is only really useful if you can be certain that all tags were produced by the same program with the same settings.

The BS17704_* tags used by Musium aim to sidestep this problem by storing the observed loudness, instead of the gain. The gain can easily be computed by the player, and the particular target loudness that is used is not important anyway for normalizing loudness in a collection of music. (It does matter when you want to match the loudness of your music to e.g. external streaming services.) Furthermore, by naming the tag after the revision of the standard, future revisions to BS.1770 will not create ambiguities in the meaning of existing tags.

Writing BS.1770-4 tags¶

The BS1770 flacgain utility can be used to analyze a collection of flac files, and to add BS17704_* tags.