6.3. Metadata-related behavior

6.3.1. Configuration

Configuring metdata-related behavior can be done in Settings / Metadata. The file metadata will always be used if available when adding external files (i.e. not from the Music Store) to Playlist. If no metadata is found, the filename will be used as a title string. In such cases the Adding files to Playlist option controls whether to use the basename only instead of the full pathname. You can utilize filename extension removal and substitution of underscores to space as well.

It is possible to select which ReplayGain tag should be used in case ReplayGain information is found in a file. This should be set according to your preference for Album-based or Track-based volume averaging. If the requested field is not found, the other one is checked and if that exists, it is used instead.

For the metadata editor (File info dialog) you can choose whether you want to set the contents of newly added frames on the basis of another existing tag's equivalent frame. For example, if this option is enabled, adding a `Title' frame to a newly added APE tag will automatically fill it in, provided that there is e.g. an ID3v2 tag in the same file that already has a `Title' frame. If this option is unset, newly added fields will be blank.

The last group of options controls the preferred set of tags in MPEG audio files. Because these files can have ID3v1, ID3v2 and APE tags in any combination, it is essential to set this to match your policy regarding the tagging of these files. These settings affect creating (via CD ripping or file export) or batch tagging MPEG audio files.

6.3.2. Precedence of metadata information

Whenever there is a generic request for some information concerning a file (e.g. `Who is the artist?') we do not want to explicitly specify which metadata tag should be used to retrieve the desired piece of data. As a consequence, all tags present have to be checked. However, since not all tags are equally capable of storing well-formed information (think of ID3v1 limitations), there should be a preference order in which the different tags are queried.

The preference order:

6.3.3. ReplayGain and RVA

Aqualung is a ReplayGain-aware music player. You have to understand and use the RVA system in order to make use of this. The RVA system relies on pre-stored loudness measurements of files, which Aqualung normally does by itself (by selecting the Calculate volume... option from the popup menus). However, for files already containing ReplayGain or RVA information, it is possible to use that as the basis of volume adjustment.

One way of doing this is importing the ReplayGain or RVA frame to the Music Store as `manual RVA' (using the File info dialog opened from the Music Store. In this case the stored value is editable via the Edit track... popup menu item, where you can also specify such a value manually, if needed.

In case there is no `manual RVA' value stored for the file, the RVA system will see if there is a previously calculated loudness value stored for this track. If yes, this will be the basis of a volume adjustment calculation, which is controlled by Settings / Playback RVA options. However, if the RVA system is enabled and neither `manual RVA', nor loudness measurement is available, Aqualung looks for ReplayGain or RVA information in the file metadata. If such information is found, it is used in the same way as `manual RVA', so the track's playback gain will be adjusted by exactly the specified amount.

There is an important difference between `manual RVA' data coming from a metadata tag or specified directly by the user, and loudness values calculated by Aqualung. The former is a volume adjustment in itself, specifying the amount of cut or boost to apply when playing the track. On the other hand, Aqualung's loudness levels merely serve as an indicator of the track's loudness, and are converted to actual volume adjustment values when adding the track to the Playlist, via Aqualung's RVA algorithm, which is a quite sophisticated tool based on listening environments, also supporting statistics-based handling of Album-based volume averaging.

6.3.4. Metadata transfer when exporting

When exporting files, Aqualung attempts to retain the metadata of the original file as much as possible by transcoding metadata into the tags supported by the output format. For each frame present in the input metadata, it is determined whether it is possible to add an equivalent frame to one of the tags supported by the output format. If yes, the frame data is converted and stored. Note that for each frame type (e.g. `Title'), only one source tag is chosen to be copied from even if it is present in more than one tag in the input metadata. In this case the tag with the highest precedence is used as input. Of course, if more instances of the same frame are present in the same tag (e.g. multiple APIC frames in an ID3v2 tag), they are all transferred if the output metadata format supports them.

This feature essentially means that files are transcoded with intelligent metadata transfer. For example, exporting a FLAC containing Ogg Vorbis comments and two FLAC picture frames to MP3 will result in an MP3 with an ID3v2 having the respective text frames, and two APIC frames for the pictures.

When assembling the metadata written to the output file, the information from the Music Store or Playlist (Title, Artist, Album, Year, Track number) will be used in case the source metadata does not contain the appropriate frames.