3.3.3.1 ID3v2 Serialization ¶

While you can of course create an <id3v2-tag> instance “from scratch” (in-memory, as a result of a call to (make <id3v2-tag> ...) you will more frequently be reading them from files on disk.

The function for doing this is read-tagset. The name is intended as a reminder that a file can have multiple ID3v2 tags, so you are in general reading a tag set, not just a tag.

scheme@(guile-user)> (define tags (read-tagset "opium.mp3"))
scheme@(guile-user)> tags
$1 = ((#<<id3v2-tag> 56188c2a3d80> 3 #f))

read-tagset returns a list of three-tuples, one tuple for each tag (so it could return '(), if the file contained no ID3v2 tags). Each three tuple contains:

1. an <id3v2-tag> instance, representing the tag
2. the ID3v2 version as which the tag was serialized (i.e. 2, 3 or 4)
3. a boolean indicating whether the unsynchronisation bit was set See The Unsynchronisation Scheme.

Once you’ve created or updated your ID3v2 tag(s), you will presumably want to write it (them) to disk, presumably in place of an existing tagset. This is done via write-tagset(tags, file, ...). tags is a list of two-tuples: the first element is always an <id3v2-tag> isntance to be written to disk & the second is the ID3v2 version under which it shall be serialized (i.e. an int, either 2, 3 or 4). file is the file into which the new tagset shall be written, replacing any tagset present therein.

write-tagset takes a few optional parameters:

1. #:apply-unsync governs whether the unsynchronisation scheme See The Unsynchronisation Scheme, should be applied when writing out the given tags: #f (the default) means never, #t means it will always be applied and 'as-needed means that it will be applied to any tag whose serialization would contain false syncs.
2. #:copy governs whether a backup copy of the target file will be made: a value of #f (the default) means that the new tagset will be written in place (moving the audio data & ID3v1 tag, if any, if needed) and a value of #t means that the target file will be copied to a backup, the new tagset will be written, and then the track data & ID3v1 tag (if any) will be copied over to the new file.