In MP3s, TXXX fields are always case insensitive

[jimbo27]

Beets seems to write replaygain tags in upper-case regardless of changelog entry 1.3.2 (https://beets.readthedocs.io/en/stable/changelog.html#december-22-2013)
There are music player in the wild which only understand lower-case ReplayGain tags (Poweramp, KODI),

Using an actual version of a of Beets I can only find upper-case ReplayGain tags in my tagged files with a hex editor.

How I reproduced it:

First of all I download a clean WAV file from bandcamp.com for example. I convert the WAV file to a mp3 via Foobar using the Lame mp3 encoder. After that this mp3 goes into Picard. This goes through processing by Beets. For this I use a Docker Beets version (https://hub.docker.com/r/linuxserver/beets). After that I get a mp3 file including replay gain tags: 03 - New Light.mp3

03.-.New.Light.zip

Using a hex editor I only find upper-case ReplayGain tags in the header of the file.

I tried beet write (files have not changed) and beet write -f (files have changed) but the mp3 files still only have upper-case ReplayGain tags.

I also tried: I have completely deleted the ReplayGain tags from a series of files with zero and checked that they are really gone in the library (info) and the mp3 files. After that I re-tagged them with replaygain (replaygain). After that I found the ReplayGain tags again, only in upper-case.

I changed the lines of code in mediafile.py (

mediafile/mediafile.py

Lines 2202 to 2209 in edc6414

	MP3DescStorageStyle(
	u'REPLAYGAIN_ALBUM_GAIN',
	float_places=2, suffix=u' dB'
	),
	MP3DescStorageStyle(
	u'replaygain_album_gain',
	float_places=2, suffix=u' dB'
	),

) to see what tags are written. After applying scrub and relaygain again, I suddenly found two tags in my file, although with different names.
So when both are named differently, all of a sudden both show up. If both have the same name but are capitalized differently, they don't show up.

[sampsyo]

Thank you for the thorough investigation. This is definitely a bug. The problem is here, in MP3DescStorageStyle:

mediafile/mediafile.py

Lines 905 to 909 in edc6414

	for frame in frames:
	if frame.desc.lower() == self.description.lower():
	setattr(frame, self.attr, value)
	frame.encoding = mutagen.id3.Encoding.UTF8
	found = True

Namely, when storing data into an ID3 TXXX field, which is disambiguated by its desc field, we currently match that desc string case-insensitively. This means that, if we first write the upper-case version of this field, then the second call to store will modify that field—rather than creating a second entry, as intended.

We should probably introduce a mode where these fields can be made case sensitive, as is required in this case.