# Subtitle field on MP4 tags for audio files (M4A)
**URL:** https://community.mp3tag.de/t/subtitle-field-on-mp4-tags-for-audio-files-m4a/43507
**Category:** Support
**Created:** [November 9, 2018, 5:56am UTC](https://community.mp3tag.de/t/subtitle-field-on-mp4-tags-for-audio-files-m4a/43507 "2018-11-09T05:56:33Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@djbronko](https://community.mp3tag.de/u/djbronko)
#### Post date: [November 9, 2018, 5:56am UTC](https://community.mp3tag.de/t/subtitle-field-on-mp4-tags-for-audio-files-m4a/43507/1 "2018-11-09T05:56:33Z")
**Short story**
Is it possible to configure MP3Tag to map the `SUBTITLE` field to the MP4 atom `---:com.apple.iTunes:SUBTITLE`?
If so, please tell me how. If not, consider this a feature request.
**Long story**
I'm having trouble with MP3Tag's [tag mapping](https://docs.mp3tag.de/mapping) for the field `SUBTITLE` to the MP4 atom `desc`.
I recently evaluated several UPnP servers ([Asset UPnP](https://www.dbpoweramp.com/asset-upnp-dlna.htm), [MinimServer](http://minimserver.com/), [ReadyMedia](https://sourceforge.net/projects/minidlna/)). They can be configured to show subtitles, but none of them has shown the subtitle fields of my M4A files (FLAC and MP3 worked, though). I then found out that according to a [iTunes Q&A](https://discussions.apple.com/thread/6605665), the `desc` atom is for video content only. I've read about a MP4 tag field `----:com.apple.iTunes:SUBTITLE` on [Hydrogenaudio](https://wiki.hydrogenaud.io/index.php?title=Tag_Mapping) and found out that is's used by some products (e.g. [Picard](https://picard.musicbrainz.org/docs/mappings/), [EZ CD AudioConverter](https://www.poikosoft.com/help/metadata-support), [JAudiotagger](https://bitbucket.org/ijabz/jaudiotagger/src/6b4d904df18b76e932085259d676c706dcc0cb2d/src/org/jaudiotagger/tag/mp4/Mp4FieldKey.java?at=master&fileviewer=file-view-default#Mp4FieldKey.java-210), [taglib-sharp](https://github.com/mono/taglib-sharp/blob/fe2cc47f0cf564a399d3ba18d6dfb0d387445d74/src/TagLib/Mpeg4/AppleTag.cs#L642)).
I tried to override the tag mapping for my M4A files in Mp3Tag by setting a custom tag mapping of
- Tag: `MP4`
- Source: `---:com.apple.iTunes:SUBTITLE`
- Target: `SUBTITLE`
With this, the `desc` atom was no longer written (good), but the tag was not shown in UPnP servers either (bad).
I then tagged my M4A files with Picard. The subtitle was shown in the UPnP servers. Using a hex editor, I saw that the Picard-written structure is different from the MP3Tag-written structure. I suppose that I created a simple text tag field with MP3Tag, and that these iTunes tags are composed ones and have to be treated specially somehow.
My conclusion is that MP3Tag's default mapping is incompatible to the UPnP servers (at least, maybe also to [other software](https://community.mp3tag.de/t/tag-feld-untertitel-bei-mp4-dateien/11728)) and that I didn't manage to change this default mapping according to my needs. Is this kind of custom mapping supported by Mp3Tag?
Christian
---
### Author:  [@Florian](https://community.mp3tag.de/u/Florian)
#### Post date: [November 9, 2018, 7:11am UTC](https://community.mp3tag.de/t/subtitle-field-on-mp4-tags-for-audio-files-m4a/43507/4 "2018-11-09T07:11:48Z")
Thanks for the detailed description and research.
There is currently now way of changing Mp3tag's internal mapping, so that `SUBTITLE` is written to an iTunes text atom instead of `desc`. This only works when using different field names.
I'm thinking about changing the mapping for the `desc` MP4 atom to `DESCRIPTION` with the next release. This way, the data entered previously via `SUBTITLE` is still accessible and compatibility with all the apps you've mentioned gets improved.
---
### Author:  [@Florian](https://community.mp3tag.de/u/Florian)
#### Post date: [November 9, 2018, 3:36pm UTC](https://community.mp3tag.de/t/subtitle-field-on-mp4-tags-for-audio-files-m4a/43507/5 "2018-11-09T15:36:08Z")
I've changed this with [Mp3tag v2.90c](https://community.mp3tag.de/t/455).
---
### Author:  [@djbronko](https://community.mp3tag.de/u/djbronko)
#### Post date: [November 9, 2018, 5:30pm UTC](https://community.mp3tag.de/t/subtitle-field-on-mp4-tags-for-audio-files-m4a/43507/6 "2018-11-09T17:30:11Z")
Thank you very much for your instant support, Florian. I downloaded v2.90c beta and made a few tests. Everything works perfectly:
- Values that were shown as `SUBTITLE` before are now shown as `DESCRIPTION`.
- Values that are put into the `SUBTITLE` field are now stored in a field `com.apple.iTunes:SUBTITLE`
- The server software is able to pick this up so that the subtitle is shown correctly.
Please don't forget to update the [mapping documentation](https://docs.mp3tag.de/mapping) once the feature is production-ready.
Thanks a lot, and keep up the good work!
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### Author:  [@Florian](https://community.mp3tag.de/u/Florian)
#### Post date: [November 9, 2018, 9:04pm UTC](https://community.mp3tag.de/t/subtitle-field-on-mp4-tags-for-audio-files-m4a/43507/7 "2018-11-09T21:04:00Z")
Great — thanks for testing the change!
I've already updated the documentation that ships with the new version and will update the online version with the v2.91 release.
---
### Author:  [@system](https://community.mp3tag.de/u/system)
#### Post date: [February 9, 2026, 12:27pm UTC](https://community.mp3tag.de/t/subtitle-field-on-mp4-tags-for-audio-files-m4a/43507/8 "2026-02-09T12:27:45Z")