[F] illegally large tag field created in m4a files: lost in itunes

mp3tag allows one to create illegally large tags in m4a files (tested with Apple Lossless):
illegal means any text field over 255 characters.

To recreate:
add a comment field (%comment%) that exceeds 255 characters to an m4a file.
It will not display in itunes (because it is illegal).

Add a comment field that is 255 chars or less.
It will display in itunes.

(This may lead to worse problems, since one can't be sure that m4a readers will handle this problem gracefully, so should be corrected.)

You may crop the comment content to the length of 255 characters using Mp3tag function $left(%comment%,255). Be aware that you might loose useful data. So you may previously copy the comment's content to another tagfield, say 'comment_save' or so, and do cropping the comment tagfield as the second step.

By the way, is there any convention on tagging especially to m4a files?


If so, please let me know (especially some documentation about the comment length limitation).

For the time being, I'm marking this as "No Bug" (at least regarding Mp3tag).

Yes, it is part of the quicktime aac and apple lossless tagging format (everything with m4a extension): 255 character max for text fields:

I do not know whether nero's tagging format imposes a 255 char max limit, but apple's tagging format is probably used by 99.9% of the aac files out there, what with itunes and ipods and what have you.

How about putting it as a default option that can be changed under mp4 preferences ("use quicktime/iTunes field limits" or such--assuming that nero doesn't impose the same limit). Just a thought. But in my opinion it's better that the default be to obey the 255 char limit for all text fields except lyrics, otherwise information might be lost (certainly appears to be lost in itunes). Especially since 99% of m4a files use the quicktime/iTunes tagging format.

There is a convention, established by Apple but not well documented, called quicktime tagging (quicktime is the engine that iTunes uses to play and encode media, on both mac and pc's). Tagging of aac audio is unfortunately not part of the mp4 standard, with the one exception of the copyright field.

(One of the unfortunate surprises is that apple does NOT support multiple artist fields, unlike wma and mp3; also itunes itself only displays certain predefined fields--custom fields are not able to be displayed by itunes, though not disturbed either. I'll suggest that to apple as an enhancement request.)

Who else but Apple could be expected to again mess everything up that can possibly be messed up … unbelievable. They insist in outdated ID3v2.2, they have MPEG encoders that mess up the header bits, they don’t adhere to long-defined standards, they »invent« non-standard tags, … and what not.

But unfortunately TechUser is right, so some of us will have to live with another crippled file format. Yuk. Not for me.

I believe apple corrected the id3v2 bugs, but they could have done a better job with the tagging format--xml would have made more sense, if that was around when the format got fixed. The incentive to depart from standards, or create something that is your own, is sometimes good, sometimes bad.

So this will get moved to the "confirmed bugs" section? Note that the 255 character limit for aaple .m4a tagging is on ALL text fields (not just "comment")--with the sole exception of lyrics.

Just to repeat: this is a confirmed bug (it does violate the spec, and going over the 255 char limit will lose information), so I think it should be moved out of the "no bug" section.

Moved and marked as confirmed bug report. Thanks for bringing this to my attention again.

I've changed this with the current Development Build so that fields exceeding 255 characters automatically get truncated.

Kind regards,

Thank you, that's much appreciated.

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