I used the replaygain to rewrite the mp3 tags.
Then its 'comment' and 'unsyncedlyrics' fields were stripped.

I can still see the original & full data in foobar (right click mp3 -> properties).
But they couldn't be full showed by windows media player 9 & windows xp explorer.

For example, the comment field is:
ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE
ABCDEABCDE

After the replaygain writing it, it's showed by windows xp explorer & windows media player 9:
ABCDEABCDEABCDEABCDEABCDEABC

In the foobar's 'properties' function is still:
ABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDEABCDE
ABCDEABCDE

My guess is that what has happened is that foobar2000 has read all the information from your id3v2, but then when it has come to write tags it has written it to an apev2 tag and stripped the id3v2 from the file.

So it can then still see the full length information because it is in the apev2 tag, but explorer and wmp don't read information from apev2 tags.

If you want to keep id3v2 tags in your files you need to select this in Preferences -> Playback -> Input -> Standard Inputs -> Tag Type To Write. I think the default there is "APEv2 and ID3v1". And it states that "other supported types will be stripped from file".

Does it stand for: If I selected the "ID3v2 and ID3v1", the tag information will be stripped ?
I don't use the default "APEv2 and ID3v1" because it will break the unicode tag info (unicode code string will become many '?' sign).

May I have any other choices about this problem?

APEv2 supports Unicode, but most programs don't support APEv2.

Could foobar2000 support the selection "APEv2 and ID3v2"?
The "APEv2 and ID3v1" will break the Unicode string in the "ID3v1" part.
I know that the APEv2 and ID3v2 support Unicode, but ID3v1 doesn't.

With all these tag combinations, it'd be probably simpler with a checkbox for each tagging format to let people make up their own combos.

...I know, I'm just daydreaming.

You do not need both APEv2 and ID3v2.

ID3 sucks. It's only there for compatibility. Yes, ID3v2 sucks too. I'd support ID3v2 at the end of the file, but that's another v2.4 feature, and the list of applications that support v2.4 at the beginning of the file is slim enough as it is.

Why APE is so good and ID3 sucks ?

Technically, I dont know, but I've heard of problems of tags being at the begining/end of the file, and it appears that one of these options sucks... But as a user, apev2 are perfect for me, because I can create ANY possible field, to fit perfectly with my needs, what is not possible with id3...

I'd be very interested to hear this also. It seems a bit foolish for me to tag all ~60GB of my music files using APEv2 for nothing except foobar to ever support them, unless there is a very good reason for me to do so. It's enough of a hassle when transferring to portable devices which don't support anything except ID3 as it is...

Is APEv2 tagging likely to become much more widely-supported?

arty

Well, I don't know much about the technical details of the tag formats, but here's what I know:

• APEv2 puts tags at the end of the file, ID3v2 at the beginning. This makes APEv2 much faster because when an ID3v2 tag is changed, usually the whole file needs to be rewritten (because if the tag's size grows, the rest of the file needs to get "shifted" to fit it in). After converting my tags to APEv2 updating them is now a fraction of that with ID3v2.
• ID3v2 has preset field names while APEv2 allows almost any kind of field name.
• APEv2 is better at handling Unicode stuff. (I'm not sure about this one, the whole Unicode mess is so confusing... )

Currently, all fields in APEv2 are UTF-8. No confusion over which encodings to support, as with ID3v2. (which supports ISO-8859-1 and UTF-16, but most implementations only support your system codepage, which is not supposed to work unless you use UTF-16. There's also UTF-8, but that was only added in v2.4, and almost nothing supports that.)