Hex viewer:
- https://hexed.it/

Convertor:
- https://numsys.ru/

Struct of ID3:
- https://habr.com/ru/post/103635/
- https://mutagen-specs.readthedocs.io/en/latest/id3/id3v2.4.0-structure.html

Tags:
- http://www.bog.pp.ru/dw/tags.html

Ascii:
- http://book.itep.ru/10/ascii.htm


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*  Warning:

Be careful when you select a file to read!
Sometimes the size of frames specified in the id3 header doesn't converge with the real size of all frames!
I don't know what this is about - either that the creator screwed up, or I didn't add something.