How ID3 tags are stored, and why editing them is lossless
An MP3 file is really two things stitched together: a block of ID3 tag data and a stream of audio frames the player decodes into sound. ID3v2 tags sit at the start of the file as a small header followed by individual frames, one per field, TIT2 for title, TPE1 for artist, APIC for the cover image. ID3v1 is different: a fixed 128-byte block at the very end of the file, with just 30 characters each for title, artist and album.
Because the tag data and the audio data live in separate parts of the file, changing a title or swapping the cover art never touches the audio frames. This editor reads the existing tag block, lets you change the fields you care about, then writes a new tag block and copies every audio frame across exactly as it was. Nothing is decoded or re-encoded, so there is no quality loss, even after editing the same file several times.
