MP3 Cutter

An MP3 is a row of frames. Each holds 1152 samples of sound, 26 milliseconds at 44.1 kHz (24 at 48 kHz, 36 at 32 kHz), and below 32 kHz half as many. Cutting an MP3 here copies whole frames into a new file. The ones you keep are the same bytes as in the original, which a test checks frame by frame, so the sound can’t get worse.

The price of that is the frame. A cut can only fall between two frames, so it lands on the one nearest your mark: at most half a frame away, 14 milliseconds or less at 44.1 kHz. That is finer than a beat of music, and far finer than the pause between two words.

The bitrate doesn’t change either. A 128 kbps MP3 stays at 128 kbps and a variable-bitrate one stays variable, so the new file is the size of the part you kept, in proportion.

MP3 frames borrow from each other. A frame can keep part of its sound in the bytes of the frames before it, so a frame cut loose from them decodes to noise. Most cutters ignore this and the new file starts with a burst of it.

This one keeps the frames the first frame reaches back into, one to three of them, with their own sound silenced and their bytes left alone, and writes in the file’s Info frame that a player should skip them. Players that read that frame, ffmpeg and the browsers we tested among them, start at the real first frame. A player that doesn’t read it plays up to 78 milliseconds of silence first.

That covers ordinary files. A very low-bitrate MP3 can reach back further than the Info frame is able to skip, and one with error-checking bits can’t be silenced this way, so a cut of either starts with a short burst of noise. Save it as WAV or MP3 instead: the sound is decoded first and the start is clean.

Choose Remove and the part between the handles is left out, with the two ends joined. Without re-encoding, the frames after the join lean on frames that are no longer there, and the join can click for a few hundredths of a second.

If it does, save as WAV or MP3 instead: the sound is decoded, the 4 milliseconds either side of the join are faded down and up, and the click is gone. MP3 encodes the song again at 128, 192 or 320 kbps, so the quality is no longer the original’s.

Title, artist, album and cover art are copied into the new file, once. The Info frame at the start of an MP3 tells players how many frames it has and where to jump to, so the new file shows its real length and seeks properly, which a plain slice of bytes doesn’t.

A fade changes the sound, so it needs the song to be encoded again: the page saves it as MP3 at the bitrate you choose, or as WAV. Without a fade nothing is encoded. A ringtone is a part kept with a short fade out, and this page makes it either way.

A 4-minute MP3 is cut without re-encoding in about 1 second, and in the same time with a fade.