Library & files
Lossy compression
Lossy compression shrinks a file by discarding audio a listener is unlikely to notice. What it removes is gone permanently, and encoding an already-lossy file removes more on top.
In practice
Every re-encode removes more, and none of it comes back — keep one lossless master and generate the rest from it.
If all you have is a lossy file, copy it rather than converting it: moving an MP3 to a stick as an MP3 costs nothing, converting it to AAC costs a generation. Where you have no choice, go up in bit rate rather than down, which does not recover anything but stops the second encoder throwing away more than it has to. And treat a file you cannot account for the history of as suspect on a big system.
Permanent
What the discarding is
Every encode
When it happens again
Copy, do not convert
The rule that saves you
Where these come from: Codec behaviour and library practice, not catalog figures. For what we can measure, see the master to generate from.
Why it matters
The word "compression" is doing two unrelated jobs and it causes real confusion. Dynamic-range compression is an effect that changes how a track sounds; data compression is a storage decision that ideally changes nothing. A track can be heavily compressed in one sense and not the other.
The rule that actually costs DJs music is the second encode. Converting a 320 kbps MP3 to another lossy format — for a stick, for a player, for a share — runs the discarding step again on material that has already been thinned, and the result is worse than either step alone suggests.
Frequently asked
What does lossy mean?+
That the encoder discarded part of the audio to make the file smaller, choosing material a listener is least likely to miss. The discarded part cannot be recovered by decoding.
Is converting MP3 to MP3 bad?+
Yes, if it is a re-encode. Every lossy encode runs the discarding step again on material that has already been thinned. Copying the file is free; converting it is not.
Is lossy compression the same as a compressor?+
No. A compressor is an audio effect that reduces dynamic range. Lossy compression is a storage technique that reduces file size. The shared word is a coincidence of English.
Related terms
Lossless
Lossless means the encoding discards nothing: decode the file and the original samples come back bit for bit. WAV, AIFF, FLAC and ALAC are lossless; MP3 and AAC are not.
Bit rate
Bit rate is how much data one second of audio uses, in kilobits per second. It decides the file size, and on a lossy format it decides how much of the original recording survives the encoding.
AAC and M4A
AAC is a lossy codec, usually delivered in an .m4a container, and it is what Apple’s ecosystem and most video use. At the same bit rate it generally holds up better than MP3.
Bit crusher
A bit crusher throws away digital resolution on purpose — fewer bits of level, a lower sample rate, or both. The result is gritty aliased distortion that gets harsher the further you push it.
Explore: Lossless · Crate to tracklist · Bit rate · All DJ terms