Library & files
Joint stereo
Joint stereo encodes what the two channels share once and only the difference separately, rather than encoding each channel in full. At normal bitrates it buys quality; at very low ones it can narrow the stereo image.
In practice
Joint stereo at 320 is what makes an MP3 good. The image-collapse warning belongs to 128 kbps and 2001.
There is nothing here to configure — you are buying finished files and the store made the choice. It is worth knowing only for the case where a file genuinely does sound narrow, which is a low-bitrate encode rather than joint stereo as such. Check the bitrate before blaming the stereo mode, and remember that a club system sums a great deal to mono anyway, which makes the whole question smaller than the arguments about it.
Shared + diff
What it encodes
320 kbps
Where it is simply better
Mono-ish
What a club system does anyway
Where these come from: How joint-stereo encoding works and where its historical criticism came from — established encoder behaviour, not catalog figures. For what we can measure, see what a club system does to stereo.
Why it matters
It is the reason a 320 kbps MP3 sounds as good as it does. Most music is highly correlated between left and right, so spending the bits on the shared content and the difference is far more efficient than encoding two nearly identical channels.
The old warning about it collapsing the stereo image comes from early encoders at low bitrates, where the difference signal was the first thing sacrificed. At the rates DJs actually buy, the effect is not something anyone reliably identifies in a room.
Frequently asked
Does joint stereo reduce quality?+
At the bitrates DJs buy, no — it improves it, because the bits go where the information is. The criticism comes from very low bitrates and early encoders.
Should I look for dual-channel MP3s instead?+
No. Fully independent channels waste data on content both sides share, which means less left for everything else at the same file size.
Why does one of my files sound narrow?+
Check the bitrate first. A genuinely narrow file is almost always a low-rate encode or a bad transcode rather than a consequence of the stereo mode.
Related terms
Mono sum and phase
Summing to mono adds the left and right channels together. Anything out of phase between them cancels rather than adding, which is why a wide stereo bass can lose most of its weight on a mono system.
Variable bitrate
Variable bitrate spends more data on complex passages and less on simple ones, so quality stays even and file size varies. Constant bitrate spends the same everywhere, so size is predictable and quality is not.
Sub stack
A sub stack is the group of subwoofers handling the lowest octaves, usually on the floor and often in a block or a line rather than spread out. It carries the part you feel more than hear.
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.
Explore: Stereo to mono · Mono sum and phase · Sub stack · All DJ terms