Oh, do tech people love their labels, or what?
Whoever came up with the concept of “joint stereo,” I don’t think he or she had getting high in mind.
Joint stereo actually means something pretty cool, but hardly necessary these days.
Back in the days when the MP3 file format was created, disk space was still fairly expensive.
So it was helpful to be able to save space wherever possible. And encoding stereo files in “joint stereo” did just that: save space.
To understand joint stereo, you have to understand the other option when creating an MP3 file, “true stereo”.
True stereo files contain every single drop of the data that describes the sound of each channel, left and right.
Which means if the strings are on the left and the woodwinds are on the right, all of that encoded data will be there.
But what if it’s a vocalist, right in the middle of the stereo space? That means that the vocalist will be on both channels, with the same data.
And that’s where joint stereo comes in.
Joint stereo collapses file to mono size when it’s appropriate (like when that solo singer is right in the middle of your headphones, holding a long note with no other instruments playing), and blooms back out to stereo size when needed.
It was the MP3 designer’s attempt at saving space by not encoding redundant data when it didn’t have to.
But it’s unnecessary today – storage space is cheap. And truthfully? So few recording situations allowed joint stereo to save appreciable space, it wasn’t all that effective anyway.
So, if you’re creating a stereo MP3 file, you can choose true stereo without fear of using up too much space.