What is the difference between Joint Stereo and Stereo?

My question, as expressed in the title, concerns the options dialogue when exporting a file as MP3. I understand what the different Bit Rate Modes are about, and what the Quality is about, but I don’t understand what the difference is between Joint Stereo and Stereo. Joint Stereo does not appear in the Manual’s Glossary and I couldn’t find a definition of it in the web page about File Export. I don’t believe the release level is particulalrly relevant to this question but I am using 2.0.5.

It’s discussed on the Manual page for the dialogue it appears in:


“Joint Stereo” is described in detail here: http://wiki.hydrogenaud.io/index.php?title=Intensity_stereo

In normal “Stereo” mode, MP3 stores a separate Left and Right channel, though bitrate can be distributed between Left/Right as needed. In “Joint Stereo” mode, there are still two channels, but they are called Mid and Side. Before the compression, Mid/Side are computed from Left/Right. While Mid stores the common portion of the Left/Right channels, Side stores the difference. After decompression, Left/Right will be reconstructed from Mid/Side.

The advantage of “Joint Stereo” is that, usually, the Left/Right channels are pretty similar, so most information will be in the Mid channel and only very few information in the Side channel. This means that, with “Joint Stereo” mode, the redundant part doesn’t have to be stored twice! This means that “Joint Stereo” can use the available bits more efficiently than normal “Stereo” mode does - provided that the Left/Right channels are somewhat similar.

The rule of thumb is that “Joint Stereo” is advantageous at lower MP3 bitrate, while it’s less helpful on higher bitrates. Also the advantage depends a lot on the content. Anyway, with the LAME MP3 encoder you don’t need to worry. It uses “Joint Stereo” by default (-m j), but still switches between Left/Ride and Mid/Side mode dynamically, i.e. it always picks the “best” mode for every frame. You can force Mid/Side mode (-m f), but that’s not recommended…

JOINT STEREO > is the default mode of encoding. jstereo means the encoder can use (on a frame by frame basis) either L/R stereo or mid/side stereo. In mid/side stereo, the mid (L+R) and side (L-R) channels are encoded, and more bits are allocated to the mid channel than the side channel. When there isn’t too much stereo separation, this effectively increases the bandwidth, so having higher quality with the same amount of bits.
Using mid/side stereo inappropriately can result in audible compression artifacts. Too much switching between mid/side and regular stereo can also sound bad. To determine when to switch to mid/side stereo, LAME uses a much more sophisticated algorithm than the one described in the ISO documentation.

Thank you all. I apologise for wasting your time but I didn’t spot the paragraph about “Joint Stereo” because I was looking for it in a bold font. Having read the information, I now realise that I want to always use the Stereo option rather than the Joint Stereo option.

“Joint Stereo” will usually produce better sound quality, so the “Stereo” option should only be used if you have a very specific reason for doing so. 15 years ago, some encoders were not very good at producing good stereo imaging using the “Joint Stereo” option, but that was fixed over a decade ago.

Right… The underlying mid/side conversion where two stereo channels (left & right) are converted to two channels of L+R and L-R is a lossless and reversible process.

The advantage of converting to mid/side before lossy encoding is that there is lots of common (L+R) information and not so much L-R information, so with mid/side you are not wasting “valuable bits” encoding the same information twice and the bits can be used to to make a smaller file of equal quality or to make a higher quality file.

The only time it makes sense not to use mid/side is when the left & right channels are totally different. For example, if you have an English dialog track on the left and a French dialog track on the right.

And, LAME’s Joint Stereo encoding is smart enough to decide frame-by-frame when mid/side is advantageous and when it’s not. So, that French-English track would probably turn-out just as good with Joint Stereo as with regular Stereo.

I believe AAC/M4A is always Joint Stereo (unless it’s a mono track).

Just to emphasize your point there Doug; in that example, “Joint Stereo” is smart enough to see if there is no common “mid” and will use “left/right” stereo.

I can’t actually think of any good reason to not use “Joint Stereo”. Providing that as an option just seems like an unnecessary complication. :confused: