stereo and joint stereo

[sporlan]

can anyone explain the difference between stereo and joint stereo that tmpg uses

[BJ_M]

Definition: Joint Stereo

Mid/Side Stereo a.k.a Joint Stereo - The encoder encodes to a mid channel, which is just one full channel by itself that is averaged out from the two original channels, and a side channel which contains all of the stereo separation. Instead of encoding two full channels like with normal stereo, the encoder only has to encode one channel and part of another. Sometimes there is too much stereo separation for JS to reproduce, if such a situation occurs, the encoder will switch back to normal stereo. This doesn't happen very often though, on most songs, JS is used on about 95% of the frames.

Uses

The main use of Joint Stereo is to cut down on the bandwidth requirements in audio files by reducing the amount of data required to encode the stereo separation of an audio file. This stereo compression mode comes in handy when dealing with low bitrates or VBR files since the VBR files allocated bandwidth experiences great variation throughout the different parts of a song/s and is greatly aided since the stereo channels, with the aid of Joint Stereo, require less bits, meaning, the bits saved can be used on other more important parts of the song/s or if it is not required in other parts of the song/s, will result in a direct file size reduction.

Definition: Stereo

Standard audio channel mode that uses no form of compression and if you use Stereo, your files will not have in any stereo separation problems if a sufficient bitrate is used, which is usually around 192-224Kbit/s+ (assumption based on the use of Lame)

Uses

The ideal use of standard stereo is for situations when a high data bandwidth is available, ideally, at least 192-224Kbit/s and higher. Normal stereo requires this extra bandwidth since it does not use any lossy compression at all. Since it does not use any lossy compression, it will not produce any stereo compression artifacts. However, one downside to its use is that since it requires a higher bandwidth, even at higher bitrates, (192Kbit/s and higher) more artifacts are introduced due to there being less bandwidth available for the other parts of the song than the possible stereo separation artifacts introduced by Joint Stereo.

[vitualis]

Using standard stereo, the MPEG encoder can still allocate MORE bandwidth to the channel that has greater need. For example, if L is silent but R has audio, then the majority of the bandwidth will go to the R channel.

Where the L and R channels are absolutely allocated half the total bitrate all the time is called "dual channel".

Regards.

[sporlan]

So if I use 128-160 I should use joint stereo and anything higher use normal stereo. I have tried both steroes but can't detect any difference
or any decrease in the file size.
thanks

[vitualis]

There will be no difference in filesize.

filesize = bitrate * time

Whether you are using JS 128 kbit/s or stereo 128 kbit/s, it is still 128 kbit/s.

However, with JS, you are essentially getting MORE bits to play with because you get compression from the redundancy between the left and right channels.

Regards.