I have wondered for awhile now why when you compress an mp3 file with joint stereo, all the higher frequencies dont all emminate from the center, and can pan from left to right? Because I thought that what joint stereo did was essentially make higher frequencies monoraul. Is this correct? How exactly does Joint Stereo work? Thanks!

at least in LAME, joint stereo means that it either gets encoded as left/right or middle/side depending on whether or not it can do a good enough job with middle/side. If the particular frame will sound too different in middle/side then it gets encoded as left/right. I don't think it alters any signal placement like you're thinking.

Joint stereo does not change the sound this way. Intensity stereo like you find in low bitrate FhG mp3 does.

Lame joint stereo is very well tuned when used with the alt presets.

Left/Right stereo is true stereo ?
Mid/Side stereo is (L+R)/2 ?

I've always had problems understanding how joint stereo works...

'Mid' channel is (L + R) / 2 and 'side' channel is L - R. This process is lossless, the original signal can be reconstructed from the mid/side information. If the music is mainly monoaural, the side channel contains very low values, which helps compression.

In Mid/Side-stereo mode the MP3 encoder encodes the sum (Mid) and the difference (Side) of the stereo channels instead of the original channels. The advantage is that if the channels are similar or even equal (mono sound) then the difference (Side) is very small or even zero, thus needing only few bits in the encoded file.

see also this thread:
http://www.hydrogenaudio.org/forums/index....t=ST&f=1&t=5542

The other joint stereo method of MP3 is called Intensity-stereo. Here the MP3 encoder assumes that the stereo have the same sound with only different intensities, for instance left is half as loud as right channel. The intensity ratio of the channels is encoded, which usually needs even less bits in the encoded file than Mid/Side-stereo. The main problem of Intensity-stereo is that it ignores the phase relation between the channels. It changes the stereo separation and mostly destroys any embedded Dolby Surround information. Therefore, it's normally used for low-bitrate encodings only. LAME up to now does not support Intensity-stereo encoding.

hey niktheblak, you were faster.

I understand 'mid', cos it's the average of the left and right channel...
BTW: How is 'mid' any different from mono?

But what is the usefulness of 'side'? What can the codec do with L-R?

In case these questions have already been answered, please ignore this post.

I don't know much about joint stereo encoding in MP3, but L+R (mono signal) and L-R (difference signal)
is how FM stereo is encoded. In the reciever the 2 signals are added and subtracted yieliding:

(L+R) + (L-R) = 2*L (left signal)
(L+R) - (L-R) = 2*R (Right signal )