Joint¿ Stereo?

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The Great Watbol!
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Joined: Mon Sep 22, 2008 5:07 am
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Joint¿ Stereo?

Post by The Great Watbol! »

Is MP3 the only thing that uses "joint" stereo¿?
What would be an advantage to useing it, as to normal stereo¿?
What are DisAdvantages¿?
Can I take a joint stereo .mp3 file and make it a Stereo .mp3¿?
If I opened a stereo .wav and saved it as .mp3,
would it make it joint stereo¿?
Can I save a MP3 as regular stereo¿?
Can I convert a joint stereo into normal stereo¿?
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DewDude420
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Post by DewDude420 »

joint-stereo is just a way of combining the stereo information to make more efficient use of an ecoder.

joint-stereo is nothing new, it's existed for YEARS. it's a good example of a simple "lossless" audio transform.

all kinds of audio stuff uses joint stereo. mp3 is the first time it was really noticed on a large scale..but it has several kinds of joint stereo encoding.

flac, a lossless encoder, uses joint-stereo....even FM stereo uses joint-stereo.

the "lossless" method of joint-stereo is known as sum/difference (or mid/side) encoding. basucally you take your two stereo tracks, add them together to get your sum, then you subtract one from the other to get a difference signal. this gives you, yeah, a stereo signal..however, the audio in that stereo signal isn't TRUE stereo, it's encoded stereo. one channel contains ALLL the audio, the other contains just the differences.

let's break it down to MP3 m/s joint-stereo (the most popular and widely used form today)

the encoder matricies the audio into it's mid/side format. the mp3 encoder then encodes the sum signal, generally giving it more bandwidth allocation than the side signal, which is supposed to be a bit more simplistic.

the trade off is while you're allocating less information to the stereo field, the entire sum of your audio gets more bandwidth, which translates into a better percieved quality. to the average ears it still sounds like the original (even though BOTH have gone through excessive processing)

in a normal stereo mp3, the audio is treated as such and each channel encoded indpendently..with the encoder handing more bits to the more complex channel on a frame by frame basis.

joint-stereo basically just allows one to more efficiently convey a stereo signal in a compressed stream.

here's a kicker. joint-stereo is used in the FM stereo broadcasting process as well. The main audio carrier on the station (called the baseband audio) is a 15khz wide signal that contains the sum signal of the broadcast. On a subcarrier 38khz into the FM stations allocation, the difference/side signal is broadcast. when a stereo reciever picks up the pilot tone on 19khz, it's then able to find the 38khz carrier, demodulate the difference signal and multiplex it with the mono sum signal and..bam..stereo. for the radios that aren't stereo, well, they just get the big mono sum signal..which contains both channels already.

to be honest, it's WAY more complicated than i put it...i watered it down a LOT. If you go on wikipedia and punch in joint-stereo, it'll give you a rather detailed and, in my opinion, correct, article about joint-stereo and how it works.

as far as the joint-stereo mp3 to normal conversion..

a) the stream is encoded as joint and would need to be decoded and re-encoded, making a bad encode
b) there's no reason.

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