this is just from the top of my head, but I'm 99% it's all correct (couple parts sourced from hydrogenaudio). basically, it ALL comes down to preference.
Stereo Vs. Joint-StereoLot of people say that JS is just as good as Stereo. What JS does is converts low & high tones (
usually inaudible to the human ear) to mono, thus saving space. The problem here is that on some tunes, there's a 'phaser' type effect that causes the high tones to come in and out throughout. More recently, JS has been improved where both channels are recorded seperately which gets rid of the problem, but the bitrate has to be high enough to work properly.
To save space and 99% of the time get a high quality mp3, use Joint-Stereo
To get the highest quality possible, use Stereo
The sidenote to this is when you encode using Lame. The way JS works in the newer versions of Lame is instead of encoding each channel (LEFT & RIGHT) separately, you can encode the two channels as "Mid" (L+R/2) and "Side" (L-R/2). Because L+R are usually more similar than not, you can allow Mid to have more bits than the Side channel. (i.e. in a 320kbps JS mp3, M gets 200kbps, S gets 120kbps). Or, depending on the song, Lame encodes it using just a straight L/R stereo.
Using straight stereo in Lame, on a 320kbps mp3 L gets 160kbps, R gets 160kbps. This
should be fine (generally), but for the highest quality you should let Lame decide what to allocate where, using Joint Stereo.
VBR vs ABR vs CBRVBR - the goal of VBR's to have a constant quality, and saving bits & space when it's possible, without lowering quality. size is less predictable than ABR & CBR, but the quality's better.
ABR - varies bits around a specified target bitrate
CBR - keeps a constant bitrate when it's not needed:
a) when it needs a higher bitrate for a more complex part, and

when it needs a lower bitrate for a simpler part
ie: pick a tune and encode them these 4 ways.
VBR - Keeps the highest sound quality by lowering the bitrate when it's not needed and boosting it when it is needed. Could take up an insane amount of space, but it all depends on the song.
ABR (at 224kbps) - Much like VBR, but you pick the average range you want, which limits the encoding. Only good aspect here is that it has the potential quality of VBR (ie on 'simpler' songs) without taking up the space VBR could.
CBR (at 320kbps) - Highest POSSIBLE sound quality, but during simpler parts where less bits are needed, it's wasteful.
CBR (at 128kbps) - Lower sound quality, although it takes up less space. During more complex parts the rate stays the same as the simple parts, making for a much lower quality than it should be.
When you use LAME, for best results use:
-V 0 --vbr-newwhich used to be
--preset fast extreme (for those people that used old versions of LAME)
its target kbps is: 245its full range is:
220kbps to 260kbpssound quality/size chart:
http://wiki.hydrogenaudio.org/index.php?ti...ame-chart-2.png