Bitrate Trouble in Audacity

I’m having trouble finding out which bit rate mode is which (Preset/Variable/Average/Constant Bitrate mode) In Audacity 1.3. I’m doing a school project on Mp3 compression, and I assumed that the bitrate mode was the same, thinking that once you set a bitrate speed, it would be the same throughout. Wich one is most popular and safest, and which one allows for a permanent bitrate speed?

“CBR” does what it says on the tin - the bit rate is constant, so if you set the MP3 Export format to 128kbps CBR, then the music will be encoded at 128bps throughout. Note that for exporting a mono file from Audacity at 128kbps will still use 128kbps, so the sound quality will be equivalent to using about 256kbps on a mono file (it’s not exactly the same because Lame can combine data that is common to both channels thus improving the quality).

128kbps CBR is probably the most widely used and the most compatible, though it is usually considered to be the minimum for reasonable quality music. Almost all players these days will support VBR, though they may report the play length incorrectly.

VBR will produce better quality than CBR for an equivalent file size until you reach the highest quality of 320kbps at which point there is no advantage in using VBR as the quality is already “maxed out” to what the MP3 format is capable of.

If reporting the correct track length is important to you, then you should use CBR, otherwise use VBR for better sound quality.

For high quality music you should be using less compression (bigger number kbps) than 128. It is common these days for people to use 190kbps or more.

The “pre-sets” in Audacity are a selection of optimum settings as devised by the developers of Lame. All of the presets are VBR except for “Insane” which is 320kbps CBR.

“Joint Stereo” may improve the sound quality at a given bit rate (file size), but may also produce a less accurate stereo effect.