MP3 Export Options

From Audacity Manual

Accessed by: File Menu > Export... > File Export Dialog > Options button

Bit Rate Mode

These offer four different ways to control the trade-off between the size and quality of the exported files.

• Preset: This gives you a choice of four presets recommended by the LAME developers. Each preset is a combination of various encoding parameters as shown in the table below. These are recommended, easy to use settings unless you have reason to experiment with other parameters. As a rough guide to the claims made for these presets, uncritical listeners with quality equipment may perceive a file encoded with Extreme preset to be indistinguishable from the original, and those with budget equipment should perceive little difference from the original file with Standard preset. The Insane preset is constant bit rate, as opposed to the variable bit rate used by the other presets (see the explanation in the following sections). Files encoded with Insane preset take a fixed 2.2 MB per minute, often double the size of Extreme preset, with very little audible improvement.

Preset	Target kbps	kbps range	-b switch (minimum kbps)$	Lowpass
Insane	320	320 CBR	-	-
Extreme	245	220-260	128	19383 Hz - 19916 Hz
Standard	190	170-210	128	18671 Hz - 19205 Hz
Medium	165	145-185	-	17960 Hz - 18494 Hz

$ Standard speed only

• Variable: This mode continuously varies the bit rate used according to the complexity of the sound, in an attempt to maintain the quality of the recording at a consistent level. The settings range from 0 (highest quality) to 9 (smallest file size). Each setting defines a range of bit rates within which it will generally operate. Level 0 is comparable to the Extreme preset mentioned above, level 2 is comparable to Standard and level 4 to Medium presets. Variable bit rate offers the best quality compared to the Average and Constant bit rate options in most circumstances (extremely quiet music is one exception). It will tend to give you somewhat smaller file sizes for a given degree of quality, but the actual file sizes can't be accurately predicted in advance.
• Average: This sets a known, average bit rate for the file, but within this average permits some fluctuations in bit rate to reflect the relative difficulty of encoding the file. Not as of so consistently high quality as variable bit rate, but generally preferable to constant bit rate if you need to know more or less how large your files will be. Offers bit rates from 8 kbps to 320 kbps.
• Constant: Sets a constant bit rate for the encoding, regardless of its complexity. Of the four bit rate modes, this usually gives the worst, least consistent quality for a given file size, but that size itself is completely predictable. Use this option if you are streaming the file over the internet, as it ensures the audio cannot stutter due to sudden short peaks in the bit rate. Offers the same choice of bit rates as average bit rate - the default Audacity rate (128 kbps) gives you a known file size of about 1 MB per minute of mono or stereo audio.

Quality

This list lets you select the bit rate (in kbps, or kilobits per second) for encoding your file. A higher bit rate always gives better quality but at the expense of a larger file size, and vice-versa. The bit rates are absolute for average and constant bit rate modes, but expressed as a range for the variable and preset modes (except for the Insane preset).

Variable Speed

Choice of variable speed is available when using variable bit rate encoding. It determines the speed and accuracy at which the encoding algorithm performs its calculations. Standard provides the best results, but will take longer to encode than the Fast option.

Channel Mode

This controls how the two channels of a stereo recording are processed during encoding:

• Joint Stereo converts the Left and Right channels into two different signals: Sum (Left plus Right, mono) and Difference (the difference between the two channels, Left minus Right). This allows recordings with relatively little stereo separation to be encoded more efficiently - more bits can be allocated to the Sum channel because there is relatively difference to encode. The higher the bit rate you use, the less the audible difference that's likely to result from using Joint Stereo.
• Stereo encodes the left and right signals independently, redistributing bits between the channels as required to get the best encoding.