|The SoX Resampler library libsoxr performs fast, high-quality one-dimensional sample rate conversion. Audacity started using libsoxr from version 2.0.3 onwards.
Four SoX Resampler quality levels are available for constant-rate resampling in Audacity; which one to use in different circumstances may be selected in the Preferences/Quality menu. Note that the names assigned to these levels differ from those used by the 'sox' command-line utility; equivalent names are shown in the following table:
|Best Quality||Very high|
See the 'rate' effect in the SoX manual for details (frequency response, etc.) of each of these quality levels.
Sample rate conversion in Audacity 2.0.3 ("Best Quality") can be compared with SoX "Very high" and numerous other audio applications in the graphs at at http://src.infinitewave.ca/. Note that in the "passband" and "transition" graphs, SoX's "steep filter" was used, which option is not available in libsoxr in Audacity. This probably accounts for those two SoX graphs being closer to the "ideal" filter.
For variable-rate resampling, libsoxr uses a single, dedicated quality level. This is selected automatically by Audacity when performing a task that requires it.
For comparison purposes, the following table attempts to place in order of increasing quality, current resampling qualities (with libsoxr), and those with resamplers available in previous/other configurations of Audacity:
|Fast Sinc Interpolation||-/14|
|High-quality Sinc Interpolation||-/15|
|Medium Quality||Fastest Sinc Interpolator||-/16|
|High Quality||Medium Sinc Interpolator||-/20|
|Best Sinc Interpolator||-/24|