From help file of standalone SoX program (ver. 14.4.0):
The simple quality selection described above provides settings
that satisfy the needs of the vast majority of resampling tasks.
Occasionally, however, it may be desirable to fine-tune the
resampler's filter response; this can be achieved using over‐
ride options, as detailed in the following table:
-M/-I/-L Phase response = minimum/intermediate/linear
-s Steep filter (band-width = 99%)
-a Allow aliasing/imaging above the pass-band
-b 74-99.7 Any band-width %
-p 0-100 Any phase response (0 = minimum, 25 = intermediate,
50 = linear, 100 = maximum)
All resamplers use filters that can sometimes create `echo'
(a.k.a. `ringing') artefacts with transient signals such as
those that occur with `finger snaps' or other highly percussive
sounds. Such artefacts are much more noticeable to the human
ear if they occur before the transient (`pre-echo') than if they
occur after it (`post-echo'). Note that frequency of any such
artefacts is related to the smaller of the original and new sam‐
pling rates but that if this is at least 44.1kHz, then the arte‐
facts will lie outside the range of human hearing.
A phase response setting may be used to control the distribution
of any transient echo between `pre' and `post': with minimum
phase, there is no pre-echo but the longest post-echo; with lin‐
ear phase, pre and post echo are in equal amounts (in signal
terms, but not audibility terms); the intermediate phase setting
attempts to find the best compromise by selecting a small length
(and level) of pre-echo and a medium lengthed post-echo.
Minimum, intermediate, or linear phase response is selected
using the -M, -I, or -L option; a custom phase response can be
created with the -p option. Note that phase responses between
`linear' and `maximum' (greater than 50) are rarely useful.
A resampler's band-width setting determines how much of the fre‐
quency content of the original signal (w.r.t. the original sam‐
ple rate when up-sampling, or the new sample rate when down-sam‐
pling) is preserved during conversion. The term `pass-band' is
used to refer to all frequencies up to the band-width point
(e.g. for 44.1kHz sampling rate, and a resampling band-width of
95%, the pass-band represents frequencies from 0Hz (D.C.) to
circa 21kHz). Increasing the resampler's band-width results in
a slower conversion and can increase transient echo artefacts
(and vice versa).
The -s `steep filter' option changes resampling band-width from
the default 95% (based on the 3dB point), to 99%. The -b option
allows the band-width to be set to any value in the range
74-99.7 %, but note that band-width values greater than 99% are
not recommended for normal use as they can cause excessive tran‐
If the -a option is given, then aliasing/imaging above the pass-
band is allowed. For example, with 44.1kHz sampling rate, and a
resampling band-width of 95%, this means that frequency content
above 21kHz can be distorted; however, since this is above the
pass-band (i.e. above the highest frequency of interest/audi‐
bility), this may not be a problem. The benefits of allowing
aliasing/imaging are reduced processing time, and reduced (by
almost half) transient echo artefacts.