Dalla man page di SOX:
codice:
rate [-q|-l|-m|-h|-v] [override-options] RATE[k]
Change the audio sampling rate (i.e. resample the audio) to any given RATE (even non-integer if this is supported by the
output file format) using a quality level defined as follows:
Quality Band- Rej dB Typical Use
width
-q quick n/a ~=30 @ playback on
Fs/4 ancient hardware
-l low 80% 100 playback on old
hardware
-m medium 95% 100 audio playback
-h high 95% 125 16-bit mastering
(use with dither)
-v very high 95% 175 24-bit mastering
where Band-width is the percentage of the audio frequency band that is preserved and Rej dB is the level of noise rejection.
Increasing levels of resampling quality come at the expense of increasing amounts of time to process the audio. If no qual-
ity option is given, the quality level used is `high' (but see `Playing & Recording Audio' above regarding playback).
The `quick' algorithm uses cubic interpolation; all others use band-limited interpolation. By default, all algorithms have
a `linear' phase response; for `medium', `high' and `very high', the phase response is configurable (see below).
The rate effect is invoked automatically if SoX's -r option specifies a rate that is different to that of the input file(s).
Alternatively, if this effect is given explicitly, then SoX's -r option need not be given. For example, the following two
commands are equivalent:
sox input.wav -r 48k output.wav bass -b 24
sox input.wav output.wav bass -b 24 rate 48k
though the second command is more flexible as it allows rate options to be given, and allows the effects to be ordered arbi-
trarily.
* * *
Warning: technically detailed discussion follows.
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
override 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)
N.B. Override options cannot be used with the `quick' or `low' quality algorithms.
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 sampling rates but that if this is at least 44.1kHz,
then the artefacts 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 mini-
mum phase, there is no pre-echo but the longest post-echo; with linear 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 frequency content of the original signal (w.r.t. the original
sample rate when up-sampling, or the new sample rate when down-sampling) 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 transient echo.
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/audibility), this may not be a problem. The benefits of
allowing aliasing/imaging are reduced processing time, and reduced (by almost half) transient echo artefacts. Note that if
this option is given, then the minimum band-width allowable with -b increases to 85%.
Examples:
sox input.wav -b 16 output.wav rate -s -a 44100 dither -s
default (high) quality resampling; overrides: steep filter, allow aliasing; to 44.1kHz sample rate; noise-shaped dither to
16-bit WAV file.
sox input.wav -b 24 output.aiff rate -v -I -b 90 48k
very high quality resampling; overrides: intermediate phase, band-width 90%; to 48k sample rate; store output to 24-bit AIFF
file.
(vedi anche: sox(1) - Linux man page )
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