"Andrew A. Grathwohl" <andrew@grathwohl.me> writes:
> Hi David,
>
> Thanks this was super-informative!
>
> We can likely rule out the idea that the smaller buffer sizes tax the
> computer further, leading to more fan/PSU noise, since the machine
> itself is not in the same room as the microphone.
>
> I am intrigued by your comments about the sampling rate on my Babyface
> Pro. I have always set it to 48kHz whenever doing low-latency audio or
> any audio that will be transmitted over a network cable, which I mostly
> do superstitiously. Is there any guidance out there about what the
> correct sampling rate would be for my device, or about how to determine
> this answer for myself?
With RME, I'd trust the soundcard, no questions asked. With other
soundcards offering substantially higher sample rates, there is a chance
that downsampling (after proper digital filtering) can lead to better
quality and/or lower latency.
Cf. the parameters in sox:
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 .na Band-width Rej dB .na Typical Use
-q .na quick n/a .na 30 @ Fs/4 .na playback on ancient hardware
-l low 80% 100 .na playback on old hardware
-m medium 95% 100 .na audio playback
-h high 95% 125 .na 16-bit mastering (use with dither)
-v .na 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 quality
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 arbitrarily.
* * *
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 .na 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 minimum 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.
* * *
The pitch and speed effects use the rate effect at
their core.
As you can see, downsampling is a science... If you sacrifice linear
phase response (which makes the main difference at very high
frequencies), you can achieve lower latency, though the difference at
lower frequencies will be comparatively minimal.
-- David Kastrup _______________________________________________ Linux-audio-user mailing list Linux-audio-user@lists.linuxaudio.org https://lists.linuxaudio.org/listinfo/linux-audio-userReceived on Wed Jan 6 04:15:02 2021
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