--- In Scott Fraser <> wr=
ote:
> This is a common misconception. Current Sample Rate Conversion
> technique does not directly divide higher rates to the target rate,
> i.e. 88.2k to 44.1k is not a case of simply dividing by two.
Right. There is no advantage in retaining an integer relation between sampl=
ing rates. At the core of this is the Nyquist frequency. A recording at 44.=
1ks/s ('ks/s' =3D kilo samples per second, to make a distinction from 'kHz'=
for the audio signal) cannot contain audio signals above 22.05kHz (the Nyq=
uist frequency for 44.1ks/s), or it will cause aliasing. A recording made a=
t 88.2ks/s will contain audio signals up to 44.1kHz, so the sample rate con=
version process MUST include a low pass filter to remove everything above 2=
Message: 2.
Subject: 05kHz *before* converting to 44.1ks/s to prevent aliasing. The
mathematic=
al process of filtering negates any theoretical benefits that an integer-re=
lated multiple sampling rate might have had.
Upsampling is a very common technique these days for many kinds of processi=
ng, including the filtering mentioned above. As I understand it [and correc=
t me if I'm wrong], it produces a cleaner result because it distributes the=
error noise through a wider bandwidth, therefore less error noise within t=
he bandwidth that we're interested in (nominally 20Hz to 20kHz). I have use=
d plug-ins from Universal Audio and similar that rely on a DSP card and off=
er upsampling to 384ks/s. When the upsampling is switched in, the sound bec=
omes noticeably smoother and cleaner when monitored through mastering quali=
ty converters, amps and speakers (Grace, Bel Canto, ATC). I am not sure if =
the differences would be apparent through every day sound systems, but it's=
all cumulative... if you have *enough* of that error noise, due to repeate=
d processing and so on, it will probably become noticeable through very ord=
inary playback systems.
FWIW, I do all of my recording at 96k 24-bit, and use iZotope's SRC and MBI=
T+ dither to downsample and reduce word size as necessary. No problems... Y=
ou can see how the iZotope SRC compares against others here:
http://src.infinitewave.ca/
And you can get it for $79 US in Sample Manager (from Audiofile Engineering=
), along with MBIT+ dither and a whole heap of other useful file processing=
tools. You can get that here:
http://www.audiofile-engineering.com/samplemanager/
- Greg Simmons
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