Hi Gianni,
Very neat test, many thanks. From what you say, it
would seem like the resampler is performing its own
anti-alias, is that so? If that were the case, would
the new sampling rate be specified in the digital
input device's specs?
Also, I was wondering whether you have graphics
depicting the resampled and non-resampled Nyquist
signal?
Thanks,
ariadne.
> I create a 16 bit stereo .wav file and I fill it
> with integer samples with values
> -32000,+32000,-32000,+32000 ... with a program
> written in Visual Basic. Such a sequence of
> samples corresponds to a perfect sinusoid at the
> Nyquist frequency (half of the sampling rate)
> that can't be recorded from an analog source.
> If you feed a non-resampling digital input the
> values are just kept and stored in a new file.
> But if the digital input device resamples the
> data stream all those value are processed with a
> resampling algorithm that produces a new series
> of samples that don't match with the original
> ones. A resampler keeps the original stream,
> upsamples to a much higher frequency, performs a
> digital filtering to cut frequencies higher than
> the new sampling rate (that could be the same of
> the input or a different one) and also attenuates
> frequencies close to the Nyquist frequency, then
> downsamples to the final rate. If your original
> stream is 48k and the output of the resampler is
> exactly 48k again, frequencies higher than
> 22-23kHz will be attenuated and the "Nyquist
> signal" will be greatly attenuated.
> In some cases the input stream has not exactly
> the rate of the sound board clock and in this
> case the "Nyquist signal" will appear at a
> frequency that is not the Nyquist frequency of the
> new file.
>
> A "no resampling" digital input is able to lock
> on the incoming rate and to transfer samples
> without any further processing. If you open with
> Audition the file created by the digital input
> device you see very clearly the difference among
> resampled and not-resampled "Nyquist signal"
>
> I hope I was clear enough,
> Gianni
>
> >Lou Judson =95 Intuitive Audio
> >415-883-2689
> >
> >On Oct 19, 2005, at 12:37 AM, Gianni Pavan wrote:
> >
> > > Create an "impossible" file with samples
> alternating among -max and
> > > +max (-32700,+32700,-32700,+32700,....)
> > > Play the file through a digital output
>
>
--------------------------------------------------------------
> Gianni Pavan
> Email
> Centro Interdisciplinare di Bioacustica e Ricerche
> Ambientali
> Universita' degli Studi di Pavia
> Via Taramelli 24, 27100 PAVIA, ITALIA
> Tel +39-0382-987874
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>
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