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procedure for testing if digital resampling occurs or not

Subject: procedure for testing if digital resampling occurs or not
From: "Gianni Pavan" gianni_pavan
Date: Sun Nov 12, 2006 10:39 am ((PST))
Hi all,
        I just tested a simple procedure to verify if digital resampling
occurs or not. I'm now willing to get feedback from you and maybe
collect information on which boards or recorders do resampling and
which don't do. Please, try to do the test and tell me if the test
works ok or not.

You need to have a sound board with digital output; here I assume the
board outputs the samples as they are in the file you play, without
any processing .
Create a wav file with an "impossible" signal just at the Nyquist
frequency, that is half the sample rate. To do this you can use Adobe
Audition. In version 2: create new file, say stereo 96 kHz 16 bit.
Then "Generate", then "Tones", in the "Generate Tones" panel set the
frequency to half the chosen sample rate (48000 for 96000), set 0 in
the modulation fields, select sine type of waveform, set start phase
90, set 100 for the 1st component, set 0 for the harmonics. Set the
output level to -6dB.

Generate the signal. Then examine the waveform by zooming at sample
level: you should see alternated positive and negative samples, all
positive samples with same height, all negative samples with same
height. Now you have a signal with exactly the frequency of half the
sample rate. (you should also see the interpolated sinusoid generated
by Audition).

Play the signal thorough the digital output and record it on the
device you want to test. Then examine the recorded file with
Audition: you should see exactly the same structure you saw after
creating the impossible wave. If resampling has occurred you should
see something completely different.

If testing a sound board, be sure to check in its settings panel how
the board manages incoming digital streams; some boards allow to
choose among adapting to the incoming rate or use internal own clock;
this second option may mean resampling. In any case repeat the test
with all options available.

Gianni Pavan
Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
Universita' degli Studi di Pavia
Via Taramelli 24, 27100 PAVIA, ITALIA
Tel         +39-0382-987874
Fax        +39-02-700-32921

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