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added a couple paragraphs for people who don't know what anti-alias filters

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Frank Mori Hess 2002-07-22 20:45:24 +00:00
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@ -494,7 +494,45 @@ command is executed with
comedi_command(). For input/output commands, data
is read from or written to the device file /dev/comedi[0..3] you are using.
</para>
</section>
<section>
<title>
Anti-aliasing
</title>
<para>
If you wish to aquire accurate waveforms, it is vital that you use an
anti-alias filter. An anti-alias filter is a low-pass filter used to
remove all
frequencies higher than the Nyquist frequency (half your sampling rate)
from your analog input signal
before you convert it to digital. If you fail to filter your input signal,
any high frequency components in the original analog signal will create
artifacts in your recorded
digital waveform that cannot be corrected.
</para>
<para>
For example, suppose you are sampling an analog input channel at a rate of
1000 Hz. If you were to apply a 900 Hz sine wave to the input, you
would find that your
sampling rate is not high enough to faithfully record the 900 Hz input,
since it is above your Nyquist frequency of 500 Hz. Instead, what you
will see in your recorded digital waveform is a 100 Hz sine wave! If you
don't use an anti-alias filter, it is impossible to tell whether the 100
Hz sine wave you see in your digital signal was really produced by a
100 Hz input signal, or a 900 Hz signal aliased to 100 Hz, or a 1100 Hz
signal, etc.
</para>
<para>
In practice, the cutoff frequency for the anti-alias filter is usually
set 10% to 20% below the Nyquist frequency due to fact that real filters
do not have infinitely sharp cutoffs.
</para>
</section>
</section>