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