From: Frank Mori Hess Date: Mon, 22 Jul 2002 20:45:24 +0000 (+0000) Subject: added a couple paragraphs for people who don't know what anti-alias filters X-Git-Tag: r0_7_21~318 X-Git-Url: http://git.tremily.us/?a=commitdiff_plain;h=add0bf082b7e42b64e389ed72f9287ad4903945d;p=comedilib.git added a couple paragraphs for people who don't know what anti-alias filters are. --- diff --git a/doc/other.sgml b/doc/other.sgml index 1b5260d..58ecdfe 100644 --- a/doc/other.sgml +++ b/doc/other.sgml @@ -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. - + + + +
+ +Anti-aliasing + + + +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. + + + +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. + + + +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. + +