Comments on: Antialiasing filters and multirate systems http://eli.thegreenplace.net/2006/05/10/antialiasing-filteres-and-multirate-systems/ Eli Bendersky's personal website Thu, 04 Dec 2008 01:36:56 +0000 http://wordpress.org/?v=2.6.2 By: John Handwork http://eli.thegreenplace.net/2006/05/10/antialiasing-filteres-and-multirate-systems/#comment-82 John Handwork Wed, 10 May 2006 17:34:13 +0000 http://eli.thegreenplace.net/2006/05/10/antialiasing-filteres-and-multirate-systems/#comment-82 Rather than thinking of the filter for the oversampled system as providing cutoff at 1/2 the higher sampling rate (Fsh), think of the filter requirements delivering a passband up to the end of your frequency of interest - 10 kHz in your examples - and a stopband beyond Fsh-10kHz (or repeating stopband locations of n*Fsh+/-10kHz). I wouldn't think it's a "common" misconception that antialiasing can be accommodated entirely in the digital domain. *Anyone* who has been introduced to sampling systems *should* know about the frequency domain "folding" that provides aliazing. Otherwize, it's a decent synopsis. One additional note: halfband filters are used heavily in digital systems such that sampling at your frequency of interest multiplied by 2^n may make the implementation easier. It is often more compact to use 3 halfband filters to decimate by 8 rather than decimating by 5 in one step. What I didn't know about halfband filters until a few days ago is that they aren't all the same; the first halfband filter only needs to pass the small bandwidth for the frequency of interest and reject the small stopband around Fsh. - John_H Rather than thinking of the filter for the oversampled system as providing cutoff at 1/2 the higher sampling rate (Fsh), think of the filter requirements delivering a passband up to the end of your frequency of interest - 10 kHz in your examples - and a stopband beyond Fsh-10kHz (or repeating stopband locations of n*Fsh+/-10kHz).

I wouldn’t think it’s a “common” misconception that antialiasing can be accommodated entirely in the digital domain. *Anyone* who has been introduced to sampling systems *should* know about the frequency domain “folding” that provides aliazing.

Otherwize, it’s a decent synopsis.

One additional note: halfband filters are used heavily in digital systems such that sampling at your frequency of interest multiplied by 2^n may make the implementation easier. It is often more compact to use 3 halfband filters to decimate by 8 rather than decimating by 5 in one step. What I didn’t know about halfband filters until a few days ago is that they aren’t all the same; the first halfband filter only needs to pass the small bandwidth for the frequency of interest and reject the small stopband around Fsh.

- John_H

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