Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!usc!ucsd!ucbvax!pasteur!bennett.berkeley.edu!parks
From: parks@bennett.berkeley.edu (Tom Parks)
Newsgroups: comp.dsp
Subject: Re: Creating Samples by stuffing & filtering
Message-ID: <8563@pasteur.Berkeley.EDU>
Date: 6 Nov 90 17:23:27 GMT
References: <16311@netcom.UUCP>
Sender: news@pasteur.Berkeley.EDU
Reply-To: parks@bennett.berkeley.edu (Tom Parks)
Organization: University of California, Berkeley
Lines: 31

In article <16311@netcom.UUCP>, mcmahan@netcom.UUCP (Dave Mc Mahan) writes:
> The Problem:
> 
> I have been given a task of smoothing an ECG (electro-cardiogram) signal that
> needs to be sent to a strip chart recorder.

[stuff deleted]

> What I have done:
> 
> To create more samples, I assumed that I could just read in one sample,
> duplicate it to the output the desired number of times for the proper
> expansion factor (currently, an expansion factor of 4 samples out for each
> sample in seems to be about right) and then pass the expanded data through
> a digital filter to low pass filter it and smooth the final output.

Instead of duplicating the samples, insert zeros between the samples. 
This way your low pass filter can be flat in the pass band, instead of
needing to compensate for the bowing introduced by duplicating samples. 
(Duplicating samples is equivalent to passing your signal through a
system that has a rectangular pulse for its impulse response.  The
Fourier transform of a rectangular pulse is sinc-like.)  Also, this
means that many of the samples you are filtering will be zero, and
multiplication by zero can be done very quickly, in zero time in fact.


Tom Parks
Electronics Research Laboratory
University of California, Berkeley
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Life is a sexually transmitted disease.