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Re: Interpolation

Started by robert bristow-johnson April 2, 2008

Eric Jacobsen wrote:
> > On Wed, 02 Apr 2008 07:27:42 -0600, jim <"sjedgingN0sp"@m@mwt.net> > wrote: > > >> > You are asking if a filter designed to prevent aliasing is really an > >> >anti-alias filter. How much of an answer do you want to a question like > >> >that? > >> > >> It's the same sort of question of whether or not an upsampling filter > >> should be called an interpolator, and you've been happy to participate > >> in that discussion. As I've already said, a common definition of > >> "anti-alias filter" is the analog filter preceding an ADC, and for a > >> lot of folks that's the only way the term is used. Using it > >> generically to describe a frequency-selective decimating filter could > >> cause the same confusion if the majority of relevant people thought of > >> the analog filter when you meant the digital. Do you not think that's > >> worth clearing up? Or do you think that the terminology confusion is > >> okay? > > > >The confusion you refer to, as far as I can tell, is not something that is > >common to the world at large. it only exists in this thread. The use of > >anti-alias filters for downsampling is not, as far as I know, something > >that causes confusion. > > We've evidently not shared the same DSP community for conversation. In > my career "anti-aliasing filter" has generally meant the analog filter > preceding an ADC. A decimating filter certainly performs an > anti-aliasing function, but in my experience they're rarely if ever > called "anti-aliasing filters", but instead called decimating filters > or downsamplers or by whatever other name that fits their function in > the system block diagram (e.g., image rejection filter, whatever). > > So the confusion of whether "anti-aliasing filter" without a > clarifying qualification wouldn't be restricted to this thread in my > experience. It's a simple thing to clear up, but seems to be > dragging on. >
What's to clear up? You think that everyone who uses the wording anti-alias filter in connection with downsampling should be required to provide a disclaimer that "this is not an analog filter". Seems easy enough - but I don't think people are going to comply. What can I do?
> I really don't mind. ;)
> > >> >> Since a fair portion of the discussion is about terminology, and a few > >> >> folks have been using "anti-aliasing" together with decimating > >> >> filters, I thought it was a pertinent question. Since "anti-aliasing" > >> >> is usually an analog filter function preceding an ADC in my > >> >> experience, I'm curious how many people also apply the term to > >> >> decimating filters? It seems unusual to me. > >> > >> > > >> >Well the point wasn't to spawn another tangential discussion about what > >> >name you give something. The point being made was why the word > >> >interpolation isn't used when the process involves reducing the frequency > >> >content. > >> > > >> >-jim > >> > >> And I think the word "interpolation" as commonly understood can > >> certainly be applied to frequency-selective filters, whether they're > >> upsampling or downsampling or preserving the sample rate. If you're > >> saying that "interpolation" should only be conditionally applied based > >> on whether or not frequency selectivity is implemented I'd be > >> interested in any arguments for why that should be the case. > >> > >> After all, linear interpolation adds distortion to a signal, so a > >> sample rate change (or even just time shifter) that uses linear > >> interpolation would change the frequency content of the signal, so is > >> it not interpolation when used in that manner? > > > > > > The process called interpolation is simply to find new points on a > >continuous curve that passes thru the input samples. > > I've said as much a few times here, and was perhaps the first one in > this thread to do so. > > >The original question > >was why the word "interpolation" is used by some to mean upsampling but > >not downsampling. A simple explanation was offered. The explanation sounds > >perfectly reasonable to me, but confusing to you. > > If you're referring to the explanations that I've given, which are > consistent with others', I don't know why you'd think I'm confused > about them. The debates seem to me to be only about the finer points > of the terminology. > > > And as near as I can > >tell there is nothing that can be done about that. > > Jeebuz, no wonder this conversation has been difficult. You might > want to re-read the thread and see who's really said what. You now > seem to be criticising me for being confused about points I made!
I have no way of knowing whether you are confused or not. But you have said you are confused by the use of term anti-alias filter when downsampling. And you take my inability to do anything about that as a criticism. There is nothing I can do about that either. -jim
> > Eric Jacobsen > Minister of Algorithms > Abineau Communications > http://www.ericjacobsen.org
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