Re: Interpolation

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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