[PD] tabread4~ "broken" interpolation algorithm - was Re: Max Smoother Audio than Pd?

Roman Haefeli reduzierer at yahoo.de
Tue Mar 30 11:58:45 CEST 2010


On Mon, 2010-03-29 at 21:06 -0400, Matt Barber wrote:
> LONG, sorry.

Thanks again for your time and patience.

> One really good way to think, then, is in terms of the continuous
> frequency response of the interpolator.  In that long, long discussion
> a couple years ago, Chuck Henry made the following post where he
> showed the impulse response of [tabread4~] vs. the [tabread4c~]
> 
> http://lists.puredata.info/pipermail/pd-list/2008-06/063408.html
> 
> (look at the graph)

Assuming, that the goal is to avoid anything above Nyquist, it seems
that [tabread4c~] does its job better (blue graph). The area below the
graph line and right of the nyquist vertical line seems smaller for the
blue graph than for the red graph. Or am I tricked by the logarithmic
view of those graphs?

> I haven't studied them in school either which is why I worry about the
> above explanation.  I think [tabread4~] is good for what it is for a
> couple of reasons, neither of them particularly compelling:
> 
> 1)  It's better than linear interpolation, and has wide use in other
> computer-music applications like csound -- people are very used to how
> it behaves.

This would apply to both, [tabread4~] and [tabread4c~], wouldn't it?

> 2)  It's similar to the alternative being discussed, but with a
> different sound; not necessarily "worse" for all sounds.

It seems intuitively logical to me that discontinuities in the first
derivative (which are basically edges in the curve) create high-frequent
noises. When listening to both (back then), I had the feeling that there
are more noises audible with [tabread~] than with [tabread4c~]. So to me
it still remains unclear in what aspect [tabread4~] is superior to
[tabread4c~], from both a theoretical and from an empirical
perspective.  

Roman



	
		
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