[PD] 4-point interpolation changes timbre depending on sample rate

[Clemens]

When there is no aliasing to worry about, i might set the cutoff to pi 
again...
On low sample rates (22kHz), the lower cutoff is definitely noticeable.

Btw. I just implemented sharing of the interpolation table of the 
delreadsinc~ object according to your suggestions.
It counts the number of references and frees the pointer when no object 
uses it anymore.

Best wishes,
Clemens

Am 29.04.21 um 21:03 schrieb Charles Z Henry:
> On Thu, Apr 29, 2021 at 12:08 PM Clemens Wegener <clemens at chair.audio> wrote:
>
>> Also: Is it really mandatory to choose a filter cutoff below pi, when the input and output Sample rates match? For a simple static delay that should be the case.
> There is an exact requirement when what you're making is an
> interpolator:  it has to pass through all the original points!
> For that to happen, your interpolating kernel i(t) must have
> i(t)=1,   t=0
> i(t)=0,  for non-zero integer values of t
>
> An interpolator will have a cutoff freq at exactly pi rad/sec
>
> So, if you're changing the cutoff frequency or amplitudes, it won't be
> an interpolator.  It will just be some kind of table reader.
> And that's good for performance, I'd say, but your users will have to
> know that (and you'll have to justify the choice if it's for an
> academic purpose)

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