[PD] object name and anti aliasing technique : WAS : Re: better tabread4~

[cyrille henry]

Charles Henry a écrit :
> On Thu, Jun 26, 2008 at 12:36 PM, cyrille henry
> <cyrille.henry at la-kitchen.fr> wrote:
> 
>> then, it's possibile to have interpolation using more points, and also
>> interpolation that offer antialiasing.
>>
>> BTW : i just commited a tabosc4aa~ (aa is for anti aliasing).
>> this is also just a test.
>> it's far from perfect, but lot's better than the original tabread4~ when
>> dealing with high pitch transposition.
>> for now, i did this with a classic 8x upsampling + biquad filter inside the
>> tabread4aa~ (ok, this is more cpu intensive : about 1 to 1.5 % cpu)
>> before i'm doing more work on this, did anyone have experience with this?
> 
> To me, the upsampling is not a good approach for anti-aliasing.  This
> means, you have to interpolate, then filter, then sample again.
yes, i know.
but it's this way i've got the best result.
if anyone want to try something else....

> 
> But you will wind up applying your filter at a lot of points, which
> will just be discarded.

the current implementation use a 4 time upsampling, so it's not so inefficient, the result are quite good.


> 
> In fewer steps, (I think) we can modify the interpolation function
> continuously as the speed changes.  Rather than filtering on a long
> signal, just filter it in place.  The part that gets tricky is that
> the DFT is a lot less relevant, and you have to rely upon continuous
> functions instead.

Shannon interpolation use sinc function.
with 16 points interpolation i was not able to have the same quality as the 4 times oversampled 4 points interpolation

plus you also have to compute a 2D table of sinc function for improving performance (so you also have to interpolate in this 2D table).

maybe it's the way to do a 128 points, very heavy, but almost perfect sounding interpolation that could be used in non real time.
i should give it an other try, but i think i'll not have the time soon.
(will finish the other one thirst).


anyway, i don't know a lot's on this subject, so any help is welcome


cyrille

> 
> I didn't make any progress today... and friday is gone for sure :)  I
> will calculate the spectrum for tabread4c~ on Saturday, I think.
> 
> I'm down with the project, but I can't really see the big picture for
> a tabread/vd/tabosc lib yet.
> 
> Chuck
> 
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