Charles Henry a écrit :
On Thu, Jun 26, 2008 at 12:36 PM, cyrille henry cyrille.henry@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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