Play~ what's the interpolation ?!

Hello maxers,
I'm Writing an external based on the simple play~ object.
Thus I need to recreate my own play~ object source code.

I decided to modify the index~ code (in the SDK) to simply add an interpolation function.
I've tried with a simple linear interpolation and a cosinus one, but the results aren't the same than in the original play~ object with a slower reading speed !

Maybe i forgot something in my own code :
- some errors or I missed something.
- or the original code implements an other interpolation algo (quad or cubic..) ?
- or there is a secret recipe ?

My object sounds better than the index~ one (thanks to the interpolation) but not like the original :
some artificial high frequencies appear, they didn't with play~.
I wonder if play~ implements a high-cut filter to disable this or if it's just due to his own interpolation algo ?

I've attached an archive file which contains my code with the builds external and its help pat which will show you better what i mean.

Thanks for read and reply.
Eliott.