Re: [PD-dev] 64-bit (and/or ARM) Pd change default cosine table?

On 05.06.2024 13:09, Matt Barber wrote:

Hi Christof,

Thanks!

Don't forget the guard point, which could also be set explicitly to 1.0

True! Thanks!

christof

Matt

On Wed, Jun 5, 2024, 4:29 AM Christof Ressi info@christofressi.com wrote:

Hi Matt,

thanks for chiming in!

...

Christof:

    This should ensure that the table is symmetric, unless the
    underlying cos() function is broken :)


I think the underlying cos() function is dependent on
architecture and compiler?

Yes. cos() is a library function, so the output depends on the
particular C library implementation, the compiler and the also
architecture. I would still expect it to be (reasonably)
symmetric, but I haven't really checked.

...

Even with the new cosine table, on my machine the zero crossings
have a (very tiny) residual, so it's sitting at a very small DC
offset.

You're right, I didn't consider the peaks and zero-crossings! The
values of 1/2 PI, PI and 3/2 PI cannot be accurately represented
as floating point numbers, so the result of cos() may be a bit off.

I think we should explicitly set these points:

cos_newtable[COSTABLESIZE / 4] = 0.0;		/* 1/2 PI */
cos_newtable[COSTABLESIZE / 2] = -1.0; 		/* PI */
cos_newtable[COSTABLESIZE / 4 * 3] = 0.0; 	/* 3/2 PI */

@Miller: what do you think? IMO we should make the cos table as
good as we can, so we won't have any regrets :)

...

The rest of the function looks symmetric, though, and certainly
much, much better than the previous one.

Thanks for checking!

Christof

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