Re: [stella] tia hue luminance

[Glenn Saunders <mos6507@xxxxxxxxxxx>]

It should also be noted that measuring the output of a particular 2600, 
especially through RF, is not the best way to judge what the chip does.

Sure, maybe it's authentic in the sense that you are getting out what you 
would have seen in the 70s, but it's not representing the raw output of the 
TIA.

There are too many ways that the signal can get altered both by the 
motherboard (the chroma knob for instance) or the signal path outside of 
the 2600 that using a capture card isn't the best approach.

So I think using a formula is better, as it represents a theoretical ideal 
at the chip level.

Once you have this ideal you have your baseline and at least for emulators, 
modify it to simulate things like signal degradation through RF or 
adjustments to the chroma knob if you really want to.

At 11:53 AM 9/27/2003, you wrote:
> The base equations are well known, but the specific scaling factors used by
> the TIA aren't as well known.  I believe what was done for the latest Z26
> was to use the RGB output of a video capture card.
>
> But if you are interested, here are the NTSC formulas.
>
> R8 = 255 * ( Y + 1.402 * Pr )
> G8 = 255 * ( Y - 0.344136 * Pb - 0.714136 * Pr )
> B8 = 255 * ( Y + 1.772 * Pb )
>
> where
> Y = ( COLU AND 15 ) / ( luminance )
> Pb = saturation * cos ( hue )
> Pr = saturation * sin ( hue )
> hue = 180 degrees - ( COLU / 16 - 1 ) * 24 degrees
> saturation = 0 if ( COLU / 16 == 0 ), otherwise some value between 0 & 0.5
> luminance = some value 14 or greater
>
> (base formula from http://www.poynton.com/PDFs/ColorFAQ.pdf)
>
> ----- Original Message -----
> From: "Adam Wozniak" <adam@xxxxxxxxxxxxxxxx>
> To: <stella@xxxxxxxxxxx>
> Sent: Friday, September 26, 2003 2:04 PM
> Subject: Re: [stella] tia hue luminance
>
>
> > On Wed, 24 Sep 2003, Adam Wozniak wrote:
> > > What equations are used to convert 4 bit hue / 3 bit luminance to
> > > RGB 8/8/8 ?
> > >
> > > I've seen tables for the conversions, but I'd like to know what
> equations
> > > are used.
> > >
> > > I am interested in both NTSC and PAL.
> >
> > Nobody knows?
> >
> > --
> > adam@xxxxxxxxxxxxxxxx        http://cuddlepuddle.org/~adam/pgp.html
> > Will code for food.          http://cuddlepuddle.org/~adam/resume.html
> > "The dinosaurs are not around today because they did not have a space
> program."
> >   -- Arthur C. Clarke
> >
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