## Re: [stella] tia hue luminance

 Subject: Re: [stella] tia hue luminance From: ecwilkso@xxxxxxx Date: Fri, 12 Dec 2003 20:33:05 -0500
```Apologies for jumping into the middle of the thread.  I'm trying to catch up.

For the time being, I'll explain the 2600 color shift circuit.  The pot
adjusts the control voltage going into a digital delay line.  The higher the
voltage, the less delay.  The input to the delay line is the color clock.  The
output of each stage is delayed from the previous one.  Each stage delay is the
same.  One of the 15 outputs is chosen by (COLU & \$f0) to be sent to the output
pin.  Color 1 has no delay...it is the same as the color burst.

color 15.  This total delay is then divided by 14 to give the inter-color phase
shift.

Which brings up a point that may be leading to some of the errors.  Most 2600s
are calibrated initially so that color 15 and color 1 are the *same*.  It's done
this way because the adjustment is visual and is quick and easy to do.  Maybe
it shouldn't be done like that...we're losing 8 colors from the palette.  But
there's no quick and easy way to precisely adjust so that color 15 has exactly
336 degrees of phase shift relative to color 1.

This being the case, a better number for the inter-color delay would be (360/14)
instead of 24 degrees.

I write this assuming that some but not all of the people doing color testing
have attempted some form of calibration on their 2600 before starting.  For
those who haven't, this is how it's usually done.

One more thing.  The numbers I've always seen for the luma equation are
y = r*.30 + g*.59 + b*.11.  (I'm not bothering with the primes, because I
don't fully remember them and I don't have any of my video books here.  Sorry.)
I believe these are the original numbers and would have been in use in 1975.

Oops.  One more thing.  When calulating Y (or Y') I believe you should use a
max value of 1.0, not 0.7.  It's true that the white level minus the blank
level is 0.7 times the amplitude of the luma video signal, but the luma content
should be normalized to 1.0 for display purposes.

-Chris

Quoting John Saeger <johnws8@xxxxxxxxxxx>:

> Yeah this is interesting.  I wonder if the color adjustment inside a
> 2600 works like that?
>
> John
>
> On Tue, 2003-12-09 at 23:50, Thomas Jentzsch wrote:
> > Adam wrote:
> > > http://cuddlepuddle.org/~adam/video.cgi
> > >
> > > Lets you tweak parameters.
> >
> > I had some fun playing with that. A phase shift of ~-40Â° makes it look
> > almost like the emulator NTSC palettes.
> >
> > Have fun!
> > Thomas
> > _______________________________________________________
> > Thomas Jentzsch         | *** Every bit is sacred ! ***
> > tjentzsch at web dot de |
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