[Coco] Artifacting (again)

[jdaggett at gate.net]

On 3 Sep 2010 at 9:21, Ciaran Anscomb wrote:

> Looking at a chart of the NTSC I/Q axes, I suppose it's the I axis
> that high-low transitions affects most, as that pushes out to red-ish, or
> cyan-ish.  AIUI, for each axis, the phase determines its "value", so as
> high-low-high is 180deg out of phase with low-high-low, you get the two
> ends of that axis.
> 
> http://en.wikipedia.org/wiki/YIQ
> 
> Disclaimer: I am not a broadcast engineer, I just hang around with them in
> the pub.



If you look at a vector plot of NTSC color you will find that the X-axis is the R-Y and the Y-
axis is B-Y. The color burst lies on the negative half of the B-Y axis. Also Red and Cyan, Blue 
and Yellow, and Green and Magenta are 180 degrees phase difference between the three 
pairs. Also the I signal is located +33 degrees rotated off the positive B-Y axis. The Q signal 
is +303 degrees rotated off the B-Y axis. 

The MC6847 uses B-Y, R-Y, and Y signals to generate one of 8 colors. I would suspect that 
the amount of artifacting is also dependant on the silicon variations as is variation of NTSC 
and the human eye. Remember the nickname for NTSC is Never The Same Color.  

The one sacred signal in the NTSC composite is the color burst frequency and phase. If it is 
off then all color and hue are off. Invert the phase (180 degrees) of the color burst and you 
essentially flip the color space. In that case instead of RGB you get CYM.

just some thoughts

james