Re: NTSC's Three Dots Per One Pixel

[mjmahon@aol.com (Michael J. Mahon)]

Bryan Parkoff wrote:

>   Do you see that Apple IIgs' RGB monitor displays one dot per pixel?  If
>you type in Applesoft BASIC like below.
>
>10 HGR
>20 HCOLOR=2
>30 HPLOT 0,0 TO 100,100
>RUN
>
>    It displays a violet line from (0,0) to (100,100).  Do you notice that a
>pair of pixels do not get close together that it only displays one dot per
>pixel?  Now, compare to NTSC.  Do you notice that it displays three dots per
>pixel?  It is because NTSC's beam to light three dots per pixel, but it
>looks like that it is really five dots or six dots per pixel.

I believe that you are confusing the dots created by the CRT's shadow
mask with pixels.

Try plotting a single point and look at the screen.  Then plot two adjacent
points (which will appear to be white) and look at the screen.

Apple pixels are, indeed, single pixels, but when sent to a monitor
using NTSC encoding, the bandwidth is limited and the dots are
smeared somewhat.  If you see multiple discrete dots in a single
Apple pixel, this is not in the video signal, but in the shadow mask
just behind the CRT's screen.

If you want to see what is actually going on, you should look at
the Apple video signal with an oscilloscope.  That will make it quite
clear that an Apple pixel is a single pulse, regardless of "color".

The color of a displayed pixel is an artifact of the phase (timing)
of the pulse with respect to the color reference burst.  The video
signal is simply a sequence of pulses, one per "on" pixel, each
of the same amplitude (even the "runt pulse" that occurs when
a pixel pulse is cut short by a change in "color set" between
video bytes).  It is the location and duration of these pulses
that produce the apparent color of the display.

-michael

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