Re: High Resolution With 16 colors?

["Lee Cremeans" <lee@lcremeans.homeip.net>]

"Bryan Parkoff" <BParkoff@satx.rr.com> wrote in message
HoOdnWmgfap_svmjXTWQlg@giganews.com">news:HoOdnWmgfap_svmjXTWQlg@giganews.com...
>     I would like to ask why Steve Worniak designed Apple II to restrict
only
> 4 colors on high resolution 280x192.  Later, he fixed a bug to add two
more
> colors.  He should add 16 colors instead of 6 or 8 colors on high
resolution
> (Not Double High Resolution).

[snip]

>     Why did IBM PC only show 4 colors (black, white, blue, violent) on
> 320x200 resolution and another 4 colors (black, white, yellow, green) on
> another 320x200 resoltuion.  Apple II's high resolution allows to combine
8
> colors together.  What is a problem with IBM PC?
>     Please explain how Steve Worniak designed low resolution and high
> resolution.

Appe II video relies heavily on quirks of the NTSC color video system that
TVs in the US use (PAL is different, but similar enough that you can do many
of the same tricks). In NTSC, color is determined by the phase and amplitude
relationships between the color signals I and Q, and a "burst" at the
beginning of each frame that sets the zero point for the color signal. What
Steve did was simplify this and take advantage of the fact that you could
get 16 colors out of NTSC (using nothing more than a shift register) if you
timed the color signal a certain way. He did it this way to save money; this
was 1977, and doing computer-generated color then was something of a black
art. The "proper" way to encode NTSC, then and now, is to feed analog RGB to
an encoder circuit (which did all the matrixing and modulation and such for
you), and send that circuit's output out the door to a TV. The output would
be beautiful, but in 1977 it would have required a lot of extra components
that were very hard-to-get or expensive...and the II was all about keeping
costs down. Also, Apple's "high" resolution wasn't all that high -- 140x192
effectively, if you were using color.

As for the PC, it follows a totally different design philosophy. Most
importantly, IBM were targeting 1-bit monochrome and 4-bit RGBI digital
monitors, which didn't need any special tricks to generate a picture; you
just dmped the bits into memory and it displayed them. The CGA could have
had 4-bit color in all modes, theoretically, it's just that it didn't have
the memory space to do it (it only had 16k RAM on board) The EGA and VGA
addressed this, eventually, having 256k or more (much more, nowadays).

-lee