harold@hp-pcd.UUCP (Harold Noyes) (12/16/83)
#N:hp-pcd:60100001:000:2355
hp-pcd!harold Dec 15 08:31:00 1983
A *very* brief treatment of a complex issue---
One of my jobs here at Hewlett-Packard has been to design a video
interface for a personal computer. Unfortunately, that project was
cancelled before it was introduced. However, I did get a lot of
exposure to the complexities of monitor designs and video interfaces.
The whole problem boils down to the frequencies needed for the whole
system to work. Lets take a case example- a 1000X1000 pixel B&W
monitor. Let's assume that you can find a tube with a long duration
phosphor that will let you run at a 30hz refresh rate (very hard to find
and expensive, by the way). Since you need some time for the vertical
retrace, usually about 10%, you need about 1100 total vertical lines.
If we assume a non-interlaced display (interlacing is a real *PAIN* to
do!), the horizontal sweep rate becomes (30*1100)=33Khz!!!!! Have you
ever tried to design the flyback transformer, the yoke, and other
circuits to run at 33Khz?! Again, you are into the bucks.
The next problem is the actual video interface to talk to this monitor.
1/33Khz=total time available for each horizontal line= 30uSec. Again,
to achieve 1000 displayed dots, you need about 10% extra for retrace.
So, your horizontal dot clock (the rate at which you shift out the info)
becomes 1/(30u/1100)=
36.3Mhz!!!!!! GOOD LUCK if you try to do this for $100.
The bottom line is:
HIGH resolution is *NOT* cheap. Not yet- hopefully soon.
As for high-res color, all of the above applies. In addition to that,
current color technology requires either continous, but DISTINCT,
bands of color (red,green, and blue) or else a matrix of colored dots
that are accessed through a shadow mask. The physical size limitations
of the shadow mask and the bands of color are such that you CANNOT get
super-high resolution on a small (<17 in) monitor. The highest that
I've seen on a 12" has been 640X240.
I have not mentioned the cost of the RAMs to store 1000X1000 worth of
black and white info (3 times that for 8 colors). Nor have I mentioned
the cost of a CRT controller to run at 36Mhz. Etc, etc.
This has been a "once over lightly" summary of a much more complex
issue. But, a high-res monitor is not cheap to build or easy to
design. Good luck if you attempt it.
Harold Noyes
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(503) 757-2000 ext.4080