Re: Laser 128 vs CGA

[david@uow.edu.au (David E A Wilson)]

Vincent Hancock <vhancock@onyx-206.southwind.net> writes:
>On 10 Apr 1997, vax2.concordia.ca wrote:
>>     CGA and EGA monitors were 15.75 kHz, same as those composite and RGB
>> screens sold for the Apple II. It's VGA and SVGA displays that started
>> using double (or more) the h-scanning frequency (also why you don't hear
>> that high-pitched squeal when they're turned on with no video source ;)

>  According to the PC Hardware FAQ, CGA uses both a different horiz
>frequency (18.43 kHz) and different voltages:

Then the PC hardware FAQ is wrong.

MDA/Hercules Mono is 18.?? kHz horizontal, 50 Hz vertical
CGA               is 15.7? kHz           , 60 Hz

>_comp.sys.ibm.pc.hardware.video FAQ, Part 1/4_
>	<http://www.faqs.org/faqs/pc-hardware-faq/video/part1/>

>Q) Can I use my CGA/EGA/VGA monitor as a TV?

>CGA and EGA monitors are digital, rather than analog like televisions
>and more modern monitors, usually making them incompatible with TV.
>Television signals contain all colour information along with syncs on
>one conductor.  In addition, there are two types of television signals
>- the RF that comes in from cable or an antenna, and composite.  The
>line-in/out on a VCR is a composite signal, and doesn't contain all of
>the different channel information that an RF cable signal does.

So far so good.

>The original CGA monitors accept a composite signal, but it is TTL,
>which uses a different voltage from composite.

Wrong - CGA monitors use a 9 pin D connector at the computer end with
discrete Red, Green, Blue, Intensity, Horizontal and Vertical wires.
The intensity signal just gives two levels of brightness to each of the
8 colors a digital RGB output can generate - hence the term RGBI.
MDA, CGA and EGA signals are all TTL (0V - 5V).

Composite video uses one wire (and ground) and modulates the color and
brightness and sync information onto that single wire. The Sync is detected
by level (0V to 0.3V is sync, 0.3V to 1.0V is picture [black to white]).
Color is done with reference to a 3.58MHz burst at the start of each scan line.

>Some CGA (and
>perhaps EGA?) monitors have composite-in jacks and circuitry inside
>them to display a composite signal.  If you have one of these, then
>you can feed it a composite video signal from a VCR, laser disc
>player or other composite video source.

>_comp.sys.ibm.pc.hardware.video FAQ, Part 3/4_
>	<http://www.faqs.org/faqs/pc-hardware-faq/video/part3/>

>Color Graphics Adapter (CGA)

>        The CGA, released by IBM, supports 4 colours in graphics mode
>and 8 in text mode at a pixel addressability of 640x200.  The CGA
>provides a vertical refresh of 60 Hz with a horizontal refresh of
>18.43 kHz.

The horizontal rate should read 15.75 kHz. I have a PC with a CGA card
and have plugged my Apple //e monitor into it and it worked fine.
--
David Wilson	Dept CompSci Uni Wollongong Australia	david@cs.uow.edu.au