16.024 What kind of RGB monitors will work with a IIc and a Laser 128?
Description
This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
16.024 What kind of RGB monitors will work with a IIc and a Laser 128?
I don't know about the Laser. The IIc doesn't have built-in RGB output. Its
video port provides several low-level timing signals which allow RGB data to be
decoded from the composite video signal (which is also provided on the port),
but this requires external hardware.
The "standard" IIc RGB adapter (assuming there was one) would probably have
produced digital RGB output, the same as the Apple III and the Apple IIe memory
expansion cards with RGB output. With a digital RGB monitor, standard digital
logic levels (TTL) indicate whether a colour (or colour weighting) is present
or absent. One wire is required for each bit of each primary colour.
The IIgs, on the other hand, produces an analog RGB signal - a voltage on the
Red, Green and Blue outputs represents the intensity of each primary colour.
Any number of shades of each colour can be supported, by providing a finer
resolution digital to analog converter within the computer. The IIgs has 4-bit
D-to-A for each primary colour. High-end video cards on the Mac and PC (SVGA)
use 8-bit D-to-A for each primary colour.
Digital RGB monitors cannot be used with an Analog RGB signal (unless
comparators are used to generate a digital signal from the analog one).
Analog RGB monitors cannot normally be used with a Digital RGB signal, but
generating an analog signal is possible with a resistor network (an example of
this is given in the Apple III Owner's Guide). In some cases, it may be
possible to plug an Analog RGB monitor into a Digital RGB output, but it won't
produce the correct colours (when compared with a Digital RGB monitor).
There are two common types of digital RGB monitor: one type will work with the
Apple III, Apple IIe (with RGB card), Apple IIc (with RGB adapter) and CGA on
an IBM PC (different cables or adapters are required). This type has intensity
and one bit each for red, green and blue (16 colours in total).
The second type is usable with EGA. This has two bits each for red, green and
blue (64 colours in total). These monitors also have a higher scan frequency
than the first type, and cannot be used with an Apple II (unless a card has
been specially designed to use them).
Analog RGB monitors are mainly classified by the scan frequency and resolution.
The IIgs RGB monitor (A2M6014X) operates at similar frequencies to television -
around 15 kHz. Macintosh and VGA/SVGA RGB monitors do not support such low
scan rates, and typically work at about 30 kHz or higher. The Mac cannot use
the IIgs RGB monitor, and the IIgs cannot use Mac/VGA RGB monitors.
Some third-party MultiSync monitors will work on the Mac/VGA and IIgs, but
these are very rare now. Most MultiSyncs do not go as low as 15 kHz.
"15 kHz" and "30 kHz" refers to the horizontal scan frequency - Apple II video
output has a horizontal retrace roughly 15,000 times per second.
Vertical retrace is a different issue (it is much slower - usually 50 to 100
retraces per second), and most monitors are very flexible in the supported
vertical retrace rate, as far as I know.
This is also where "interlacing" comes in. Interlacing is a technique which
doubles the effective vertical resolution of the monitor, by performing two
vertical scans (fields) per frame, with a slight vertical shift in the second
field. The scan lines for the second field are interleaved between the scan
lines for the first field.
An interlaced display has more noticeable flicker than a non-interlaced display
with double the frame rate, because the phosphor is only lit half as often.
For example, the Second Sight card will support a 400 line interlaced mode with
the IIgs RGB monitor. There will probably be noticeable flicker in this mode
(especially out of the corner of your eye).
This mode will have 60 fields (i.e. 30 frames) per second, whereas the standard
IIgs video output is non-interlaced with 60 frames per second (but only 200
lines vertical resolution).
(I'm assuming 60 Hz mode - the IIgs also support 50 Hz mode, for use in
countries with 50 Hz mains supplies and TVs.)
Television also uses interlacing - with NTSC, there are 525 lines per
interlaced frame and 30 frames per second, with alternating lines being scanned
on each pass of the electron beam (262.5 lines per field, 60 fields per
second).
PAL uses 625 lines per frame, usually at 25 frames per second (312.5 lines per
field, 50 fields per second).
By: David Empson
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