Re: //gs video and keyboard anomalies

[dempson@actrix.gen.nz (David Empson)]

Jon Bettencourt <processoroverclocked@33mhz.cjb.net> wrote:

> In article <39EC83CA.7386EA42@topmail.de>, Patrick Schaefer
> <patrick.schaefer@topmail.de> wrote:
> 
> > Roy and/or Janet Miller schrieb:
> > 
> > > The 12" RGB has exactly the same screen resolution as the original B&W
> 9" monitors
> > > of the compact Macs. something like 512 x 348 (?) that is certainly a
> bit different
> > > from the IIGS 640 x 200.
> > 
> > The frequencies are more interesting. IIGS has fh=15.625kHz, fv=60Hz;
> 
> Actually, IIgs has fv=15Hz.

No it doesn't.  The IIgs refreshes the screen at either 50 Hz or 60 Hz,
depending on the setting chosen in the Control-Option-Reset menu.
Patrick's numbers are approximately right.

A standard PAL TV signal has 25 frames per second, with two fields per
frame (thus 50 fields per second). The fields are vertically interlaced,
i.e. offset from each other by half a line.  This doubles the vertical
resolution of the image, but the odd and even numbered lines are only
generated on alternating fields.  The phosphor on a TV retains the image
long enough that this doesn't cause noticeable flicker.  There are 625
lines of video in a PAL frame (312.5 per field).  PAL has a horizontal
scan frequency of 15.625 kHz (not sure if this figure is exactly right,
and I don't have a detailed reference handy).

NTSC is exactly the same principle, but it runs at 29.97 frames per
second, has 525 lines per frame (265.5 per field), and the horizontal
scan frequency is 15.734 kHz.

Apple II video output is designed to be compatible with TV, but it is
not a perfect match.  The main difference is that the video signal is
not interlaced: in effect, the Apple II produces an independent frame in
place of each field of the normal TV signal.  This means that the frame
rate is double that of a TV, but the vertical resolution is about half:
192 lines (plus border) for classic Apple II video modes, 200 lines
(plus border) for the IIgs Super Hi-Res mode.

The vertical and horizontal scan frequencies are close enough to a TV
for the image to be displayed correctly.  The Apple II video output is
not good enough for equipment which requires strict adherence to a
standard TV signal.

> > Mac 12" RGB has fh=15.???kHz, fv=60Hz. 

A 13" RGB monitor (640x480) has a horizontal scan frequency in the
ballpark of 30 kHz.

The 12" monitor (512x384) would require at least 25 kHz, assuming it
refreshes at 60 frames per second.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P O Box 27-103, Wellington, New Zealand