Monitor refresh rate query
jnc at mercury.lcs.mit.edu
Fri Jun 17 10:36:14 CDT 2016
So I'm using an old (very old! :-) laptop for a console for some of my
computers. The built-in screen on it is this miserable 800x600 thing (I said
it was _old_ :-), so I'm trying to hook up an old 15" display (of which I have
tons) and run it at 1024x768 at least (the highest resolution the old display
hardware in the laptop will support). However...
The first couple of LCD's I plugged in, they worked fine, but when I went to
change from 800x600 to 1024x768, I got error messages about 'can't handle
that display format' (or words to that effect). Nothing about what parameter
was out of bounds, alas. Finally I found one that _did_ give me the numbers,
and it said the retrace was 35.6 Khz / 44 Hz.
OK, looking at some specs, it makes sense those LCD monitors didn't work - 44
Hz vertical retrace (which I gather is the same as the refresh rate, in things
like 1024x768/60Hz - sorry about the newbiesh questions, but I want to make
sure I'm not making a bad assumption) is _below_ the input specs on them.
(Although why the refresh rate on an _LCD_ would have a lower bound that high
doesn't make sense to me - so the screen updates less often, what's the
problem? It's not like a CRT, where it could cause flicker.)
So I finally found an old CRT monitor that worked - but now I have another
problem! It reports the video as being 35.6 Khz / 87 Hz! Which makes me
worry, because I'm not sure that particular monitor can handle an 87 Hz
(Parenthetically, what exactly is the mechanism that causes damage if you run
an old CRT monitor at too high a refresh rate? I assume the excessive speed
generates too much heat somewhere, and causes transistors to fail, or
something like that?)
Anyway, back to the monitor - I'm wondering if that monitor is reporting a bad
number on the vertical retrace, and in fact it actually is 44 Hz? Because I
found this equation to calulate the vertical retrace frequency from i) the
horizontal retrace frequency, and ii) the number of lines. So I plug 35.6 Khz
(which agrees with the other monitor, note) and 768 into that formula, and
get... 44 Hz!
So is there some bizarre interlace mode, or something, that could
legitimately cause confusion over the vertical retrace? Or is the second
monitor just confused?
I tried to look up the video controller chip (a Cirrus CL-GD 7543 'Viking'),
to see if it could even _do_ 88 Hz, but I was unable to find any
documentation about it online.
Also, if it really is 44 Hz, can anyone point me at an old LCD display that
will handle that? (The CRT I'm currently using - another one that supposedly
handles up to 100 Hz vertical retrace - takes up too much room! And there's
no point getting something new - and larger - since the machine won't do
anything past 1024x768 anyway.)
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