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Re: IIgs Monitor Repair

ferdimh@gmx.de wrote:

On 10 Jan., 22:43, "Michael J. Mahon" <mjma...@aol.com> wrote:


Right.  It's a shame that digital TVs have become so intolerant
of distorted or non-standard signals.  It's not really that hard
to make the sync algorithm adaptive...

I still remember the original RCA 630TS color receiver--it would
sync solidly to a signal with such low signal-to-noise ratio that
you could barely make it out in the "snow"!  Of course, it had 30
tubes, too.  ;-)

"Analog emulation" would be a nice mode for digital TVs.  ;-)



Not a good idea... They would blur the picture and wouldn't fix their
sync problems.


Sync emulation is part of the emulation--it would work fine if
there were any DSP engineers today who understood how TV sets
used to work.  ;-)


BTW, Grundig used analog sync and color decoders before a digital
double-scan is performed...
This yields a picture quality I've never seen on a double-scan TV.
I don't think an analog sync decoder is very expensive.


It's easy in one chip, but that's an *extra* chip for a digital TV.

The DSP code is pretty straightforward, but it's extra code.  It's just
cascaded RC filters and thresholding.


BTW, why do many color TVs choose the color system based on the
vertical refresh rate?! I have seen that many times with a multisystem
TV on a 50Hz Apple. If it is forced to NTSC it will work. Otherwise it
won't.


Because it's simple and the first idea that comes to mind.  ;-(


Another think is the extreme sensivity of today's color TVs. Why did
they turn it up that far?! The TV I use on the Apple ][ syncs to 10mV
color burst. The problem is, that such a low level is always present,
even in text mode. This will cause VERY annoying color patterns in
text mode, much worse than the little green/violet shadows. I solved
it by detuning the Apples oscillator so that the TV won't lock to it
anymore.


I think there is a spec for the level of burst required to switch to
color, but it would be easily ignored...

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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