Re: Analog NTSC to RGB converter?

[Linards Ticmanis <ticmanis@gmx.de>]

On 10/30/2012 01:27 AM, Alex Freed wrote:

> I only want to add that some 30 years ago I had an Apple connected to a 
> TV and discovered that the color saturation was way too high. Not a big 
> surprise: consider a 01010101... pattern. It is a 3.58 MHz square wave 
> rail to rail. The first harmonic of that is about 63% of that - a lot 
> more than the normal chroma signal. So I added a shallow notch filter - 
> a low Q LC tank in series with the signal and it actually looked better. 
> That's an LC tuned to 3.58 and shunted with a resistor. I definitely 
> don't remember the values after all these years :) If the colors are 
> oversaturated you may want to give it a try.

Thanks for the hint, but the 1084 has a fairly standards-conforming
saturation when the knob is in the middle position, which means it's
lower than on most TV sets (which tend to be over-saturated from the
factory, since - as I've read somewhere - this supposedly boosts sales
in the showroom). So even with the Apple II the colors are not all that
garish, and with a C64 connected they're even somewhat dull.

One more chapter to the saga: I didn't notice until a bit later that
when the multi-standard decoder chip can't detect any color encoding at
all - such as when an Apple II in text mode is connected - it will
continuously cycle through its four color systems while trying to find a
subcarrier. That means that the NTSC output line gets activated for
short time about twice per second, resulting in an ugly "pulsing" effect
of the external LC filter, with short drops in sharpness. I solved it
with a large capacitor and two resistors, now the NTSC setting of the
filter doesn't kick in until the NTSC output pin is stable at high level
for about one seconds.

Also I had a rather unpleasant fight with "Hanover Bars" in orange or
greenish-blue screen areas in PAL modem (around the U = -V points).
Correctly adjusting the PAL delay line is quite tricky in this monitor
as the setting doesn't seem to be entirely stable. I got it into a state
where at least at standard saturation the bars are no longer visible.

-- 
Linards Ticmanis