Re: Analog NTSC to RGB converter?

[Kevin <kevin@hackaday.com>]

On Friday, October 26, 2012 12:04:46 AM UTC-5, Michael J. Mahon wrote:
> Linards Ticmanis <ticmanis@gmx.de> wrote:
> 
> > On 10/22/2012 05:18 PM, Linards Ticmanis wrote:
> 
> > 
> 
> >> I'll report on my results as soon as I have some - in case anybody is
> 
> >> interested.
> 
> > 
> 
> > OK, the main part is done. It even does auto-detection of PAL or NTSC
> 
> > mode, which I didn't know was even possible until Alex Freed rightly
> 
> > suggested that I read the data sheet more attentively. All in all it's
> 
> > working really well.
> 
> > 
> 
> > There are two remaining issues: First, the filter that removes the
> 
> > chroma info from that signal which goes down the luma path still has to
> 
> > be re-tuned by hand for either PAL or NTSC. Not too convenient as it is
> 
> > deep inside this beast. As Alex has suggested, this needs to be switched
> 
> > between two differently tuned filters just like the two quartzes are
> 
> > switched; but currently I'm having a hard time finding out what exactly
> 
> > to get for the second filter. According to the schematic the filter is a
> 
> > tunable inductor and a capacitor in series (inside a grounded metal can,
> 
> > as a single component), but no pF or µH values are given and I don't
> 
> > understand the inscription on the component itself either.
> 
> > 
> 
> > Second, no matter how you tune the filter, the Apple II signal seems to
> 
> > be a bit too "violent" for it. With the beam moving from white or black
> 
> > into a colored area, it takes about 2-3 chroma cycles until the
> 
> > filtering really gets going. No such problem when watching DVB TV, a DVD
> 
> > or VHS through it, so I guess the fact that an Apple color signal is
> 
> > more or less "pure" color carrier, highly saturated and instant-onset,
> 
> > defeats the circuit until it's had time to adjust. I guess it's a trade
> 
> > off with the fact that the monitor has a much higher bandwidth than a
> 
> > normal video monitor - 80 char text is quite legible on it.
> 
> > 
> 
> > I'll stop replying to myself now.
> 
> 
> 
> The chroma trap is, as you've observed, a series-tuned circuit. If there is
> 
> a PAL/NTSC level generated by the decoder IC, you can use it to control a
> 
> relay that would switch in an (adjustable) additional capacitance in
> 
> parallel with the trap capacitor. Hopefully, the signal goes to the
> 
> inductor and the capacitor goes to ground, but it will work either way. 
> 
> 
> 
> Align it by tuning for PAL chroma rejection with the relay contact open,
> 
> then adjust the additional trimmer capacitor for NTSC chroma rejection with
> 
> the contact closed. 
> 
> 
> 
> Alternatively, if you don't mind seeing the pixel structure of colors (like
> 
> most RGB monitors), just remove the chroma trap entirely.  That will
> 
> improve the bandwidth even more. (This option won't work if the chroma
> 
> signal is taken from a second winding on the chroma trap.)
> 
> 
> 
> The only time that it can get annoying seeing the chroma signal in the
> 
> luminance is in fairly large areas of relatively saturated colors (like an
> 
> Apple II display ;-)--but RGB monitor users seem to get used to it. 
> 
> 
> 
> -michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon

I cant wait to see what happens to a fairly high speed analog signal as it passes millimeters away from a giant inductor though creappy contacts...

really? a relay for a no current 1 volt signal?

gets popcorn =)