Re: Analog NTSC to RGB converter?

[Michael J. Mahon <mjmahon@aol.com>]

Kevin <kevin@hackaday.com> wrote:
> 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 =)

Well, the "signal" we're talking about is the unneeded chroma channel being
shunted to ground, and a low impedance path is important. 

A small relay should work well in such a case, while providing relatively
little stray capacitance when it's open. 

If the "giant inductor" you refer to is the relay coil, no worries--the
coil is driven by DC and is well isolated from the contacts. 

-michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon