[Coco] [coco] video signal generation

[Gene Heskett]

On Thursday 04 January 2007 22:15, Gene Heskett wrote:
>On Thursday 04 January 2007 21:25, RJRTTY at aol.com wrote:
>>People
>>
>>I have  been struggling with a problem with my converter for a while
>>and I need some  information.
>>Does anybody know the details of how the GIME  generates a
>>RGB video signal?   The problem I am having is that  while 80 col
>>text on the green channel is clear and sharp, 80 col text on the  red
>> shows some video degradation and 80 col text on the blue channel is
>> completely illegible.
>>
>>What I am thinking is that the GIME generates the  green channel first,
>> the red channel second and the blue channel  last.   Is this correct?
>> Another strange thing is that  80 col text in any color combination
>> that has even a little green in it shows  up well but if you look at
>> 80 col. text using pure red or blue as the  foreground or backround
>> you get a wierd kind of video smearing.
>>
>>I  already have a solution.  If a low pass filter is placed in the
>> input channels
>>the 80 col text shows up well in all color combinations.    In other
>> words I had
>>to DECREASE the bandwidth of the converter in order to  get acceptable
>>results.   It seems that the quality of the video  signal coming out of
>> the GIME in the coco3 is only slightly better than the  old monitors
>> could display and the high bandwidth of my converter is just too  much
>> for the signal showing it "warts and all".
>>
>>I don't know about the  rest of you but purposely degrading the
>> converter to approximate the  perfomance of the old monitors just
>> seems wrong to me.  The image does  soften a tiny bit with the filters
>> but not enough to notice if you were not  already looking for it or
>> directly comparing it to an  unfiltered converter.
>>
>>Does anybody know what is going on with the  conversion process in the
>>GIME to cause this?
>>
>>TIA
>>Roy
>
>I have also looked at this with my own 100mhz dual trace scope, Roy, and
>found the rise times out of the gime to be rather slow considering what
>its supposed to be doing, and this is directly at the gime pins, not
>after the coco buffers those signals.  I also didn't see any great
> amount of HF noise either there or at the input connectors of your
> board, so I came to the conclusion the gime wasn't that fast even with
> no loading by your board.

But, I just had another ugly thought.  The gime is a cmos chip, and as 
such needs a seriously low impedance bypass capacitor right at its supply 
pins to give its best performance vis a vis rise & fall times.  If that 
capacitor was say a 1 uf or thereabouts electrolytic, there is no way in 
all creation that this capacitor is still good after what, 19 to 20 
years?  The next time I have the top off mine, I WILL check that.  
Anything more than a few millivolts of noise there while its running 
should condemn that capacitor to the out bin.  I'd be tempted to replace 
it with a big low voltage paper or mylar of half a mic or so, which 
should last at least till the rapture.  Electrolytics in that position 
(or any similar such place in a circuit) are a ticking time bomb.

>I suspect when you apply the low pass filter, you are actually creating
> a pi network that while its top end isn't what you'd think, is also
> creating a peaking network that is giving a rise in the HF response in
> the area that counts.
>
>Assuming you have:
>
>input from gime->c1 & and inductor -> c2 and output to your circuit, and
>that the opposite side of both c1 and c2 are connected to ground.

Correction:
Tell me the L in microhenries, and the C1 and C2 values please,
including estimated circuit strays for C2 in particular.  Somewhere I have 
a formula to calculate such a peaking network that I can probably invert 
to see what effect you are getting.

But I like the bad (open) cap idea, really I do.

>--
>Cheers, Gene
>"There are four boxes to be used in defense of liberty:
> soap, ballot, jury, and ammo. Please use in that order."
>-Ed Howdershelt (Author)
>Yahoo.com and AOL/TW attorneys please note, additions to the above
>message by Gene Heskett are:
>Copyright 2007 by Maurice Eugene Heskett, all rights reserved.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Yahoo.com and AOL/TW attorneys please note, additions to the above
message by Gene Heskett are:
Copyright 2007 by Maurice Eugene Heskett, all rights reserved.