[Coco] [coco] video signal generation

[Gene Heskett]

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.

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.

Tell me the L in microhenries, and the C1 and C2 values please, including 
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.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
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Copyright 2007 by Maurice Eugene Heskett, all rights reserved.