[Coco] [coco] video signal generation

[jdaggett at gate.net]

On 4 Jan 2007 at 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.
> 
*****************

My first question is to ask if the RGB signals are of the same amplitude?

It almost appears from your description that the Red and Blue levels are to high. 


> 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.
> 
*************

My belief is that the GIME produces weighted signals for the RGB lines. Though I 
maybe wrong. That would  need to be verified. The CM8 monitor may do that 
after the RGB signals are received. 


> 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".
> 
**************
What is the bandwidth of the filters that make t hem work? If they are around 3.2 
MHz then that might help explain something. 
 
> 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?
> 
***********

The GIME chip is not doing a conversion. It generates both RGB and composite 
signals. 

james