[Coco] New 64-color RGB test chart program (re: RGB2VGA adapter)

Sat Nov 29 23:29:47 EST 2014

Hello,

Today I took the 64-color demo program and rewrote it to have a color layout
which is much nicer for RGB monitors.  I put both this new program and the
original one on a disk image for everyone to use.  It's a useful test program
for calibrating your monitor color settings.  You can download it here:

http://www.fascinationsoftware.com/media/rgb2vga/CC3COLRS.DSK

Once I ran this on my real Coco, I found that my CM-8 was adjusted totally
wrong.  It's quite difficult to adjust the monitor so that you get decent
contrast in both the darkest 2 shades of each channel, and the lightest 2
shades.  My digital camera didn't capture the image exactly as I saw it (the
contrast was a little bit better in the dark colors than what's shown here),
but this is what a CM-8 looks like when adjusted as well as I could get it:

http://www.fascinationsoftware.com/media/rgb2vga/RGBChart_CM8.jpg

There is still room for improvement with the colors on the RGB2VGA adapter.
The contrast problem is worse here, and pretty much no matter how you adjust
the 10k pot, either the darkest 2 shades look the same, or the lightest 2
shades look the same:

http://www.fascinationsoftware.com/media/rgb2vga/RGBChart_RGB2VGA_Dark.jpg
http://www.fascinationsoftware.com/media/rgb2vga/RGBChart_RGB2VGA_Light.jpg

Could you put an 8-entry LUT on each color channel (one for R, one for G, and
one for B) in between the input and output sections on this video adapter?  If
so, we might be able to tweak it to get even better color reproduction than
what is possible with the CM-8.  The output circuitry on the coco must have a
very non-linear voltage response.

I will try your suggestions to see if they clean up the noise.  I can desolder
the unnecessary components.

Richard

On 11/29/2014 09:04 AM, Luis Antoniosi (CoCoDemus) wrote:
> well 24 inches is a pretty long cable. I'm using 12 inches maximum.
> 
> Did you check all the groundings on the board ? check the coco3 pins 1&2
> and also altera's pins GIPO0 12 & 30.
> 
> in my prototype the lm1881 is detachable, so it's never plugged when I use
> the coco3.
> 
> Try assembling another board without the lm1881, 680K, and the 2 caps, or
> remove them for your current board.
> 
> 
> Luis Felipe Antoniosi
> 
> 
> 
> On Sat, Nov 29, 2014 at 11:30 AM, Richard Goedeken <
> Richard at fascinationsoftware.com> wrote:
> 
>> I probably need to change the front porch delay, because the left-most
>> column
>> of characters is cut off in 80-column mode.
>>
>> I think the noise is from a ground loop, but I don't know where.  The Coco
>> is
>> plugged in with a 2-prong plug.  I was powering the DE0-Nano board from a
>> USB
>> port on a laptop which was plugged in with a 3-prong.  But I changed that
>> to
>> use a 2-prong USB power adapter instead.   The noise didn't go away.  The
>> monitor has a power brick which uses a cable with a 3-prong plug, so I
>> guess
>> my next test will be to block the ground connection on the monitor and see
>> if
>> that helps.
>>
>> I also accidentally discovered that the sync resistors weren't right.
>> When I
>> was building up the board, at first I couldn't find the 82-ohm resistors
>> in my
>> pile of parts from digikey.  I thought that I had forgotten to order them,
>> so
>> I looked at the schematic to see what they were being used for.  I saw that
>> they were for the H/VSYNC going to the VGA, so I searched online to find
>> the
>> input impedance of these on a VGA monitor.  I found something which said
>> that
>> most VGA cables have a characteristic impedance of 60 ohms in the lines
>> used
>> to carry the sync, so drivers are supposed to use 60 ohm resistors.  I
>> looked
>> around and found some 68 ohm resistors and used those.  Anyway, the VGA
>> output
>> is perfect, because the test pattern looks super sharp and clean.  The
>> noise
>> is definitely coming in on the input ADC side.
>>
>> I actually bought all the parts before you put the digi-key parts list on
>> your
>> blog.  I was looking at a BOM that you had posted here on the mailing list,
>> and selected all of the parts on the digi-key website.  For some reason I
>> ended up with two 4-position DIP switches instead of one 8-pos, and the
>> particular 4-position DIP switches that I got are too wide to fit next to
>> each
>> other.  That's why I don't have any switches mounted yet.
>>
>> I do have other Cocos that I could test with.  The RGB cable that I'm
>> using is
>> a plain flat ribbon cable, so not shielded.  It's not very long, maybe 20
>> or
>> 24 inches.
>>
>> Richard
>>
>> On 11/28/2014 08:37 PM, Luis Antoniosi (CoCoDemus) wrote:
>>> The customization is about the front porch, the code now is pretty stable
>>> to be generic. But yours seem to work. Steven Hirsch had to change on
>> him.
>>> He didn't report about noise. So I can't tell if is a particularly from
>>> yours or if this board has indeed noise. Until my mules decide to cross
>> the
>>> border I can't help you on this.
>>>
>>> BTW, Steven has found out theat the correct values for the sync resistors
>>> are 47 ohms instead of 82. I doubt this could be the source of problem.
>>>
>>> You don't need the LM1881, the 680K and the 2 caps if you're not planning
>>> to connect any other computers on it.
>>>
>>> The scalines help improve the colors. You may turn them on, use a dip
>>> switch. I have changed the dip switches, the scanline is now the label
>>> shrink.
>>>
>>> You don't have another coco to test right ? Also, how long is your cable
>>> from the coco3 ? Is it shielded ? Do you have anything else that could be
>>> potentially noisy ? Tried connecting the altera's usb cable in a
>> different
>>> power source, like iphone or samsung usb chargers ?
>>>
>>> Luis Felipe Antoniosi
>>>
>>>
>>
>> --
>> Coco mailing list
>> Coco at maltedmedia.com
>> https://pairlist5.pair.net/mailman/listinfo/coco
>>
> 