[Coco] [!! SPAM] Re: Coco compatible monitors...

[John Kent]

On 2/07/2011 2:45 PM, John Kent wrote:
>
>
> On 2/07/2011 7:34 AM, Joel Ewy wrote:
>>
>> I wouldn't mind having an LCD monitor that syncs to 15KHz.  One 
>> problem I've seen using an ordinary SVGA LCD monitor with CoCo3FPGA 
>> is that since the monitor's "native" resolution is not the same as 
>> what the FPGA board puts out (640x480 I suppose) there are vertical 
>> bands of fuzziness that run through the screen.  Not a big problem, 
>> but it slightly mars an otherwise beautiful picture.  I suppose most 
>> LCD monitors meant for SVGA will suffer from similar issues.
>>
>> JCE
>
> Yes is does sounds like an aliasing problem. 640x480 works out well 
> for a 25MHz pixel clock. The FPGA boards normally have a 50MHz clock 
> which you just divide by 2. 640 horizontal display pixels works out to 
> 800 pixels or clock cycles per horizontal scan line. 25MHz / 800 = 
> 31.25KHz which is pretty typical for PC monitors. 31.25KHz / 60Hz 
> frame rate is about 521 scan lines, of which say 480 are displayable. 
> I use an old CRT display for the FPGA monitor, so aliasation (sp?) is 
> not a problem. If you are using a LCD monitor is say 1280 pixels 
> across then you will get aliasation. 640 display pixels is 80 
> characters x 8 pixels / character which is a multiple of the 40 
> character low res display.
>
> Some FPGA boards such as the XESS XST-4.0/XSA-3S1000 combo, the Altera 
> DE2 / DE2-70 have video digiziters on that than allow you to capture 
> composite video and store it in a frame buffer. You can then read it 
> back and display out the VGA connector at higher speeds. They are a 
> very expensive way of building a scan rate converter though. The 
> Spartan3E starter board from Digilent also has a VDEC-1 video 
> digitizer available for it, but it only has SDRAM on the board which 
> is difficult to interface to.
>
> This is a 15KHz RGB / YUC component video scan rate converter here
>
> http://www.ambery.com/rgbcgatovgac.html
>
> If the CoCo video is composite though, you'd need a TV composite 
> decoder chip in front of it.
> You could probably do it with a large CPLD and a TV decoder chip.
> I think some one on the list was designing a scan rate converter so I 
> guess it's a matter of waiting for that.
> I'm not sure it will overcome the aliasing problem.
>
> John.
>
Is the CoCo interlaced ? If it is then you'll probably need an entire 
frame store the buffer the frame to convert it to progressive scan for 
the monitor. If it's not interlaced, then you might get away with a 2 
scan line buffer.

John.

-- 
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