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

[gene heskett]

On Saturday, July 02, 2011 01:21:54 AM John Kent did opine:

> 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.

It is not interlaced John.

Cheers, gene
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