[Coco] CoCo RGB video timing and levels

[jdaggett at gate.net]

James

In 80 character text mode the CM8 is displaying  480 pixels. Each 
horizontal line is 114 characters times 6 pixels or 684 pixels. Take 684 
times 15701 and you get a pixel clock of 10.739484 MHz. 

In 640 pixel graphics mode I would figure that the pixel clock is closer to 
12.56 MHz. That is if the GIME is actually doing 800 pixels total for ech 
line. In that case then the access time would be 80nS for the memory . 
150nS fast page mode dram will handle that speed barely. Based on the 
CM8 specs, at 59.7 Hz field frequency the nominal number of lines per 
field will be 263. Maximum would be 276 lines and the lowest would be 
251. That is taking the min and max field frequency and dividing it into 
the allowable tolererance of the line frequency. 

Field frequency and the nu mber of lines in a field will determine the line 
frequency. In standard defined VGA with a field frequency of 60 Hz and  
525 lines per field requires 31.5 KHz line frequency. Each line requires 
800 pixels total for display, sync and border. 800 times 31.5 KHZ results 
in a 25.2 MHz pixel clock.

Basic equations:

1) Pixel clock is equal to the line frequency times the # of pixels per line

2) Line frequency is the field frequency times the # of lines per field. 

3) Pixel clock establishes how fast the video ram needs to be.

4) Field frequency can be whatever the monitor can handle. This ranges 
from about 30 Hz to 125 Hz.

The RGB levels out of the Coco 3 are 0.8 to 2.0 VDC and for the syncs, 
they are TTL level. My guess is that they are four voltages of  1.85VDC, 
1.55VDC, 1.25VDC and 0.95VDC. If you are  interested, by altering the 
three resistors in the base of the drive transistors one could make them 
also TTL levels. 

james


On 23 Aug 2004 at 11:37, James Dessart wrote:

Date sent:      	Mon, 23 Aug 2004 11:37:36 -0400 (EDT)
From:           	James Dessart <james at skwirl.ca>
To:             	coco at maltedmedia.com
Subject:        	[Coco] CoCo RGB video timing and levels
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> I'm trying to do some calculations to figure out how to interface with
> the CoCo's RGB port. Now, on an NTSC CoCo, we have a vertical retrace
> of 59.95 Hz, and a horizontal sync of 15kHz. Based on this, and the
> standard size of an NTSC frame, it seems that the pixel clock would be
> 10MHz, am I right in that calculation? or around 11 MHz for a 720
> width NTSC frame.
> 
> Also, what are the levels for the RGB lines? I know there are only 4,
> since the CoCo can only output two bits per component.
> 
> James
> 
> 
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