[Coco] Coco 3 pixel clock

Robert Gault robert.gault at worldnet.att.net
Mon Oct 8 23:01:04 EDT 2007

Christopher Hawks wrote:
>     Does anyone know the pixel clock rate for the RGB output?
>     and if it can be programmatically changed??

Assuming I understand the question, and that might not be true, the 
pixel clock rate ought to be approximately the horizontal sync rate 
times the number of pixels per line. The answer would be need to be 
corrected by the width of the horizontal pulse.

Since the RGB and Composite signals are locked, the horizontal rate must 
be about 15.7KHz NTSC. Ignoring the pulse width, the pixel clock at a 
640 width should be about 10MHz.

So, the pixel clock rate can be changed by changing the number of pixels 
per horizontal line. I'd bet that's not what you meant however. I don't 
think it is possible, keeping the number of pixels per line constant, to 
change the size of the pixels via software. In that sense, there is no 
method for changing the pixel clock short of changing the master crystal.

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