[Coco] Q: CoCo2 border

[jdaggett at gate.net]

The MC6847 chip is NTSC only and is non interlace format.

The border is a color and is actaully not part of the vertical blanking or 
retrace.

Flicker rate is the rate at which the human eye and brain can 
consciously distinguish the time between frames or about 20 frames 
per second. Movement is distinguished at 15 frames per second. 
Movie projectors are 24 frames per second and are just barely able to 
detect flicker. Interlaced NTSC is at 30 frames per second. While at 
the this frame rate some flicker can be detected in very bright scenes. 
This rate is good for television but not so good for monitors and 
computer use. The faster the frame rate the less strain on the eyes. 
Modern video chips will run frame rates from 50 to 85 or more.

Horizontal sweep frequency is dependant on the Frame rate and the 
number of lines per frame. Pixel clock is dependant on line frequency 
and the # of dots between the horizontal blanking pulses. For 1024 x 
768 monitor at 75 Hz noninterlace refresh rate you need a horizontal 
line frequnecy of 57.6 KHz. and with 1024 pixels per line a pixel clock 
around 59MHz.

NTSC interlaced as well as other interlaced system require long 
persistance phosphers. This is the so that it will still be at near full glow 
until the electron beam can get back around to reenergize the 
phospher group. The size of the phospher dot alone is not an measure 
of whether it will work properly in an interlace system. 

james


On 11 Jan 2004 at 12:36, Alex wrote:

From:           	Theodore (Alex) Evans 
<alxevans at concentric.net>
Subject:        	Re: [Coco] Q: CoCo2 border
Date sent:      	Sun, 11 Jan 2004 12:36:03 -1000
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<coco at maltedmedia.com>
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> On Jan 11, 2004, at 12:03 PM, jdaggett at gate.net wrote:
> 
> > Alex
> >
> > The MC6847 chip is noninterlace.The chip is doing 262 vertical
> > lines. One field per frame. Therefore the frame rate is ~ 60 frames
> > per second. Well above the flicker rate. Movie projectors are 24
> > frames per second.
> >
> > Per the 6847 spec
> >
> > 13 vertical blanking
> > 26 top border
> > 192 visable
> > 25 bottom border
> > 6 vertical retrace.
> 
> So there are 39-45 lines of border at the top and 25-31 lines at the
> bottom.  And PAL is 25/50 Hz as is SECAM.  The flicker rate is
> dependent on a number of environmental conditions.  They tend to be
> exacerbated in interlaced video on computers (i.e. Amiga NTSC/PAL lace
> modes or interlaces 1024x768 VGA) because there is a great deal of
> contrast between adjacent lines and computer displays tend to use
> fairly fast phosphors.
> 
> 
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