[Coco] COCO Video

[Mike Pepe]

The logic of the 6847 is relatively simple and constructing a replacement
for it isn't outside of the realm of possibility. The thing was breadboarded
with discrete logic while being designed, after all.

The main problem with this, engineering wise, is that the 6847 and 6883 are
tied together very closely, and the character rate is tied to the bus speed
of the CoCo. The data is fed into it at 1/4 the colorburst frequency
(.89Mhz) and is clocked out of the device with the CLK input at the
colorburst frequency (3.8MHz) - you can't get any more data out of the
device other than an 8 bit input every E clock period. In other words, you
get 32 bytes into and 256 dots out of it per line at best.

It may be possible to scan-double at the device level to drive the output at
VGA frequency, but we can do that with existing analog scan doublers that
exist today.

It also would be possible to add an enhancement like a 32x192 mode in 256
colors, 64x192 in 16 colors.. something along those lines, which would
adhere to the 32 bytes per scanline max that the 6847/6883 supports.

If connecting a CoCo 1/2 to a modern TV is the goal, I think an easier way
to go would be to feed off the luma and two chroma outputs that the 6847 has
and use them to drive the component inputs of a TV (G, R-Y, B-Y) - one of
these days I'll get around to giving that a try and seeing if it actually
works.

Anyway, interesting to think about what's possible.

> -----Original Message-----
> From: coco-bounces at maltedmedia.com [mailto:coco-
> bounces at maltedmedia.com] On Behalf Of Louis Ciotti
> Sent: Tuesday, June 18, 2013 6:45 AM
> To: CoCoList for Color Computer Enthusiasts
> Subject: [Coco] COCO Video
> 
> All this recent talk about graphics has me thinking that in my view I
would like
> to create some thing to "replace" the VDG IC in the coco so that it
connects
> directly to a VGA monitor.  Not having a COCO 3, this means a replacement
> for the MC6847.  In my mind I envision a daughter card the connects pin
for
> pin where the MC6847 is and the output would be a VGA or HDMI connector.
> The daughter card would mimic all the functionality of the MC6847, no more
> no less, and allow modern TV/monitor to be utilized.
> 
> Sure I realize that it is easy to add functionality, more colors better
> resolution, but then the CoCo is no longer a CoCo IMHO.
> 
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