[Coco] composite / s-video to hdmi

[Gene Heskett]

On Sunday 16 December 2012 12:46:09 Frank Swygert did opine:

> Date: Sat, 15 Dec 2012 10:21:16 -0500
> From: James Hrubik<jimhrubik at earthlink.net>
> 
> Anyone looking for a converter might be interested in this:
> 
> http://www.kvmswitchtech.com/scs.asp?ProductID=46410
> 
> =============================
> 
> It's a bit expensive on sale at $75, especially at the regular price of
> $120!
> 
> This converts to VGA, but is more cost effective at $32
> 
> http://www.monoprice.com/products/product.asp?c_id=101&cp_id=10114&cs_id
> =1011407&p_id=4722&seq=1&format=2
> 
> Same company has one that converts composite or S-video to HDMI also,
> only $38.28!
> 
> http://www.monoprice.com/products/product.asp?c_id=101&cp_id=10114&cs_id
> =1011406&p_id=7111&seq=1&format=2
> 
> I have to wonder about the picture quality, someone will have to try
> one!

Frank, and other readers here.  The loss of ntsc capable display devices 
from old age continues apace.  It would be nice if modern tv's still had 
the ever present phone jack for accepting a cable from your equally elderly 
VCR.  But I suspect that likewise is going away because its a complex 
circuit, much like these adapters that converts that low res signal into 
something that displays relatively flicker free on a 1920x1250 screen.
That jack WILL go away for cost reasons before the last of us falls over, 
bet the farm on that.

Re s-video or s-vhs as its sometimes called.  This is also a dead ended 
tech.  It helped in the decline of the NTSC days by offering a signal 
transmission method up the cable that was the NTSC sharpness B&W signal,
separate from the narrower banded, much lower sharpness color signal, and 
was really only capable of any improvement over regular NTSC if the color 
signal was kept separate from the camera output ALL the way to the monitor 
input.

If they were mixed together by being combined into a compliant, 
broadcastable signal, then the chroma dot crawl and all the other mixing 
artifacts were in it forever despite the best efforts of some very good 3 
line deep "comb" filters to separate them again.  The top 30% of the 
luminance sharpness was lost forever in such circuitry.

For the best results, driving vga or better monitors, you must stay in the 
RGB domain all the way to the db9 (or better) jack on the monitor.  That 
leaves us with scan doublers to at least bring the 15.75 NTSC time signal 
we have, up to at least 31.5 kilohertz for the vga monitor.

There, the improved sharpness available makes it very obvious that the gime 
chips relatively slow rise & fall times that its totally puny output 
drivers enforce, is the major limiting factor in how sharp it can get.  

That thing was under so much pressure from the power budget it was allowed, 
that we got screwed by a digital chip that was both restricted in the 
voltage swing to .7 volts, and in rise & fall times in the 240 to 180 ns 
territory.  It needed to be faster than 90 ns, but the cmos world was still 
crawling on its belly in those days.  Today, if we started from scratch, 10 
million dollars later we could have an almost drop in replacement gime with 
a 2 watt power budget with .09 ns rise & fall times.  The "almost" is 
because it would need a 1.65 volt power rail, 5 volts would kill it in half 
a microsecond.

The alternative to that would be a FPGA gismo that dropped into the gime 
socket and emulated it with better stuff.  But then you are up against that 
granite cliff face of a 2 watt power budget.  It would need its own, 
probably 25+ watt, 3.3 volt supply, something the transformer in the coco 
couldn't do without letting out all the smoke that makes it work.

Doing such modifications to a coco motherboard are restricted to those of 
us comfortable with a hot soldering iron that probably costs over a $100 
bill sitting on our workbench.  I paid $190 for my last one.

Someday, when I've nothing better to do, I am going to see if a better 
buffer stage, and possibly a little external pullup can improve that.  But 
I don't have another 1987 gime to replace that one if I blow one of those 
ultra puny output stages these have.

But the bottom line is that while s-vhs was a good idea, and since it is 
generated that way in the coco's then mixed in the modulator cage,
the Hawks adapter hooked in ahead of that signal mixer travesty can no 
doubt make a pretty decent pix, but the time is approaching when there will 
not be an s-vhs connector on a monitor.

Cheers, Gene
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