[Coco] Trying to find out what signal a composite adapter is using.

[gene heskett]

On Sunday, December 11, 2011 04:03:42 PM Mark Ormond did opine:

> Ok, here is a link for the schematic. As for the caps used, it's what is
> on the original board.
> 
> http://www.svfdtn.com/cbm/coco2comp.jpg
> 
Humm, that is a bit convoluted, and the only way I can make any sense of it 
is if the R(ed wire?) terminal is being fed with a +5 volt supply rail, at 
which point the video output then is a mix, using the R1 resistor to 
develop a mixed Y & G signal to the base of the 2n3904 which is functioning 
as an emitter follower to drive the video output.  There is quite a bit of 
high peaking going on in the series'd networks, with 2 different turnover 
frequencies in that circuit connecting the emitters of the NTE123AP 
transistors together.

It took some thought, and some calculator time to build this circuit 
originally, and I have an idea that for a black and white composite output 
to a wide band video monitor it probably works rather well, modulating the 
Y signal as a function of the G signal for additional grey scale tones.  If 
the G signal is the color subcarrier output, so much the better for this 
circuit, but then the color separation in the monitor steals 70% of the 
potential sharpness too.

If you have a wide band scope, it would be educational to look at the 
signal on the G(reen wire?) to see if it has color subcarrier on it, which 
would change my analysis slightly.

Interesting circuit.

For the coco's, I have always felt that the svhs style output conversion 
would be a natural, where the color is never mixed into composite, but fed 
to the monitor/tv as a separate signal for lumninance and a separate signal 
for the chroma, thereby removing the sharpness killing color separation 
circuitry from the signal path from the coco to the users eyeballs.

This one, to paraphrase the commercial, so easy even a caveman can do it.  
Just buffer the two signals & send to the 4 pin jack!  Then you can watch 
the sharpest color picture ever from a coco on your new 48" tv!

> 
> -----Original Message-----
> From: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com]
> On Behalf Of gene heskett Sent: Sunday, December 11, 2011 1:08 PM
> To: coco at maltedmedia.com
> Subject: Re: [Coco] Trying to find out what signal a composite adapter
> is using.
> 
> On Sunday, December 11, 2011 01:00:16 PM Mark Ormond did opine:
> > I'm copying the coco2 composite video adapter from John's coco 2 and I
> > was wondering if anyone knew what is located on the left side of c32
> > (from the front) on a 8709416 board?
> > 
> > Here is what I have so far.
> > 
> > [cid:image002.png at 01CCB7FA.DB50E2C0]
> 
> Unforch, linux firefox STILL doesn't know what to do with a cid:
> address, so I can't help there.
> 
> > Parts list.
> > 
> > 
> > 
> > 
> > 
> > C1 1uf 50V
> > 
> > C2 1uf 50V
> 
> These 2 capacitors will have about 2 years shelf life if they are
> electrolytic, 5 volts or less of bias across them allows them to deform
> and lose capacitance rapidly.  At 2 yo age, they will likely be less
> than .1 uf in actual capacity.  Use the most compact mylar types you
> can find, they will last at least till the rapture.
> 
> > C3 30pf
> > 
> > C4 30pf
> > 
> > C5 223Z (22nf)
> > 
> > T1 NTE123AP
> > 
> > T2 2N3904
> > 
> > T3 NTE123AP
> > 
> > R1 2.2K
> > 
> > R2 1K
> > 
> > R3 200K
> > 
> > R4 41 ohm
> > 
> > R5 220 ohm
> > 
> > R6 200K
> > 
> > R7 2.2K
> > 
> > R8 3.3K
> > 
> > 
> > 
> > 
> > 
> > GND = GND
> > 
> > G = Pin 8 of MC1372P
> > 
> > Y = Y on Modulator
> > 
> > S = S on Modulator
> > 
> > R = Left side of C32 (Looking from front)
> > 
> > 
> > 
> > Later,
> > 
> > dabone
> 
> Cheers, Gene


Cheers, Gene
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