[Coco] S-Video mod for Coco1

[Tormod Volden]

On Wed, Oct 9, 2013 at 4:45 PM, Gene Heskett wrote:
> On Wednesday 09 October 2013 10:36:35 Ken H. did opine:
>
>> I tested LM1889 pin 13 on my Dragon with a multimeter and it's 10V :(
>>
>> THANKS Luis!  Your persistent warnings probably saved my
>> monitor/composite adapter.
>>
>> It's strange though because the Spectrum guys are clearly sending LM1889
>> pin 13 directly to the s-vga chroma input.
>> http://www.retrowiki.es/fororw/viewtopic.php?f=667&t=29522
>
> They probably got by with that because the monitor has a coupling capacitor
> in its input circuitry.  The chroma input is only interested in the chroma,
> which at this point is an AC signal with a nominal 3.58 Mhz center
> frequency, and an occupied bandwidth of perhaps 1 Mhz either side of that.

Actually S-video can use a 5V DC offset on the chroma channel to
signal a squeezed 16:9 image is transferred and should be expanded on
16:9 screens. So we can safely assume any chroma decoding circuitry
has a coupling capacitor blocking DC. High DC might destroy the 16:9
detection but I would guess it is pretty robust.

I also tried connecting the LM1889 pin 13 to my S-video chroma input,
both directly and with a 0.1 uF capacitor, with no luck. I also tried
tying the pin with a 1.8kOhm resistor to ground. However I see that in
most examples it is followed by an amplifying stage. These stages are
always inverting but that should not matter for the color signal.
However a impedance matching stage is required to make a 75 Ohm output
resistance. I'll try that later. Not sure how the Spanish Spectrum
guys got away without it (on the diagram with the red marking).

I think the DC voltage you will measure on pin 13 is not very well
defined or important. Principally it is an input pin for a video luma
signal going to the internal RF converter (which is not used on
Dragon) and it is more or less happens to be where the internal color
signal is added. Taking the color signal out from here is not
mentioned in the datasheet and is maybe sort of a hack.

Tormod