oracle_jedi wrote:Hey guys,
Can someone explain Oge's comment to me:
You could try amplifying the Chroma and then 0,1uF/1uF bipolar capacitor plus 470/1000 Ohm resistor between VIC and TV in order to protect the VIC itself. And a 75 Ohm (Chroma to GND) resistor as a final touch.
I've followed the practice described by Bear of a 0.1uF capacitor off Pin 2, followed by a 75 Ohm resistor to Pin 5 for Chroma. So far it has worked well. But this comment has me concerned I am stressing the VIC chip.
I've seen several suggestions of larger resistors. 360 Ohm has been suggested as a better option than the 75. But is the suggestion here that an additional resistor be added than connects the chroma line to a local GND on the VIC's motherboard via a 75 Ohm resistor too? I'm not sure what is being advised here.
I'd like to see the above comment drawn as a schematic if anyone has one.
Thanks!
In theory, a device which generates composite video or S-Video signal should have an output impedance of 75 Ohms. In practice, this would result in the amplitude of the chroma signal coming out of your VIC-20 having an excessive amplitude relative to RS-170A specifications.
A 75 Ohms resistor will provide a signal that's adequate if you use a Commodore monitor with separate luma/chroma RCA jacks. This system was a precursor to real S-Video with 4 pin mini DIN jack and had slightly different electrical specs.
If you want to reduce the amplitude of the chroma to acceptable S-Video specs, use a 360 Ohms resistor. This resistor will interact with the 75 Ohms input impedance of the display device and act as a voltage divider.
There are a few TV sets which have a higher input impedance (up to 1k Ohms) and they may require a higher resistor value.
IMHO, a correct way to protect this new output would be to use a pair of diodes the same way as on the IEC port on the C64c.
Be normal.