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PostPosted: Wed Jul 25, 2018 1:19 pm 
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NewRisingSun wrote:
Any idea about the console and TV combination resulting in what I have observed?
Not really... if you can dig up a very long cable (at least 10m) to run from the Famicoms to the TV, that would be informative. At that length, the spacing between reflections should be comparable to a pixel width, making reflections visible as luma artifacts instead of doing something to chrominance.

You could also try using cable with explicitly wrong impedance instead. CVBS is almost always over 75Ω cable, but twisted pair as used in ethernet is 110Ω instead.

NewRisingSun wrote:
Reflections themselves may not cause differential phase distortion. But different reflections can cause differently-spaced comb filtering, which will cause different cancellations of original signal components, in turn causing different changes in amplitude, causing different phase distortion in any powered stage on the part of the TV set that displays the signal.
Sure. Reflections could tickle clipping behavior (and thus phase distortion) at this stage, but...

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I don't know how to measure the impedance of any input or output, but from what I have read, it's not as simple as holding an Ohmmeter to it.
For something like a TV, where we really do expect a properly impedance matched input, we don't usually need to worry about reactances or nonlinear stages. They're almost always a high-input-impedance amplifier in parallel with proper termination, and so measuring the input impedance is just measuring the resistance.

Measuring the output impedance is more complicated, especially since we're anticipating something changing the impedance as a function of voltage. I can't think of a good way to do this without an oscilloscope.


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PostPosted: Fri Jul 27, 2018 5:35 am 
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lidnariq wrote:
You can see the 2C04's DACs in the upper left corner of the die shots here. I'm having a hard time reading it, but I don't see any reason to think there's a nonlinearity in the die.
Non-linearity may not be the right word. What I mean is that many arcade PCBs use resistors for RGB bits that are not perfect multiples of each other. For example, the original Donkey Kong used 1k/470/220 ohm resistors, which yields slightly different bit weights than one would get when merely multiplying values 0-7 with 255/7. It's also not uncommon to use different resistors for blue than for red and green.


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PostPosted: Fri Jul 27, 2018 8:15 am 
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I was mostly referring to the fact that levels 0-1 and 6-7 on a Mega Drive/Genesis VDP are spaced much farther apart than, say, 3-4.


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PostPosted: Fri Jul 27, 2018 11:40 am 
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NewRisingSun wrote:
Non-linearity may not be the right word. What I mean is that many arcade PCBs use resistors for RGB bits that are not perfect multiples of each other. For example, the original Donkey Kong used 1k/470/220 ohm resistors, which yields slightly different bit weights than one would get when merely multiplying values 0-7 with 255/7. It's also not uncommon to use different resistors for blue than for red and green.
Fortunately, non-linearity is the right word here also.

On the 2C04's die, you can see eight total MOSFETs per output, so much like the 2A03's audio DACs, they're as linear as they could manage. (The MOSFET gates are the vertical red lines surrounded by vertical white lines on both sides)

The design is unipolar and appears to be sinking current, so unlike the pull-up nMOSFETs and external the pull-down resistor with the 2A03's audio path there's no obvious source of nonlinearity here.

The PNP-based emitter-follower is probably actually just serving as a straight current amplifier on the RGB PPUs.


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