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 Post subject: Buffered video circuits
PostPosted: Tue Jul 01, 2014 7:38 pm 
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I was reading up on various A/V mods for the Atari 2600, and I came across something interesting.

The TIA has 3 pins for luma and 1 pin for sync, and you're supposed to feed these into a resistor ladder to create an analog signal. The console already does this, and a really simple A/V mod takes the luma from right before it goes into the RF modulator and uses that. A more complicated mod grabs the signals straight from the TIA and puts them through its own resistor ladder. However, if you feed them into a buffer first (the schematic said CD4050, I guess a 74904 would be the same thing?) before the resistor ladder, the video signal is much sharper/cleaner visually.

Why? Is the TIA's output noisy or slow or something that the buffer filters it into a sharper 0/1 binary signal? The signals are open collector, so I dunno if that makes a difference.

I'm curious because I think up circuits all the time, and one of the things I was working out is how to drive a plain arcade RGB monitor using a DAC, but now I don't know if it's necessary to buffer the digital part of the signal first, before sending it to the DAC. Were buffered video circuits common in arcade games too?


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PostPosted: Tue Jul 01, 2014 8:01 pm 
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Joined: Sun Apr 13, 2008 11:12 am
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Drag wrote:
resistor ladder
I'd usually call it a DAC ;) Or at least the luma portion.
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Why? Is the TIA's output noisy or slow or something that the buffer filters it into a sharper 0/1 binary signal? The signals are open collector, so I dunno if that makes a difference.
Yup, open collector outputs are lousy for this reason; all noise from any source anywhere will couple directly into the logic 1 bits, so the only clean luma will be black. (In practice, that mostly means φ2 and audio)
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I don't know if it's necessary to buffer the digital part of the signal first, before sending it to the DAC.
Well, what's generating the digital signals in the first place? E.g. lft's wild demos (Craft, Phasor, and Bit Banger) just use the atmega's internal output drivers (although Phasor buffers the output using BJTs to hack chroma)


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PostPosted: Wed Jul 02, 2014 12:06 pm 
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Thanks for the response, I'd suspected that it might've been the open collector that was the issue. There's still a lot that I don't know about electronics, despite taking two courses of it (once in highschool and once in college), so I dunno what the best way to generate a binary signal is. I guess transistors are the way to go? Maybe I ought to look at some schematics to see what other VDCs do.

I guess I should mention that one of my goals in life is to design and build a game system from scratch. :P When I talk about thinking up circuits and the like, it's because of that interest, so I was planning on building the VDC myself. :P


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PostPosted: Wed Jul 02, 2014 12:40 pm 
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There's a bunch of random "use an atmega as a video game platform" thing I've seen people do. They almost all just use the microcontroller's internal digital drivers to directly drive the DAC.

There's an annoying thing about resistors that you can't get better than 4-6 bits of precision in your DAC using ordinary resistors. (5% resistors = 1/20th; they not only are random but additionally will change resistance over time and as a function of temperature, not all the same way; 1% resistors get you safely to 6 bits. For more bit depth you should probably use something like one of Analog Devices' video DACs).

In college, for microcontroller project class, I built a simple video game console that did was also a video overlay system. Low resolution due to memory constraints (96x64), memory mapped, each byte corresponded to independent transformations on input luma and chroma. Kinda fun, but the low resolution made it look bad even in comparison to a 2600.


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PostPosted: Wed Jul 02, 2014 2:37 pm 
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I actually imagine something like the fairchild channel F when you mention that. :P

Of course, if I wanted really precise color control, I'd indeed use an actual DAC IC. I was actually wondering why the 2600 A/V mods didn't use one, but it seems to not be necessary since it's just 3 bits plus the sync bit.

I guess I could get an off-the-shelf RGB->YIQ modulator if I wanted to stick with RGB, but I was also thinking of doing it the NES/2600 way.


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