Atari 2600

[kripp]

Spent a couple of hours this weekend adding composite video output to my Atari 2600. Dead simple modifications with fairly good results, I've never been a huge fan of the Atari's but I could get use to this. The image looks like ass on my Commodore 1702 monitor, the colors are all wrong. But on the Commodore 1084S it looks pretty good, much better than the 1702.



Now no 20' RF cable to deal with!

Thanks.

[kripp]

No love for my 2600 modifications?

I had hand drilled the holes for those RCA plugs, took me 30 minutes to find the drill bits and didn't feel like spending another 30 minutes looking for the drill.

Enjoy.

[Endymion]

Nice clean work. I've not looked up the mod for this in a while though--isn't it just as simple a matter to make an s-video (y/c) mod for the 2600? Maybe it is the 2600 jr system I am thinking of. Got any screenshot comparisons?

[kripp]

I just took a couple of signals (5 or so, can't remember) off the Stella chip and wired them to one RCA jack, no resistors or potentiometers on any of the signal lines (despite what the tutorial had said). It is a very dirty modification, but it does work.

I was not worried about image quality at all, the stock RF signal was the worst I've ever seen from a console. I couldn't find the piece needed to hook up the Atari RF line to my TV, so no comparison pictures.

I have one more Atari 2600 sitting in a box, I was planning this modification for that one as well. What I would like to do with that one is take the time and add the resistor(s) and pot's to the signal lines, and add a third RCA jack for RF output.

Thanks.

[viletim!]

That's the untra-crude way to do it...and it doesn't work very well, unfortunately. Maybe it's ok if you don't care to much about video quality though.

I think the atari 2600 is probably the trickiest 8 bit console to get composite video out of partly because there are so many different varieties/revisions out there (so you'll find no good, easy to follow instructions) and partly for technical reasons. Last time I checked there were loads of really awful step-by-step guides that yeilded horrible resaults even if followed through precisely. It seems people don't care if the colours are wrong or if there are only half the luminance shades that there should be, they just want the RF noise gone. That was a while ago...4/5 years maybe...

So, just in case anyone wants to know, is what I came up with. From memory...
The way the atari generates video is the TIA (a/v generator chip) generates a chrominance signal, 4 (they vary with pal/ntsc version) digital luminance signals, a composite sync signal, and an audio signal. The 4 luminance outputs are open collector digital outputs (they cannot output any voltage, only suck it in). Each has a pull up resistor to Vcc and is buffered by a 4050 (with the exception of the balck model, I think it's kripp's one, which is why that model has such piss poor RF video). At the outputs of the 4050 is where the analog to digital conversion takes place with some kind of resistor ladder. Right at this point the chrominance is added and the audio is mixed in too with a mixing oscillator thing. Then this luma/chroms/audio/er....sync (I forget how it gets in) signal passes to the modulator box.

At no point in the chain is there a actual 'composite video' signal because the audio is mixed in at the same spot the chroma is added so there's more to it than simply tapping a signal and amplifing it. Though not too much more. Basicaly what needs to be done is...
> disable the audio mixer (pull out the transistor)
> disconnect the modulator (it loads the video signal)
> take the video signal from the 'magic spot' (right after the luma ADC (resistor ladder)) and put it through a simple emitter folower amplifier

The resault, while not standard 75 ohm video (due to the use of a simple emitter folower amp), is pretty good, and will work consistantly well on all monitors.

The other method is to attack the TIA directly, take it's digital inputs, make them analog, buffer them, etc. It gets pretty complicated and the end resault is basicaly a duplicate of what's already on the board anyway.

There are circuits for both these methods in the atari faq (just looked it up now) and there are circuit diagrams for the atari floating around too (probably necessary to make any sense from what i've just written). I can expand on the above...i've got some notes somewhere too...I just don't what to type if no-one's listening.