Need help with my Atari 2600 A/V mod

[tonki]

I'm in the process of modding my 2600 jr to output composite video and stereo audio. So far the audio is great and the video works, but I'm not too happy with the colours when I compare them to the original rf. I figured I'd drop a note here to see if anyone could help me out. I also have to say thanks to the various members here who have posted info about the mods I'm attempting.

 

I'm using a transistor circuit, and have tried both the 4050 mod w/o 4050 chip (http://www.atariage.com/2600/faq/index.htm...=2600#composite), as well as the circuit designed by Zybex (http://www.atariware.cl/pixel/sec/eng/2600av.htm).

 

I changed the 4050 mod by taking the chroma signal directly off the resistor on the 2600 and adding it to the combined luma signal (kinda like what was done in the Zybex's mod), then just passed it into an rca jack for composite.

 

In both mods the colours seem to be off. I'm comparing Adventure on two 2600's hooked up to the same tv simultaneously (one unmodded rf, the other one modded composite). I find the greens to be off by quite a bit (more yellowish), and the blues to be a lot lighter, especially for Zybex's mod. I've tried dropping the 3 luma signals in an exact 1:2:4 ratio but it still didn't do the trick. If I bump the 2.7 kOhm resistor in Zybex's mod to 3.7 the blues/purples get better, but the greens/oranges are still off by a lot imo.

 

I can't remember the guy's name, but I've looked at the Video Mod comparison page (quite useful btw!), and I'm definitely not getting the same colours seen in the colour bar pictures (again, I'm comparing the first few screens of Adventure using both rf and a modded 2600).

 

I know this probably all sounds confusing, but I'm just wondering if anyone here has successfully modded an Atari (jr or otherwise) for composite using either of the above mods, and if you had to change anything to get the colours right. Also, if anyone could explain to me what the 3 luma signals actually are I'd also be quite happy. Finally, is there somewhere where I can obtain a couple of copies of this Color Bar Generator? Thanks in advance!

 

Toni

[+batari]

I did the second mod (by Zybex) on a 4-switch model and I wasn't satisfied with the results (colors were dim and whites were way too bright.) I experimented with changing the resistors, and I found that making R8=250 ohms helped a lot. The colors are now brigher and the whites aren't so bad. I'm now satisfied with the mod, although it is definitely not perfect.

 

BTW, the guy with the video mod page is Nathan Strum: http://www.cheeptech.com/2600mods/2600mods.shtml

Edited June 23, 2005 by batari

[tonki]

If it's just a matter of the colors not being bright enough you can just change R4. This doesn't sound like it's for you though, as the whites were too bright. What game(s) do you use to see whites? I just found out that there's an adjustable color pot on the Atari itself so I'll go back to Zybex's original circuit and play around with the pot first.

[jsoper]

I'd say the 4050 type mode is the one to stick with, it always worked great to me. You can use it without a 4050 or tap off the one that is already on the 2600jr board. Just make sure you don't have the luma lines swapped, some diagrams are wrong. I always put my resistors in the same order as on the motherboard, smallest to largest.

[A.J. Franzman]

I've tried dropping the 3 luma signals in an exact 1:2:4 ratio but it still didn't do the trick.  If I bump the 2.7 kOhm resistor in Zybex's mod to 3.7 the blues/purples get better, but the greens/oranges are still off by a lot imo.

<snip>

if anyone could explain to me what the 3 luma signals actually are I'd also be quite happy.

879121[/snapback]

The luma signals are... luma signals! In other words, brightness. The 3 resistors form a digital-to-analog converter, combining the binary-weighted outputs (1, 2, 4) to produce a single level in the range of 0 to 7. In the absence of a chroma signal, this should produce black, 6 levels of gray, and white. Taking chroma into account, this allows each hue to have 6 levels of brightness (luma 0 and luma 7 should still be black and white respectively, regardless of the chroma value.)

 

A good way to tweak the resistor values might be to use a game screen known to have all 8 black-gray-white levels (Air-Sea Battle/Target Fun in B&W mode? - not sure since I don't presently have it) and adjust for the smoothest transition from black to white. Theoretically the exactly doubling values should be best, but the reality is that the lowest-valued resistor allows some of the other signals to be pulled down when the "1" level pin is low. Adding diodes might help, but would probably attenuate the signals enough to reduce the screen contrast.

 

For tweaking the 2600's color pot, I use Pitfall and adjust for the best contrast between Harry's shirt and the green background between the trees, while keeping the green as pure as possible; not too yellow or too blue.

[tonki]

Thanks to everyone here, I was able to successfully mod my 2600 Jr. this past weekend. I ended up using Zybex's mod and making the 3 luma resistors and exact 1:2:4 ratio, as well as bumping the next (attenuating?) resistor up to 10 kOhm. I did end up using Air-Sea Battle in B&W mode to see all the luma levels, it's pretty useful actually! The luma level separation still wasn't 100% what I'd like it to be, but it was pretty darn close and you wouldn't notice the difference unless you were comparing the two outputs side by side. I'm going to mod my friend's Atari next, and I think I'll try out the 4050 mod w/o the 4050 chip.

 

I had to play around a bit to get the stereo audio to work with the channel 2/3 switch (for stereo/mono), but now that's up and running as well. I remember seeing a post about how to get stereo, I'll go back to that one and add what I had to do. Thanks again!

Edited June 27, 2005 by tonki