A few quick questions... (Atari modding related)

[Dragnerok X]

As mentioned in an earlier thread, I'm working on modding my atari 2600 for composite a/v, and right now, am in the middle of it. I've gotten some of it to work just by sheer experimentation, but right now I have a few questions so here they are...

 

1. What is the best way (cleanest, least interference from rf, etc.) to get an audio signal off the board?

 

2. In theory at least, couldn't it just be possible to pick off the video signal before it combines with the audio and goes into the rf modulator, routing such signal into the center tap of an rca plug (for video)? If not, then why not?

 

BTW: The system is a 4-switch woodgrain

Edited January 15, 2007 by Dragnerok X

[Dragnerok X]

*bump*

[djmips]

If you are doing a composite mod then you ARE picking off the vide signal before it combines with the audio. The audio is combined IN the RF modulator. Composite means a composite of LUMA and CHROMA in one signal (but no audio). This should be obvious when you think about it. The composite input on your TV (yellow) is separate from the red/white audio inputs.

 

I haven't done this mod.

 

http://www94.pair.com/jsoper/2600_stereo.html

[Dragnerok X]

If you are doing a composite mod then you ARE picking off the vide signal before it combines with the audio. The audio is combined IN the RF modulator. Composite means a composite of LUMA and CHROMA in one signal (but no audio). This should be obvious when you think about it. The composite input on your TV (yellow) is separate from the red/white audio inputs.

 

Wow, actually, that was horrendously obvious. I guess what I am trying to say is that is it possible to take the video signal before it goes into the modulator and directly pick it off a single point (that point being just before the modulator) instead of taking the luma, sync, etc. and combining. This, to me, seems feasible because, unlike other similar mods (Chris Cracknell's, for example) it uses the "already there" video circuitry to display the image (versus taking something very similar and building it on proto-board).

 

I haven't done this mod.

 

http://www94.pair.com/jsoper/2600_stereo.html

 

I think I've seen that before. It looks good, but what I'm looking for is more of a monaural audio mod, though I can see how dodging the Atari's circuitry altogether (by bending up the TIA's pins, which I DEFINETLY wouldn't do) could literally "cleanse" the signal.

[djmips]

I understand what you are saying. It seems logical that inside the modulator, a composite signal must be created before it is modulated at radio frequencies for output to an old style TV.

 

Recreating the circuitry that creates the composite signal is how a normal composite mod works. It takes the sync and the chroma and luma and combines them.

 

I had assumed that the Ben Heckendorn mod was tapping the composite directly from the 2600 circuit board. That's why it is simpler.

 

I had also assumed that the mods that recreate the composite signal themselves directly from the digital outputs of the TIA would have a cleaner signal.

[djmips]

I think I've seen that before. It looks good, but what I'm looking for is more of a monaural audio mod, though I can see how dodging the Atari's circuitry altogether (by bending up the TIA's pins, which I DEFINETLY wouldn't do) could literally "cleanse" the signal.

 

Just wanted to point out that the stereo audio mod has a MONO pushbutton. Otherwise, see Ben Heckendorn's or other mod online to find out how to tap the mono audio from the 2600 circuit.

[Dragnerok X]

I understand what you are saying. It seems logical that inside the modulator, a composite signal must be created before it is modulated at radio frequencies for output to an old style TV.

 

Recreating the circuitry that creates the composite signal is how a normal composite mod works. It takes the sync and the chroma and luma and combines them.

 

I had assumed that the Ben Heckendorn mod was tapping the composite directly from the 2600 circuit board. That's why it is simpler.

 

I had also assumed that the mods that recreate the composite signal themselves directly from the digital outputs of the TIA would have a cleaner signal.

 

Yeah, exactly. My theory on the heckendorm mod (according to this source, it may be 7800 related, but it uses the same concepts) is that all ben's mod is functioning as is a simple, adjustable, digital to analog converter. That "converter" appears to be present already in the system. Take a look at this. If you look over towards the middle, left-hand side you'll notice the chroma and luma signals. Notice how each luma signal at the end, has a resistor, going up from 27 to 56 to 110 (roughly double of each) in value? That, as it appears to me, is mimicking the function of such a converter, which would mean that ben's mod is doing something which is already done! Also note, with a little further trace following, you will see that the chroma signal is combined in, before it reaches the modulator. That is where I presume the composite video signal can be obtained (however, the components that follow could be "tweaking" the signal further). I'm not 100% sure of this, but it's just an idea.

Edited January 15, 2007 by Dragnerok X

[djmips]

I guess what I am trying to say is that is it possible to take the video signal before it goes into the modulator and directly pick it off a single point (that point being just before the modulator) instead of taking the luma, sync, etc. and combining.

 

I just took a brief glance at all of the mods at http://www.cheeptech.com/2600mods/2600mods.shtml and it looks like all of them are different variations on creating a composite signal from the either the chroma and luma present or by regenerating from the digital outputs.

 

If you refer to http://www.atariage.com/2600/archives/sche...c_2600_Low.html you will see an M in a Pentagon. This point has the composite video PLUS audio, and is the input for the RF modulator. It does seem feasible to cut out the audio, and massage the composite video directly from this point. I don't know the characteristics of this signal. Proper (modern) NTSC composite video is noted here in this TEKTRONICS White Paper. as 1.0 Volts peak to peak with 714 mV peak video, zero volts blanking, 0.0536 volts black and sync tip at -286 mV.

 

If you could measure the output at M, perhaps one could determine if it was suitable for direct use as a composite signal. I'm guessing that it isn't somehow or why else would it not have been used before now?

[Dragnerok X]

I guess what I am trying to say is that is it possible to take the video signal before it goes into the modulator and directly pick it off a single point (that point being just before the modulator) instead of taking the luma, sync, etc. and combining.

 

I just took a brief glance at all of the mods at http://www.cheeptech.com/2600mods/2600mods.shtml and it looks like all of them are different variations on creating a composite signal from the either the chroma and luma present or by regenerating from the digital outputs.

 

If you refer to http://www.atariage.com/2600/archives/sche...c_2600_Low.html you will see an M in a Pentagon. This point has the composite video PLUS audio, and is the input for the RF modulator. It does seem feasible to cut out the audio, and massage the composite video directly from this point. I don't know the characteristics of this signal. Proper (modern) NTSC composite video is noted here in this TEKTRONICS White Paper. as 1.0 Volts peak to peak with 714 mV peak video, zero volts blanking, 0.0536 volts black and sync tip at -286 mV.

 

If you could measure the output at M, perhaps one could determine if it was suitable for direct use as a composite signal. I'm guessing that it isn't somehow or why else would it not have been used before now?

 

Yes, I agree. My idea seems, so simple and yet... I'll test it tonight to see what I can get. Anyone else, feel free to share your opinion(s).

[djmips]

OOPS - double post somehow.

 

Here's the delta from my above post

 

See this link for information on how these mods work (this is for 7800 but similar to 2600)

 

Here's another story from someone else who was modding their 2600. He has come up with a slightly different variation on the mods that are out there. It seems to be fairly simple yet of a good quality. Perhaps Nathan Strum should test it.

Edited January 15, 2007 by djmips

[Dragnerok X]

Here's another story from someone else who was modding their 2600. He has come up with a slightly different variation on the mods that are out there. It seems to be fairly simple yet of a good quality. Perhaps Nathan Strum should test it.

 

I've also seen that one before and believe it or not, even sent him (nathan strum) a link to it via email, but you know spam filters these days, so who knows!

 

*cough* *cough* Nathan, are you reading this?!? [/cough]

 

Edited January 16, 2007 by Dragnerok X

[Dragnerok X]

...I'm guessing that it isn't somehow or why else would it not have been used before now?

 

I think Atari has been planning this for a LOT longer than you may think...

 

 

(this came from the rf modulator schematics, by the way)

[djmips]

I think Atari has been planning this for a LOT longer than you may think...

 

Well, as I said before, you need composite video before you can modulate it to broadcast frequencies. That reference to the composite bus is the 'M' inside the pentagon, that I mentioned, and connects from the other schematic. It already has audio mixed in and may not be the correct levels. Also note that you would may have to disconnect the RF modulator from the composite input or the signal would be too weak to drive both your own composite output and the RF modulator.

[A.J. Franzman]

Your idea has been thought of before. I did a lot of research before doing a mod, and I only found one mention of anyone having tried it using the already-combined luma+chroma signal from the 2600 motherboard, and he said it didn't work. I didn't believe him, so after a bit more research and experimentation, I came up with a version that does work. The problem is that the signal on the motherboard is too weak due to the influence of the RF modulator, which can be helped by disconnecting the modulator. It also needs buffering and amplification, as well as impedance-matching to the video cable. I accomplished this with a simple one-transistor amplifier.

 

Try my composite video mod: A.J. Franzman's Atari VCS Composite Video Modifications.

[djmips]

Try my composite video mod: A.J. Franzman's Atari VCS Composite Video Modifications.

 

Wow! That's got to be the most detailed yet simple mod yet! It reminds of the quality of instructions for modding a PS1.

 

When did you get that up? I hadn't heard of it before? Have you received any feedback yet?

 

I'll probably try this out on my 4 switcher in the near future.

 

Did you know that the six switcher heavies have a seperate printed circuit board from the switches / RF section? This would make for a much easier to assemble portable project! No cutting.

[Dragnerok X]

Your idea has been thought of before. I did a lot of research before doing a mod, and I only found one mention of anyone having tried it using the already-combined luma+chroma signal from the 2600 motherboard, and he said it didn't work. I didn't believe him, so after a bit more research and experimentation, I came up with a version that does work. The problem is that the signal on the motherboard is too weak due to the influence of the RF modulator, which can be helped by disconnecting the modulator. It also needs buffering and amplification, as well as impedance-matching to the video cable. I accomplished this with a simple one-transistor amplifier.

 

Try my composite video mod: A.J. Franzman's Atari VCS Composite Video Modifications.

 

Nice! Here's what I've got going so far...

 

I've disconnected the rf modlator(well... snipped it's main pins) along with seperating the audio & video signals (at least I THINK they're separated) by removing the audio resistor (as Ben Heckendorn descirbed in his second mod) of which the audio and video intersect. I'm using ben's old audio spot, though, however, have yet to come across any difficulties sound-wise. As for video... well I might want to try your Idea, A.J., but I have (yet another) quick question first. Has anyone ever heard of this before, sounds almost like a "Cracknell" approach to me. Maybe having already separated the audio/video and having removed the rf circutry maybe this could work? Just an idea.

[djmips]

Has anyone ever heard of this before, sounds almost like a "Cracknell" approach to me. Maybe having already separated the audio/video and having removed the RF circuitry maybe this could work? Just an idea.

 

That doesn't sound very promising. It really does seem like the "Cracknell" mod. Having to find a 'good' TV is tip-off of a bad mod with questionable output levels. I would recommend to steer clear and stick with AJ's mod. It's not a lot of soldering and the part count is low. Snipping pins is easier than soldering.

 

A good RF cable is better than a bad mod. I have my heavy connected using a cable TV coax cable from TV to circuit board so it's shielded the whole way. It gets an excellent picture that rivals a modded 2600.

[Dragnerok X]

That doesn't sound very promising. It really does seem like the "Cracknell" mod. Having to find a 'good' TV is tip-off of a bad mod with questionable output levels. I would recommend to steer clear and stick with AJ's mod. It's not a lot of soldering and the part count is low. Snipping pins is easier than soldering.

 

Well... I'll at least try it anyways.

 

A good RF cable is better than a bad mod. I have my heavy connected using a cable TV coax cable from TV to circuit board so it's shielded the whole way. It gets an excellent picture that rivals a modded 2600.

 

I realize that... but what can I do now that the rf circutry is out!?! Just saying, you could have mentioned that earlier!!!

[Dragnerok X]

Well... I'll at least try it anyways.

 

Just tried it, except I kinda modified it to be a mix of Cracknell's and the one I posted, with a few improvisions. Well... contrary to popular belief it, thus far, has turned out pretty good. The color seems to be a bit off on some areas, but otherwise works just fine. As expected, the picture quality is phenominal (compared to rf) and unlike most of the screenshots Nathan posted on his site, I have yet to expieriance any distortion what-so-ever. Thus far I have tested this on ~8 different carts on a single T.V., but will likely try others to verify.

 

 

 

BTW: My cruddy digital camera seems to be phosphor happy, lately.

Edited January 17, 2007 by Dragnerok X

[A.J. Franzman]

Has anyone ever heard of this before, sounds almost like a "Cracknell" approach to me.

Nope. I've never seen that one before, and hope I never do again. I can't even make sense of where he's saying to make all of the connections, but since he doesn't mention adding or disconnecting anything, I would guess it's either a lot like the Cracknell method, or takes the unamplified signal from the motherboard and makes it the s-video or composite out (this explains the problem with some TVs -- the signal from the motherboard is much weaker than standard, but some TVs will work OK with it.) The audio signal will still be present in the video with this mod regardless of whether one uses the s-video or composite version, so some picture degradation is unavoidable.

Edited January 17, 2007 by A.J. Franzman

[A.J. Franzman]

Try my composite video mod: A.J. Franzman's Atari VCS Composite Video Modifications.

Wow! That's got to be the most detailed yet simple mod yet! It reminds of the quality of instructions for modding a PS1.

 

When did you get that up? I hadn't heard of it before?

Oh, I don't remember, I guess close to a year now. I posted it on Ben Heckendorn's site when I first came up with it, and I've revised it slightly a few times since then. If you find the post where I announced it you can come back and tell me when it was!

 

Have you received any feedback yet?

No, none yet.

 

I'll probably try this out on my 4 switcher in the near future.

 

Did you know that the six switcher heavies have a seperate printed circuit board from the switches / RF section? This would make for a much easier to assemble portable project! No cutting.

Both the "heavy sixer" and "light sixer" consoles have similar internal construction - it's the bottom part of the plastic casing that makes a "heavy", "HEAVY"! There's over half a pound of plastic in the heavy sixer case bottom; it's about 3 times thicker than the light sixer bottom.

 

Using a six-switch console is only for people who want a BIG portable - using Ben's method on a 4-switch motherboard, you can get it down to 4" x 4", or less if you go to the trouble of putting the cart port on ribbon cable. My work-in-progress portable is 4 3/8" x 3" -- the cart port is on ribbon cable, but my board is a little wider than Ben usually makes because I cut to the outside of the shielding instead of the inside so I could keep most of the shielding in place. You'll never get near that small using a six-switch motherboard, mainly because of the extra chip (4050 hex buffer) and space on the board for a deleted chip (originally Combat or another game was going to be built into every console).

[Dragnerok X]

Well... I'll at least try it anyways.

 

Just tried it, except I kinda modified it to be a mix of Cracknell's and the one I posted, with a few improvisions. Well... contrary to popular belief it, thus far, has turned out pretty good. The color seems to be a bit off on some areas, but otherwise works just fine. As expected, the picture quality is phenominal (compared to rf) and unlike most of the screenshots Nathan posted on his site, I have yet to expieriance any distortion what-so-ever. Thus far I have tested this on ~8 different carts on a single T.V., but will likely try others to verify.

 

 

 

BTW: My cruddy digital camera seems to be phosphor happy, lately.

 

Just messed with this a bit more... tried it on 3 t.v.'s (all different sizes/manufacturers) as of yet, not a single picture problem. I also figured out wht's wrong with the color, I used the wrong resistor for the luma signals! I post further results later...

 

I think I'm on to something...

Edited January 17, 2007 by Dragnerok X

[Dragnerok X]

Well... It didn't turn out to be the wrong resistor, but for sure it's the wrong luma levels. The colors are just perfect, along with the sync, but some of the color's brightness doesn't look right. Right now I'm hooking up the "luma" to the correct resistors, though I have a feeling that where I placed the wire, (on the intersection of the original signals and said resistors), *could* have something to do with it (maybe I have to place the wire after the resistors, though I don't think resistors are polarized, are they?). Just want a, quick, easy-to-follow answer so I can go ahead and keep tweaking!

Edited January 18, 2007 by Dragnerok X

[Dragnerok X]

Question: Does anyone have a working link to where I could find Chris C's mod?

 

[djmips]

I usually use archive.org when I can't find a page

So if you start at Nathan's page and use that link on archive.org you'll get this page http://web.archive.org/web/*/http://www.hw...29/2600vidm.htm

You can then select a date and check out the contents.

 

Nevertheless, there is probably another mirror somewhere.