Remove "hard-wired" B/W-mode of SECAM Atari 2600

[patrickvogt]

Hi,

 

I own a SECAM Atari 2600 system and did a video mod since it has the C011903(PAL)-Chip. The picture looks good. The only problem is that the picture is in B/W and the switch doesn't seem to change anything. I think I've read that the SECAM atari is 'hard-wired' to B/W mode.

 

Does somebody know where this 'hard-wired' B/W-mode on the PCB can be disabled?

 

Best Regards and thanks in advance

[+SpiceWare]

The B&W switch doesn't do anything in Ms. PacMan, it'll output color regardless of the setting. This is true of a lot of games.

 

That tells me either:

1. your mod isn't right
2. the game is a SECAM specific version of the game

The SECAM Atari uses different color values than the PAL (and NTSC) Atari. You can see in this post about Draconian how selecting the SECAM version of the game results in a B&W image on a PAL Atari.

 

My Atari NTSC vs PAL vs SECAM page might help you understand what's going on with the colors.

[patrickvogt]

Hm,

 

The Mrs. Pacman game cartridge is definitely a PAL cartridge, so this can't be the problem. I will desolder the video mod circuit and solder it into my woodgrain PAL Atari to check the mod.

 

Thanks for the clarification on the switch (that this switch is read by the software and no hardware switch at all). I found this post and this guide on the internet and didn't know that the switch position has to be read by the software itself and just will react to the switch position if the software is programmed that way:

 

http://atariage.com/forums/topic/10800-secam-2600/?p=104949

http://www.alienbill.com/2600/101/docs/stella.html#palsecam

[patrickvogt]

Hm,

 

desoldered the mod circuit from the SECAM Atari and soldered it into PAL Atari. The circuit works fine with the PAL Atari. I also posted the question about the B/W video at the German forum where I got the video mod circuitry from. One guy there suggested I should compare the circuitry between PIN 8 and PIN 12 of the SECAM/PAL TIA with the scheamtic of the PAL motherboard here on AtariAge. Definitely the quartz crystal between those pins of the SECAM TIA has a different frequency (PAL(Y201) has 4.43MHz and SECAM(Y201) seems to have 6.5MHz). But maybe I am wrong. I will further investigate this issue.

 

But another question: Even if I would get colour with this SECAM console: A software which uses the B/W switch to change between color and B/W mode (like the Activision Tennis International Edition) will be only shown in B/W on this Atari, right? Since the SECAM console is hardwired to the B/W position and if the software support B/W mode it will always use B/W mode.

 

But probably better to take one step after another.

[+SpiceWare]

But another question: Even if I would get colour with this SECAM console: A software which uses the B/W switch to change between color and B/W mode (like the Activision Tennis International Edition) will be only shown in B/W on this Atari, right? Since the SECAM console is hardwired to the B/W position and if the software support B/W mode it will always use B/W mode.

That's right, Space Invaders would always look like this:

 

 

But probably better to take one step after another.

Yep - best figure out why you're not getting color when you should, then worry about the hardwired B&W switch.

[alex_79]

The problem isn't the b/w switch, but the fact that SECAM 2600 generates colors differently. While PAL and NTSC 2600 can be modded for composite or s-video using the same circuit (you just have to adjust for the different pinout of the TIA), you can't do the same for SECAM.

On PAL and NTSC, the 3 digital LUMA outputs of the TIA are converted into an analog LUMA signal which is then mixed with CHROMA and SYNC to generate composite video. The TV TYPE switch on the console is just an input read by software. Early games uses this for its intended purpose, but later it was either ignored or used for other functions (e.g. to pause the game).

On SECAM the chroma output of the TIA is unused, so using PAL or NTSC TIA makes no difference and in fact there's a different SECAM revision board that uses a NTSC TIA.

The TV TYPE switch on SECAM 2600 is an hardware switch that controls how the 3 LUMA outputs are used to generate the video. The input which on PAL and NTSC is wired to the switch, is tied to GND so that it's always set to B/W from the software point of view (and that can cause incompatibilities with later games that need the switch for gameplay).

With the physical TV TYPE switch in B/W position, the 3 LUMA outputs are converted to an analog LUMA signal just like in NTSC and PAL consoles, which produces a B/W video.
With the TV TYPE switch in COLOR position, the 3 LUMA outputs are used as digital Red, Green and Blue signals which are then converted into a composite SECAM signal by a dedicated circuitry on the board.

Even if your console has a PAL TIA, it lacks the additional oscillator circuit needed to genereate the PAL chroma signal, that's why you don't get color out of your mod. The only way to make your mod to work is to actually turn the console into a PAL unit: you'll end with the full PAL color palette, but that would be a quite extensive modification and probably not worth the effort as PAL 2600 are more common than SECAM ones.

If you're OK with the 8 color SECAM palette on the other hand, and just want a better quality video output, then you can easily mod your console to have a true RGB scart output.

Here is an example:
http://eddystoys.blogspot.fr/2014/08/atari-2600.html

That mod bypasses all the video circuitry on the 2600 board, including the "hardware" TV TYPE switch, so you will always have a color image (if you want to keep the ability to switch to a b/w image when a game looks especially bad with SECAM palette you'll need a bit more complicate mod). Note that the output will be RGB only, no composite, and it won't show the artifact described here (no commercial games nor homebrews took advantage of that to increase the number of colors, anyway.)

As for enabling the "software" TV_TYPE switch to play games that require it, the easiest way is to isolate pin 21 of the 6532 ic from the board and install a simple on-off switch between that pin and ground (OFF = COLOR, ON = B/W).

[patrickvogt]

Wow, thanks for all the information and espacially the link to the french blog article.

 

I ordered the parts and will do the RGB mod first then. If that works, I will take care of the TV TYPE switch and will try to get "James Bond 007" to pause on this Atari if TV TYPE is set to COLOR.

 

But there is still one question about the circuitry of the RGB mod: Why is he putting the Red (Rouge), Green (Vert), Blue (Bleu) and Sync signal trough an AND gate each? I mean from a boolean/logical point of view, if A is a fixed variable, then A /\ A <=> A (of course this is electronics and not a real boolean environment, where the FALSE and TRUE state are completly isolated/separated). Is this to stabilize/normalize the voltage level of those signals? Just want to understand that part of the circuitry.

 

Best Regards

Patrick

Edited May 23, 2015 by patrickvogt

[alex_79]

Is this to stabilize/normalize the voltage level of those signals?

I'm not an expert, but I think that's the purpose. By connecting the two inputs together, the AND gate acts as a buffer for the signals (similar to the CD4050 used in other mods).

[patrickvogt]

So, the parts arrived and I made an RGB scart cable and the RGB mod for the console.

 

First image is Keystone Kapers (PAL) on the SECAM Atari 2600.

 

Second image is Medieval Mayhem (https://atariage.com/software_page.html?SoftwareID=4081) as referenced by SpiceWare before in his Atari NTSC vs PAL vs SECAM page.

 

So next step would be testing e.g. James Bond about the Black and White switch to be used as pause button. But I will probably wait a few days before I investigate this problem.

[alex_79]

The video quality looks really good. I have a SECAM 2600 too (mine is the revision which uses a NTSC TIA) and the only TV set which was able to tune the SECAM RF signal stopped working, so I'll probably install the RGB mod too.


If you look at the 2600 (NTSC and PAL) schematics, the TV TYPE switch is just connected between pin 21 (PB3) of the 6532 chip and GND.

On Secam, pin 21 is hardwired to GND so, if you want a working TV TYPE switch you must isolate that pin (by cutting the traces around it, for example) and then wire a new switch. If you don't want to modify the case you can hot glue a small slide switch in the hole for the channel select switch (which is not present on SECAM consoles). You can of course also rewire the existing TV TYPE switch, altough that would require more work on the board to isolate it from the existing ciruit.

If you do the mod, you'll need to keep the switch in B/W position when playing some of the early games, else you might have problem distinguishing all the elements on screen. For example the 2 players in Basketball will appear in the same color with the switch in COLOR position.

(that's the reason why Atari decided to hardwire the switch to B/W, so the console could play the already released PAL games without the need to program a SECAM version)

Edited June 4, 2015 by alex_79

[+SpiceWare]

Second image is Medieval Mayhem (https://atariage.com/software_page.html?SoftwareID=4081) as referenced by SpiceWare before in his Atari NTSC vs PAL vs SECAM page.

 

The colors look much better than in the emulated screenshot!

[patrickvogt]

@alex_79: Thanks for all the useful information about the SECAM atari :-)

 

Ok, today I got some time and looked a bit closer for the hard wired B/W mode of my SECAM atari.

 

1. I tried the basketball game and looked at the colours in B/W mode (see first picture).

 

Then I had a closer look at the traces from PIN 21 of the 6532. It is directly connected to the GND pin of the cartridge port (see second picture).

 

The good thing is that all the ICs on my SECAM atari are socketed (I hope this english word even exists). So I got the IC6532 out of the socket, then bent PIN 21 about 90 degree to be horizontal to its prior position and put it back in (see third picture).

 

2. I tried the basketball game again and looked at the colours in Colour mode (see fourth picture).

 

So this all looks exactly as you said. Somehow an inner voice said I shouldn't cut any traces. This way the mod is reversible again. I mean I could also repair a cutted trace but this way it is much easier to return to the initial setting of the SECAM atari.

 

So after that I had a closer look at the area around TV TYPE switch with my multimeter to check for continuity. It seems that on my SECAM atari the lower four pins are connected to each other and go to GND. The upper two pins only are connected to GND if the switch is in its upper position (see fifth picture). So I just soldered PIN 21 of the 6532 to one PIN of the top row of the TV TYPE switch and connected one of the bottom four pins of the TV TYPE switch to GND (as I am writing now this should be unnecessary since the lower four pins of the switch are already conected to GND) (see sixth picture).

 

That will do the job but the only problem is that the switch is inverted now. So Colour position (switch) is B/W position since then pin 21 of 6532 is connected to GND in this position and vice versa. So I'll try to find a spare 4x NOR gate, so I can negate the singal by using the same signal for the same NOR gate since NOT(A \/ A) <=> NOT(A), where A is the position of the TV TYPE switch.

 

Without that James Bond was not playable since it will automatically pause after starting the game.

 

Edit: Hm, after thinking about it. The inverted switch shouldn't be that much of a problem, since it doesn't have the meaning of B/W and Colour on a SECAM atari. And if I pause the James Bond Game (e.g.) with the B/W position of the switch or the Colour position of the switch should be irrelevant. The important thing is that I can at least switch now between those both positions.

 

So last steps are only to remove the flux residue from the PCB and make the mod final (maybe also using a custom PCB, if I have enough time)

Edited June 5, 2015 by patrickvogt

[Andromeda Stardust]

Interesting info. Man, it really looks like SECAM regions got the short end of the stick. I can imagine the SECAM Atari consoles were probably mocked in the graphics depatrment compared to Intellivision, Oydessy II, or even the FairChild Channel F.

 

Another observational note, the 8-colors of the SECAM unit would be very difficult to distinguish in grayscale, especially whichever color got the LSB, and PAL or NTSC games being assigned seemingly random color values based on Luminosity would look putrid in comparison.

 

And no pause or alt functionality with the B/W switch...

[patrickvogt]

Ok, so I think this will be my last 2 cents about this topic. As I already said above: You dont have to wire one of the four bottom pins of the TV TYPE switch to GND since they are already connected to GND (see first picture).

 

And unfortunately I didn't had a spare NOR-Gate, just a spare NAND-Gate . But fortunately I have been attentive during my logic lecture, so I know that NOT(A \/ A) <=> NOT(A) <=> NOT(A /\ A). So if I negate a signal via a NOR- or NAND-Gate is the same piece of cake. (Just put the same signal into both inputs of the NAND-Gate and the result will be the inverted signal)

 

So I used a CMOS IC 4093 (Quad NAND-Gates) and as VCC I used the same +5VDC from the voltage regulator which is used for the AND-Gates in the RGB mod circuitry. Then pin 21 of 6532 goes to the output of the NAND and as input I used the orange wire which I wired to one of the top pins of the TV TYPE switch.

 

So that should help people which doesn't want to have the flaw with the inverted TV TYPE positions for colour and b/w on SECAM ataris.

 

So only thing left is a crappy demonstration video. Here you go: