Atari TIA chip question

[Longhorn Engineer]

I have been looking at the schematics on this site for the 2600, 2600A, and 7800 and I noticed something odd about the TIA chip. On the 2600 the 6th pin (BLANK) is connected to the circuit while in the 2600A and 7800 it is not. I have noticed when this pin is connected to the composite output of my video mod (right before the 75ohm resistor) it cuts any bleeding out of the picture and results in a better image overall.

 

Question is, what does this pin do and why isn't it hooked up in the 2600A and 7800?

[A.J. Franzman]

That pin carries the horizontal blanking signal -- a period of "blacker than black" (the NTSC "front porch" and "back porch" periods which lie on either side of the "sync" pulse) which is supposed to let the TV determine the correct DC levels for the AC-coupled video data.

 

I find that some TVs and monitors display a better picture without this signal, but apparently most do better with it -- or at least, they did when the 2600 was current. The 2600's timing for this signal is apparently much longer than it should be, so for example some LCD screens will mistake the "blank" level for actual black picture info, and everything that's supposed to be black in the picture becomes a shade lighter and the whole active display area is surrounded by a black frame.

[Defender II]

Good catch.

[Longhorn Engineer]

That pin carries the horizontal blanking signal -- a period of "blacker than black" (the NTSC "front porch" and "back porch" periods which lie on either side of the "sync" pulse) which is supposed to let the TV determine the correct DC levels for the AC-coupled video data.

 

I find that some TVs and monitors display a better picture without this signal, but apparently most do better with it -- or at least, they did when the 2600 was current. The 2600's timing for this signal is apparently much longer than it should be, so for example some LCD screens will mistake the "blank" level for actual black picture info, and everything that's supposed to be black in the picture becomes a shade lighter and the whole active display area is surrounded by a black frame.

 

Thanks AJ! That was exactly the information I was looking for.

 

I found if you place a 600ohm resister in series with the blank pin and tie it to the output of the composite output it cuts the bleed of the image. Have to do some more testing to be sure though. I will be sure to look out for the lighter black backgrounds.