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Cosmic Ark stars different on different consoles!

Andrew Davie

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I've just found out, testing Boulder Dash®, that the Cosmic Ark starfield effect behaves differently on the two consoles I have.

One is a stock-standard Australian PAL '2600, and the other is a "Video Game System" clone marked "2600 B".

Surprisingly, it's the clone that behaves "properly" like the emulator. The real thing is totally different.

Looks like some users will be getting pretty stars and the others will be getting pretty stripes. Interesting.



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That happens when your console (PAL or NTSC) has a TIA chip that was made in China after 1989. Unfortunately this includes all PAL 7800s.


As you know, the starfield effect works by tricking the TIA chip to keep sending extra clock pulses to the position counter for one of the movable objects every four pixels during the invisible part of the scanline. The thing is that these extra clock pulses also happen during the visible part of the scanline. But here they usually overlap perfectly with the normal clock pulses that advance the position counters once every pixel, and therefore don't have any effect.


On the TIA chips from 1989 the timing for the extra clock pulses is slightly off. Therefore they connect two of the normal clock pulses effectively canceling out one of them. Therefore you are seeing a shift to the right instead of the expected shift to the left, because the position counter doesn't get the normal 160 clock pulses it's supposed to get.


If you play Kool Aide Man on these consoles, the player sprite will keep bouncing back and forth the the upper left corner, making the game unplayable. This is because the player objects in the score display overlap which constantly triggers the collision detection. The way Kool Aide Man positions the player objects for the score also accidentally triggers the extra clock pulses.

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Stella doesn't (yet) properly handle the problems with Kool Aid Man, so I suspect that some other ROMs won't look right for similar reasons. Also, the current emulation of these weird effects isn't accurate for every machine anyway. A few releases back, I release a version of Stella with tweaks that made Cosmic Ark and Stay Frosty look like they do on my light sixer. I figured it was representative of the systems out there, but received much feedback that things didn't look right on other systems. So I reverted the changes for the subsequent release. I'm wondering, are those initial changes somehow the 'more correct' way of doing things, and the newer consoles are at fault? Perhaps I'll make it a configurable option at some point??

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