Troubles

[Littlewass]

Hi,

having some trouble with my Intellivision.  When you are playing a game (I’ll use Triple Action tank battle as an example), when you drive your tank to the top of the screen, a second tank appears at the bottom.  Both turn and shoot at the same time and both can kill the other tank by shooting it.  Any ideas would be appreciated.

thanks, Marc 

[mr_me]

I'm surprised nobody else here has had this problem, it is a weird one.  My initial guess is a faulty stic chip.  Normally you can't drive your tank into the top border, is there still a small gap there?  Does it happen with both both red and blue tanks?  Do other cartridges have this behaviour?

[Littlewass]

Hi Mr_Me

thanks for the reply.  So I’ve checked it some more to get better specifics for you and will add some pictures. Can’t drive through the top.  Does happen with both tanks.  Same with all games.  Its not at the very top of the screen.  It’s when you drive your tank or car or cursor or whatever about a little ways down from the top to about a third of the way down that the ghost appears.  I’m trying to add pictures for you but it won’t let me.

[Littlewass]

[Littlewass]

The tank ones show what’s happening.  If you’ll notice in B17 bomber the cursor is doubled.  Also in Utopia the cursor is doubled and the rain clouds are also.  Any ideas? 
‘thanks

marc

[+intvsteve]

Wow, this is a cool problem! Not that it's cool that the problem is happening... but it's certainly a fresh one!

 

It's like at least one MOB is being double-drawn. Do the duplicated guys flicker? I wonder if it's got to do w/ the bits that form the Y position for a particular MOB - or a bug in the register(s) themselves? It could be the system RAM too - the addresses that map to the MOB's position registers, for example.

 

It appears that the vertical distance between the duplicated MOBs is fixed, but I'm just eyeballing it. It's almost like a single line is unstable or something. With some pixel counting and whatnot I bet someone might even be able to figure out which pins to look at. I.e. is the pixel distance some power of 2, like 128 pixels or 64 pixels or so?

[Littlewass]

They don’t flicker. 

[Littlewass]

Hi River Patroller,

Thanks for your response.  they don’t flicker. Yes, the distance is fixed. 

[Zendocon]

On 4/22/2020 at 10:21 PM, intvsteve said:

Wow, this is a cool problem! Not that it's cool that the problem is happening... but it's certainly a fresh one!

 

It's like at least one MOB is being double-drawn. Do the duplicated guys flicker? I wonder if it's got to do w/ the bits that form the Y position for a particular MOB - or a bug in the register(s) themselves? It could be the system RAM too - the addresses that map to the MOB's position registers, for example.

 

It appears that the vertical distance between the duplicated MOBs is fixed, but I'm just eyeballing it. It's almost like a single line is unstable or something. With some pixel counting and whatnot I bet someone might even be able to figure out which pins to look at. I.e. is the pixel distance some power of 2, like 128 pixels or 64 pixels or so?

That's what I'm thinking.  Given that the vertical position of MOBs is 96 pixels, the two images have to be 64 pixels apart.  That would mean during the VBLANK period that the STIC is disregarding Bit 6.  Do you have any indie games to try?  If it doesn't happen on those, then it could be a problem with a bit in the 16-bit RAM that the EXEC uses to feed MOB data to the STIC.

 

If that "problem" could somehow be duplicated, it would open up new programming techniques similar to what we see on Atari 2600 games.  Then we could have the same kind of sprite multiplexing seen mostly on Activision Atari games.