Space Rocks "Comb" line on 7800?

[AtariLeaf]

So I was playing my new copy of Frenzy for the 7800 (Awesome BTW) and since I had it out, I thought I'd pop in Space Rocks. Title screen was fine but once you actually start playing, you can see this noticeable line down almost the middle of the screen as you can see in the picture. Now it's hard to tell from that picture but the reason I believe it's a comb line or Hmove line I believe it's called, is that there are little gaps in the line that move with the objects on screen.

 

This doesn't happen with other 2600 games on this 7800 and it doesn't happen with space rocks on a 2600. Tried 3 different 7800's with two different power supplies. Some kind of RF interference? If it is a comb line then why is it so close to the middle of the screen and not off to the side?

 

I've read that homebrews don't have the shielding older retail carts had. Any ideas?

 

[+save2600]

Try turning down your brightness! Outer space is black, not gray!

[AtariLeaf]

Try turning down your brightness! Outer space is black, not gray!

 

I think that's mostly from the flash of the camera, plus I did turn up the brightness a bit so it would show in the picture.

[RevEng]

It looks like RF interference to me.

 

There was a thread somewhere here where a few Harmony users that saw this effect, and it was eventually reduced/went-away when they upgraded to better RF cables. Since Melody boards are based on Harmony tech, I wouldn't be surprised if the same RF effects happen on some users' systems.

 

See if moving the RF cable or putting your finger on the shield portion of the cable makes the effect go away or lessen.

[AtariLeaf]

It looks like RF interference to me.

 

There was a thread somewhere here where a few Harmony users that saw this effect, and it was eventually reduced/went-away when they upgraded to better RF cables. Since Melody boards are based on Harmony tech, I wouldn't be surprised if the same RF effects happen on some users' systems.

 

See if moving the RF cable or putting your finger on the shield portion of the cable makes the effect go away or lessen.

 

I'll try that thanks. Was also thinking of getting a better quality gold cable. Would adding a ferrite bead to the cable help too?

[RevEng]

Usually they're great for this sort of thing, but one of the guys in the original thread said it didn't help. I'd still give it a shot, if you happen to have one handy. (or have an old vga cable or usb cable with one on it)

[+SpiceWare]

Now it's hard to tell from that picture but the reason I believe it's a comb line or Hmove line I believe it's called, is that there are little gaps in the line that move with the objects on screen.

Just ran across this - you were seeing RF interference, most likely from the extra hardware found in the Harmony cartridge.

 

HMOVE comb/line/bar appears on the left, not in the middle. They're a side affect of strobing the HMOVE register, which is used during object repositioning. The HMOVE lines are not normally noticeable on a black background, though if you crank up the brightness and/or contrast of your display they will become visible as seen below.

 

In the menu the HMOVE is hit just before each menu entry in order to reposition the BALL object (used for one of the two colors on each scanline).

 

During the game itself the HMOVE is hit on every line of the gameplay area, so it's not noticeable until you get to the score.

 

Hitting the HMOVE on every scanline in order to hide it is a trick that Activision is well known for. That's why their games often appear to be off-centered in screenshots:

 

The name "Comb" comes because they take on the appearance of a comb in games like Missile Command where it's hit every-other-scanline:

[SoundGammon]

Darrell, any chance of you doing a version of Space Rocks for the 7800?

[alex_79]

An additional note about the comb lines:
They're visible on a black background if you turn up the TV brightness because they're at "blanking" level, just like the area on top and below the active part of the picture. NTSC has black voltage level which is slightly above than blanking one. PAL, on the other hand, (and the NTSC variant used in Japan) uses the same level for black and blanking. So on PAL console you cannot see the comb lines or blanking areas on a black background even if you crank up the brightness.

Here are pictures of the PAL60 version of space rocks played on a PAL console and TV:


According to the 2600 Service manual, at least a couple of NTSC 2600 revision boards have the "blank" pin of the TIA unconnected (2600A revisions 14 and 15) Also the NTSC 7800 schematic on the main AA has that pin unused (don't know if other revisions are different). Those consoles won't show the comb lines on a black background just like PAL ones. I assume the atari 2800 (for Japan market) does the same too, but there aren't schematics of that console available.

PAL TIA doesn't have a blank pin at all.

Edited November 27, 2016 by alex_79

[+SpiceWare]

Darrell, any chance of you doing a version of Space Rocks for the 7800?

While I do enjoy playing them on occasion, including Bob's awesome version of Asteroids Deluxe, I'm not interested in writing 7800 games. I'm interested in how far we can push the 2600:

[+Zeptari]

OMG!! Ultima on the 2600!!!