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Vertical hold issue with RS Volleyball?


AtariLeaf
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I use Commodore 1702's as my primary gaming monitors and I noticed today after playing a few 2600 games just fine, I decided to pop in RS Volleyball and the screen rolled so I adjusted the vertical hold. Strange thing is I didn't have to do this with any other game. Is there something different with RS Volleyball? I tried two carts and two different consoles and for good measure, two different 1702's. Same thing - RS Volleyball needs very specific VH requirements.

 

For example I can play space invaders or ms pacman and I have some play with the vertical hold knob before the screen starts rolling. With RS Volleyball there's a very small narrow turn of the dial to keep it steady. Is there a scanline difference with this game? I'm sure it's NTSC.

 

EDIT - according to my research it has 262 scanlines so normal NTSC. Weird.

Edited by AtariLeaf
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Based on a quick check, it asserts the sync line for a really short period: cumulatively, only slightly more than a line and a half — slightly more than half the NTSC specification. I'll wager the 1702's tolerances require a little more.

 

Of other titles I tried randomly:

  • Combat, Pitfall, Night Driver, Pengo, River Raid: identical periods around 2.8 lines;
  • Solaris: more like 2.6 lines;
  • Pacman: just one colour cycle longer than Combat et al;
  • Miner 2049er: around 2.4 lines;
  • Joust: around 2.7 lines;
  • Video Life: 4.6 lines (!).

So they're mostly closer to spec or over it.

 

I think, despite being an analogue dunce so cut me down if you have to, that a common technique for discerning sync in the analogue days was allowing the sync level to charge up a capacitor and triggering a vertical sync if it fills up and hence discharges. So over-extending sync likely wouldn't be as problematic as cutting it short.

Edited by ThomH
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Composite video is a one-dimensional signal. It contains horizontal and vertical synchronisation marks. Horizontal marks tell the beam to move back to the left, vertical marks tell it to move back to the top.

 

The TIA automatically handles horizontal duties. Vertical are the responsibility of the programmer.

 

To communicate a vertical sync you need to enable a particular signal, wait an appropriate amount of time, then disable it. The NTSC standard says* you should leave the signal active for three lines.

 

This game appears not to be leaving the proper amount of time. So the monitor doesn't realise what it's trying to communicate.

 

* more or less.

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Is there a reason why they violated the spec?

The programmers made a mistake. From the second paragraph on page 1 of the Stella Programmer's Guide:

All horizontal timing is taken care of by hardware, but the microprocessor must manually control vertical timing to signal the start of the next frame. When the last line of the previous frame is detected, the microprocessor must generate 3 lines of VSYNC, 37 lines of VBLANK, 192 lines of actual TV picture, and 30 lines of overscan. Fortunately, both VSYNC and VBLANK can simply be turned on and off at the appropriate times, freeing the microprocessor for other activities during their execution.

The guide's from December 3, 79. Per the copyright in the manual, Canyon Bomber's a 79 game so it would have been finished before the guide. As such, they may not have had a clear understanding on that requirement.

 

On the other hand, Real Sports Volleyball's is from 86. So no excuse.

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