Why am I seeing a black gap on the bottom despite being 262 vertical lines?

[tschak909]

Hi everybody, I have a stable kernel that works, and generates 262.00 NTSC lines ,but am wondering why I am getting a black gap at the bottom of the emulated display?

 

-Thom

 

dodgeball.asm

dodgeball.bin

[gauauu]

I'm no expert, but I think it's normal. I think maybe it gives space for games that draw during overscan?

 

(Hopefully some better informed person will provide us a proper answer soon)

[Mr SQL]

 

[Andromeda Stardust]

The actual screen size for a standard spec NTSC Atari game is 192 horizontal scanlines in the display area, and 262 scanlines total. PAL normally uses 312 scanlines. I forget the PAL playfield size though. Most games use the blacked out areas above and below the playfield for calculations and controller polling but the programmer can set the screen size to whatever he/she wants. You could design a game with 500 scanlines if you wanted, but no CRT in the world would sync to such a slow refresh rate.

[alex_79]

That's normal and the result of the fact that the vcs video signal is mostly under software control and there's not a fixed vertical resolution, nor fixed scanline for the start and end of the picture.
Not all of the 262 scanlines are visible on a TV screen and the actual visible area (number of scanlines) varies from one TV to another, as the vertical allignment does (first visible scanline).
Stella uses default values so that in most games the image isn't cropped, which means that you often see black borders on top and/or bottom of the image, but you can change those in the "display" tab inside the "game properties" menu or through the command line.

On a standard 525 lines NTSC signal there are 486 active scanlines that can be used for the picture (on 625 PAL signal, they are 576). Action-safe area is about 90% of that value and title-safe area is 80% (see here).
Since the vcs video is not interlaced, you have to divide those scanlines counts by 2, so the title-safe area (that is, the one garanteed to be visible even on very old crts which crop the image a lot on the borders) is about 194 scanlines for NTSC, which is close to the suggested vertical height in the stella programming guide (192).

Safe areas apply also on the horizontal direction, but the TIA already generates a signal which is narrower than the max display area. The result is that you can see black borders also on the sides of a game picture depending on the TV you're using. Since that part of the signal is not under software control and therefore is the same for every game, stella (correctly) never shows black borders on the sides of the emulated display.

[Thomas Jentzsch]

Additional info:

 

The black gap is created by Stella. Use Tab/Game Properties/Display and modify YStart and Height. The default for Height is 210 lines, because not all games start at line 34, and many display more than 192 lines.

 

Modify the values to get rid of any black space.

[+Random Terrain]

Speaking of YStart, to simulate what a batari Basic game will look like on a real Atari 2600, YStart needs to be changed to 28. I wonder if any Assembly Language programs need a similar setting?

[Thomas Jentzsch]

Yes, but Stella has a growing built-in database (source code: DefProps.hxx) . Also it remembers your personal settings per game.

 

E.g. Check Artillery Duel, the notorious only 241 scanlines version. Here Stella starts at 20.

Edited June 10, 2016 by Thomas Jentzsch

[tschak909]

Yup, am _very_ aware of the software defined nature of VCS video output. Mr. Jentzsch pointed me in the right direction.

 

-Thom

[iesposta]

I hacked the batariBasic DPC+ kernel again to have 199/200 visible scan lines. (No score).

Had to get rid of 11 overscan lines to keep 262 60fps.

That will probably lead to running out of cycles down the line.

 

And our DK Arcade 2600 is 272 scan lines and 58.22fps.

Haven't seen any problems with old or new displays, but maybe someone will have problems.

Believe me, the 1st stage with the barrels, and during the hammer routine and tune we chose a non-standard display because screen jitter is worse than flickering.

Depends on the TV also. My Sony WEGA is rock solid on the build that jumps and jitters like crazy on the RCA.

[Thomas Jentzsch]

Thrust is running at 270 scan lines for over 10 years now without any complaints.

 

Back then NTSC games like Desert Falcon displayed up to 281 lines.

[Mr SQL]

Safe areas apply also on the horizontal direction, but the TIA already generates a signal which is narrower than the max display area. The result is that you can see black borders also on the sides of a game picture depending on the TV you're using. Since that part of the signal is not under software control and therefore is the same for every game, stella (correctly) never shows black borders on the sides of the emulated display.

 

 

I agree it's a good choice for emulation to stretch the screen to utilize all of the screen real estate, but this can be done vertically as well as horizontally - the emulator could scale even 500 scanlines (StarDust's example and I think Stella used to do this, allowing emulator-only games) to full screen height.

 

If the goal is precise emulation of the hardware then we should see borders on all sides not just the bottom for a 192 visible scanline game, and not have to set custom settings.

 

 

Depends on the TV also. My Sony WEGA is rock solid on the build that jumps and jitters like crazy on the RCA.

 

Agree - the Sony has extra circuitry so it won't roll even if you vary the scanlines by a considerable margin from frame to frame, the RCA is right on the spec and rolls if the scanlline count varies even by 1 (great choice for testing).