Adjusting scan lines to test CRT fault tolerance

[Andromeda Stardust]

Since the Atari display updates on every scanline rather than every screen refresh, every piece of software written for it has to count scanlines to ensure that the screen is displayed properly. I was wondering if there exists a homebrew app to "screw" with our TV sets (whether CRT or flat screen) by manually adjusting the number of scan lines. Alternating the scanline count between frames would also be cool too.

 

For example, the program could manually adjust the number of scanlines based on user input, until the TV loses hold and starts "rolling". I've got three CRTs in my house and one HDTV. If I play a PAL ROM on my Harmony cart, my CRT television "rolls", but my HDTV simply cuts off whatever is below the bottom of the screen. Maybe I could get the HDTV to "roll" if I increase the display rate above 60Hz?

 

I've read that many poorly programmed games have scanline issues that affect some TVs differently than others. I thought it would be fun to deliberately screw with the TVs to see how fault-tolerant they are before they start "rolling". With the exception of certain vintage rear-projection sets which used mirrors to focus the light beam, I'm pretty sure that the chances of actually damaging good old tubes and/or solid state electronics are pretty null.

 

Or if anyone has certain roms or can suggest specific games that cause issues due to scan-line counts, I'd like to try them out and see what happens. Especially if anyone knows of ROMS or demos that speed up the display beyond 60Hz (less than 262 scanlines) to "overclock" the refresh rate, that should be interesting too.

 

I have a Harmony cart, so feel free to post "problem" ROMs or recommend "badly" programmed games or demos.

[Rybags]

There was a thread about games that used off-spec # of scanlines, fairly sure it was in the 2600 section some time in the last few years.

There's a fair amount of leeway, some consoles/computers used 262/312 and others used 263/313 scanlines Pal/NSTC.

 

In the thread mentioned above, I remember some games being several scanlines out of spec.

 

HDTV might or might not roll. There's digital processing and often buffering of entire fields, often the solution to a way off-spec signal is to just blank the screen.

My old HDD/DVD recorder didn't like the non-interlaced signal coming from Ataris and would roll the display as if the VSync was missing altogether.

 

An app to vary the # of scanlines would be interesting - in fact the 2600 is probably better qualified than practically any other old home system to do it since VBlank/VSync is totally under software control and you can even generate HSync on demand.

[Mr SQL]

I've read that many poorly programmed games have scanline issues that affect some TVs differently than others. I thought it would be fun to deliberately screw with the TVs to see how fault-tolerant they are before they start "rolling". With the exception of certain vintage rear-projection sets which used mirrors to focus the light beam, I'm pretty sure that the chances of actually damaging good old tubes and/or solid state electronics are pretty null.

 

Or if anyone has certain roms or can suggest specific games that cause issues due to scan-line counts, I'd like to try them out and see what happens. Especially if anyone knows of ROMS or demos that speed up the display beyond 60Hz (less than 262 scanlines) to "overclock" the refresh rate, that should be interesting too.

 

I have a Harmony cart, so feel free to post "problem" ROMs or recommend "badly" programmed games or demos.

 

Hi stardust4ever,

excellent topic!

 

BREAKANOID is stable on most displays but rolls on JVC televisions and on SR3000 composite monitors, you can download the demo here:

 

http://www.atariage....0-order-thread/

 

I have a hotfix version available but there is a tad less performance without pushing the spec.

[SeaGtGruff]

The size of the display area (active lines) is normally controlled by a counter, so it should be pretty simple to make a program that would let the user increase or decrease the number of active lines by moving the joystick without changing the lengths of the front porch ("overscan") and back porch ("vblank").

[alex_79]

I did a very basic demo some time ago to test the behaviour of my TV sets. You just use joystick right/left directions to increase/decrease the scanline count. Keep the firebutton pressed to speed-up the process. The vertical stripes represent the scanline count in binary notation.

scanline_test.bin

[Omegamatrix]

Here is a rom that should do the trick. You press up to increase the scanlines, and down to decrease them. All I'm adjusting is the amount of scanlines in the kernel below the digits that are drawn. I made it so you can add or subtract quite a few, but eventually it will crash the rom.

 

 

Should be more than enough to do some simple tests though.

 

 

TestScanlines.zip