Commodore cartdriges anti-screen flutter key?

[nippur72]

In Jupiter Lander cartdrige manual, there is a small note:

Eliminating Screen Flutter
On some TV sets with automatic fine tuning your screen may "flutter" up and down when using a computer or video game. To correct this, hold down the [SHIFT] key and hit the [F7] key. (NOTE: Doing this with a standard TV set may cause the screen to flutter).

Does anybody know what SHIFT-F7 actually does? I know CRSR keys are used to center the screen, but this one is new to me. Perhaps it has something to do with the "interlace" bit of the VIC chip?

[Mike]

The interlace bit only affects the operation of the NTSC VIC-I. Some TV sets might have been confused when they received a non-interlaced signal and added 'extra' screen flutter because of this. Switching to interlace produces a more conformant video signal for these and thus reduces the flutter (it's still there but not as apparent).

On the other side, monitors usually cope well with a non-interlaced signal and show a 'rock-steady' picture. Switching on interlace there actually makes the picture worse, as now the monitor also outputs an interlaced picture with the electron beam strokes of the odd frames wedged in between the strokes of the even frames. As monitors usually have a shorter phosphor decay time, and the electron beam is more focussed, that makes the picture jump up and down half a line between frames.

With PAL, the VIC chip only outputs a non-interlaced video signal.

BTW, the correct spelling of such a expansion module is cartridge, not "cartdrige".

[tokra]

Hmm, I tried to find the check for SHIFT+F7 in the code, but could not find it. The manual also says you can move the screen drown with the cursor-down key. This also does not work, at least for the version on Zimmers. Do you have a version where moving the screen down works? This may also be the one where SHIFT+F7 works.

[nippur72]

I have this file for the cartdr... cardtr... cartridge which has the cursors keys thing:

Jupiter Lander.zip

(16.95 KiB) Downloaded 37 times

I apprehended only recently about the interlaced vs non-interlaced mode, I realized it's the actual reason why we are able to see the scanlines, it's because we are only seeing odd frames, right?

My understanding is that for an interlaced signal, the video chip (or whatever is generating the signal) adds an extra half line at the end of the "odd" frame telling the receiver the "even" is coming. There is a nice web page that helped me a lot when I had to write the video generator for an FPGA: http://martin.hinner.info/vga/pal.html

[Vic20-Ian]

The interlace bit only affects the operation of the NTSC VIC-I. Some TV sets might have been confused when they received a non-interlaced signal and added 'extra' screen flutter because of this. Switching to interlace produces a more conformant video signal for these and thus reduces the flutter (it's still there but not as apparent).

On the other side, monitors usually cope well with a non-interlaced signal and show a 'rock-steady' picture. Switching on interlace there actually makes the picture worse, as now the monitor also outputs an interlaced picture with the electron beam strokes of the odd frames wedged in between the strokes of the even frames. As monitors usually have a shorter phosphor decay time, and the electron beam is more focussed, that makes the picture jump up and down half a line between frames.

With PAL, the VIC chip only outputs a non-interlaced video signal.

BTW, the correct spelling of such a expansion module is cartridge, not "cartdrige".

The phrase "auf der andere Seite" translates better to "on the other hand" in English.

A German manager I worked for always used to combine the two into "on the one hand side"

By the way - the spelling of focused has one s too

[tokra]

I have this file for the cartdr... cardtr... cartridge which has the cursors keys thing:

Jupiter Lander.zip

No, sorry, none of the 4 files in the ZIP can control the image vertically, just horizontally as the Zimmers-one.

[nippur72]

No, sorry, none of the 4 files in the ZIP can control the image vertically, just horizontally as the Zimmers-one.

no this is the only one I have. BTW is there a theory why Commodore cartridges did worry to allow screen alignment? Why not just use the register values in the ROM kernal?

[Mike]

[...] I realized it's the actual reason why we are able to see the scanlines, it's because we are only seeing odd frames, right?

That, in combination with a more focussed electron beam with monitors, right.

BTW is there a theory why Commodore cartridges did worry to allow screen alignment? Why not just use the register values in the ROM kernal?

I'd presume no one bothered to tell the game programmers that PAL and NTSC variants of the VIC chip actually need different defaults for the XPOS and YPOS registers. Without that knowledge, neither was there any incentive to use the ROM init values directly, or recenter the display window by applying offsets. The programmers then just used values that worked on the machine and TV system with which they could test the result.

...

That being said, on NTSC systems interlace can be used to increase the vertical resolution if one manages to properly discriminate between even and odd frames and change between two different displays on alterate frames as well. Some of the new graphics modes constructed by tokra do this, most notably "Retina Display" (192x416i) and "NIFLI" (88x400i, with 8x1 colour RAM attributes and background/border/auxiliary colours redefined for each raster).