Do PAL and NTSC 2600's run at different speeds?

[Jonesyy]

While playing around with Stella emulator I noticed some PAL games have much slower music than the NTSC equivalents. Now this reminds me of the PAL sega megadrive and the NTSC sega genesis, where the PAL megadrive games run at 50 mhz, and the NTSC genesis games run at 60mhz. This was so annoying as a PAL megadrive owner that myself and countless other owners ended up modding the megadrive with a switch so that we could play our PAL games at the originally intended 60mhz instead of the slowed down 50 mhz.

 

So, if I'm looking to buy a real 2600, I would like to know if pal machines are in fact slowed down to 50mhz compared to NTSC version, as was the case with the megadrive/genesis?

 

Thanks

[Thomas Jentzsch]

The consoles are running at (about) the same speed. And they are running the games at 60 or 50 Hz (not MHz). These are the frame rates of the TV systems. Since many Atari 2600 games are clocked by the frame rate, everything moves, plays, counts etc. by this frame rate. If such a NTSC 60Hz game is converted to PAL 50Hz, everything moves slower.

 

Atari advised in its own developer guide, not to use the frame rate here. Instead they urged the developers to use fractional math. And therefore for their own games, most PAL conversion have no slowdown. However, some 3rd party companies (e.g. Activision) did not know or follow this advise (it requires more resources), and therefore their PAL conversions are noticeably slower.

 

Today we have a popular workaround for this problem, because most modern PAL TVs support 60Hz too. There is a dedicated thread in this forum for these so called "PAL-60 conversions", which fix the problem for the original games. And most homebrews are developed for PAL-60 or take care of Atari's advise.

[KevKelley]

After reading about this, is it possible to use this to slow a game or speed a game up? Can a programmer make a NTSC game but somehow use this to change the speed of there game? Is there a way to go slower or faster? Or do I have a simplistic and complete misunderstanding of how this works?

[+Karl G]

29 minutes ago, KevKelley said:

After reading about this, is it possible to use this to slow a game or speed a game up? Can a programmer make a NTSC game but somehow use this to change the speed of there game? Is there a way to go slower or faster? Or do I have a simplistic and complete misunderstanding of how this works?

If by the "speed of the game" you mean the framerate, then yes. As mentioned above, the game logic does not have to be tied to the framerate, and shouldn't be if you are releasing a game in formats that use different framerates. Otherwise, you can speed up your game's framerate by having less total scanlines per frame, and slow it down by having more. I don't know what the effective upper and lower limits on these would be before TVs start to freak out, though. I've wondered about a title screen at 120Hz that uses flicker for extra colors, but the flicker isn't visible due to the very high framerate.

[KevKelley]

I was thinking about either something like that, or if it was possible to use this as a way to increase difficulty in a game, but I don't know if this would either be more effective or more efficient than other ways. 

 

How would one go about less total scanlines or increasing/decreasing the Hz?

[+SpiceWare]

18 minutes ago, KevKelley said:

How would one go about less total scanlines or increasing/decreasing the Hz?

 

Don't change the Hz, the further you get from spec the more likely the picture will become unstable on a real display.  Do some research on fractional or subpixel movement which lets you move an object 0.7, 1, 1.2, etc. pixels per frame.  Example would be this topic:

 

 

 

[KevKelley]

I was just curious if at some point changing the Hz would be stable (like if 120hz would be okay but 112hz might cause issues) or if you could get the same effects as using fractional or sub-pixel movement while using a different method and possibly freeing up something.

 

Again, my understanding is probably very simplistic and I was just trying to understand a little more of the hardware and possible methods. I read about how some games may be slower in a different region and it just got me wondering. Thanks for the suggestion 

[orange808]

240p@120Hz refresh rate would push the horizontal scan rate too high.

 

We can remedy the situation by cutting the scanlines in half, but I doubt consumer televisions would sync to ~120p.  Even if they did, what would that look like?

[+SpiceWare]

To get 120 Hz you would have to cut the total scanlines in half to 131.  I doubt any display will handle that very well.  You can do some experiments using Jitter.bin from this blog entry, which should also answer your Jittery Recover question.

 

Jitter's default screen size is 262 with 128 rainbow lines being drawn.  Use joystick to change rainbow line count, then hold fire for it to take affect.

 

My C=1084S can handle a variety of screen sizes, I frequently use it for PAL games. It can handle down to 51 just fine (185 scanlines @ 85.0 Hz):

 

 

At 50 and 49 the 1084S becomes unstable.  From 48 down to 1 it draws the screen twice.

 

 

 

[KevKelley]

Thank you!

 

I will have to look into all of this. I definitely want to have a good understanding of all the workings of the Atari and seeing how impressive the programs coming out are makes me want to think outside of the box to understand the limits.

[Mr SQL]

On 5/8/2020 at 5:35 PM, KevKelley said:

I was just curious if at some point changing the Hz would be stable (like if 120hz would be okay but 112hz might cause issues) or if you could get the same effects as using fractional or sub-pixel movement while using a different method and possibly freeing up something.

 

Again, my understanding is probably very simplistic and I was just trying to understand a little more of the hardware and possible methods. I read about how some games may be slower in a different region and it just got me wondering. Thanks for the suggestion 

Yes changing the Hz can be stable and it is recommended by the programming manual using fractional math -

many games show 4 player sprites rendered at 30 Hz by displaying them every other frame instead of every frame at 60 Hz.

 

This is because the nonstandard video signal gives us twice the Hz of a 30 Hz NTSC broadcast signal to work with by throwing entire frames 60 times pre second instead of fields (half frames) to assemble 30 frames per second.

 

There is no way however to get 120 Hz because you only start with 60, however fractional math to divide the signal can look very cosmetic in a game, here's an example with KC Monster Maze:

 

The Maze is displayed at 60 Hz

The four characters are displayed at 30 Hz (divide by 2,  both character sprites every other frame)

The 6 dancing dots and power pellet are displayed at 20 Hz (divide by 3, both missle sprites every third frame)  

 

When the screen scrolls all of those elements get divided by two again -

The Maze is displayed at 30 Hz

The four Characters are rendered at 15 Hz

the 6 Dancing Dots and power Pellet are displayed at 10 Hz  

 

The frame division has to be perfect (no calling draw screen to load balance if you are using bB) when dividing the Hz but the results can look great on a classic Television.

 

Stella emulation merges some of the frames by default on the latest Retron77 community build which can reduce flicker for games that don't rely on fractional math, but it makes a mess of the frame division used in games like KCMM and also some classics like ESB:

 

[KevKelley]

Thank you for your explanation. When reading about it before, I kept picturing that one could just incrementally adjust the Hz and speed or slow things down. I think I have a better understanding now. 

[alex_79]

11 hours ago, KevKelley said:

When reading about it before, I kept picturing that one could just incrementally adjust the Hz and speed or slow things down

You can in fact, (that's exactly what the jitter.bin test rom in the above post by Spiceware is doing) but as stated,  it's not recommended, as there will be incompatibilities with TVs and monitors the more you diverge from the standard values of 50Hz or 60Hz. Moreover varying the scanline count during the game might cause the image to jitter, so that should be avoided too.

 

The TIA automatically handles the horizontal sync, so the time it takes to draw a scanline is fixed, while the program controls the vertical sync, and by increasing or reducing the number of scanlines in a frame, you change the time it takes to draw a frame, and so the frequency (more scanlines, lower frequency, less scanlines, higher frequency).

But, again, you shouldn't do that to adjust the speed, but use fractional math instead, as explained Thomas, karl G and Spiceware in the above posts

[KevKelley]

Okay. That clarified things further (more scanlines, lower frequency, etc).

 

Thanks