# Can 20fps be smoothly produced on interlaced displays *spawn

Discussion in 'Console Technology' started by milk, Jul 26, 2018.

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*20fps.

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2. ### Squeak Veteran

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How would that happen? Some kind of frame pulldown?
You have to have to half the frame rate every time you want to go lower. Otherwise you’d get tearing. So for NTSC it goes 60, 30, 15. PAL is 50, 25 and 12.

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3. ### TheAlSpark Moderator ModeratorLegend

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For double buffer, you're adding one additional frame delay for every missed refresh, so it's 1/(n+1).

For 60Hz, if you miss 16.66ms, you have to wait until the next refresh @ t= 33.33ms ("30fps"). If you miss that, the update is @ t= 50ms ("20fps"). etc.

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4. ### HTupolev Regular

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To get evenly-timed frames, you don't need to halve the rate with each drop, you can divide the refresh rate by any integer to get a valid steady framerate. 60, 30, 20, 15, 12, 10, 8.571 etc.

A steady 20fps would mean that your frames would each double one of their fields of an SD CRT (even-odd-even, odd-even-odd, etc). In practice, no problems really arise from this; 60fps games also alternate fields (even, odd, even, odd, etc) and it looks great.

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5. ### Squeak Veteran

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Any integer? I think you need to go back to the fifties and explain your scheme to the guys who first attempted telecining film.
How would you make those 20 fps fit within a second of NTSC? How many sequential frames, or even fields would you display the single frame for to make it fit without tearing?

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6. ### milk Like Verified VeteranRegular

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For 20fps you simply need to display every frame 3 times on a 60fps display. It's trivial. You can check on google if you don't trust us that OOT as well as many ps1/n64 games ran at 20fps.
It's not the same as film, because they are trying to fit 24fps into 60. It's a non intiger division of 60. THAT is a pain, but 20fps isn't.

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7. ### milk Like Verified VeteranRegular

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How does that translate to PAL you might ask? Well, the answer sadly is OOT ran at 12,5 fps there. At least that's what I've seen onine. I don't know how europeans can endure that.

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8. ### Shifty Geezer uber-Troll! ModeratorLegend

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HTupolev didn't say you could create any integer framerate; he said you could take the 60 Hz refresh and divide it by any integer (duplicate a frame over multiple screen refreshes) and get a constant framerate, and then he listed the available framerates.

Create a new frame every refresh, you get 60 fps. Create a new frame every other refresh and get 30 fps. Create a new frame every third refresh and get 20 fps. Every four refreshes == 15 fps, etc.

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9. ### swaaye Entirely Suboptimal Legend

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Was F-Zero the only 60fps game?

I feel like the typical frame rate range was 20fps or less.

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10. ### Reznor007 Regular

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Smash Bros was 60fps also.

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11. ### Squeak Veteran

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What you’re forgetting is that it’s sixty fields, not frames we’re dealing with here.
Two silky smooth frames followed by an interlaced frame is not kosher.
Sure you can do it, but it will look like arse. But then, I never played the NTSC version.

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12. ### damienw Regular

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Or triple buffer and not worry about it.

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13. ### HTupolev Regular

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Interlacing means that the display is alternating between drawing even lines and drawing odd lines. A 60Hz interlaced CRT spends 16.67ms drawing the even lines, then it spends 16.67ms drawing the odd lines, then 16.67ms drawing the even lines again, etc. There's no such thing as "silky smooth frames" and "interlaced frames"; there are even fields and odd fields. No magical bad effects crop up as a result of a frame using more odd fields than even or vise versa; a 20fps video feed will have more obvious combing artifacts than a 30fps video feed, but that's because the framerate is lower, not because of the field "imbalance."

Also, consoles from that era - including the N64 - supported modes to force televisions to display all fields as even or all fields as odd. This is what causes the "scanlines" effect: the intermediate lines simply aren't being drawn. So, for many games, an SD CRT was literally a 240p60 display. I don't have an N64 to plug in and check, but I suspect that OoT/MM fall into this category, and so interlacing really doesn't need to enter the discussion anyway.

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14. ### swaaye Entirely Suboptimal Legend

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Yeah I think most games are progressive, but the ones with "high-res" switch to various interlaced modes.

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15. ### Shifty Geezer uber-Troll! ModeratorLegend

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The image construction is no different and you get no irregularities in framerate or interlacing different to 60 or 30 fps.

At 60 fps, you get to see half of one frame, and then the other half. In certain circumstances you may see interlacing. At 30 fps, you get to see half of one frame, then the other half of the same, and no interlacing. At 20 fps, you see half the first frame, other half the first frame, first half of the first frame again, and then second half of the next frame. You may see interlacing artefacts if the difference between frames 1 and 2 are that way inclined, but in practice it's no worse than interlacing at 60 fps.

It's really no different to progressive displays except you only see half the frame each refresh. And interlacing can affect static images too (alternating horizontal lines for example). The time delta between objects moving on screen is constant, so the framerate is constant. The only thing 20 fps et al are bad for is the janky framerate; image quality didn't look any worse on CRTs than 30 fps and 60 fps.

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16. ### see colon All Ham & No Potatos Veteran

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His example is right. If you divide the refresh rate of each field (60 in the case of NTSC) by an integer you can get evenly paced fields. 60/2 is 30, so you would display 2 identical fields back to back. 60/3 is 20, so you would display 3 fields in a row. 60/4 is 15 so 4 fields in a row, and so on.

Film was traditional 24 FPS, and you can't get to that number by dividing 60 by an integer, which is the problem with transferring film to television. The same is true with 50hz formats like PAL.

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17. ### Shifty Geezer uber-Troll! ModeratorLegend

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PAL movies ran the film at 25 fps - we never experienced the horror of 3:2 pulldown!

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18. ### Squeak Veteran

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So basically it has the same problems as when pulldown converting 24 fps film to NTSC (some of the frames are static images and others are composites of two interlace fields). That is, a beating, irregular looking frame rate.
Barely acceptable with film, and I can only imagine how bad looking it is with a camera that is constantly swinging and panning at high speed.

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19. ### Squeak Veteran

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What I balked at was the "any" part of "any integer".

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20. ### Shifty Geezer uber-Troll! ModeratorLegend

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No. A 3:2 pulldown has frames shown for different durations. Frame 1 is shown for 3/60ths of a second, and frame 2 is shown for 2/60ths of a second. That's forcing a 24fps from intervals of 60. 20 fps is achieved by showing every frame for 3/60ths of a second. The composition of two different images for each field leads to interlacing artefacts, not motion judder, and you have that problem with 60 fps material. 20 fps has interlacing artefacts like 60 fps, and a lower framerate, but it has no motion issues or other visual issues. The end result is uniform motion and as clean an image as you get with 60 fps.

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