Posted: 8/13/2018 3:52 PM
Post subject: Platform frame rates? (SNES vs NES dot skipping)
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Dwedit
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Joined: 3/24/2006
Posts: 692
Location: Chicago
There is a page on the wiki called "Platform Frame Rates", http://tasvideos.org/PlatformFramerates.html The NES frame rate (60.098813897441) seems to be calculated perfectly: 
Master clock =  236.25 * 1,000,000 / 11 (21477272.7272727...)
PPU clock = master clock / 4 (5369318.18181818)
scanlines = 262
dots per scanline = 341
dots per frame = scanlines * dots per scanline (89342)
framerate (no skipping dots) = PPU clock / dots per frame (60.0984775561123)
framerate (skipping 1 dot every other frame) = PPU clock * 2 / (dots per frame * 2 - 1)  (60.0988138974405)
The NES uses dot skipping every other frame to remove one PPU dot, so the frame is one dot faster than otherwise. This is done to make the colors look better. In order for the SNES frame rate to match the NES frame rate, it would need to also use dot skipping. However, I cannot find anything suggesting that the SNES uses dot skipping. This would make the frame rate be the "No Skipping Dots" value (60.0984775561123) rather than the "Overall" value (60.0988138974405). This is a difference of 0.02s per hour.
Posted: 8/13/2018 4:42 PM
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natt
Editor, Emulator Coder, Site Developer
Joined: 5/11/2011
Posts: 1108
Location: Murka
sounds about right. I think the snes gets more complicated in interlace mode, but few games use that. Whatever, all of those digits are just mathematical masturbation. Would need hours of gameplay to drop or dup one frame. More stated accuracy than the accuracy numbers on the crystals, too, so better play in a temperature controlled 68 deg f room.
Posted: 8/14/2018 1:02 AM
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creaothceann
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Editor
Joined: 4/7/2005
Posts: 1874
Location: Germany
Here are my notes for the SNES: https://pastebin.com/W6KfC7Sr Basically, the NTSC color subcarrier is 5*7*9 / 88 * 1,000,000 = 3,579,545.{45} Hz. The master clock frequency is 6 times of that, ca. 21.477 MHz. Scanline duration is 1,364 master clock cycles except in certain conditions. b/w TV refresh rate is the same as the power line, i.e. 60.0 Hz for the US. NTSC slows it down to 60/1.001Hz to encode color. US SD TV is 262.5 lines per field, 525 per frame. That half line shifts the next field between the lines of the previous field. SNES progressive mode ends the fields early, so there are no half lines - you get 262 lines per field and 524 lines per frame (with black scanline gaps). The duration of a field decreases, so the number of fields per second increases to ca. 60.098. Note that a game could make any frame progressive or interlaced, so the number of frames doesn't strictly determine the duration of a TAS. Most games don't use interlaced mode though.