tepples wrote:
tomaitheous wrote:
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(Square pixels would be 135/22 = 6.136 MHz.)
My calculations put it at ~6.075mhz. If you use the 13.5mhz = 0.9 PAR.
The source I found while doing research for the
wiki article about overscan and PAR treats 135/10 MHz as 10/11 PAR (for a 704x480 pixel 4:3 clean area) and 135/11 MHz as square.
Oh, nice link (the first one)
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Then the NES and SNES are 8/7 PAR.
That would be a little too much. And here's why.
There are a number of scanline timing layouts, but most of them have the exact ratios (or close). Ideally, you would have to sample the scanline on an oscilloscope to get the exact PAR (from the ratio of scanline length and active period length), but you can use a pre-existing popular model for an easy example.
63.56us length for scanline. 53.33us for the active part. Now, the active part has to be an frame aspect ratio of 4:3.
4/3 = 1.3333333333333333333333333333333
That means for 480 scanlines at the correct frame aspect ratio, the square pixels would have to be 640. 480 x 4/3 = 640. Ideally, those would be square pixels.
63.56 / 53.33 = 1.1918244890305644102756422276392
1.1918244890305644102756422276392 *640 = 762.76767297956122257641102568911
762.76767297956122257641102568911 * 15734 = 12001386.6mhz dot clock.
Now, the BT.601 spec says there are 720 samples in 53.33us. Or 13.5mhz
640/720 = 0.888888888888888888888888888889 PAR
Less than 0.909090 PAR or even the straight 0.9 PAR I've seen stated.
We can test is with:
13500000 / 15734 = 858.01449091140205923477818736494
858.01449091140205923477818736494 / 1.1918244890305644102756422276392 = 719.91681561209993421948899830367
I'd say that's pretty darn close given I did it all in decimal instead of leaving it in fraction form.
We can now use 13.5mhz as a base for a ratio for comparing other dot clocks against.
13.5 / 5.3693 = 2.5142942282979159294507663941296
But, we don't have 480 pixel rows, we have 240 pixel rows. So you divide that by 2..
2.5142942282979159294507663941296 / 2 =
1.2571471141489579647253831970648 <- that's our ratio.
Now apply the PAR of 13.5 to get the PAR of the ratio..
1.2571471141489579647253831970648 * 0.8888888888888888888889 =
1.1174641014657404130892434768026 PAR for NES/SNES.
8/7 would give you 1.1428571428571428571428571428571 PAR. A little too much.
If 13.5mhz was exactly 0.9 PAR, that would put the NES/SNES PAR at 1.13. But the problem with that is, if 13.5mhz is 0.9 PAR - then frame aspect ration isn't exactly 4:3. You can't have both. So, if the PAR is going to be 0.9 for 720 samples for the active part of the scanline, the dot clock is going to be less than 13.5mhz. And I think this where why I saw reference to 13.4234mhz (which puts NES par at 1.125). It was an old scan - probably pre 90's. 4:3 frame ratio is fixed, everything else changes around it. 13.4234mhz also corresponds to almost
exactly half the Genesis dot clock for mid res mode (6.71165mhz).
Might just be coincidence, but it just doesn't feel like it
(or (5.36932 * 10) / 8 = 6.71165)
But like I said, you'd have to get the exact scanline and active period values. I've seen 63.5/52.6, 63.55/52.9, etc. And then set your set properly for 4:3 (but regardless, everything will fall into place). But nothing I've seen gives as much as 8/7 PAR. We're talking in terms of absolutes, but all things are relative on that set for PAR (even if incorrect frame aspect ratio) - if you're measuring against other res' of other systems on the same TV set.
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Apple II also uses twice the NTSC subcarrier.
Wasn't that so they could exploit a side effect of that? I.e. to get more colors or such? Like CGA and CoCo computers did? PCE doesn't have the same artifacts (it keeps shifting the color burst signal 180 degrees every other line, not to mention it does half pixel shifting every other line and then the next set on the next frame - if you turn on that filter).
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I've noticed that one digital video recorder I own won't record 240p. (The other one works.) Does this mean that if I ever get into SNESdev, I need to give games an option to run in interlace mode? It's jumpy, but at least it's closer to spec.
Dunno. I've heard of people running the composite signal through certain VCRs and outputting to the DVR to get the 240p signal capture-able. My analog capture card doesn't care. And it will capture the frame as 480i if you set it to that high of a res. That's how I capture at 240p 60fps from my consoles. I just separate the fields back into frames afterwards in post.
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I just happen to have a Street Fighter II CE board right here. It's 3 boards stacked, but on the bottom main board I see 3 oscillators. 16mhz near the C.P.S.-A-01 chip, 12mhz near the 68K, and 3.579Mhz of course near the Z80/YM2151. It was cool to unstack it and see these chips, I hope that helps.
PCE mid res has a PAR of ~0.8381. But the problem is, I don't think CPS-1 arcade system use NTSC. But the PAR of 8mhz is like 0.75. That's huge. It's possible, but that means pictures should be really stretched out on the PCE when they used direct tiles/sprite from CPS-1 games (pixel for pixel minus some master palette depth reduction 12bit -> 9bit).
A full 4:3 correct NTSC frame for PCE 7.159090mhz dot clock is ~382 pixels (that's max overscan, but still 4:3). That comes pretty close to the CPS1 assuming the res shown by the game system is full res (edge to edge max overscan). So maybe the 3.579Mhz for the audio chips, is up clocked (maybe internally by the video chipset). Or maybe it is just 8mhz and I'm missing something.