What's the reason for the missing PPU cycle on even frames?

Discuss technical or other issues relating to programming the Nintendo Entertainment System, Famicom, or compatible systems.

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psycopathicteen
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Re: What's the reason for the missing PPU cycle on even fram

Post by psycopathicteen » Wed Mar 21, 2018 9:24 pm

Is it a coincidence that 1.3Mhz at -3dB is the standard chroma bandwidth for NTSC, and it just so happens that a basic (-1/4, 0, 1/2, 0, -1/4) FIR filter with a sample rate of 14.32 Mhz gives a -3dB level at 3.58 +/- 1.3 Mhz?

lidnariq
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Re: What's the reason for the missing PPU cycle on even fram

Post by lidnariq » Thu Mar 22, 2018 7:04 pm

Yes.

You can painstakingly work out the math longform if you'd rather. Nothing magical will show up about a number that works out to very approximately roughly Fs/11.

Is there possibly some special relationship between the sampling rate—4x the colorburst frequency—and 1.3MHz? Yes, that I think might be deliberate. But your choice of discrete-time filter? Absolutely not, discrete-time stuff was basically not a practical option during NTSC's genesis.

psycopathicteen
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Joined: Wed May 19, 2010 6:12 pm

Re: What's the reason for the missing PPU cycle on even fram

Post by psycopathicteen » Wed Mar 28, 2018 1:02 pm

This thread has turned into a "ask questions about analog TVs" thread.

Is there any research about the SNES's RF modulator circuit? I know that most composite encoder chips have a max of 133 IRE, and RF has a smaller range of 120 IRE. Does the SNES in RF have a smaller chroma amplitude, or does the SNES clamp yellows and cyans?

lidnariq
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Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: What's the reason for the missing PPU cycle on even fram

Post by lidnariq » Mon May 28, 2018 9:08 pm

lidnariq wrote:I should remember to plug the RF output of my SNES into my oscilloscope and get a spectrum plot. It's no spectrum analyzer, but I bet even with crappy resolution I'll see a few interesting things.
Connecting my oscilloscope directly to the RF output of various things. No cart, so solid screen and no sound, scope set to 250MS/s and 50mV/div, the lowest setting that's not additionally bandwidth limited.
NES-001, channel 4:
Primary video carrier image at 67.25MHz; ≈-47dB during vsync/hsync/blanking and ≈-54dB during whatever color it randomly chose.
Audio carrier visible at 71.75MHz, ≈-65dB
Extra modulated signal visible at 62.25MHz, comparable in magnitude to the audio carrier.
Main 21.47MHz system clock visible; ≈ -65dB

NES-001, channel 3:
Primary video carrier image at 61.25MHz, ≈-45dB and ≈-53dB as above
Audio carrier visible at 65.65MHz (should be 65.75), ≈-61dB
Extra modulated signal visible at 66.65 ≈-68dB
Extra modulated signal visible at 56.70MHz, also ≈-61dB
Main 21.47MHz system clock visible; ≈ -65dB

SNS-001:
Weird ringing even when no power adapter plugged in at 66.75MHz, regardless of RF channel setting. ≈-64dB. Image disappears if via RF switch and console is off.
SNS-001, channel 4:
Primary video carrier image at 67.25MHz, ≈-46dB
Audio carrier visible at 71.75MHz, ≈-64dB
Extra modulated signal visible at 62.8MHz, ≈-61dB

SNS-001, channel 3:
Primary video carrier image at 61.30MHz, ≈-44dB
Audio carrier visible at ≈65.80MHz, ≈-60dB
Extra modulated signal visible at 56.80MHz, ≈-59dB

Unfortunately, the SNR of my scope isn't good enough to see anything quieter, and the whole FM radio band is coupling into everything.

psycopathicteen
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Joined: Wed May 19, 2010 6:12 pm

Re: What's the reason for the missing PPU cycle on even fram

Post by psycopathicteen » Wed Aug 15, 2018 7:51 am

I've been looking at docs for ntsc encoder and decoder chips and something I noticed that most of the "1.3 Mhz" chroma filters have zeros at ~2 Mhz, which is a much faster roll off than either Gaussian or Hamming filter. Does this mean there's chroma ringing on these filters?

lidnariq
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Location: Seattle

Re: What's the reason for the missing PPU cycle on even fram

Post by lidnariq » Wed Aug 15, 2018 11:47 am

Depends on the specific transfer function, whether it's continuous time or discrete, and what you mean by "2MHz".

Oh, of course. And zeroes never cause ringing, only poles do.

psycopathicteen
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Re: What's the reason for the missing PPU cycle on even fram

Post by psycopathicteen » Thu Aug 16, 2018 2:07 pm

Is there a limit to how fast the roll off without ringing?

lidnariq
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Joined: Sun Apr 13, 2008 11:12 am
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Re: What's the reason for the missing PPU cycle on even fram

Post by lidnariq » Thu Aug 16, 2018 2:33 pm

Not really.

Just increased group delay.

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