Composite Video on the NES: Why's it so wobbly?

The Nintendo Entertainment System. Is it the platonic ideal of an 8-bit video game system? Well, only because it’s so prominent and successful– it’s actually kind of an oddball in its expandability and design. But there’s something else about it. The picture is a bit… wobbly. Well, over composite video anyway. Let’s dig in and learn a little big more about the nitty-gritty of composite video.

This whole blog post is only going to talk about NTSC encoding and timing. PAL encoding is its own thing, and the signal timings on 50Hz are different in significant ways. I don't have many consoles that output PAL and I'm less familiar with it.

Not so mighty now, are you

You don't have a video tag support or something? So you can't see this footage of the *Mighty Bomb Jack* title screen? Ah too bad.

Mighty Bomb Jack, the most iconic bomb-collection platform taking place in a pyramid for the Nintendo Entertainment System. The title screen here, like a lot of title screens, is entirely static. But here, I’m using my composite-modded system, which just amplifies the video signal coming right off of the PPU, and the picture is wobbly.

And you might be tempted to say, now wait a second Nicole, this is composite video. You wrote a whole post on it! Composite video is inferior to other signals because it’s basically a form of video signal compression. And that’s true; but there’s no reason a composite frame should differ from frame to frame, is there? Here’s my beautiful Apple ][ running Shanghai (as referenced in a recent post).

You don't have a video tag support or something? So you can't see this footage of the *Shanghai* title screen? Ah too bad.

That’s incredibly static!

Explanation Attempt 1: Interference

Interference isn’t actually as big of a problem as you’d expect. It’s probably something you’re thinking about, but interference would generally present as a random pattern, not the cyclic noise seen on the NES screenshot. In fact, the Apple video was captured using a much longer cable, being strewn over my desk where several electronics were running, and you can actually see some noise in the white parts of the image. But it’s noise; it’s uncorrelated to the input signal.

This is the letter “B” from Mighty Bomb Jack above, crapped from the raw capture. Notice the repeating jagged pattern on the left side of the letter “B” where the white meets the blue of the sky. If this was just signal interference from outside, there’d be no reason to expect it to follow the images that closely.

Explanation Attempt 2: The colorburst

If we divide the colorburst frequency of ~3.57 MHz by the NTSC linerate of 15.734kHz, we get a near-exact 227.5. That’s no coincidence; RCA deliberately aimed to have the color carrier a half-integer multiple of the linerate. That’s because if one scanline is similar to the next line, it’ll be a signal with a frequency similar to the linerate. It’s very rare, on the other hand, for the television signal to have components that are a half-integer multiple; so this makes it easier to filter out the color signal. I talked about this a bit in my 2021 post.

You might notice something here. The colorburst is expected to just be a portion of a continuous 3.57 MHz signal. That’s signal doesn’t fit evenly into the line frequency; so as a result, the colorburst should not be the same on every scanline. Could that be the cause of our wobble?

On the Apple, on the other hand, Woz noted it’d be very hard to use his HIRES mode if the pixel arrangements different on every other scanline; differing in every other byte on a single scanline is hard enough. So in this rare case, he took pity, and included extra hardware to stretch the clock cycle; on the Apple, the line rate is altered such that it has 228 color cycles. That is to say, it breaks RCA’s optimization, and allows the luminance to more easily interfere with the chrominance. Which is what artifact color is, after all.

So the Apple was a bad example. Another console with 228 color cycles per scanline is my Japanese Sega Master System, even though it’s seen as a more compliant signal in other ways. It was actually surprisingly hard to find a Master System game with a static title screen without a black background; I went with Ace of Aces.

Black backgrounds are only to be used to represent space, Atari said so.

You don't have a video tag support or something? So you can't see this footage of the *Ace of Aces* title screen? Ah too bad.

Where you do see variation in scanline artifacts is in scrolling. Look at the ground in the Alex Kidd in Miracle World shot above. Look at the ground, and the artifacts on the ground in particular– where the artifacts are varies from frame to frame; or it’s more accurate to say, remains constant, while the ground moves, producing a shimmering effect.

What does a system that actually has 227.5 color cycles per scanline look like? Well, the PC Engine! (or, in this...