File:NTSC video ragged box.png

How an NTSC video signal gets generated in the PPU and decoded by the TV

Horizontal scale: 1 diagram pixel = 1 NTSC master clock (21.5 MHz) cycle; 4 diagram pixels = 1 NES pixel; 6 diagram pixels = 1 color subcarrier cycle

Top row: what goes on in the NES PPU

1. Generate the subcarrier for a solid red screen (color $16).
2. A shape drawn in this color, including a 2-pixel-wide vertical line.
3. Multiply it by 0 outside of the shape and 1 inside the shape. Notice how the subcarrier protrudes into the shape.

Bottom row: what goes on in the TV when separating luma from chroma

1. Incoming picture signal on the composite
2. Impulse response of the low-pass filter
3. Picture signal convolved with the low-pass filter, used as luma. Notice the ragged left and right sides of the vertical line.

The filter in this diagram is an FIR filter [1 4 7 8 8 8 7 4 1]/48, which factors to [1 1][1 1][1 1][1 1 1][1 0 0 1]/48. A real TV might use a Bessel filter (near-linear-phase IIR filter), but the principle is the same: filter out anything above 3 MHz.

Permission is granted to use this copyrighted illustration under the WTFPL.