NTSC video
Note: This data is preliminary and still being reviewed.
Basics
Master clock is 21.47727273 MHz. Each PPU pixel lasts four clocks. $xy refers to a palette color in the range $00 to $3F.
Scanline Timing
Values in PPU pixels (341 total per scanline).
| sync | 25 |
| black | 4 |
| colorburst | 15 |
| black | 5 |
| pulse | 1 |
| left border (background color) | 15 |
| active | 256 |
| right border (background color) | 11 |
| black | 9 |
Brightness Levels
Voltage levels used by the PPU are as follows - absolute, relative to synch, and normalized between black level and white:
| Type | Absolute | Relative | Normalized |
| Synch | 0.781 | 0.000 | -0.359 |
| Colorburst L | 1.000 | 0.218 | -0.208 |
| Colorburst H | 1.712 | 0.931 | 0.286 |
| Color 0D | 1.131 | 0.350 | -0.117 |
| Color 1D (black) | 1.300 | 0.518 | 0.000 |
| Color 2D | 1.743 | 0.962 | 0.308 |
| Color 3D | 2.331 | 1.550 | 0.715 |
| Color 00 | 1.875 | 1.090 | 0.397 |
| Color 10 | 2.287 | 1.500 | 0.681 |
| Color 20 | 2.743 | 1.960 | 1.000 |
| Color 30 | 2.743 | 1.960 | 1.000 |
$xE/$xF output the same voltage as $1D. $x1-$xC output a square wave alternating between levels for $xD and $x0. Colors $20 and $30 are exactly the same.
Color Phases
111111------ 22222------2 3333------33 444------444 55------5555 6------66666 ------777777 -----888888- ----999999-- ---AAAAAA--- --BBBBBB---- -CCCCCC-----
The color generator is clocked by the rising and falling edges of the ~21.48 MHz clock, resulting in an effective ~42.95 MHz clock rate. There are 12 color square waves, spaced at regular phases. Each runs at the ~3.58 MHz colorburst rate. Color $xY uses the wave shown in row Y from the table. Color burst uses color phase 8 (with voltages listed above).
Color Tint Bits
There are three color modulation channels controlled by the top three bits of $2001. Each channel uses one of the color square waves (see above diagram) and enables attenuation of the video signal when the color square wave is high. A single attenuator is shared by all channels.
| $2001 | Active phase | Complement |
|---|---|---|
| Bit 7 | Color 8 | Color 2 (blue) |
| Bit 6 | Color 4 | Color A (green) |
| Bit 5 | Color C | Color 6 (red) |
When signal attenuation is enabled by one or more of the channels and the current pixel is a color other than $xE/$xF (black), the signal is attenuated as follows (calculations given for both relative and absolute values as shown in the voltage table above):
relative = relative * 0.746
normalized = normalized * 0.746 - 0.0912
For example, when $2001 bit 6 is true, the attenuator will be active during the phases of color 4. This means the attenuator is not active during its complement (color A), and the screen appears to have a tint of color A, which is green.
Example Waveform
This waveform steps through various grays and then stops on a color.
1.0 +--+
0.9 | |
0.8 | |
0.7 +--+ | +-+ +-+
0.6 | | | | | |
0.5 | | | | | |
0.4 +--+ | | | | |
0.3 +--+ | | | | |
0.2 | | | | | |
0.1 | | | | | |
0.0 . +--+ . . . . . +-+ +-+ + . .
-0.1 --+
0D 0F 2D 00 10 30 11
The PPU's shortcut method of NTSC modulation often produces artifacts in which vertical lines appear slightly ragged, as the chroma spills over into luma.
Interactive Demo
The following C source code implements the above described algorithm and displays it on screen with interactive mouse control of phase using SDL.
- nes_ntsc_waveform.c
- Windows executable (requires SDL.dll)
- nes_ntsc_palette.c - a sample NTSC palette generator program
