NTSC video

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Note: This data is preliminary and still being reviewed.

Contents

Basics

NTSC Master clock is 21.47727273 MHz and each PPU pixel lasts four clocks; PAL master clock is 26.6017125 MHz, and each PPU pixel lasts five clocks. $xy refers to a palette color in the range $00 to $3F.

Scanline Timing

Values in PPU pixels (341 total per scanline).

Rendering scanlines (n=240):

name start duration row number
short sync 280 25 261, 0-238
black (back porch) 305 4 261, 0-238
colorburst 309 15 261, 0-238
black (the rest of back porch) 324 5 261, 0-238
pulse (background color in grayscale) 329 1 261, 0-238
left border (background color) 330 15 261, 0-238 14 columns on end of row 261 for odd frames, if either background or sprite rendering is enabled
active 4 256 0-239 if background rendering is disabled, border will be rendered instead
right border (background color) 260 11 0-239
black (front porch) 271 9 0-239

Post-render scanlines (n=2):

name start duration row
short sync 280 25 239-240
black (back porch) 305 4 239-240
colorburst 309 15 239-240
black (the rest of back porch) 324 5 239-240
pulse (background color in grayscale) 329 1 239-240
bottom border (background color) 330 282 239-240
black (front porch) 271 9 240-241

Blanking scanlines (n=3+14):

name start duration row
short sync 280 25 241-243, 247-260
black (back porch) 305 4 241-243, 247-260
colorburst 309 15 241-243, 247-260
black 324 297 241-243, 247-260

Vertical sync scanlines (n=3):

name start duration row
long sync 280 318 244-246
black (sync separator) 257 23 245-247

For a total of 262 scanlines.

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.

When grayscale is active, all colors between $x1-$xD are treated as $x0. Notably this behavior extends to the first pixel of the border color, which acts as a sync pulse on every visible scanline.

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. On NTSC, color $xY uses the wave shown in row Y from the table. NTSC color burst (pure shade -U) uses color phase 8 (with voltages listed above); PAL color burst is believed to alternate between 7 (-U+V) and A (-U-V), so hue is rotated by 15° from NTSC. PAL alternates the broadcast sign of the V component, so on PAL every odd scanline will use the appropriate opposite phase—e.g. phases 5-C are respectively replaced with C-5.

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.

Note that on the Dendy and PAL NES, the green and red bits swap meaning.

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.

Generation and demodulation of a red corner



Emulating in C++ code

Calculating the momentary NTSC signal level can be done as follows in C++:

// pixel = Pixel color (9-bit) given as input. Bitmask format: "eeellcccc".
// phase = Signal phase (0..11). It is a variable that increases by 8 each pixel.
float NTSCsignal(int pixel, int phase)
{
    // Voltage levels, relative to synch voltage
    static const float black=.518f, white=1.962f, attenuation=.746f,
        levels[8] = {.350f, .518f, .962f,1.550f,  // Signal low
                    1.094f,1.506f,1.962f,1.962f}; // Signal high

    // Decode the NES color.
    int color = (pixel & 0x0F);    // 0..15 "cccc"
    int level = (pixel >> 4) & 3;  // 0..3  "ll"
    int emphasis = (pixel >> 6);   // 0..7  "eee"
    if(color > 13) { level = 1;  } // For colors 14..15, level 1 is forced.

    // The square wave for this color alternates between these two voltages:
    float low  = levels[0 + level];
    float high = levels[4 + level];
    if(color == 0) { low = high; } // For color 0, only high level is emitted
    if(color > 12) { high = low; } // For colors 13..15, only low level is emitted

    // Generate the square wave
    auto InColorPhase = [=](int color) { return (color + phase) % 12 < 6; }; // Inline function
    float signal = InColorPhase(color) ? high : low;

    // When de-emphasis bits are set, some parts of the signal are attenuated:
    if( ((emphasis & 1) && InColorPhase(0))
    ||  ((emphasis & 2) && InColorPhase(4))
    ||  ((emphasis & 4) && InColorPhase(8)) ) signal = signal * attenuation;

    return signal;
}

The process of generating NTSC signal for a single pixel can be simulated with the following C++ code:

void RenderNTSCpixel(unsigned x, int pixel, int PPU_cycle_counter)
{
    int phase = PPU_cycle_counter * 8;
    for(int p=0; p<8; ++p) // Each pixel produces distinct 8 samples of NTSC signal.
    {
        float signal = NTSCsignal(pixel, phase + p); // Calculated as above
        // Optionally apply some lowpass-filtering to the signal here.
        // Optionally normalize the signal to 0..1 range:
        static const float black=.518f, white=1.962f;
        signal = (signal-black) / (white-black);
        // Save the signal for this pixel.
        signal_levels[ x*8 + p ] = signal;
    }
}

It is important to note that while the NES only generates eight (8) samples of NTSC signal per pixel, the wavelength for chroma is 12 samples long. This means that the colors of adjacent pixels get mandatorily mixed up to some degree. For the same reason, narrow black&white details can be interpreted as colors.

Because the scanline length is uneven (341*8 is not an even multiple of 12), the color mixing shifts a little each scanline. This appears visually as a sawtooth effect at the edges of colors at high resolution. The sawtooth cycles every 3 scanlines.

Because also the frame length is uneven (neither 262*341*8 nor (262*341-1)*8 is an even multiple of 12), the color mixing also changes a little every frame. When rendering is normally enabled, the screen is alternatingly 89342 and 89341 cycles long. The combination of these (89342+89341)*8 is an even multiple of 12, which means that the artifact pattern cycles every 2 frames. The pattern of cycling can be changed by disabling rendering during the end of the pre-render scanline; it forces the screen length to 89342 cycles, even if would be 89341 otherwise.

The process of decoding NTSC signal (convert it into RGB) is subject to a lot of study, and there are many patents and different techniques for it. A simple method suitable for emulation is covered below. It is not accurate, because in reality the chroma is blurred much more than is done here (the region of signal sampled for I and Q is wider than 12 samples), and the filter used here is a simple box FIR filter rather than an IIR filter, but it already produces a quite authentic looking picture. In addition, the border region (total of 26 pixels of background color around the 256-pixel scanline) is not sampled.

    float signal_levels[256*8] = {...}; // Eight signal levels for each pixel, normalized to 0..1 range. 
Calculated as above.

    unsigned Width; // Input: Screen width. Can be not only 256, but anything up to 2048.
    float phase;    // Input: This should the value that was PPU_cycle_counter * 8 + 3.9
                    // at the BEGINNING of this scanline. It should be modulo 12.
                    // It can additionally include a floating-point hue offset.
    for(unsigned x = 0; x < Width; ++x)
    {
        // Determine the region of scanline signal to sample. Take 12 samples.
        int center = x * (256*8) / Width + 0;
        int begin = center - 6; if(begin < 0)     begin = 0;
        int end   = center + 6; if(end   > 256*8) end   = 256*8;
        float y = 0.f, i = 0.f, q = 0.f; // Calculate the color in YIQ.
        for(int p = begin; p < end; ++p) // Collect and accumulate samples
        {
            float level = signal_levels[p] / 12.f;
            y  =  y + level;
            i  =  i + level * cos( M_PI * (phase+p) / 6 );
            q  =  q + level * sin( M_PI * (phase+p) / 6 );
        }
        render_pixel(y,i,q); // Send the YIQ color for rendering.
    }

The NTSC decoder here produces pixels in YIQ color space.

If you want more saturated colors, just multiply i and q with a factor of your choosing, such as 1.7. If you want brighter colors, just multiply y, i and q with a factor of your choosing, such as 1.1. If you want to adjust the hue, just add or subtract a value from/to phase. If you want to see so called chroma dots, change the begin and end in such manner that you collect a number of samples that is not divisible with 12. If you want to blur the video horizontally, change the begin and end in such manner that the samples are collected from a wider region.

The YIQ colors can be converted into sRGB colors with the following formula, using the FCC-sanctioned YIQ-to-RGB conversion matrix. This produces a value that can be saved to e.g. framebuffer:

    float gamma = 2.0f; // Assumed display gamma
    auto gammafix = [=](float f) { return f <= 0.f ? 0.f : pow(f, 2.2f / gamma); };
    auto clamp    = [](int v) { return v>255 ? 255 : v; };
    unsigned rgb =
        0x10000*clamp(255.95 * gammafix(y +  0.946882f*i +  0.623557f*q))
      + 0x00100*clamp(255.95 * gammafix(y + -0.274788f*i + -0.635691f*q))
      + 0x00001*clamp(255.95 * gammafix(y + -1.108545f*i +  1.709007f*q));

The two images below illustrate the NTSC artifacts. In the left side image, 12 samples of NTSC signal were generated for each NES pixel, and each display pixel was separately rendered by decoding that 12-sample signal. In the right side image, 8 samples of NTSC signal were generated for each NES pixel, and each display pixel was rendered by decoding 12 samples of NTSC signal from the corresponding location within the scanline.

Per-pixel rendering: 12 samples of NTSC signal per input pixel; the same 12 samples are decoded for each output pixel
Per-scanline rendering: 8 samples of NTSC signal per input pixel; 12 samples are decoded for each output pixel
Same as above, but rendered at 256x240 without upscaling
Same in grayscale (zero saturation). This illustrates well how the different color values have exactly the same luminosity; only the chroma phase differs.
Same as above, but rendered at 256x240 rather than at 2048x240 and then downscaled
Same in grayscale

The source code of the program that generated both images can be read at [1]. Note that even though the image resembles the well-known Philips PM5544 test card, it is not the same; the exact same colors could not be reproduced with NES colors. In addition, some parts were changed to better test NES features. For example, the backgrounds for the "station ID" regions (the black rectangles at the top and at the bottom inside the circle) are generated using the various blacks within the NES palette.

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.

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