YouTube video of the filter running in an NES emulator: https://www.youtube.com/watch?v=giML77yy7To

The result of going down a very deep rabbit hole. I learned a lot about analog signal processing, television, and the NTSC standard in the process. Written to be compatible with C89.

Just like King's Crook (my from-scratch 3D game), this code follows the same restrictions:

1. Everything must be done in software, no explicit usage of hardware acceleration.
2. No floating point types or literals, everything must be integer only.
3. No 3rd party libraries, only C standard library and OS libraries for window, input, etc.
4. No languages used besides C.
5. No compiler specific features and no SIMD.
6. Single threaded.

This program performs relatively well and can be easily used in real-time applications to emulate NTSC output. It is by no means fully optimized (mainly for readability), so
there is a lot of room for performance gains.

• Relatively accurate composite NTSC image output
  -- with bandlimited luma/chroma
  -- color artifacts (extends to being able to show specially patterned b/w images as color)
  -- accurate dot crawl
• VSYNC and HSYNC
• Signal noise (optional)
• Interlaced and progressive scan
• Monochrome and full color
• Highly customizable signal encoding (see crt_template.h to make your own)
• NES decoding support

The command line program provided does not let you mess with all the settings like black/white point, hue, brightness, saturation, and contrast.

In the crt_main.c file, there are two main()'s. One is for a command line program and the other uses my FW library (found here https://github.com/LMP88959/PL3D-KC) to provide real-time NTSC emulation with adjustable parameters.

The famous waterfall 'rainbow' effect created as a result of dithering will show if it is compiled with CRT_CHROMA_PATTERN set to 0. Specially patterned black and white images can be encoded/decoded with color just like a real composite NTSC display.

NES Mode is a separate version of NTSC-CRT that has adjusted timings to match the non-standard NES specifications.
https://www.nesdev.org/wiki/NTSC_video
These timings and extra NES specific features were incorporated into the NES version by Persune.
Extra features include the NES-specific NTSC frame pulses, dot skipping every odd frame, and border colors.

cd NTSC-CRT

cc -O3 -o ntsc *.c

or using CMake on Linux, macOS, or Windows:

Note: There are 3 available flags / variables:

• LIVE (default: off) - Set to on to enable rendering to a video window from an input PPM/BMP image file
• VIDEO (default: off) - Set to on to enable rendering of sequence of frames. See video_convert.c for details
• CRT_SYSTEM (default: 0) - 0 - CRT_SYSTEM_NTSC (standard NTSC), 5 - CRT_SYSTEM_NTSCVHS (standard NTSC VHS). See crt_core.h for details

cmake -B build -DLIVE=off -DVIDEO=on -DCRT_SYSTEM=5
cmake --build build --config Release

The default command line takes a single PPM or BMP image file and outputs a processed PPM or BMP file:

usage: ./ntsc -m|o|f|p|r|h|a outwidth outheight noise artifact_hue infile outfile
sample usage: ./ntsc -op 640 480 24 0 in.ppm out.ppm
sample usage: ./ntsc - 832 624 0 90 in.ppm out.ppm
-- NOTE: the - after the program name is required
	artifact_hue is [0, 359]
------------------------------------------------------------
	m : monochrome
	o : do not prompt when overwriting files
	f : odd field (only meaningful in progressive mode)
	p : progressive scan (rather than interlaced)
	r : raw image (needed for images that use artifact colors)
	a : save analog signal as image instead of decoded image
	h : print help

by default, the image will be full color, interlaced, and scaled to the output dimensions

If -DVIDEO=on is specified, the command line output will look like this:

NTSC/CRT v2.2.1 by EMMIR 2018-2023
This program does not operate on video files, only sequences of
images. Please make sure you have the FFMPEG command line tools
installed and follow these instructions to convert a video
using the NTSC/CRT library:
  mkdir frames
  mkdir output
  ffmpeg -r 1 -i your_video.mov -r 1 ./frames/$frame%06d.bmp
  ./ntsc_video.exe <arguments>
  ffmpeg -r 30 -f image2 -s 640x480 -i ./output/%06d.bmp -vcodec libx264 -crf 10 -pix_fmt yuv420p out.mp4

------------------------------------------------------------
usage: ntsc_video.exe -m|o|p|s|h num_frames outwidth outheight noise
sample usage: ntsc_video.exe -oa 5000 640 480 0
sample usage: ntsc_video.exe - 1400 832 624 12
-- NOTE: the - after the program name is required
------------------------------------------------------------
        m : monochrome
        o : do not prompt when overwriting files
        a : mess up the bottom of the frame (useful for the VHS look)
        s : fill in gaps between scan lines
        p : progressive scan (rather than interlaced)
        h : print help

by default, the image will be full color and interlaced

Global variables:

#include "crt_core.h"

static struct CRT crt;
static struct NTSC_SETTINGS ntsc;
static int color = 1;
static int noise = 12;
static int field = 0;
static int raw = 0;
static int hue = 0;

In your initialization function:

/* pass it the buffer to be drawn on screen */
crt_init(&crt, screen_width, screen_height, CRT_PIX_FORMAT_BGRA, screen_buffer);
/* specify some settings */
crt.blend = 1;
crt.scanlines = 1;

In your drawing loop:

ntsc.data = video_buffer; /* buffer from your rendering */
ntsc.format = CRT_PIX_FORMAT_BGRA;
ntsc.w = video_width;
ntsc.h = video_height;
ntsc.as_color = color;
ntsc.field = field & 1;
ntsc.raw = raw;
ntsc.hue = hue;
if (ntsc.field == 0) {
  ntsc.frame ^= 1;
}
crt_modulate(&crt, &ntsc);
crt_demodulate(&crt, noise);
field ^= 1;

Check out crt_template.h and crt_template.c
Most modifications should only be to the constants defined in crt_template.h

These emulators have this NTSC filter as an option:
puNES: https://github.com/punesemu/puNES
BeesNES: https://github.com/L-Spiro/BeesNES

Web version by @binji (might not be up to date):
https://binji.github.io/NTSC-CRT/
To use the web version, drag a PPM image into the web browser.

https://github.com/LMP88959/PAL-CRT - my PAL modem written in a similar fashion to this library
https://github.com/F33RNI/NTSC-VHS-Renderer - Intuitive rendering and GUI for this project
https://github.com/svofski/CRT - PAL/SECAM modem in Python/GLSL by @svofski

If you have any questions feel free to leave a comment on YouTube OR join the King's Crook Discord server :)

YouTube: https://www.youtube.com/@EMMIR_KC/videos

Discord: https://discord.gg/hdYctSmyQJ

itch.io: https://kingscrook.itch.io/kings-crook

Feel free to use the code in any way you would like, however, if you release anything with it, a comment in your code/README saying where you got this code would be a nice gesture but it’s not mandatory.

Thank you for your interest!