Editing NTSC (section)

=== Color encoding ===
{{More citations needed|section|date=February 2024}}
{{See also|YIQ}}

NTSC uses a [[Luma (video)|luminance]]-[[chrominance]] encoding system, incorporating concepts invented in 1938 by [[Georges Valensi]]. Using a separate luminance signal maintained backward compatibility with black-and-white television sets in use at the time; only color sets would recognize the chroma signal, which was essentially ignored by black and white sets.

The red, green, and blue primary color signals <math>(R^\prime G^\prime B^\prime)</math> are weighted and summed into a single [[Luma (video)|luma]] signal, designated <math>Y^\prime</math> (Y prime)<ref>{{cite web |title=Poynton's Color FAQ by Charles Poynton |url=http://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/POYNTON1/ColorFAQ.html#RTFToC9 |website=Homepages.inf.ed.ac.uk}}</ref> which takes the place of the original [[analog television|monochrome signal]]. The color difference information is encoded into the chrominance signal, which carries only the color information. This allows black-and-white receivers to display NTSC color signals by simply ignoring the chrominance signal. Some black-and-white TVs sold in the U.S. after the introduction of color broadcasting in 1953 were designed to filter chroma out, but the early B&W sets did not do this and [[chroma dots|chrominance]] could be seen as a [[dot crawl|crawling dot pattern]] in areas of the picture that held saturated colors.<ref>{{cite book |last1=Large |first1=David |url=https://books.google.com/books?id=QOFWx2umt8sC&dq=black+and+white+dot+crawl&pg=PA55 |title=Modern Cable Television Technology |last2=Farmer |first2=James |date=January 13, 2004 |publisher=Elsevier |isbn=978-0-08-051193-1}}</ref>

To derive the separate signals containing only color information, the difference is determined between each color primary and the summed luma. Thus the red difference signal is <math>R^\prime - Y^\prime</math> and the blue difference signal is <math>B^\prime - Y^\prime</math>. These difference signals are then used to derive two new color signals known as <math>I^\prime</math> (in-phase) and <math>Q^\prime</math> (in quadrature) in a process called [[Quadrature amplitude modulation|QAM]]. The <math>I^\prime Q^\prime</math> color space is rotated relative to the difference signal color space, such that orange-blue color information (which the human eye is most sensitive to) is transmitted on the <math>I^\prime</math> signal at 1.3 MHz bandwidth, while the <math>Q^\prime</math> signal encodes purple-green color information at 0.4 MHz bandwidth; this allows the chrominance signal to use less overall bandwidth without noticeable color degradation. The two signals each amplitude modulate<ref name="Monochrome and Colour Television">{{cite book |last1=Gulati |first1=R. R. |url=https://books.google.com/books?id=SHM47MKmGXkC |title=Monochrome and Colour Television |date=December 2005 |publisher=New Age International |isbn=978-81-224-1607-7}}</ref> 3.58&nbsp;MHz carriers which are 90&nbsp;degrees out of phase with each other<ref>{{cite book |url=https://books.google.com/books?id=Tm_U-Qd58RgC&dq=ntsc+i+and+q+carrier&pg=PA226 |title=Newnes Guide to Digital TV |date=November 17, 2002 |publisher=Newnes |isbn=978-0-7506-5721-1}}</ref> and the result added together but with the [[Double-sideband suppressed-carrier transmission|carriers themselves being suppressed]].<ref>{{cite book |url=https://books.google.com/books?id=xwMZw4UewuUC&dq=ntsc+i+and+q+carrier&pg=PA8 |title=Digital Television: Satellite, Cable, Terrestrial, IPTV, Mobile TV in the DVB Framework |date=February 20, 2024 |publisher=Taylor & Francis |isbn=978-0-240-52081-0}}</ref><ref name="Monochrome and Colour Television"/> The result can be viewed as a single sine wave with varying phase relative to a reference carrier and with varying amplitude. The varying phase represents the instantaneous color [[hue]] captured by a TV camera, and the amplitude represents the instantaneous color [[Colorfulness#Saturation|saturation]]. The {{Fraction|3|51|88}}&nbsp;MHz [[subcarrier]] is then added to the Luminance to form the composite color signal<ref name="Monochrome and Colour Television"/> which modulates the video signal [[Carrier wave|carrier]]. 3.58&nbsp;MHz is often stated as an abbreviation instead of 3.579545&nbsp;MHz.<ref>{{cite book |url=https://books.google.com/books?id=AvQAa5Zfuj0C&dq=ntsc+3.58+MHz&pg=PA123 |title=Modern Television Practice Principles, Technology and Servicing 2/Ed |publisher=New Age International |isbn=978-81-224-1360-1}}</ref>

For a color TV to recover hue information from the color subcarrier, it must have a zero-phase reference to replace the previously suppressed carrier. The NTSC signal includes a short sample of this reference signal, known as the [[colorburst]], located on the back porch of each horizontal synchronization pulse. The color burst consists of a minimum of eight cycles of the unmodulated (pure original) color subcarrier. The TV receiver has a local oscillator, which is synchronized with these color bursts to create a reference signal. Combining this reference phase signal with the chrominance signal allows the recovery of the <math>I^\prime</math> and <math>Q^\prime</math> signals, which in conjunction with the <math>Y^\prime</math> signal, is reconstructed to the individual <math>R^\prime G^\prime B^\prime</math> signals, that are then sent to the [[Cathode-ray tube|CRT]] to form the image.

In CRT televisions, the NTSC signal is turned into three color signals: red, green, and blue, each controlling an electron gun that is designed to excite only the corresponding red, green, or blue phosphor dots. TV sets with digital circuitry use sampling techniques to process the signals but the result is the same. For both analog and digital sets processing an analog NTSC signal, the original three color signals are transmitted using three discrete signals (Y, I and Q) and then recovered as three separate colors (R, G, and B) and presented as a color image.

When a transmitter broadcasts an NTSC signal, it amplitude-modulates a radio-frequency carrier with the NTSC signal just described, while it frequency-modulates a carrier 4.5&nbsp;MHz higher with the audio signal. If non-linear distortion happens to the broadcast signal, the {{Fraction|3|51|88}}&nbsp;MHz color carrier may [[Beat (acoustics)|beat]] with the sound carrier to produce a dot pattern on the screen. To make the resulting pattern less noticeable, designers adjusted the original 15,750&nbsp;Hz scanline rate down by a factor of 1.001 ({{Fraction|100|1,001}}%) to match the audio carrier frequency divided by the factor 286, resulting in a field rate of approximately 59.94&nbsp;Hz. This adjustment ensures that the difference between the sound carrier and the color subcarrier (the most problematic [[intermodulation]] product of the two carriers) is an odd multiple of half the line rate, which is the necessary condition for the dots on successive lines to be opposite in phase, making them least noticeable.

The 59.94&nbsp;rate is derived from the following calculations. Designers chose to make the chrominance subcarrier frequency an ''n'' + 0.5 multiple of the line frequency to minimize interference between the luminance signal and the chrominance signal. (Another way this is often stated is that the color subcarrier frequency is an odd multiple of half the line frequency.) They then chose to make the audio subcarrier frequency an integer multiple of the line frequency to minimize visible (intermodulation) interference between the audio signal and the chrominance signal. The original black-and-white standard, with its 15,750&nbsp;Hz line frequency and 4.5&nbsp;MHz audio subcarrier, does not meet these requirements, so designers had to either raise the audio subcarrier frequency or lower the line frequency. Raising the audio subcarrier frequency would prevent existing (black and white) receivers from properly tuning in the audio signal. Lowering the line frequency is comparatively innocuous, because the horizontal and vertical synchronization information in the NTSC signal allows a receiver to tolerate a substantial amount of variation in the line frequency. So the engineers chose the line frequency to be changed for the color standard. In the black-and-white standard, the ratio of audio subcarrier frequency to line frequency is {{frac|4.5&nbsp;MHz|15,750&nbsp;Hz}}&nbsp;{{=}}&nbsp;{{Fraction|285|5|7}}. In the color standard, this becomes rounded to the integer 286, which means the color standard's line rate is {{frac|4.5&nbsp;MHz|286}}&nbsp;&asymp;&nbsp;{{Fraction|15,734|266|1,001}}&nbsp;Hz. Maintaining the same number of scan lines per field (and frame), the lower line rate must yield a lower field rate. Dividing {{frac|4,500,000|286}} lines per second by 262.5&nbsp;lines per field gives approximately 59.94&nbsp;fields per second.<!--There are two reasons why this is important. First, the chrominance signal was interpreted as part of the luminance by monochrome TV sets that were in use at the time of the introduction of color TV (which didn't have notch filters to remove the chrominance carrier), causing dots to appear on strongly contrasting edges (which are high-frequency video information.) The chosen line rate causes the dots to move, which makes them harder for the human eye to follow. The effect is still noticeable on close examination, however, and is referred to as [[dot crawl]]. A second benefit of the chosen field rate was realized much later: The phase difference of the interference pattern on successive lines makes it very easy to design a simple [[comb filter]] to separate chrominance and luminance information to a greater degree.-->