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YIQ is the
color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represen ...
used by the analog
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
color TV system, employed mainly in
North North is one of the four compass points or cardinal directions. It is the opposite of south and is perpendicular to east and west. ''North'' is a noun, adjective, or adverb indicating direction or geography. Etymology The word ''north ...
and
Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...
, and
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
. ''I'' stands for ''in-phase'', while ''Q'' stands for ''quadrature'', referring to the components used in
quadrature amplitude modulation Quadrature amplitude modulation (QAM) is the name of a family of digital modulation methods and a related family of analog modulation methods widely used in modern telecommunications to transmit information. It conveys two analog message signa ...
. Other TV systems used different color spaces, such as
YUV YUV is a color model typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, compared to a "direct" RGB-representation. H ...
for
PAL Phase Alternating Line (PAL) is a colour encoding system for analogue television. It was one of three major analogue colour television standards, the others being NTSC and SECAM. In most countries it was broadcast at 625 lines, 50 fields (25 ...
or YDbDr for SECAM. Later digital standards use the
YCbCr YCbCr, Y′CbCr, or Y Pb/Cb Pr/Cr, also written as YCBCR or Y′CBCR, is a family of color spaces used as a part of the color image pipeline in video and digital photography systems. Y′ is the luma component and CB and CR are the blue-diff ...
color space. These color spaces are all broadly related, and work based on the principle of adding a color component named ''
chrominance Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying luma signal (or Y' for short). Chrominance is usually represen ...
'', to a black and white image named ''
luma Luma or LUMA may refer to: Arts * La Trobe University Museum of Art, Melbourne, Australia * LUMA Projection Arts Festival, an annual event featuring building-scale projection mapping and light installations in Binghamton, NY * LUMA Foundation, ...
''. In YIQ the Y component represents the luma information, and is the only component used by black-and-white television receivers. I and Q represent the chrominance information. In YUV, the U and V components can be thought of as X and Y coordinates within the color space. I and Q can be thought of as a second pair of axes on the same graph, rotated 33°; therefore IQ and UV represent different coordinate systems on the same plane. The YIQ system is intended to take advantage of human color-response characteristics. The eye is more sensitive to changes in the orange-blue (I) range than in the purple-green range (Q)—therefore less
bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
is required for Q than for I. Broadcast NTSC limits I to 1.3 MHz and Q to 0.4 MHz. I and Q are frequency interleaved into the 4 MHz Y signal, which keeps the bandwidth of the overall signal down to 4.2 MHz. In YUV systems, since U and V both contain information in the orange-blue range, both components must be given the same amount of bandwidth as I to achieve similar color fidelity. Very few television sets perform true I and Q decoding, due to the high costs of such an implementation. Compared to the cheaper R-Y and B-Y decoding which requires only one filter, I and Q each requires a different filter to satisfy the bandwidth differences between I and Q. These bandwidth differences also require that the 'I' filter include a time delay to match the longer delay of the 'Q' filter. The Rockwell Modular Digital Radio (MDR) was one I and Q decoding set, which in 1997 could operate in frame-at-a-time mode with a PC or in realtime with the Fast IQ Processor (FIQP). Some
RCA The RCA Corporation was a major American electronics company, which was founded as the Radio Corporation of America in 1919. It was initially a patent trust owned by General Electric (GE), Westinghouse, AT&T Corporation and United Fruit Comp ...
" Colortrak" home TV receivers made circa 1985 not only used I/Q decoding, but also advertised its benefits along with its
comb filter In signal processing, a comb filter is a filter implemented by adding a delayed version of a signal to itself, causing constructive and destructive interference. The frequency response of a comb filter consists of a series of regularly space ...
ing benefits as full "100 percent processing" to deliver more of the original color picture content. Earlier, more than one brand of color TV (RCA, Arvin) used I/Q decoding in the 1954 or 1955 model year on models utilizing screens about 13 inches (measured diagonally). The original
Advent Advent is a Christian season of preparation for the Nativity of Christ at Christmas. It is the beginning of the liturgical year in Western Christianity. The name was adopted from Latin "coming; arrival", translating Greek '' parousia''. ...
projection television used I/Q decoding. Around 1990, at least one manufacturer ( Ikegami) of professional studio picture monitors advertised I/Q decoding.


Image processing

The YIQ representation is sometimes employed in color
image processing An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensio ...
transformations. For example, applying a
histogram equalization Histogram equalization is a method in image processing of contrast adjustment using the image's histogram. Overview This method usually increases the global contrast of many images, especially when the image is represented by a narrow rang ...
directly to the channels in an RGB image would alter the
color balance In photography and image processing, color balance is the global adjustment of the intensities of the colors (typically red, green, and blue primary colors). An important goal of this adjustment is to render specific colors – particularly ne ...
of the image. Instead, the histogram equalization is applied to the Y channel of the YIQ or YUV representation of the image, which only normalizes the brightness levels of the image.


Formulas

These formulas allow conversion between YIQ and RGB color spaces, where R, G, and B are gamma-corrected values. Values for the original 1953 NTSC colorimetry and later SMPTE C FCC standard. The following formulas assume: :R, G, B, Y \in \left 0, 1 \right \quad I \in \left 0.5957, 0.5957\right \quad Q \in \left 0.5226, 0.5226\right/math> The ranges for I and Q are a result of the coefficients in the 2nd and 3rd rows of the RGB-to-YIQ equation matrix below, respectively.


NTSC 1953 colorimetry

These formulas approximate the conversion between the original 1953 color NTSC specification and YIQ.


From RGB to YIQ

: \begin Y \\ I \\ Q \end \approx \begin 0.299 & 0.587 & 0.114 \\ 0.5959 & -0.2746 & -0.3213 \\ 0.2115 & -0.5227 & 0.3112 \end \begin R \\ G \\ B \end


From YIQ to RGB

: \begin R \\ G \\ B \end = \begin 1 & 0.956 & 0.619 \\ 1 & -0.272 & -0.647 \\ 1 & -1.106 & 1.703 \end \begin Y \\ I \\ Q \end Note that the top row is identical to that of the
YUV YUV is a color model typically used as part of a color image pipeline. It encodes a color image or video taking human perception into account, allowing reduced bandwidth for chrominance components, compared to a "direct" RGB-representation. H ...
color space * \begin R \\ G \\ B \end = \begin 1 \\ 1 \\ 1 \end \implies \begin Y \\ I \\ Q \end = \begin 1 \\ 0 \\ 0 \end


FCC NTSC Standard (SMPTE C)

In 1987, the
Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers (SMPTE) (, rarely ), founded in 1916 as the Society of Motion Picture Engineers or SMPE, is a global professional association of engineers, technologists, and executives working in the m ...
(SMPTE) Committee on Television Technology, Working Group on Studio Monitor Colorimetry, adopted the SMPTE C.Society of Motion Picture and Television Engineers (1987–2004): Recommended Practice RP 145-2004. Color Monitor Colorimetry.Society of Motion Picture and Television Engineers (1994, 2004): Engineering Guideline EG 27-2004. Supplemental Information for SMPTE 170M and Background on the Development of NTSC Color Standards, pp. 9 The previous conversion formulas were deprecated, and the NTSC standard contained in the FCC rules for over-the-air analog color TV broadcasting adopted a different matrix:


From RGB to YIQ

::: \left \{ \begin{array}{ccl} E_Y^\prime&=&0.30E_R^\prime+0.59E_G^\prime+0.11E_B^\prime\\ E_I^\prime&=&-0.27(E_B^\prime-E_Y^\prime)+0.74(E_R^\prime-E_Y^\prime)\\ E_Q^\prime&=&0.41(E_B^\prime-E_Y^\prime)+0.48(E_R^\prime-E_Y^\prime) \end{array} \right . in matrix notation, that equation system is written as: : \begin{bmatrix} E_Y^\prime \\ E_I^\prime \\ E_Q^\prime \end{bmatrix} = \begin{bmatrix} 0.30 & 0.59 & 0.11 \\ 0.599 & -0.2773 & -0.3217 \\ 0.213 & -0.5251 & 0.3121 \end{bmatrix} \begin{bmatrix} E_R^\prime \\ E_G^\prime \\ E_B^\prime \end{bmatrix} Where: * E_Y^\prime is the gamma-corrected voltage of luma. * E_R^\prime, E_G^\prime and E_B^\prime are the gamma-corrected voltages corresponding to red, green, and blue signals. * E_I^\prime and E_Q^\prime are the amplitudes of the orthogonal components of the chrominance signal.


From YIQ to RGB

To convert from FCC YIQ to RGB: :E_R^\prime = E_Y^\prime + 0.9469 E_I^\prime + 0.6236 E_Q^\prime :E_G^\prime = E_Y^\prime - 0.2748 E_I^\prime - 0.6357 E_Q^\prime :E_B^\prime = E_Y^\prime - 1.1 E_I^\prime + 1.7 E_Q^\prime


From YUV to YIQ and vice versa

\begin{bmatrix} Y' \\ I \\ Q \end{bmatrix} = \begin{bmatrix} 1 & 0 & 0 \\ 0 & -\sin(33^\circ) & \cos(33^\circ) \\ 0 & \cos(33^\circ) & \sin(33^\circ) \end{bmatrix} \begin{bmatrix} Y' \\ U \\ V \end{bmatrix} \approx \begin{bmatrix} 1 & 0 & 0 \\ 0 & -0.54464 & 0.83867 \\ 0 & 0.83867 & 0.54464 \end{bmatrix} \begin{bmatrix} Y' \\ U \\ V \end{bmatrix} Due to orthogonal symmetry (symmetry is not required or enough) of the matrix the same matrix can be used for YIQ to YUV conversion.


Phase-out

For broadcasting in the United States, it is currently in use only for
low-power television Low-power broadcasting is broadcasting by a broadcast station at a low transmitter power output to a smaller service area than "full power" stations within the same region. It is often distinguished from "micropower broadcasting" (more commonly " ...
stations, as full-power analog transmission was ended by the
Federal Communications Commission The Federal Communications Commission (FCC) is an independent agency of the United States federal government that regulates communications by radio, television, wire, satellite, and cable across the United States. The FCC maintains jurisdicti ...
(FCC) on 12 June 2009. It is still federally mandated for these transmissions as shown in this excerpt of the current FCC rules and regulations part 73 "TV transmission standard":


References


Further reading

*Buchsbaum, Walter H. ''Color TV Servicing, third edition''. Englewood Cliffs, NJ: Prentice Hall, 1975. {{DEFAULTSORT:Yiq Color space