The technique of compatible colour television utilizes two transmissions. One of these carries information about the brightness, or luminance, of the televised scene, and the other carries the colour, or chrominance, information. Since the ability of the human eye to perceive detail is most acute when viewing white light, the luminance transmission carries the impression of fine detail. Because it employs methods essentially identical to those of a monochrome television system, it can be picked up by black-and-white receivers. The chrominance transmission has no appreciable effect on black-and-white receivers, yet, when used with the luminance transmission in a colour receiver, it produces an image in full colour.

Historically, compatibility was of great importance because it allowed colour transmissions to be introduced without obsolescence of the many millions of monochrome receivers in use. In a larger sense, the luminance-chrominance method of colour transmission is advantageous because it utilizes the limited channels of the radio spectrum more efficiently than other colour transmission methods.

To create the luminance-chrominance values, it is necessary first to analyze each colour in the scene into its component primary colours. Light can be analyzed in this way by passing it through three coloured filters, typically red, green, and blue. The amounts of light passing through each filter, plus a description of the colour transmission properties of the filters, serve uniquely to characterize the coloured light. (The techniques for accomplishing this are described in the section Transmission: Generating the colour picture signal.)

The fact that virtually the whole range of colours may be synthesized from only three primary colours is essentially a description of the process by which the eye and mind of the observer recognize and distinguish colours. Like visual persistence (the basis of reproducing motion in television), this is a fortunate property of vision, since it permits a simple three-part specification to represent any of the 10,000 or more colours and brightnesses that may be distinguished by the human eye. If vision were dependent on the energy-versus-wavelength relationship (the physical method of specifying colour), it is doubtful that colour reproduction could be incorporated in any mass-communication system.

By transforming the primary-colour values, it is possible to specify any coloured light by three quantities: (1) its luminance (brightness or “brilliance”); (2) its hue (the redness, orangeness, blueness, or greenness, etc., of the light); and (3) its saturation (vivid versus pastel quality). Since the intended luminance value of each point in the scanning pattern is transmitted by the methods of monochrome television, it is only necessary to transmit, via an additional two-valued signal, supplementary information giving the hue and saturation of the intended colour at the respective points.

Chrominance, defined as that part of the colour specification remaining when the luminance is removed, is a combination of the two independent quantities, hue and saturation. Chrominance may be represented graphically in polar coordinates on a colour circle (as shown in the diagram), with saturation as the radius and hue as the angle. Hues are arranged counterclockwise around the circle as they appear in the spectrum, from red to blue. The centre of the circle represents white light (the colour of zero saturation), and the outermost rim represents the most saturation. Points on any radius of the circle represent all colours of the same hue, the saturation becoming less (that is, the colour becoming less vivid, or more pastel) as the point approaches the central “white point.” A diagram of this type is the basis of the international standard system of colour specification.

In the NTSC system, the chrominance signal is an alternating current of precisely specified frequency (3.579545 ± 0.000010 megahertz), the precision permitting its accurate recovery at the receiver even in the presence of severe noise or interference. Any change in the amplitude of its alternations at any instant corresponds to a change in the saturation of the colours being passed over by the scanning spot at that instant, whereas a shift in time of its alternations (a change in “phase”) similarly corresponds to a shift in the hue. As the different saturations and hues of the televised scene are successively uncovered by scanning in the camera, the amplitude and phase, respectively, of the chrominance signal change accordingly. The chrominance signal is thereby simultaneously modulated in both amplitude and phase. This doubly modulated signal is added to the luminance signal (as shown in the diagram of the colour signal wave form), and the composite signal is imposed on the carrier wave. The chrominance signal takes the form of a subcarrier located precisely 3.579545 megahertz above the picture carrier frequency.

The picture carrier is thus simultaneously amplitude modulated by (1) the luminance signal, to represent changes in the intended luminance, and (2) the chrominance subcarrier, which in turn is amplitude modulated to represent changes in the intended saturation and phase modulated to represent changes in the intended hue. When a colour receiver is tuned to the transmission, the picture signal is recovered in a video detector, which responds to the amplitude-modulated luminance signal in the usual manner of a black-and-white receiver. An amplifier stage, tuned to the 3.58-megahertz chrominance frequency, then selects the chrominance subcarrier from the picture signal and passes it to a detector, which recovers independently the amplitude-modulated saturation signal and the phase-modulated hue signal. Because absolute phase information is difficult to extract, the hue signal is made easier to decode by a phase reference transmitted for each horizontal scan line in the form of a short burst of the chrominance subcarrier. This chrominance, or colour, burst consists of a minimum of eight full cycles of the chrominance subcarrier and is placed on the “back porch” of the blanking pulse, immediately after the horizontal synchronization pulse (as shown in the diagram).

When compatible colour transmissions are received on a black-and-white receiver, the receiver treats the chrominance subcarrier as though it were a part of the intended monochrome transmission. If steps were not taken to prevent it, the subcarrier would produce interference in the form of a fine dot pattern on the television screen. Fortunately, the dot pattern can be rendered almost invisible in monochrome reception by deriving the timing of the scanning motions directly from the source that establishes the chrominance subcarrier itself. The dot pattern of interference from the chrominance signal, therefore, can be made to have opposite effects on successive scannings of the pattern; that is, a point brightened by the dot interference on one line scan is darkened an equal amount on the next scan of that line, so that the net effect of the interference, integrated in the eye over successive scans, is virtually zero. Thus, the monochrome receiver in effect ignores the chrominance component of the transmission. It deals with the luminance signal in the conventional manner, producing from it a black-and-white image. This black-and-white rendition, incidentally, is not a compromise; it is essentially identical to the image that would be produced by a monochrome system viewing the same scene.

The television channel, when occupied by a compatible colour transmission, is usually diagrammed as shown in the figure. The luminance information modulates the chrominance subcarrier in the form of two orthogonal components, the I signal and the Q signal. This form of quadrature modulation accomplishes the simultaneous amplitude and phase modulation of the chrominance subcarrier. The I signal represents hues from the orange-cyan colour axis, and the Q signal represents hues along the magenta-yellow colour axis. The human eye is much less sensitive to spatial detail in colour, and thus the chrominance information is allocated much less bandwidth than the luminance information. Furthermore, since the human eye has more spatial resolution to the hues represented by the I signal, the I signal is allotted 1.5 megahertz, while the Q signal is restricted to only 0.5 megahertz. To conserve spectrum, vestigial modulation is used for the I signal, giving the lower sideband the full 1.5 megahertz. The quadrature modulation used for the chrominance information results in a suppressed carrier.

When used by colour receivers, the channel for colour transmissions would appear to be affected by mutual interference between the luminance and chrominance components, since these occupy a portion of the channel in common. Such interference is avoided by the fact that the chrominance subcarrier component is rigidly timed to the scanning motions. The luminance signal, as it occupies the channel, is actually concentrated in a multitude of small spectrum segments, by virtue of the periodicities associated with the scanning process. Between these segments are empty channel spaces of approximately equal size. The chrominance signal, arising from the same scanning process, is similarly concentrated. Hence it is possible to place the chrominance channel segments within the empty spaces between the luminance segments, provided that the two sets of segments have a precisely fixed frequency relationship. The necessary relationship is provided by the direct control by the subcarrier of the timing of the scanning motions. This intersegmentation is referred to as frequency interlacing. It is one of the fundamentals of the compatible colour system. Without frequency interlacing, the superposition of colour information on a channel originally devised for monochrome transmissions would not be feasible.