hue (or tint, or phase) and chroma (or colour, or saturation) controls properly apply only to NTSC/PAL decoding. They have no place in modern component video systems. See Composite decoder adjustment using colourbars, on page 422, and NTSC and PAL Chroma modulation in Composite NTSC and PAL: Legacy Video Systems.

to form luma (Y’) and colour difference signals (U and V, or in certain NTSC systems, I and Q).

•U and V (or I and Q) colour difference signals are modulated onto a pair of intimately related continuouswave colour subcarriers, typically at a frequency of about 3.58 MHz in 480i29.97 or 4.43 MHz in 576i25, to produce a modulated chroma signal, C. (See the left side of Figure 14.1 overleaf.)

•Luma and modulated chroma are summed to form

a composite NTSC or PAL signal. (See the right side of Figure 14.1.) Summation of luma and chroma is liable to introduce a certain degree of mutual interference, called cross-luma and cross-colour; these artifacts can be minimized through frequency interleaving, to be described.

NTSC and PAL decoding

NTSC or PAL decoding involves these steps:

•Luma and modulated chroma are separated. Crude separation can be accomplished using a notch filter. Alternatively, frequency interleaving can be exploited to provide greatly improved separation; in NTSC, such

a separator is a comb filter. (In an S-video interface, luma and modulated chroma are already separate.)

•Chroma is demodulated to produce UV, IQ, PBPR, or CBCR baseband colour difference components.

•If R’G’B’ components are required, the baseband colour difference components are interpolated, then luma and the colour difference components are dematrixed.

S-video interface

S-video involves NTSC or PAL chroma modulation; however, luma and modulated chroma traverse separate paths across the interface instead of being summed; the S-video interface bypasses the third step of NTSC and PAL encoding above. Cross-luma and crosscolour artifacts are avoided. Figure 14.2 sketches the encoder and decoder arrangement. The S-video interface is widely implemented in consumer video equipment.

Frequency interleaving

When luma and modulated chroma are summed,

a certain amount of mutual interference is introduced.

Y’

C

Y’

C

Y’/C

(S-video)

In PAL, all but the most sophisticated comb filters separate U and V, not luma and chroma. See “NTSC and PAL Chroma modulation,” Chapter 5 of Composite NTSC and PAL: Legacy Video Systems.

Interference is minimized by arranging for frequency interleaving, which is achieved when the colour subcarrier frequency and the line rate are coherent – that is, when the unmodulated colour subcarrier is phaselocked to a carefully chosen rational multiple of the line rate – an integer multiple of half the line rate for NTSC, and an integer multiple of 1⁄4 the line rate in PAL. Coherence is achieved in the studio by deriving both the sync and colour subcarrier from a single master clock.

In NTSC, frequency interleaving enables use of

a comb filter to separate luma and chroma: Adjacent lines are summed (to form vertically averaged luma) and differenced (to form vertically averaged chroma), as suggested at the bottom right of Figure 14.1.

In industrial and consumer video, the subcarrier often free-runs with respect to line rate, and the advantages of frequency interleaving are lost. Most forms of analog videotape recording introduce timebase error; left uncorrected, this also defeats frequency interleaving.

Composite analog SD

Composite analog 480i NTSC and 576i PAL is widely deployed in consumer equipment (such as television receivers and VCRs) and was used for terrestrial VHF/UHF broadcasting and cable television for many decades. Details are found in “Analog SD broadcast standards,” Chapter 12 of Composite NTSC and PAL: Legacy Video Systems.