ANSI/EIA-189-A, Encoded Color Bar Signal (formerly denoted EIA RS-189-A).

SMPTE EG 1, Alignment Color Bar Test Signal for Television Picture Monitors.

pluge is pronounced ploodge.

Figure 36.1 The SMPTE EG 1 SD colourbar test signal is represented here as an image; however, it is standardized as a signal in the R’G’B’ domain. Its colour interpretation depends upon the primary chromaticities in use. The corresponding Y’CBCR or Y’PBPR waveforms depend upon luma coefficients and scaling.

SD colourbars

Figure 36.1 below is a sketch of an image produced by the classic SMPTE colourbar test pattern. The upper 2⁄3 of the image contains a 100% white bar followed by primary and secondary colours of 75% saturation. The narrow, central region contains “reverse bars”; this section enables setting composite NTSC or PAL decoder hue and chroma. The bottom 1⁄4 of the image contains subcarrier frequency at -I phase, a white bar, subcarrier frequency at +Q phase, and (at the right) the pluge element, which I will describe in a moment.

Figure 36.2 overleaf shows the R’G’B’ components that produce the upper 2⁄3 of the frame of SMPTE colourbars. Each scan line is based upon a binary sequence of red, green, and blue values either zero or unity. The components are arranged in order of their contributions to luma, so that the eventual luma component decreases from left to right. (The narrow band 2/3 of the way down SMPTE bars has its count

Figure 36.2 Colourbar R’G’B’ primary components in SMPTE colourbars have amplitude of 75 IRE, denoted 75/0/75/0. A variation denoted 100/0/75/0, whose R’, G’, and B’ waveforms are sketched here, places the white bar at 100 IRE. Other variations have different amplitudes for the uncoloured and coloured bars.

100/0/

75/

R’

0

G’

B’

See the section UV components of

Composite NTSC and PAL: Legacy Video Systems. Strictly speaking, owing to negative AM video modulation, an NTSC transmitter would undermodulate if presented with composite video level exceeding 120%.

sequence reversed and its green component forced to zero.)

In studio equipment, in component video, and in PAL broadcast, the processing, recording, and transmission channels can accommodate all encoded signals that can be produced from mixtures of R’G’B’ where each component is in the range 0 to 1. The 100% colourbar signal exercises eight points at these limits.

Fully saturated yellow and fully saturated cyan cause a composite PAL signal to reach a peak value 4/3

(133 1/3%) of reference white. However, an NTSC transmitter’s composite signal amplitude is limited to 120% of reference white. If 100% bars were presented to an NTSC transmitter, clipping would result. To avoid clipping, 75% bars are ordinarily used to test NTSC transmission. The white bar comprises primaries at 100%, but the other bars have their primary components reduced to 75% so as to limit their composite NTSC peak to the level of reference white. The “75% bars” convention was adopted for good reason, but analog NTSC transmitters have been decommissioned; the convention remains in use for no good reason.

R’G’B’ components of 100% colourbars take R’G’B’ (or RGB) values of zero or unity, independent of the chromaticity of the primaries: Owing to differences in primary chromaticities, the exact colours of the bars are not identical among SMPTE, EBU, and HD standards.

ITU-R Rec. BT.471, Nomenclature and description of colour bar signals.

BLACK

-4 IRE

576i (±2%)

+14 mV BLACK

-14 mV

Figure 36.3 The PLUGE element of the colourbar signal enables accurate setting of black level. The 14 mV excursion in 576i PLUGE is equivalent to ±2 units.

SD colourbar notation

I have referred to 100% and 75% colourbars. Several additional variations of colourbars are in use, so many that an international standard is required to denote them. A colourbar signal is denoted by four numbers, all in units (formerly, IRE units), separated by slashes. The first pair of numbers gives the maximum and minimum values (respectively) of the primary components in uncoloured bars – that is, the black or white bars. The second pair gives the maximum and minimum primary values (respectively) in the coloured bars.

The 100% bars, described earlier, are denoted 100/0/100/0. That variation is useful in the studio, in all forms of component video, and in PAL transmission. In legacy 480i composite NTSC systems where 7.5% setup is used, 100% bars refers to 100/7.5/100/7.5. That variation was once useful in the studio. However, as I explained on page 420, terrestrial analog NTSC transmission cannot handle 100% bars. NTSC transmitters are tested using 75% bars with setup, denoted 100/7.5/75/7.5. Japan uses 480i video with zero setup; there, 75% bars, denoted 100/0/75/0, are used.

pluge element

The lower-right quadrant of the colourbar pattern contains elements produced by picture line-up generating equipment (pluge); see Figure 36.3. The acronym originates with the “generating equipment,” but nowadays pluge signifies the signal element. Superimposed on reference black are two elements, one slightly more negative than reference black, the other slightly more positive.

A display’s black level is adjusted so that the first (negative-going) element is just barely indistinguishable from reference black. (The second element should then be barely visible.) Details are found in Black level setting, on page 56. The negative-going element of pluge cannot be represented in positive R’G’B’.

In SD, ±4% pluge was standardized in the now-with- drawn SMPTE EG 1. However, SMPTE RP 219 standardizes pluge for HD with 8-bit interface codes 44, 64, 84, and 104 – that is, approximately -2%, +2%, and +4%.

If the red and green guns at the display cannot be turned off,

a similar effect can be accomplished by viewing the CRT through a blue gel filter.

Composite decoder adjustment using colourbars

When composite NTSC or PAL colourbars are decoded, the amount of blue decoded from the white, cyan, magenta, and blue bars should ideally be identical. Any chroma gain (saturation) error will affect the signal decoded from blue, but not the blue decoded from white. Chroma phase error will cause hue errors of opposite direction in the blue decoded from cyan and the blue decoded from magenta. To manually adjust

a decoder’s hue and chroma (or tint and colour, or phase and saturation) controls involves displaying composite SMPTE colourbars, and disabling the red and green components at the decoder output. The amount of blue decoded from each of cyan and magenta is equalized by adjusting the decoder’s hue control. The amount of blue decoded from each of grey and blue is equalized by adjusting chroma. The comparison is facilitated by the reversed bar portion of SMPTE colourbars. Figure 36.4 shows a representation of the colourbar image, showing the bars that are visually compared while adjusting hue and chroma.

Adjusting hue and chroma controls in this way is only meaningful to compensate errors in NTSC and PAL decoding. In component video such as R’G’B’ and Y’CBCR, no recording or transmission impairment rotates hue or alters chroma (saturation): Using the scheme described above is nonsensical.