The color bar test signal is just about the most

familiar sight in studio, maintenance and

transmission environments. An extremely useful

tool, it is needed for phasing components into a

system, to adjust monitors and to pinpoint many

signal difficiencies at a glance. Color bars are

recorded as the “leader” before program material

to facilitate checks and adjustments of signal

processors. Let’s take a look at some basic

aspects of color bars.

Color Sequence

The left-to-right sequence of colors is no accident.

Refer to Figure 8-1. Notice that every other bar,

starting with white (actually gray) has blue in it —

white (RGB), Cyan GB), magenta (RB) and blue

(B). Thus, the blue signal is formed of a

squarewave whose half period is equal to the bar

width. Red appears in pairs of bars — white

(RGB)-yellow (RG), magenta RB-red. This

waveshape is the blue signal frequency divided by

two. Finally, green appears as a single

squarewave, half the red signal frequency, since

green appears in the first four bars and absent in

the last four. Thus, the RGB signals are very easy

to generate. Furthermore, since the luminance (Y)

signal is developed from the recipe Y = .3R +

.59G + .11B, the Y signal forms a descending

staircase as shown in the figure. Looks like a

linear staircase but it isn’t; the “risers” are not

equal. Setup has been omitted from Figure 8-1 for

simplicity.

75% or 100% Bars

A common misconception is to take the

percentage number applied to color bars as color

saturation. It isn’t. Color bar signals are always

100% saturated. That is, yellow is red and green

only.

VIDEO TEST SIGNALS PART 2

TELEPRODUCTION TESTVOLUME 1 NUMBER 8

Figure 8-1. Generating RGB for the color bar signal. Y isalso shown.

There is no blue which would add some white(RGB) to dilute saturation. The number refers tothe a m p l i t u d e of RGB into the encoder. Onehundred percent color bars put an undue andunnecessary strain on transmission systems inthat the subcarrier peaks for yellow (which setsatop the highest luminance value) go to 130 IRE.Blue, the lowest in brightness, has subcarrierthat s inks too far below blanking. But thearithmetic for 75% amplitude color bars worksout to a signal that reaches to just 100 IRE forthe positive peaks of the subcarrier signal forboth yellow and cyan. This turns out to be a veryuseful attribute as you will see.

Types of Color Bars

Full field bars are continuous from the top to thebottom of the picture. They are often used invisual phase matching between two camerasusing an SEG wherein a vertical wipe permitscolor bars from each camera to appear one

above the other on the monitor screen.

Full field bars are narrower than other types to

allow a black bar (at setup level) to appear to theright of the blue bar. Fig. 8-2 shows the 2Hwaveform monitor display for full field bars. Notethe black bar at setup and the positive tips ofyellow and cyan subcarrier reach to 100 IRE.

EIA bars comply with the EIA standard RS-189A.These are split-field 75% bars in which color barsoccupy the top 3/4ths of the picture and a 100%white chip and a sample of -I and Q signals inthe bottom 1/4th. Refer back to Figure 8-1. Note

the p lacement of the 100% whi te chip. Itstraddles yellow and cyan and gives a built-inreference point at 100 IRE. This enables you totell at a glance if the luminance/chrominance(Y/C) ratio is correct. Figure 8-3 shows the

correct waveform for EIA bars. Note that thesubcarrier peaks for yellow and cyan (the first two

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Figure 8-2. Full-field color bars include black (setup) tothe right of the blue.

Figure 8-4. EIA bars, low chroma.

Figure 8-5. EIA bars, chroma too high.Figure 8-3. EIA bars.

“color” bars) just kiss the bright l ine thatrepresents the 100 IRE chip. Note also that thespace to the right of the +Q signal is at setup andgives the bright line at 7.5 IRE.

Figure 8-4 shows the effects of low chroma; inthis case by a receiver monitor that does notterminate the coax properly. Excessive chroma isshown in Figure 8-5, caused in this case, by anovercompensated distribution amplifier.

A word about the 75% gray bar. Its luminancevalue works out to 77 IRE. The reason is setup. Ifwe subtract setup from 100 this leaves 92.5actual dynamic range. Take 75% of this (69.4)and add it back to the setup floor and you get69.4 + 7.5 = 76.9 = 77 IRE. The waveformmonitor is marked at 77 IRE with a dashed linefor this purpose.

SMPTE bars conform to SMPTE Standard ECR1-1978. They are identical to EIA bars except fortwo major additions. First, there is an extra band

of color bars just above the horizontal split line.See Figure 8-6. Note the colors in the addedband with respect to the colors just above them.For example, gray contains blue (RGB) and thecolor below it is blue. Yellow contains no blue, theextra band in black. Cyan contains blue and thechip below it also contains blue, magenta. Greenhas no blue nor does red and the SMPTE band isblack in both cases. Magenta and blue containblue so the chips below them are cyan and gray,both of which contain blue.

The SMPTE bars permit precise adjustments ofchroma phase and saturation in monitors thathave a “blue only” switch (red and green are cutoff). In some cases, the blue signal is applied toall three channels (RGB) to give a white displayof the blue-only signal. The operator simplyadjusts phase and saturation so that thebrightness of the bars match that of the addedband, both at those bars that contain blue andthose that do not. The latter should be at visualextinction. Adjustments made using SMPTE barsrule out improper settings due to variations incolor vision and subjective judgement. SeeFigure 8-8.

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Figure 8-6. SMPTE bars.

Figure 8-9. Pluge off screen. Brightness set too high toshow the three bars.

Figure 8-8. SMPTE bars off screen, blue only; (A) correct(B) wrong phase and saturation.

Figure 8-7. SMPTE bar waveform.

(A) (B)

The other added feature of SMPTE bars is theso-called “PLUGE” signal in the lower quarter,centered in the space to the right of the +Qsignal. This is a 4 IRE unit minus and plusexcursion from setup and appears as that wigglearound setup in the waveform of Figure 8-7. ThePluge is used to adjust monitor brightness. Whenadjusted properly, the -4 zero IRE bars should bevisually extinguished but the +4 IRE bar shouldstill be visible. Figure 8-9 shows the Pluge signalwith brightness set too high so that all three barsare visible.

Other forms of color bars include split field bars inwhich the luminance (Y) signal appears in thelower half. See Figure 8-10. Note that the

luminance “framework” appears as a brightstairstep superimposed over the color bars. Thissignal is useful as it permits monitor gray scaletracking to be checked without turning color off.

Split field bars with a fully saturated red in thelower half appears in Figure 8-11. The red is thesame value as the red color bar. This signal isuseful in judging the effects of chroma noise inVTR/VCR playback.

F i n a l l y, reverse bars are split-field bars in whichthe bars in the lower half are reversed from left toright. Some appear to prefer split/reverse bars toSMPTE bars for monitor adjustments. In anycase, they are pretty.

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Figure 8-10. Split bars, Y (luminance in lower half). Figure 8-11. Split bars, red in lower half.