Transition to Digital: Inside color bars

Inside color bars

By Michael Robin

In last month's column, we introduced color-bar signals and took a look inside matrixed color-bar signals. This month concludes the two-part series on color bars with a look inside encoded color-bar signals.

Figure 1. The NTSC color-bar signal displayed on a vectorscope.

Encoded composite color-bar signals

The NTSC and PAL encoding processes use identical matrixing coefficients to obtain the luminance (E´Y) and color-difference (E´B-Y or E´U and E´R-Y or E´V) signals. NTSC 100 percent luminance has a value of 714.3 mV (100 IRE) and 7.5 percent black-level pedestal (setup), while PAL has 700 mV and no setup. The E´Y signal is given by the expression:

E´Y = 0.587 E´G + 0.114 E´B + 0.299 E´R

The amplitudes of the color-difference signals are reduced by specific scaling factors to avoid overmodulating land-based AM video transmitters using negative modulation. The scaled color-difference signals are given by the following expressions:

E´B-Y = 0.493 (E´B - E´Y), also known as E´U in PAL

E´R-Y = 0.877 (E´R - E´Y), also known as E´V in PAL

The NTSC and PAL encoding processes are similar. In the process, each of the two scaled color-difference signals amplitude modulates a dedicated subcarrier. The two dedicated subcarriers are equal in frequency but quadrature in phase, and are subsequently suppressed. The NTSC encoding process uses two 3.579MHz subcarriers and results in a half-line-offset interleaved chrominance/luminance spectrum. The PAL encoding process uses two 4.43MHz subcarriers. The V subcarrier phase alternates line by line, resulting in a more complex quarter-line-offset interleaved chrominance/luminance spectrum.

Figure 2. PAL color-bar vectorscope display showing line-sequential alternating phase change

The NTSC and PAL vectorscope displays feature vector amplitude and phase reference graticules, selectable for 100 percent or 75 percent color bars. They allow you to adjust the vector phase and amplitude of the encoder under test within accepted tolerances.

Figure 1 shows a vectorscope display of an NTSC color-bar signal. Note the presence of the color-burst signal. This display of the six reference color signals and the burst signal is obtained by decoding the NTSC signal and feeding the decoded color-difference signals to the horizontal and vertical amplifier or the vectorscope. Figure 2 shows a vectorscope display of a PAL color-bar signal. Two line-sequential displays of line “n” and line “n+1” show the alternating phase process of PAL. Usually, the two displays are superimposed.

Figure 3 shows the relationship between 100 percent and 75 percent PAL and NTSC color-bar signals and AM modulation percentage of land-based transmitters. From the 100 percent PAL and NTSC color-bar signals on the left side of the figure, you can see that the yellow and cyan colors would overmodulate the carrier. However, 75 percent color bars, with either 100 percent or 75 percent white level, would not overmodulate the transmitter. It is important to note that saturated yellow and cyan colors don't occur in nature, so camera-generated signals will not overmodulate. But high-amplitude synthetic signals, such as those generated by a character generator, would create problems.

Figure 3. This oscilloscope display shows original and scaled-down color-bar signals as a percentage of AM modulation.

The right side of Figure 3 shows reduced-amplitude color bars with 100 percent white as well as 75 percent white (indicated by the dotted line). Note that the NTSC signal has a black-level pedestal (setup) and the amplitudes are expressed in IRE units. The PAL signal has no setup and the amplitudes are expressed in mV. In both standards, the lowest permissible carrier modulation by the white level is 12.5 percent. The only exception is the UK, where the lowest permitted level is 20 percent, allowing for the transmission of 95 percent color bars, identified as 100/0/100/25. (So much for uniformity.)

The United States, Canada and some other NTSC countries use a general-purpose color-bar signal that is seldom encountered elsewhere. Its most usual variety is known as EIA Standard RS-189-A. A seven-segment 75/7.5/75/7.5 color-bar display occupies the upper 75 percent of the picture. The lower 25 percent contains, from left to right, a -I bar, a 100 percent white bar, a +Q bar and a black bar. The top part of Figure 4 illustrates this arrangement. Some variations include an additional two bars, one slightly whiter than black and the other slightly blacker than black, inside the black bar area. These signals are an adaptation of a monitor's black-level alignment signal, known as picture line-up generator (PLUGE).

Figure 4. RS-189-A NTSC color bars

While the PLUGE is helpful for adjusting color monitors, the -I and +Q components are a heritage of the early NTSC encoding practices and, in today's age, serve no useful purpose. The center part of Figure 4 shows the waveform that creates the upper 75 percent of the display, and the bottom part of the figure shows the waveform that creates the lower 25 percent of the display. Figure 5 shows the vectorscope display of this EIA RS-189-A color-bar signal, which is similar to a simple NTSC vector display with the addition of the -I and +Q vectors.

Are color bars fading away?

In analog composite or component television, a color-bar signal is used as a leader to a recorded tape. This allows the VTR playback operator to optimize the signal characteristics before the program material begins. In an analog camera, you can use the color-bar signal to verify and optimize the encoder. In early, unstable receivers, the color-bar signal was used to adjust contrast, saturation and hue. Current receivers are very stable and don't require such adjustments. So, the question arises: Why do we need to transmit a color-bar signal anyway? In a digital studio, you really don't need to use a color-bar signal unless you want to verify the performance of an ADC or a DAC. Otherwise, you don't need it in daily operations. And, since modern receivers are stable, they don't need a color-bar signal. Contemporary television transmissions, with very few exceptions, run 24 hours a day, seven days a week.

Figure 5. Vectorscope display of RS-189-A color-bar signal

So even if you wanted to transmit color bars, you would have difficulty finding a time slot in which to do so. In the 21st century, color-bar signals are not used for operations. Instead, they are used for maintenance activities such as verifying the performance of a piece of equipment, the performance of a telco network or, perhaps once a month, the performance of an analog NTSC transmitter.

One area where color-bar signals, and the expert knowledge of how to use them, will be required is in a multiformat SDTV and HDTV teleproduction operational environment. In the near future, 4:3-format SDTV signals will be upconverted to 16:9-format HDTV signals, and vice versa. The situation is complicated by the fact that the matrixing coefficients are quite different in the two worlds: Digital SDTV signals are governed by ITU-R BT.601, and HDTV signals are governed by ITU-R BT.709. The SMPTE and ITU are currently developing a color-bar signal concept that will allow signal characteristics common to both formats and specific to each, located in a 4:3 area, to be translated into the alternate format. Additional HDTV-specific content will be located in the remaining 16:9 space. So a new generation of color-bar signals will be used in an entirely different working environment. Stay tuned.

Michael Robin, former engineer with the Canadian Broadcasting Corp.'s engineering headquarters, is an independent broadcast consultant located in Montreal, Canada. He is co-author of Digital Television Fundamentals, published by McGraw-Hill.

To read part one of Michael Robin's article click The color bars puzzle.

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