Figure 1. Analog component GBR signal characteristics for 100 percent color bars. Click here to see an enlarged diagram.
Component video describes a system in which a color picture is represented by a number of video signals, each of which carries a component of the total picture information. In a component video facility, the analog component video signals are processed separately. Ideally, encoding into a transmission format occurs only once: prior to transmission.
All color television systems use the principle of additive colors with green, blue and red (GBR) as primary colors. GBR signal amplitudes are best described using a color bars signal representation. In this article, we will make reference to a standard set of color bars with well-defined characteristics. They feature a full-screen sequence of vertical bars showing the saturated primaries (green, blue, red) and their complements (magenta, yellow, cyan) as well as black and white. The active line is thus divided in eight equal parts. The first is occupied by a luminance reference white bar, i.e. a white bar of a standard amplitude. The last bar is a black bar; that is, it is black level only.
The GBR component signals
GBR component signals are essentially three monochrome video signals, each representing one of the primary colors. Possible sources of GBR signals include cameras, telecines, composite video decoders, character generators and graphics systems. Figure 1 presents several sets of GBR signals encountered in practice. The signal amplitudes are typical of 100 percent color bars. All signals are shown with sync added. Some sets of signals have sync added to the green component only, whereas others carry sync on a separate (fourth) wire.
The distribution channels need to have identical bandwidths, gains as well as controlled (small) differential delays with respect to the reference (green) signal. Loose channel gain and frequency response tolerances result in corrupted (colored) whites. Differential delays result in poor registration, leading to funny-paper-like colored-fringe pictures. GBR component signals have limited applications in a teleproduction environment, mainly because there are no GBR component video VTRs available on the market.
The Y, B-Y, R-Y component video signals
Y,B-Y,R-Y signal components are linear combinations of signals representing the three primary colors: green, blue and red. Possible sources of Y,B-Y,R-Y signals are cameras, analog component videocassette recorders and composite video decoders. GBR signals are combined to form a full bandwidth luminance signal (Y) and two narrowband color-difference signals (B-Y and R-Y). Because the human eye relies on luminance to convey picture detail, much less resolution is needed in the color information. Some savings in bandwidth can thus be obtained by using color-difference signals. One-half or one-quarter of the Y bandwidth is usually acceptable, depending on the application. Y, B-Y and R-Y component analog signal outputs are available with most contemporary analog cameras.
Table 1. The color-difference scaling factors Click here to see an enlarged diagram.
The mathematical expression for the luminance component is:
E'Y = 0.587 E'G + 0.114 E'G + 0.229 E'B
where E represents a voltage, and the prime sign indicates that the signal has been gamma-corrected.
The chrominance information is conveyed by two of the primary signals minus the brightness component. These signals are known as the blue color-difference and red color-difference signals. They are:
E'B - E'Y = - 0.587 E'G + 0.889 E'B - 0.299 E'R
and
E'R - E'Y = - 0.587 E'G - 0.114 E'B + 0.701 E'R
There have been limited attempts at standardizing analog component video signals in North America. Currently, a number of de-facto proprietary “standards” coexist, making interconnection of equipment difficult. The difficulties encountered are mainly due to the non-standard scaling factors of the color-difference signals. These are scaled in amplitude by suitable multiplication factors. The scaling factors depend on the application. Table 1 shows the various scaling factors used. The scaled color-difference signals are identified as E'B-Y and E'R-Y to avoid confusion.
Figure 2 shows the typical waveforms of Y,B-Y,R-Y signals encountered in practice and their characteristics. The signal amplitudes are typical of 100 percent color bars. Normally, the Y signal has sync added. The color-difference signals are bipolar and symmetrical about the reference axis.
The first column of Figure 2 shows the characteristics of NTSC-related signals as would be obtained at the output of an NTSC decoder. Note that the color-difference signals have unequal p-p amplitudes as determined by the scaling factors in Figure 1.
The second column of Figure 2 shows the amplitudes of signals as per EBU N-10. This standard specifies the characteristics of the European Y,B-Y,R-Y component analog video signals. There is no equivalent North American SMPTE standard. Note the equal 700mV p-p signal amplitudes (ignoring the sync) of the three component signals. The color-difference scaling factors are listed in the third column of Table 1. Identical scaling factors are specified in the ITU-R BT.601 (formerly CCIR 601) component digital standard. ITU-R BT.601 calls the scaled color-difference signals E'CB and E'CR. The same signals are known in North America as PB and PR.
The third column of Figure 2 shows the signal amplitudes of signals typical of Sony Betacam VTRs and related products marketed in North America. The fourth column of Table 1 lists the color-difference signals scaling of the North American versions of the Betacam component analog VTR format as well as the component analog outputs of color cameras of Japanese manufacturers marketed in North America. Note the 933mV p-p color-difference signal amplitude and the 714.3mV (including setup) p-p luminance signal amplitude (ignoring sync). It is to be noted here that the same products marketed in 625/50 countries have scaling factors as per EBU N-10 and signal characteristics as shown in the second column of Figure 2. The same products marketed in Japan are similar to the North American version except that the luminance signal has no setup.
Figure 2. Analog component Y,B-Y,R-Y signal characteristics for 100 percent color bars Click here to see an enlarged diagram.
The fourth column in Figure 2 shows the signal amplitudes of signals typical of Panasonic MII component analog VTRs marketed in North America. The fifth column in Table 1 lists the color-difference signals scaling of the North American version of the MII component analog VTR format marketed in North America. Note the 648mV p-p color-difference signal amplitude and the 700mV (including setup) p-p luminance signal amplitude (ignoring sync). It is to be noted here that the same products marketed in 625/50 countries have scaling factors as per EBU N-10 and signal characteristics as shown in the second column of Figure 2. The same products marketed in Japan are similar to the North American version except that the luminance signal has no setup.
Potential problems
Both component analog recording formats feature analog composite and analog component in/out ports. The composite in/out ports are mutually compatible. Consequently, connecting equipment using analog composite NTSC signals should cause no problems other than the unavoidable accumulation of analog NTSC impairments.
The color-difference scaling factors are different in the four standards discussed above Strictly speaking, these “standards” and their component analog in/out ports are signal-level incompatible. An ideal approach is the normalization of the component analog signal levels in the component analog signal distribution path to EBU N-10 specifications and the use of signal level adaptors to match the component analog inputs and outputs of non-standard equipment.
Michael Robin, a fellow of the SMPTE and 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, and recently translated into Chinese and Japanese.
Send questions and comments to:michael_robin@primediabusiness.com
