Mike Robin MIKE ROBIN S COLUMN SEPTEMBER Introduction. Generation of a color bars signal

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1 MIKE OIN S COLUMN SEPTEME 1999 Mike obin, a 2-year veteran of the Canadian roadcasting Corporation Engineering Headquarters, is an independent broadcast consultant located in Montreal, Canada. He is the co-author of Digital Television Fundamentals, published by Mcraw-Hill and a contributing writer to roadcast Engineering Magazine. mrobin@miranda.com Introduction The color bars signal is widely used and often misinterpreted. There are many variations of the color bars signals in general use. Many of them are application-specific, that is, reflect the operational requirements of the specific organization, like color bars optimized for use with amplitude modulated transmitters. Others are typical of the respective analog composite encoding standard and contain, in addition to the color bars, various signals serving the purpose of color monitor alignment or various performance measurements. They all share a common overall form. In its common form, the color bars signal provides a sequence of vertical bars in the picture displaying the The Color ars Signal Mike obin saturated primaries and their complements as well as black and white. The active line is, thus, divided in eight equal parts. The first is occupied by a luminance reference bar, that Figure 1 The position of the eight bars on the television screen is a white bar of a standard amplitude. The last is a black bar, that is black level only. In between there are six bars representing the three primary colors and their complements. They are, in order, yellow, cyan, green, magenta, red, and blue. The standard order of presentation was chosen to give a descending sequence of luminance values. Figure 1 shows the position of the eight bars on the television screen. This technical note describes the common type of color bars and serves as a reference for other technical notes. eneration of a color bars signal A color bars generator has three outputs corresponding, respectively, to the green, blue, and red primary color signals (E,E, E ). The letter E designates a gamma-corrected voltage as generated by a television camera, however, test signal generators do not gamma-correct the test signals. The color bars test signals consist of a sequence of flat-top pulses. y a suitable overlap of the pulses in certain portions of the raster and non-overlap in others, the three saturated complementary colors are produced. These signals may be used in their original form, matrixed into E Y,E -E Y and E -E Y or encoded into an analog (NTSC, PAL, SECAM) or digital (component or composite) signal. Two groups of saturated color bars signals are described here. They reflect the peculiarities of the two conventional television scanning standards (525/6 and 625/5) in terms of white and black signal levels. Each group comprises a full-amplitude (1) and a reduced-amplitude (75%) color bar signal. The various pulse levels are described in percentages of the white level. Each color bar is identified with four numbers with an oblique stroke between them as follows: The first number describes the primary color signal level during the transmission of the white bar, that is, the maximum value of E,E, and E.

2 The second number describes the primary color signal level during the transmission of the black bar, that is, the minimum value of E,E, and E. The third number describes the maximum level of the primary color signal during the transmission of the colored color bars, that is, the maximum value of E,E, and E. The fourth number describes the minimum level of the primary color signal during the transmission of the colored color bars, that is, the minimum value of E,E, and E. Figure 2-a shows a fully saturated color bars signal with maximum signal levels of 1 and minimum signal levels of. This type of color bar signal is called 1//1/. Figure 2-b shows a fully saturated color bar signal with maximum signal levels of 75% and minimum signal levels of. These types of color bars signals are typical of those used in 625/5 countries. They are representative of the signals used to feed a PAL encoder and would be obtained at the output of a properly adjusted PAL decoder. Figure 2-c shows a fully saturated color bar signal with maximum signal levels of 1 and minimum signal levels of %. This type of color bar signal is called 1//1/. Figure 2-d shows a fully saturated color bar signal with maximum signal levels of 75% and minimum signal levels of %. These types of color bar signals were used in 525/6 countries except Japan. They are representative of the signals used to feed an NTSC encoder using the original philosophy behind the 1953 NTSC standard. In those days the primary signals (E,E, and E ) had the black level set at % of the peak white level ( IE setup) and their peak level was mv (1/ IE). Current NTSC encoders use E,E, and E signals without setup and, very often, with a SINAL WY CMK SINAL WY CMK a 1//1/ COLO AS b 75//75/ COLO AS Figure 2 elative amplitudes of components for four types of colors bars SINAL WY CMK WY C MK WY C MK WY CMK WY CMK WYC MK c 1//1/ COLO AS WYCMK WY CMK WYCMK WY CMK d 75//75/ COLO AS

3 ADDE 11.4% % Figure 3 raphic representation of the formation of 1//1/ color bars Y signal from the primary reen, lue and ed signals % INVETE ADDE % 7.1% 41.3% 29.9% 11.4% Y SINAL peak amplitude of 7 mv, leaving it to the encoder to normalize the encoded signal to NTSC specifications. This allows for standard camera circuit designs irrespective of the analog composite encoding standard. The signal at the output of an NTSC decoder will, however, be as shown in Figure 2-c or 2-d. In NTSC regions it is customary to use 75% color bars while non-ntsc regions prefer the use of 1 color bars. A subset of the color bars signal is known as 1//75/ or 1//75/. Here the white bar of a 75% color bars signal is raised to full amplitude (1) and the luminance and chrominance part of the signal have an amplitude of 75%. This type of signal is used with older analog-type VTs and analog transmitters. Some test signal generators offer only the subset color bars signals which are referred-to erroneously as 1 color bars. Contemporary analog and digital studio equipment is designed to handle 1 color bars signals without difficulties. It is customary to use such signals to verify the quantizing range of A/D converters and ensure that it meets the relevant standards requirements with respect to signal overhead. It is, therefore, ill-advised to use 75% color bars in a modern studio. The use of various types of 75% color bars may still need to be used with analog television transmitters due to their reduced headroom. 88.6% % 29.9% INVETE -Y SINAL Figure 4 raphic representation of the formation of 1//1/ color bars blue colordifference signal from the primary reen, lue and ed signals -88.6% %

4 The,, component color bars signals The signal structure and levels described above serve to derive component analog E,E,E signals. SMPTE standard 253 M specifies the characteristics of component analog E,E,E signals which are embodied by the 1//1/ color bars signals. Specific characteristics of all three signals are: Maximum (white) signal level: 7 mv 1 1 INVETE Minimum (black) signal level: mv 1 1 lack level setup: mv Each of the three signals carries the sync information. The sync information is similar to that specified by SMPTE standard 17M with the exception that the sync amplitude is -3 mv instead of mv (-4 IE) and there is no subcarrier burst. With some types of equipment only the green signal carries the sync information. Various types of equipment use slightly different signal characteristics making equipment interconnection difficult. Among the variations encountered, either separate or combined, are the following: % 1 7.1% INVETE ADDE -Y SINAL Figure 5 raphic representation of the formation of 1//1/ color bars red-difference signal from the primary reen, lue and ed signals 11.4% 7.1% -7.1% - The sync signal is carried by a separate (fourth) conductor and its amplitude can vary from -3mV to -4 V. The maximum (white) signal amplitude is mv (1 IE) The black setup level is mv ( IE)

5 Figure 6 NTSC 1//1/ color bars signal waveform a b -4 ( ) 4 1 (714.3) 89.5 (639.29) 72.3 (516.43) 61.8 (441.43) 45.7 (326.43) 35.2 (251.43) 18. (128.57) (53.57) W Y C M LK LUMINANCE COMPONENT LEVELS IN IE mv IN ACKETS 14 IE (1 Vp-p) CHOMINANCE COMPONENT p-p SUCAIE LEVEL IN IE The matrixed color bars signals,, component color bars signals have relatively limited practical uses. They are mainly used to verify and adjust the gain of component analog,, distribution systems and serve as a signal source for matrixed and encoded color bars signals. The matrixed color bars signals are derived from component analog E,E,E signals by a specific process. The process consists of matrixing the three component signals into a new set of component signals, a luminance signal (E Y ) and two color-difference signals ( E -E Y and E -E Y ) COMPOSITE COLO SINAL SUCAIE ADDED TO LUMINANCE LEVELS IN IE 93.6 The E Y signal is given by the following expression: c IE (1.22 Vp-p) E Y =.587 E E E Figure 3 shows a graphical representation of the formation of the 1//1/ color bars E Y signal from the primary E,E,E signals. The amplitudes are expressed in percentages of the full-amplitude signal (7 mv) Figure 7 NTSC 75//75/ color bars signal waveform a -4 1 (714.3) 76.9 (549.29) 69 (492.86) 56.1 (4.72) 48 (344.29) 36 (258.57) 28.2 (21.43) 15.4 (11) W Y C M LUMINANCE COMPONENT LEVELS IN IE mv IN ACKETS (53.57) LK 14 IE (1 Vp-p) The color-difference signals are bipolar and have equal positive and negative excursions. Their amplitude is reduced by scaling factors to meet specific signal amplitude range requirements. When the allowed amplitude range of these signals is 7 mv ( 35 mv), as specified by the EU N1 standard and the CCI 61 recommendation, the color-difference signals are given by the following expressions: b -4 ( ) CHOMINANCE COMPONENT p-p SUCAIE LEVEL IN IE E C =.564 (E -E Y ) (also known as P in North America) and E C =.713 (E -E Y ) (also known as P in North America) COMPOSITE COLO SINAL SUCAIE ADDED TO LUMINANCE 72.1 LEVELS IN IE c IE (1 Vp-p) -4

6 Figure 4 shows the graphical representation of the formation of the 1//1/ color bars E -E Y color-difference signal. Figure 5 shows the graphical representation of the formation of the 1//1/ color bars E - E Y color-difference signal. Their amplitudes are expressed in percentages of the fullamplitude signal (7 mv). Different scaling factors are used by North American versions of the ETACAM and MII component analog Videotape recording formats. The dominant European version of the ETACAM format and the MII format use EU N1 scaling factors. a b W 62 Y C M LUMINANCE COMPONENT LEVELS IN mv LK 1 Vp-p CHOMINANCE COMPONENT p-p SUCAIE LEVEL IN mv Encoded composite color bars signals The NTSC,and the PAL encoding process uses identical matrixing coefficients to obtain the encoded luminance (E Y ) and color-difference (E -Y or E U and E -Y or E V ) signals. The E Y signal is given by: E Y =.587 E E E The color-difference signals are bipolar and have equal positive and negative excursions. Their amplitudes are reduces by specific scaling factors aimed at avoiding the overmodulation of land-based AM video transmitters using negative modulation. The scaled color-difference signals are given by the following expressions: c COMPOSITE COLO SINAL SUCAIE ADDED TO LUMINANCE LEVELS IN mv Vp-p Figure 8 PAL 1//1/ color bars signal waveform E -Y =.493 (E -E Y ) (Called E U in PAL) and E -Y =.877 (E -E Y ) (Called E V in PAL)

7 The encoding process of both television sys- a W 465 Y C M LUMINANCE COMPONENT LEVELS IN mv LK 1 Vp-p CHOMINANCE COMPONENT p-p SUCAIE LEVEL IN mv tems is similar. It consists of a process of suppressed carrier amplitude-modulation of two dedicated equal-frequency subcarriers ( MHz in NTSC and MHz in PAL), in quadrature-phase, with the two scaled color-difference signals. The NTSC encoding process results in a half-line-offset interleaved chrominance/luminance spectrum. In PAL, the V subcarrier phase alternates line-by-line resulting in a more complex quarter-line-offset interleaved chrominance/luminance spectrum. b c Figure 9 PAL 75//75/ color bars signal waveform COMPOSITE COLO SINAL SUCAIE ADDED TO LUMINANCE 491 LEVELS IN mv Vp-p Figure 6 shows the waveform of a 1//1/ (1) NTSC color bars signal resulting from the addition of luminance and chrominance components. Note that the peak positive signal excursion is 13.8 IE which is beyond the overload level of an analog television transmitter. This signal can, however, be accommodated by modern studio equipment. Figure 7 shows the waveform of a 75//75/ (75%) NTSC color bars signal resulting from the addition of luminance and chrominance components. The dotted outline of the luminance bar represents a 1//75/ color bars signal. The signal in Figure 7 can be accommodated by television transmitters. Figure 8 shows the waveform of a 1//1/ (1) PAL color bars signal resulting from the addition of luminance and chrominance components. The peak positive signal excursion of this signal is beyond the transmitter overload level of most 625/5 scanning standard television transmitters with the exception of the CCI-I transmission system as used in England. This signal can, however be accommodated by modern studio equipment. Figure 9 shows the waveform of a 75//75/ color bars signal resulting from the addition of luminance and chrominance signals. The dotted outline of the luminance bar represents a 1//75/ color bars signal. The signal in Figure 9 can be accommodated by television transmitters.

8 1 TECHNICAL NOTES TECHNICAL Figure 1 shows the relationship between the NTSC encoded composite signal amplitude and the percentage of modulation of the video carrier. The FCC transmission standards have established the minimum video carrier level, corresponding to peak white (1 IE), at 12.5%. It is evident that 1 color bars signals will overmodulate the transmitter and this is the reason for the use of 75% color bars signals for transmission tests. It is important to note that excessive positive signal amplitudes, corresponding to saturated yellow and cyan hues, are unlikely to occur with camera generated signals and this explains the FCC tolerances. Problems can occur with synthetically generated signals such as 1 color bars, character generators or digital video effects (DVE) generators which can produce signals causing video transmitter overload. VIDEO LEVEL SYNC TIP LANKIN -4 IE IE IE 1 IE 12 IE 12.5% 7.3% 75% Figure 1 Significant video signal levels show as a percentage of carrier amplitude in negative amplitude modulated sys 1 Printed in Canada Miranda Technologies inc. 2323, Halpern, St-Laurent (Québec) Canada, H4S 1S3 Tél Fax