Teleproduction Test Volume 1 Number 4
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TELEPRODUCTION TEST VOLUME 1 NUMBER 4 A PRIMER IN THE USE OF VECTORSCOPES PART 1 The vectorscope deals with the color aspects of modulation is used to carry two color signals on the composite video signal only and ignores the same 3.58 MHz subcarrier signal. Tw o luminance (black and white, sync). It is designed modulators operating with subcarrier signals that to evaluate color using the standard color bar are 90° apart in phase are fed with the color signal (75% amplitude, fully-saturated colors as difference signals R-Y and B-Y. (Holdoff on I and per EIA RS-189A or SMPTE ECR1-1978 Q for a wee dab.) The outputs of the two standards). Color bars form the basic color test modulators are simply added together and to find signal. For this reason, they are often transmitted the resultant, a vector plot is made. Figure 4-2 by broadcasters prior to the start of the program shows the basic vector diagram. It happens to be day and are always recorded on the leader of the same diagram that is plotted on the screen of taped productions so that equipment can be the vectorscope. In the diagram, B-Y (blue checked and reset, if needed, for proper subtracted from the luminance or Y signal) is operation. In addition to providing the means for plotted horizontally. The modulators are balanced monitoring the quality or the NTSC encoded color and the carrier itself is suppressed so that positive signal, the vectorscope is also a powerful tool for values of the B-Y signal are plotted to the right (3 phase-matching video sources when setting up o’clock) and negative values, -(B-Y), to the left (9 multiple-source systems. o’clock). Subcarrier for R-Y signal is at 90·° from the B-Y signal and is plotted at 90° as shown. What the Vectorscope Shows Positive R-Y is up at 12 o’clock and negative R-Y The subject of previous issues, the waveform is down at 6 o’clock. m o n i t o r, is basically an oscilloscope. It draws a O k a y, let’s plot a color. For a simple illustration, graph of voltage (up and down) versus time (from we’ll plot red and we’ll ignore the complications left to right). The vectorscope is quite different. It that are applied for purely practical reasons. For too draws a graph but this time the amplitude of fully saturated, 100% red R=1, B=0 and G=0. The the 3.58 MHz chroma signal is plotted as a radius Y and color difference signals then work out to Y = whose amplitude is plotted outward from center 0.3*, R-Y = 0.7 and B-Y = -0.3. If we lay out the screen and whose phase angle is measured amplitudes for R-Y and B-Y on the vector plot of around the circle much as time is marked off on a Figure 2, we get the vector for red as shown. clock face. Engineers call this graph a polar plot. To get an idea of how a vectorscope works you need a gist of the concept of vectors. Consider a simple example. Figure 4-1 shows a boat moving directly across a river (90° to the flow) at a speed of 4 mph. The stream flows at 3 mph as shown. We can plot the boat’s position after 1 hour graphically and determine speed over the bottom by plotting the vectors as shown. To do this, a rectangle is plotted as shown by the dashed lines and the resultant vector, in this case the actual path of the boat, is constructed as shown. The actual speed over the bottom can be measured off the resultant (5 mph) or simple illustration of the use of vectors to deal with actions at an angle to one another. Vectors are also used to solve problems of two AC voltages that have the same frequency but d i ffer in phase. In the NTSC system, two-phase Figure 4-1. A vector solution to a simple piloting problem. Figure 4-2 Red plotted as a vector using simplified values for B-Y and R-Y. Figure 4-3 Red plotted using corrected values for B-Y and The complications alluded to earlier include R-Y. attenuation factors for B-Y and R-Y to squash down the subcarrier signal and keep it within 100 subcarrier sample that the color decoder in the IRE for most signals and to take care of the receiver or monitor uses to regenerate the carrier e ffects of setup which subtracts from the total needed for the demodulation process. B y available signal swing. When these corrections convention, burst phase is at 180°, that is on the are applied, R-Y and B-Y for fully saturated 75% -(B-Y) axis at 9 o’clock. Figure 4-4 shows the (amplitude) red turn out to be about 0.426 and burst vector at the indicated position. When 0.103 respectively. These are plotted in Figure 4- viewing any NTSC signal, the front panel PHASE 3. Simple trigonometry allows the solution for the control is adjusted to put the burst vector at the angles as shown. All angles are measured position in Figure 4-4. The graticule is also counter clockwise from zero, which by marked for proper burst amplitude (distance from engineering convention starts at 3 o’clock, and the origin). The burst dot is somewhat dimmer this puts the red vector at 104°. because the vector stays at the burst value for a So much for pencil drawings — let the short time (the duration of the burst). vectorscope do it. Figure 4-4 shows how fully saturated 75% red appears on the screen of the vectorscope. The arrow tips of the pencil vector diagram shows up as a bright dot on the vectorscope. The dots are bright because they represent the signal dwelling at one value for a period of time. The signal for Figure 4-4 is a full red raster so the vector remains at the same location for the entire active period of the raster. Somewhat dimmer dots show up at each of the colors for the color bar signal because the vectors stay at one location for the duration of each bar and repeat for 3/4ths of each field for split-field bars. Burst Burst also shows up as a vector (dot) on the vectorscope. It is the phase reference and Figure 4-4. Red plots as a bright spot on the screen of the *Luminance (Y) = 30%R + 59%G + 11%B vectorscope. 14 Figure 4-6. Electronic targets on Leader Vectorscopes. Figure 4-5. Detail of tolerances for the vectorscope targets. Color Bars So far we’ve shown one color, red, on the vectorscope. The standard color bar signal provides eight colors if we include white and black. These are, in order from left to right: white, y e l l o w, cyan, green, magenta, red, blue and black. White and black are balanced colors. That is, the color difference signals go to zero for Figure 4-7. Centering isn’t critical because the targets neutral white, gray or black. There is no output move with the origin. from the encoder modulators in these cases and show tight and loose tolerances respectively. See the 3.58 MHz subcarrier also goes to zero. Thus, any neutral gray, white or black has no effect on Figure 4-5. The smaller boxes outline a spread of the vectorscope display. But the remaining colors, ±2.5° in phase and ±2.5 IRE units in amplitude. the primaries red, green and blue and the The larger boxes outline a spread of ±10° in complements cyan, magenta and yellow all phase and ±20% in amplitude. You can accept a product vectors that can be plotted as was shown c a m e r a ’s encoder as being accurate if its color for red. The vectorscope plots these vectors and bar signal puts each of the vector dots in the is basically a very simple device. It simply smaller boxes. decodes the input composite signal into R-Y and Leader’s Electronic Graticule B-Y components and applies them to an X-Y An attractive feature of Leader’s vectorscopes is display so that R-Y deflects the election beam up an electronically-generated graticule in addition to and down, B-Y left and right. It’s doing this job the graticule that is etched internally on the CRT. accurately that causes vectorscopes to be See Figure 4-6. The bright targets can be seen somewhat expensive. They require dedicated from a distance and they also reduce the fussing cathode ray tubes and precision decoders. with centering controls that is required with The Vectorscope Graticule conventional vectorscopes. The electronic targets At this point, you should be convinced that the (boxes) move with the display. See Figure 4-7. So vectorscope plots the color bar signal. In fact, the it isn’t really necessary to put the electronic origin scale or graticule plots the correct locations for smack on the cross hairs engraved on the CRT. each of the six colors. The correct spots are The electronic graticule is also more accurate marked with a small cross. But these are because it is processed by the same circuits and surrounded by both small and large boxes that CRT that handle the input signal; the target signal 15 Figure 4-9.