TABLE OF CONTENTS
2. VIDEO REVIEW
2.1. A LITTLE HISTORY
2.5. IRE UNIT
3.1. STANDARD PATTERNS
3.1.1. SMPTE Bars
3.1.2. 100% White Full Field Bars
3.2. ENHANCED PATTERNS
3.2.2. Cable Sweep
3.2.3. NTC 7 Combination
3.2.4. Stair Case
3.2.5. Modulated Ramp
3.2.6. Cross Hatch with Dots
3.2.7. Center Cross with Safe Area
3.2.9. Test Signal Matrix
Have you always wanted a video test pattern generator for your tool box but the cost or quality kept you waiting for the right unit. The video generator in the following article is a low cost, high quality instrument which can produce a variety of helpfull test patterns as well as source identification or information in text and can even generate a audio tone. The design allows you to build only what you need so it is very flexible. This article first discusses the basics of color video then why do we need a video test pattern generator then how the hardware and software functions in this design (see the VG10 operations manual for circuit theory).
Lets discuss some of the basics of video before we get into detail circuit operation. What do this signals do and why do we need video test signals?
Color television was first developed in the
A black and white TV set has only one electron gun. This single electron beam scans the picture tube in a interlaced fashion, from left to right and top to bottom., for 252 ½ lines called a field , then repeats the process to interleave the next 252 ½ lines to create a 525 line interlaced frame. The synchronizing information is a series of pulses which tell the horizontal deflecton section when to return to the left of the screen to start a new line, and the vertical deflection section when to return to the top of the screen to start a new frame. This is done by scanning the horizontal at approximately 15,750 lines per second, and the vertical at 30 frames per second (the vertical scan rate is actually 60 Hz, but it takes two trips down the screen to complete one frame). The process of returning to start a new scan is called retrace or fly back.
The voltage level of the luminance signal determines the instanteous brightness of the image on the screen. The negative signal extremes correspond to the dark areas of the picture and the positive signal extremes correspond to the bright areas of the picture. . Figure 1 shows the video signal during the time that it takes the electron beam to make one horizontal scan across the screen. Now lets look at the evolution of the blank and white signal to create color video. The NTSC committee came up with a ingenious way to maintain compatibility with the existing black and white system and add color. A color subcarrier signal was added to the luminance signal.
A color picture tube has three electron guns, red, green and blue. Virtually any color can be created, as well as black and white, by correctly controlling the intensity of each primary color. The color subcarrier is used to encode the red, green and blue information on the camera side and is decoded at the TV to recover the primary colors. The red, green and blue signals are used to modulate the color sub-carrier (which is ignored in a black-and-white TV) to produce the “color difference” signals, designated R-Y, B-Y and G-Y, which has a frequency of 3.58 Mhz in the NTSC system.
Although the type of modulation used on the sub-carrier is of a complex nature if boils down to a simple result:
1. The instantaneous phase of the 3.58 MHz signal determines what color will be displayed (called hue or tint).
2. The instantaneous amplitude of the 3.58 MHz signal determines how much color will be displayed (called saturation).
An obvious question is the phase and amplitude of the 3.58 MHz signal relative to what? The answer is a short burst 3.58 MHz (simply called the burst) which has constant phase and amplitude. The burst will be used to determine the tint and saturation of the color to be displayed. For the wave form shown in Figure 1(d) each bar would have a different saturation.
Before we get into the details of the test signals we need to define a few TV terminology terms. This is a arbitrary unit used to describe the amplitude characteristic of a video signal. Television Engineers find it more convenient to specify signal levels in IRE rather than milivolts. Pure white is defined as 100 IRE and the blanking level is 0 IRE. NTSC video has 714 mV between blanking and peak white so 1 IRE is 7.14 mV.
Video Test signals are very helpfull to evaluate a video processing system. A few of the uses are, televison monitor setup and alignment, a test pattern to be recorded at the head of a video tape production so the playback can be adjusted accurately to match how it was recorded, or it could be used as a constant signal on a video transmission link when no live video is needed. The best and easiest way to evaluate video equipment is with a well-defined, highly stable test signal having known characteristics. All video testing is based on the simple principle of applying a known test signal to the video system or equipment input and observing the signal at the output. The output could be your oscilloscope or a picture monitor. Any distortion or impairment caused by the system is observed and measured on the output signal or seen on the monitor. If there are distortions, the equipment is adjusted to eliminate or minimize them or faulty components need to be replaced and repaired. The end result is that if the system can pass the test signal properly it can cleanly pass picture signals as well. The signals necessary for such testing are obtained from a test signal generator. This instrument produces a set of precise video signals with carefully defined and controlled characteristics. Each signal is ideal for verifying one or more specific attributes of the video system under test. Just like anything else in life each test pattern has its job it does well. Here are a few applications and uses for the patterns available on the video generator.
SMPTE Bars are split field bars composed of standard EIA 75% amplitude white bars for the top 2/3 of the field, reverse blue bars for the next 1/12 of the field, and the IYQB or “PLUGE” signal for the remainder of the field. This split-field arrangement allows adjustment of color saturation or color intensity and hue or tint on a color monitor that has the feature to allow the blue gun only. The monitor is set to blue only and the hue or phase is adjusted on the monitor until there is no discernable intensity difference between the reversed blue bars and their adjacent color bars. The IYQB section of the bottom pattern consists of a 7.5 IRE (black level) pedestal with a 40 IRE “-I” phase modulation, a 100 IRE white pulse, a 7.5 IRE (black level) pedestal with a 40 IRE “+Q” phase modulation, and a 7.5 IRE pedestal with 3.5 IRE, 7.5 IRE, and 11.5 IRE pedestals. -I and +Q phase modulation signals are helpful to assure the subcarrier processing in correct. PLUGE stands for (Picture Line-Up Generating Equipment). This pattern, at the bottom and to the right side of the SMPTE bars is used to set the brightness of the monitor. The monitor is adjusted so that the black and blacker than black areas are indistinquishable from each other and the lighter than black area is slightly lighter (the contrast should be at the normal setting).
The 100% white full field bars are the same as the EIA color bars , except a 100 IRE white level is used. This test signal is helpful to check chroma amplitude versus overall video level. If the system under test is setup properly for chroma gain the tips of the yellow and cyan bars should be at 100% level matching the white bar peak amplitude.
The EIA color bars are a part of the EIA-189-A standard. The seven bars (gray, yellow, cyan, green, magenta, red, and blue) are at 75% amplitude,100% saturation. Each color bar uses 1/7 of the image area. 3.1.4. Window The window pattern consists of a white rectangular area in the middle of the screen surrounded by black. This pattern is good for testing low frequency response and video tilt as well as the performance of video clamping in the video processing system. 3.1.5. Red, Green, Blue and Black Full Field These patterns are full image screens of red, green, or blue. These are helpfull in television monitor testing to see if there are any purity problems. If there are problems, you would see off color areas rather than full saturated vivid colors throughout the screen.
This pattern is great for checking the low frequency response of the video system. The signal is best looked at on a oscilloscope at the output of the video system. Be sure that the window waveform is flat across the oscilloscope display at both the horizontal or vertical sweep rates.
These full raster field colors are great for monitor purity verification and adjustments. If you see the color saturation or hue vary across the screen the purity needs adjustment.
The multiburst signal is great for quick measurment of the frequency response of the system. The signal typically includes six packets of discrete frequencies which fall withing the TV passband. The packet frequencies usually range from 0.5 Mhz to 4.1 or 4.2 Mhz, with frequency increasing toward the right side of each line.
Cable Sweep is another frequency response measurement signal. Rather than discrete packets as in multiburst this signal has a continous sweep of frequencies from 1 to 4.5 Mhz. There are frequency markers on lines toward the bottom of the screen. These are helpful to determine where your rolloff occurs.
The NTC (U. S. Network Transmission Committee) developed a combination test signal that may be used to test several NTSC video parameters, rather than using multiple test test signals. This test signal is cleaverly called the NTC-7 Combination Test Signal and consists of a white flag, a multiburst, and a modulated pedestal signal. The white flag has a peak amplitude of 100 IRE and a width of 4 us. The multiburst has a 50 IRE pedestal with peak-to-peak amplitudes of 50 IRE. The starting point of each frequency packet is at zero phase. The width of the 0.5 Mhz packet is 5 us; the width of the remaining packets is 3 us. The 3-step modulated pedestal is composed of a 50 IRE luminance pedestal with three amplitudes of modulated chrominance (20, 40, and 80 IRE peak-to-peak). The rise and fall times of each modulation packet envelope are 400 ns.
The 5-step staircase signal, shown in the figure below, consists of 5 luminance steps. The peak-to-peak modulated chrominance is 40 IRE. The modulated chromaninance has a phase of 0 relative to burst. Note a 0 IRE setup is used.. This test signal can be used for measuring non-linear luminance variations in a system.
The modulated ramp test signal is composed of a luminance ramp from 0 IRE to either 80 or 100 IRE. The 80 IRE ramp provides testing of the normal operating range of the system; a 100 IRE ramp may be used to optionally test the entire operating range The peak-to-peak modulated chrominance is 40 IRE. The modulated chromaninance has a phase of 0 relative to burst. Note a 0 IRE setup is used. The rise and fall times at the start and end of the envelope is 400 ns. This test signal is can also be used to measure differential gain and is good for measuring analog to digital bit errors in digital video systems.
This pattern generates a matix of horizontal and vertical lines helpfull for adjusting monitor convergence. Since a white line on the screen is made up of red, green and blue components, each picture tube electron gun must have their respective beams aligned perfectly overlayed on each other or you get hallows around the misaligned image areas.
This signal is simalar to the Cross Hatch but is used to define the safe image. Your video production should not contain any picture information outside of the safe area or your viewers may not see it. The televison monitor should show the entire safe area or it needs adjustment.
This test signal is great to test the low frequency and clamp response of the system. The video signal will vary from 0 IRE to 100 IRE at a 1 second rate. The video signal should not distort or clip under either condition and maintain a constant sync tip level if the clamp circuitry is functioning properly. The television monitor should not change brightness or width with the widely varying average picture level of the bounce signal.
The Matrix pattern is a combination of the patterns we have discussed previously. There are approximately 48 lines of each test pattern to make one image comprised of 5 different patterns. The 5 patterns that make up the matrix are, Color Bars, Red, Green, Blue and 50 IRE flat signal.
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