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~Television Display Calibration Page~
Looks as good as new!
|Introduction:||Know how to get the most out of any old TV set, and make it look even better than those new "flatties"!|
The sad fact is that many hard-working reliable television sets across the country (and the world) are being frowned upon by viewers who demand more in picture quality and who are told by the salesmen that their flawless sets are "old". Just because cathode ray tube (CRT) televisions may not look like the slickest thing on the market today, it doesn't mean they should be banished if they can still operate to a quality standard. Besides, the picture quality of CRT TVs are in many ways superior to that of any new pixel-based digital LCD or plasma TV, as explained in this article from Aerials and TV (ATV). The problem is that there is a common misunderstanding of the five basic television picture adjustment controls and how to properly set them to an internationally-recognized quality picture standard. Frequent maladjustment renders these TVs displaying pictures so bright that black is only seen when the TV is off and white patches swell obtrusively while skin tones are unnaturally red or even green. Don't blame the TV set for being "old"! It just needs a few minutes of proper adjustment and the pictures will look as good as new.
You will need a source from which to feed standardized video test patterns to your television set, whether it be a broadcast station, VHS/DVD player, or professional pattern generator (harder to get your hands on). It is ideal to use a pattern from the medium which you will most frequently be watching (i.e. broadcast TV). Unfortunately, test patterns are rarely broadcast nowadays as informercials and 24-hour programming keep the TV busy day and night. For a calibration DVD instead, I recommend Digital Video Essentials from Joe Kane Productions. The only pattern you really need is the ubiquitous SMPTE Color Bars, though it may be easier to use the larger grayscale bars on the PLUGE (picture line-up generation equipment) test pattern to calibrate luminance. You will also need a blue filter, a piece of transparent material (usually a film or lens) which allows only blue light to pass through. (Do not use the color bars displayed by clicking on the link above. Digital image formats do not store sub-black luminance information and therefore cannot be used as accurate test pattern sources.)
Bonus: Oscilloscope-Based Video Monitor
Introduction/Background: Oscilloscope Basics
Besides duct tape, an engineer's other best friends are the multimeter and the oscilloscope. While a multimeter provides a quantitative reading of the electrical voltage or current in a wire, an oscilloscope provides a qualitative display, projecting the actual shape of the signal's waveform on-screen. An oscilloscope is a device (usually found in laboratories and in ham radio shacks) used to observe the waveforms of an electrical signal on a wire. To see what the oscilloscope can do, a novice user can plug the audio output from a headphone jack into the oscilloscope's "vertical input" jack (grounded, of course). It's screen will then display the shape of the sound waves (waveforms) in time with the music. High-pitched sounds will appear as narrow waves (high frequency) and low-pitched sounds will appear as wide waves (low frequency). Loud sounds will make tall waves (high amplitude) and quiet sounds will make short waves (low amplitude). Silence will make the oscilloscope display just a horizontal line with zero amplitude. If you consider the display as a coordinate graph, the vertical axis is the amplitude of the wave and the horizontal axis is time.
You can adjust the relative amplification of the signal (how tall the tallest waves appear on-screen) with the vertical sensitivity knob. This is typically calibrated in millivolts per division (a unit on the graticule) with a continuously-variable knob concentrically stacked in the middle. On older units, the knob may be labeled as an uncalibrated ratio, such as 1:1 or 100:1. You can also adjust the rate at which the oscilloscope "paints" the waveform (from left to right) with the timebase knob, typically calibrated in milliseconds per division. Older units have the "frequency" knob calibrated in horizontal sweeps per second (hertz), or "cycles", the unit for frequency used before 1960. These make up the basic controls on an oscilloscope. All words specifically referring to oscilloscope controls will be in bold.
An oscilloscope is helpful in many scenarios dealing with the testing and troubleshooting of electronic circuits. Looking at the shape of a signal can tell you a lot about the kind of circuit you're working with or the causes of its possible malfunction. For example, an oscilloscope can show you the quality of the circuitry in a cell phone's MP3 player compared to that of a PC's sound card. If you use an oscilloscope to "read" a sawtooth wave (example of a test signal which can tend to challenge or stress electronic circuitry) played by the PC and by the cell phone, the one from the cell phone will look ragged (distorted signal), whereas the one from the PC will look like an actual sawtooth wave. The differences are too subtle to detect by ear with speakers. Given a test signal, the oscilloscope can also show you the impact of distortion on the original audio signal caused by speaker impedance loading.
Furthermore, you can use an oscilloscope to track the changes in a signal as it passes through the components of a circuit. In practice, no "perfect" undistorted signal exists. But we can get it as close to perfect as possible by isolating the sources of distortion and making suitable adjustments.
The oscilloscope's X-Y mode graphs two signals against each other (parametrically), most commonly used to compare phase differences between them. A common application is to graph the left and right channels of an audio signal against each other to determine the amount (if any) of stereo separation present in the audio. A mono signal would produce a line with a slope of 1. A signal in which the two channels are 180░ out of phase would produce a line with a slope of -1. A signal in which the two channels are 90░ out of phase would produce a circle (for sine waves), a square (for square waves), a double triangle (for sawtooth waves), etc. A variety of Lissajous curves can be produced by graphing a wave against its harmonics. The X-Y mode can also make the oscilloscope behave in a vectorscope fashion to analyze video signals before they reach their display monitors.
Advanced: Video Display Technique
For all its contributions to the dry analysis of electronic circuitry, the oscilloscope can also behave like a common form of twentieth-century entertainment, the television. I have discovered a rarely-known but logical technique for displaying a composite video signal on any basic oscilloscope (provided it has an external trigger/sync input and a sweep rate of up to 15.7 kHz), effectively turning it into a video monitor. The method lies in a fundamental understanding of how television monitors work. There are three world standards for video signals and displays: NTSC, PAL, and SECAM. North America uses only the NTSC (National Television System Commitee) protocol. Television pictures are displayed in a raster format, which means that a rectangular "canvas" is created on which to "paint" a picture. A television "paints" a picture by starting in the upper left corner, going one line at a time from left to right and down the screen, until all 525 of them are scanned, forming one still picture frame. Then the whole process repeats for the next picture frame, and so on. This all happens so fast that our eyes see a moving image on the screen.
Normally an oscilloscope's CRT uses a vector scan (i.e. it draws a circle like you would on a sheet of paper without picking up your pencil). However, in order to display a video signal it must employ a raster-scan technique. (By analogy, imagine drawing many close light parallel lines on a paper, only darkening your stroke on the points which represent a circle.) How exactly does one go about creating this raster on an oscilloscope? Well, the NTSC standard calls for a horizontal refresh rate of about 15,734.267 Hz (15.75 kHz ¸ 1.001) and a vertical refresh rate of about 59.94 Hz (60 Hz ¸ 1.001). The horizontal refresh rate describes how fast the beam of electrons travels from left to right and the vertical refresh rate describes how fast it travels from top to bottom to cover the screen with a raster. Setting the horizontal sweep is relatively simple with the timebase knob. But setting the vertical sweep is a bit trickier because there is no "vertical timebase knob"; a vertical timebase signal must be input. This is where a quality signal generator comes in handy, or if you haven't got one, use the free software program, Audacity, with your computer's sound card. You need to generate a wave of 59.94 hertz for the oscilloscope's vertical input. But what kind of a wave must it be? Think about the way a raster is scanned vertically over time. The electron beam starts at the top of the screen and smoothly (linearly) makes its way down to the bottom, then instantly jumps back to the top for the next picture frame. This kind of a pattern traces an inverse sawtooth wave. This technique won't work unless your signal generator is good at generating true sawtooth waves. Just to be sure, observe the wave itself before proceeding. If it looks like a sawtooth wave (no curves), you're good to go. (You could salvage a vertical sweep generator from an old NTSC TV if you're lucky. Better yet, if you have the time and the know-how you could even build a circuit specifically for this purpose.) Essentially, the oscilloscope's internal timebase component serves as the video monitor's horizontal sweep circuit, while the external signal generator (or computer sound card) serves as its vertical sweep circuit. This is the purpose of "vertical hold" and "horizontal hold" controls on a TV. If your picture rolls up or down, it means that the TV's vertical scanning frequency got knocked off a bit from 59.94 Hz and should be readjusted.
Next we must figure out a way to vary the brightness of each spot on the raster in order to form a picture. The oscilloscope circuitry takes on quite a complicated procedure to accomplish this, and even I don't quite know how it works, but through experimentation I know that the following method does work. After the vertical and horizontal input jacks, the only remaining jack on my basic oscilloscope is for the external trigger, sometimes labeled "Ext. Sync.," "Ext. Input," or "Z-Axis Input". (This jack might be on the back of the oscilloscope.) Normally this is used to steady the display of waveforms rather than having them uncontrollably roll across the screen. (The oscilloscope detects spikes (steep slopes of relatively large amplitude) in the external synchronization signal to trigger the scan of each cycle of the vertical input signal across the screen.) But we're in luck because somehow it works just as well for a composite video signal! Actually, the video signal must first be inverted, otherwise the picture's luminance gets inverted (black becomes white and white becomes black). This is accomplished by switching the connections between the external trigger input and its respective ground input. So, just connect your video signal to the external trigger jack and, voila, you have a picture on the oscilloscope screen! What a thrill! Well, it is an ugly dim monochromatic black-and-green picture on a tiny screen rife with chroma dots, but it's an oscilloscope! Stare at the screen in amazement for a moment before moving on to the relatively easy task of calibration.
Say hello to your new tabletop TV! The oscilloscope's controls now have new names. "Intensity" becomes "brightness", "focus" is still "focus" (or what I like to call "actual sharpness"), "vertical sensitivity" becomes "vertical size", "horizontal sensitivity" becomes "horizontal size", "frequency/timebase" becomes "horizontal hold", and the horizontal/vertical position knobs still do their usual jobs. Here's a convenient recap of the six (or seven) steps required to set up your oscilloscope as a video monitor:
And that's all there is to it! Realize that a TV is just a refined oscilloscope with its vertical and horizontal controls locked and hidden inside. Also realize that an oscilloscope is just a more flexible, more versatile kind of TV (like a low-level programming language, for computer geeks). So the next time your television quits on you, turn to your trusty oscilloscope to catch that emergency weather report, or the latest episode of Jeopardy! If anyone knows the mathematics behind how the external trigger magically varies the brightness of each spot on the screen, please explain it to me!
Note: As far as I know, this technique will not work on any digital sample-based oscilloscopes, as the persistance of phosphor CRTs is a necessary ingredient. A digital scope will instead display a raster of widely-spaced pixels and is not capable of varying its beam intensity as a function of the external trigger signal.
Double Bonus: Waveform Monitor
A waveform monitor is as much a friend to the video engineer as the oscilloscope is to electrical engineers in general. This device was made for graphically monitoring the luminance of video signals and is designed for use in tandem with the SMPTE Color Bars to calibrate a video source. A waveform monitor displays the actual waveform of the video signal in a special manner. The x-axis corresponds to the horizontal positioning of objects in the video and the y-axis corresponds to the brightness of said objects. The vertical position of said objects cannot be distinguished on a waveform monitor as all horizontal lines are superimposed. For example, if there is a bright window in the left side of your video, it will appear as a peak in the left portion of the waveform on the waveform monitor. Technicians use a waveform monitor to ensure that on average the brightest components of a video signal do not surpass a level of 100 IRE (1 volt, full white) and the darkest components do not go below 7.5 IRE (0.339 volts, full black). 0 IRE (0.285 volts) is used as a blanking interval to switch off the electron beam as it sweeps back to start scanning a new line and -40 IRE (0 volts) is used as a sync pulse to signal the start of a new line or frame. (The IRE is the unit of brightness in composite video; it appears in a marked scale on the waveform monitor.)
Since professional waveform monitors were originally derived from oscilloscopes, you can make any oscilloscope behave like a waveform monitor (in "line mode") by simply connecting a composite video signal source to the vertical input of the scope and setting the horizontal timebase to as close as possible to 15,734.266 Hz, the NTSC horizontal scan frequency. The oscilloscope is painting the waveforms that represent each of the 525 lines of the image, one at a time in sequence, each for only 1/15,734.266 of a second. Because this is too fast for the human eye to detect and because of the persistance of the CRT phosphor, we perceive this as a display of all 525 lines continuously superimposed. Furthermore, 54.94 of those superimposed "frame signals" must pass by our eyes each second. The resolution of the displayed waveform is dependent on the bandwidth of your oscilloscope. For example, since my economy-grade Heathkit oscilloscope was made in 1960, its bandwidth is only 200 kHz. The vertical amplifier cannot react properly to frequencies above this limit and therefore such waveforms are rounded off.
To make the oscilloscope function in "field mode," set the horizontal timebase to 59.94 Hz, the NTSC vertical scan frequency. In this case, the brightness of all 525 lines are not superimposed, but adjacently positioned such that the left side of the scan corresponds to intensity at the top of the image and the right side corresponds to intensity at the bottom of the image. To magnify the waveform, simply increase the horizontal timebase as desired. This will cause the waveform to "stretch" horizontally. Of course, the effectiveness of this capability is dependent on the bandwidth of your oscilloscope.