|Hawes Mechanical Television Archive||by James T. Hawes, AA9DT|
Color Wheel FAQs
The station doesn't transmit hue and saturation directly. Instead, the station transmits two vectors. The I vector translates to the range of colors between orange and cyan. The Q vector translates to the range between purple and greenish-yellow. (Notice: No red, green or blue!) The vectors are AM signals. Here's how each signal indicates hue and saturation...
The two vectors also coexist on the same subcarrier at the same time. Not a bad trick! To prevent interference with the picture and sound signals, the station suppresses the subcarrier.
Without a subcarrier, of course, your TV set can't accurately detect colors. The set does detect the signals, though. Since detection (or demodulation) requires a subcarrier, your set generates one. Of course, not just any subcarrier will do. The subcarrier must synchronize with the station's signal. Here, the station helps out. Between video lines, your set blanks out the picture for just an instant. During this blanking interval, the station transmits a signal that we call the color burst. The burst is about eight to 11 cycles of unmodulated subcarrier. Your TV color circuits use these few cycles as a reference or sync signal. This signal keeps your set's subcarrier oscillator on track.
Earlier, we mentioned Messrs. I and Q. These guys are really eccentric. In fact, they have several unusual characteristics. For one, vectors I and Q insist on occupying the same band at the same time. The station needs to keep these vectors from canceling each other. To prevent cancellation, the station delays the Q signal by 90 degrees. Afterward, neither signal has any significant effect on the other. Long ago, clever scientists discovered that they could interleave signals in this way. Engineers call this technique "quadrature transmission." In fact, Q stands for "quadrature," while I stands for "in phase." Another fancy term for this transmission mode is "orthogonal transmission."
Why are engineers so proud of quadrature transmission? Here's why: Since I and Q are 90 degrees apart, they can also describe both the length and breadth of a plane. Imagine I and Q as city map directions. I covers the north-south addresses. Q covers the east-west addresses. Television uses I and Q directions slightly differently, but the idea is the same. In TV, I and Q plot a position on a circle. The color wheel is just such a circle. (If you had "polar coordinates" in high school trig, then you recognize the idea.) By comparing I and Q values, TV can locate any color on the color wheel.
The TV station transmits I and Q values to your set. The burst signal keeps your set's color electronics in sync with the station camera. Your TV set decodes the transmission and paints realistic colors on its screen.
QUESTION. How does my TV reproduce color?
ANSWER. Your TV receiver compares the I and Q vectors and the burst signal. For those who tuned in late: Burst is a color sync signal. Burst keeps your set in step with the TV camera in the studio. The comparison process allows your receiver to reconstruct R, G and B data.
For example, let's assume that our circuit syncs normally with the burst. We have a positive I signal and a negative Q signal. Then the picture tube reproduces a yellowish color. How did it get yellow? Here's how...
Still in the dark? Really, this is all high-fallutin' language for a very simple idea: Remember those spinning pointers on children's board games? We kids used to flip a pointer with our fingers. The pointer would stop on a position on a circle. This position told us where to move on the board.
Your TV set does the same thing, only in a controlled way. Comparing the two color vectors is like spinning the pointer. In this case, the pointer spins around the color wheel. When the pointer stops, it rests on the color that your TV reproduces.
Color or hue isn't the end of the story, though. Your TV also tracks two other values: Saturation and brightness. Let's return to the spinner idea. Saturation is the length of the spinner. Saturation tells your TV how much of a hue to add to the picture. Brightness (or luminance) is the black and white part of the picture.
Your TV applies hue, saturation and brightness values to every part of the picture. The TV starts with the smallest part of a picture, a pixel. Then the TV builds the entire picture from there. Every pixel has a brightness value. Yet several pixels in the same line (about eight) share the same hue and saturation values.
QUESTION. What are hue, saturation and brightness, or HSB?
ANSWER. HSB stands for hue, saturation and brightness. HSB is one of the color models, or ways of thinking about colors.
Hue is the color: Red, green, blue or some other color from the TV color wheel.
Saturation is sort of a ratio: The ratio expresses how much color we have vs. how much brightness data. For example, TV can make red and pink from the same hue. Why do these colors look different? Because red is more saturated than pink is. That is, pink has more white than red does. In television terms, white isn't a color at all. Instead, it's a brightness value.
Brightness is the black and white picture information. Some people think of brightness as the average of the color data. This idea works for the real world, but not necessarily for TV. In TV, the colors have less detail than the monochrome data has. TV sends monochrome data separately. That is, brightness or "Y" exists separately from color or "C." You can have black and white TV without ever having a color picture. On the other hand, color pictures include black-and-white pictures.
QUESTION. What do the "hue" and "color" controls do?
ANSWER. The color TV hue control shifts the phase of the burst signal. When you shift the burst signal, you change all the colors. By the way, burst is what the TV's reference oscillator syncs to. While decoding the chroma, the demodulators use the oscillator signal as a beat frequency.
The TV reference oscillator is where the famous 3.58 MHz color crystals come from. We hams use those for transmitting on the 80 meter band. Of course, you need one for a color adapter, too. Some TVs call "hue" something else. "Tint," usually.
The color or "saturation" control in a color TV is a contrast control for the chroma signal. The control usually adjusts the level of the chroma signal into the demodulators. The demodulators get the signal from the last chroma amplifier. Col-R-Tel doesn't have chroma amplifiers before the demodulator. In Col-R-Tel, the saturation control is at the demodulator stage. Because Col-R-Tel's field-sequential, it only has one demodulator instead of two.
QUESTION. How does TV make green from the two colors that it receives? Does it use the Y signal to make green?
ANSWER. The two, incoming chroma signals are really two, low-detail pictures. In the US, we call these incoming signals I and Q. Positive I and Q are close to red and blue. By combining I and Q, the TV set obtains the third chroma signal. This third signal is a low-detail picture in a greenish hue.
In Europe and elsewhere, U and V are the two, low-detail color signals. The rest of the process is about the same as with I and Q.
The Y signal is what engineers call "luminance." That's a $64 term for brightness. No, you don't need Y to get G-Y. The Y signal does something else. Y indicates the picture brightness.
QUESTION. What are the world TV standards?
ANSWER. In the analog realm, there are three major color systems: NTSC, PAL and SECAM. You'll also find every imaginable combination of the three. Combined systems tend to be incompatible with the major three, or with each other. Some system details appear in the table.
The first-invented system was NTSC (Never Twice the Same Color). PAL (Pay and Learn) operates slightly differently. SECAM (System Essentially Contrary to the American Method) operates way, way differently. You can't blame the Europeans. They tried to invent NTSC, but found that RCA had already invented it. So what did they do? They shelled out a lot of money and time. After about 13 years, they nearly got NTSC all over again. The results are more complicated, noisier and less efficient than NTSC. The European versions also have half of NTSC's vertical color resolution. A shame. Yet the "new" television systems offer a huge advantage: These systems allow different people to collect patent royalties.
Excuse me for joking so much. NTSC really stands for National Television System Committee. PAL really stands for Phase Alternation Line. SECAM really stands for Sequential Color and Memory. I like NTSC, especially now that the VIR signal controls the wandering color. VIR stands for vertical interval reference. During vertical blanking, TV stations transmit a color correction signal on Line 19. This is the VIR signal. VIR has been a part of NTSC for decades. My set also includes automatic fleshtone correction and automatic color correction. Yet another circuit corrects for the aging of the CRT.
Back in the 1960's, we didn't have color TV. I recall visiting neighbors' homes and watching their color sets. In those days, TV watching truly involved the viewer: I rode the color and hue controls constantly. Every commercial and every station seemed to maintain a different color balance! But technology marches on. (Well, it doesn't exactly march...) Anyway, despite what other Web sites say, differing colors aren't an NTSC problem anymore. At least, not for this viewer. Back in 1985, we bought our new RCA set. Since then, I haven't tweaked my color or hue control even once. That's a span of over 20 years. In fact, I don't know where the controls are. So there, you other color TV sites! Get with the program! The only problem NTSC has is slander from PAL land, just over the pond. (Hop the English channel, and you hear a different tune. The French know the quality of modern NTSC. They make our RCA sets. Viva RCA! Viva la France!)
On the other hand, PAL and SECAM produce excellent pictures. Yes! I admit it! Oh, and reinvention is a perfectly valid form of invention. In fact, most invention is reinvention. That's only natural. Ideas spring from other ideas. Besides, all the television systems involve original ideas from around the world.
This link provides more detail about world TV standards.
QUESTION. What is a flyback transformer, and what does it do?
ANSWER. Who says that US engineers don't appreciate poetry? Swallows fly back to Capistrano. And flyback transformers— Well, they don't really fly to Capistrano or anywhere else. That's where the poetry comes in. Your TV's flyback transformer (or just "flyback") is a high-frequency step-up transformer. The term "flyback" refers to a pulse that occurs at the end of every TV line. This pulse causes the scanning beam to fly back to the left side of your picture tube. At the end of a video field, the scanning beam flies back to the top of the screen. Without flyback action, we simply wouldn't have a picture.
FLYBACK FUNCTION. Poor Europeans use a much more prosaic term for the flyback transformer. They call it the "high tension transformer." Flyback or HT, this transformer performs a fantastic chore: It takes AC voltage from the horizontal output transistor and steps up this voltage to 25 kilovolts. This 25 kV drives the second anode or ultor of your color picture tube (CRT). Larger sets might use an even higher ultor voltage.
FLYBACK HISTORY. Flybacks have been a part of electronic TV sets since about the 1940s. The earliest sets powered their CRT ultors with huge transformers. These transformers stepped up the power line voltage. Some oscilloscopes still use such dangerous and extremely heavy transformers. Did your TV repairman walk with a limp? He'd been lugging one of those early oscilloscopes!
Back when TVs used line transformers for high voltage, the CRTs were small. Small CRTs require comparatively low ultor voltages. One rule of thumb estimates the ultor voltage to be about 1,000 volts per inch of screen. For instance, a 25-inch screen requires 25 kV. This rule of thumb tends to overestimate the necessary voltage for small screens. Some five-inch screens only need 2,000 volts.
WHY FLYBACK? Anyway, as screens grew larger, line transformer power supplies became impractically heavy and expensive. The only way to reduce transformer weight is to reduce the amount of iron in the laminations. Here's a transcript of the very meeting where that fact came out...
MANAGER. (Ecstatic) "Great! Less iron means lower cost! Chop out the iron, boys and girls!"
ENGINEER GROUCH. (Grumbling) "Not so fast. Do that, and our transformer melts!"
Fortunately, most companies listened to Engineer Grouch. Companies that didn't aren't around anymore. Fact: In order to reduce the iron without overheating the transformer, you must increase the power frequency. Fortunately, TV sets include a generator of much higher frequencies: The horizontal oscillator. Our savvy engineer also realized that a lightweight transformer could step up the horizontal frequency. Here's a transcript of the introductory meeting for the new transformer...
MANAGER. "Do you realize what we have here? Why, this will revolutionize television as we know it! The name associated with this device will go down in history! By the way, Engineer Grouch, what do we call this thing?"
ENGINEER GROUCH. "I don't know, Mr. Freibach."
Of course, some engineers don't spell very well. Grouch was among this distinguished group. He scribbled his manager's name in the title block of his new part release. Well, sort of. Anyway, that's how the "flyback" debuted.
LIGHT & CHEAP. The horizontal frequency and the flyback were a match from— Well, not heaven, but a pretty good lab. We were talking about reducing iron in the core. The horizontal frequency is 15 kHz. Contrast that with 50 or 60 Hz for the power line frequency. The considerable difference cuts way down on the amount of transformer iron that we need. In fact, most flybacks have ferrite cores instead of iron cores. At 15 kHz, ferrite is a more efficient core material than iron. Filter capacitors for 15 kHz are also much smaller than line voltage filter capacitors. In this case, "smallness" refers both to size and microfarad value.
DOUBLER OR TRIPLER. In addition to the flyback transformer, some solid state sets use a doubler or tripler circuit. This circuit consists of diodes and capacitors. The doubler or tripler further reduces flyback size and cost. In some sets, the doubler or tripler is inside of the flyback assembly. In more serviceable sets, the doubler or tripler is a separate module. This tripler evolved from the high voltage rectifier in tube sets.
For more about flyback history and facts, click... Flyback history and facts
QUESTION. What is the the function of a scan-derived power supply?
ANSWER. The picture tube ultor supply is an example of a "scan-derived power supply." That is, the power comes from the horizontal oscillator, rather than from the power line. At the horizontal output transistor, the AC voltage is about 600 to 1,200 volts. The flyback transformer boosts the horizontal output voltage to tens of thousands of volts. Now, the ultor voltage is very high, and a special case. Yet in most TV sets and some monitors, the flyback also sources the lower voltages.
For example, your TV set might have a video amplifier that runs on 24 volts DC. The 24 volt source is an extra flyback winding. An outboard rectifier and filter converts the flyback AC to DC. Again, the filter circuit can be very small and efficient. Imagine a comparable, line-operated power supply. It would require a separate transformer and rectifier, plus large, heavy and expensive filter capacitors. Just a few turns of wire on the flyback can provide equivalent output. Add a half-wave rectifier and a small filter capacitor, and you have a useful 24-volt source.
Modern flybacks also contain high voltage rectifiers. Taps produce voltages for the focus and screen grids.
QUESTION. Since scan-derived power supplies power themselves, how do they start up?
ANSWER. Scan-derived power supplies sort of pull themselves up by the bootstraps. In many cases, horizontal oscillator power comes from the horizontal output transistor and flyback. While scan-derived circuits power themselves, they can't start without help.
Here's the trick: When you switch on your set, a startup circuit pulses the horizontal oscillator. The startup circuit is like a defibrillator for a robot. In this case, the "robot" is the horizontal oscillator. The startup pulse pulses provides him with temporary power until his oscillations become self-sustaining.
Without a startup pulse, the horizontal oscillator is dead. Also, without horizontal oscillations, the horizontal output transistor can't provide drive current to the flyback. Of course, a flyback without drive means no power from scan-derived power supplies. Plain and simple: In a normal set, the startup pulse wakes up the entire circuit. In a moment, you have a picture.
QUESTION. What does the shutdown circuit do?
ANSWER. The shutdown circuit is a solid-state set's overvoltage protection. Excessive voltages lead to excess heat, arcing, component breakdowns and even fire. A more insidious result of overvoltage is excessive x-radiation from the picture tube.
The overvoltage circuit takes a voltage measurement near the horizontal output transistor. If voltage at this point measures high enough to switch a trigger diode, an SCR fires. This SCR is usually part of a crowbar circuit. This circuit blows a line fuse or disables the horizontal oscillator circuit. In either case, the TV set powers down. Before the set can operate normally again, it requires service.
Overvoltage projection is also one reason why manufacturers allow no parts substitution in certain circuits. Among these circuits are...
QUESTION. What does the tristable switch (tritch) do?
ANSWER. Tristable switches are part of some color adapters, including Colordaptor. Such a switch is really a flip-flop with an extra stage. When such circuits toggle (switch), either one stage comes on, or two stages come on. If you invert the circuit output, one stage at a time passes current. (What you really have is a basic shift register.) Anyway, here's how Colordaptor uses its three-way switch...
Instead of the tritch, Col-R-Tel uses a fancier commutator. This commutator can detect which color gel is before the CRT. The commutator then selects one of three diodes that each switch one color subcarrier. Each subcarrier decodes one of the three primary color signals. Spectrac, a two-color system, probably uses a bistable switch.
Still, switching isn't just a simple matter of following the disc. Jay Stanley's color adapter articles drive this point home. Stanley stresses that color shifts must also sync with the vertical signal. The problem is more complex than syncing the CRT gun with the disc color wedge. Stanley adds more signals that must match up. Suppose that we're reproducing the red signal. We toggle the tristable switch. At that point, here's what must coincide...
Each new color should begin painting at the top of the CRT face. Syncing color wedge start with vertical blanking achieves that effect.
Also, the circuit must detect disc wedge color. The adapter must display only red video when the red wedge covers the CRT. The same goes for green and blue video. Otherwise, you might match the red video with the green gel. Then your picture wouldn't be lifelike.
Copyright © 2006 by James T. Hawes. All rights reserved.
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|WARNING. If you take apart the family TV, your dad will yell at you. Pursue your experiments at your own risk. I take no responsibility for your results. Expenses, losses, injuries or damages that you incur are your responsibility. I offer no guarantee as to the accuracy of the information on this or succeeding pages. Sometimes I slip on my calculator.|