In Plain English. Okay, so you aren't a geek. Col-R-Tel still interests you, and you want to learn more about it. Congratulations. You know a good thing when you see it. I'll cut the nonsense and speak English.
Let's start at the TV station. Tonight, the station is broadcasting your favorite color show. But the TV signal isn't really just one signal. It's several. These signals come to you over a "carrier frequency." Inside the TV channel, many signals "modulate" or ride on this carrier. Each signal takes up part of the channel. In the center of each channel is another carrier that we call a color subcarrier. All color signals modulate this subcarrier.
The color part of your show seems to require four signals...
- One signal for each of the three primary colors
- One signal to synchronize the color data with the monochrome picture
Clever engineers came up with a way to transmit the three primary colors on two signals. This method reduces our requirement from four to three color signals. As we'll see, we can economize even further. Our second point above is the synchronizing signal. This synchronizing signal is invisible to the viewer. Yet we can't get rid of it. Without the invisible signal, our colors would go completely haywire.
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Chroma. The two color signals produce visible effects on your screen. In a feat of engineering finesse, these signals actually occupy the same subcarrier. With this trick, we fit three colors on two signals, but combine these two into one! I'll spare you the details, because they get a little hairy. Anyway, each signal determines the saturation value for one picture hue. We refer to the combined color signals as chrominance or just "chroma." Saturation. In case you're wondering, "saturation" means how much of a color mixes with the black and white signal. For example, red is a saturated color. Pink is the same color, but not so saturated. |
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Burst. Remember the invisible signal? Let's take a closer look at this mystery. For a short time between video lines, the normal color signals don't transmit. In their place on the color subcarrier is the color sync signal. Engineers call this signal the "color burst" signal. The burst assures a reasonable match between your TV set's colors and the station colors. Burst signals determine picture hue.
Luminance. In the background, the regular black and white picture keeps on transmitting. It even has its own sync signals. Technicians call the black and white part the "luminance" signal. Really, luminance is a $64,000 term for brightness.
Diagram. So now, here's what we have: Two color signals carry color saturation data. The station combines these two signals into one chroma signal. Between lines, the TV station transmits a color sync signal that we call burst. Burst enables the TV set to detect hues. The normal TV signal carries brightness. Our diagram shows the major Col-R-Tel parts. You can see how the parts relate inside the Col-R-Tel system.
The broadcast goes on the air and arrives at your home antenna. You've just installed a Col-R-Tel disc in front of your TV set. What was a monochrome receiver is now a field-sequential color TV. So much for the mechanical part of the Col-R-Tel converter. What is a "field," you ask? A TV field is a complete picture from top to bottom. Your TV set draws pictures from horizontal lines. As they scan across the set, the lines vary in gray content. Each field contains only every other picture line. The TV set builds up a detailed picture from two fields. The second field fills in the gaps of the first one. The resulting detailed picture is what we call a "frame." In field-sequential color, each field indicates saturation values for one of three primary colors. The same field still conveys gray tones.
Col-R-Tel chassis. But wait! With the disc out there, your set isn't compatible with regular TV (NTSC) broadcasts anymore. You can fix the problem. You get out a tool box and install the Col-R-Tel electronics. Now everything should work just fine. The electronics come in two chassis. Let's take a look at what's going on in those two chassis.
Size Box. The first chassis is simple. It's a size box. Your color wheel has six, transparent, colored wedges. As the wheel spins, you must watch the picture through these wedges. The wedges impart "hue" or color to the monochrome picture. If your TV picture is larger than one of the color wedges, you adjust the size box. Nothing complicated here. The size box reduces the picture window on the CRT until the picture fits the wedge. Size reduction involves loading the yoke coils. The yoke coils scan the electron beam across your picture tube. Loading reduces the extent of the scanning. Loaded yoke coils can only move the beam a short distance from center screen. Result: A smaller picture. Theory wise, that's all there is to it.
Color Processor Box. The second box is an analog color processor. In the usual way, the converter synchronizes, detects and reproduces color TV. Inside the converter are most of the parts you'd find in a conventional color TV. In the block diagram above are several blocks. We'll go over these blocks.
Demodulator Block. Our demodulator detects each chroma signal in turn: Red–Blue–Green. To do its job, the demodulator requires two inputs. The reference signal derives from the burst signal. This reference signal determines hue. The second signal is chroma. Chroma is a combination of two different color signals from the station. After the demodulator, the chroma signals again separate and become "color difference signals." These signals contain color information, but not brightness information. Recall that the brightness information is the black and white signal. This brightness signal is an average of all the color values.
After detection, the color difference signals arrive at the final amplifier. This amplifier drives your picture tube. At the CRT, the signals mix with luminance. Mixing makes color saturation values add and subtract from the luminance signal. Again, the exact details are a little hairy. You can find more theory on the main Col-R-Tel site and at the Col-R-Tel FAQs.
Burst Block. The Col-R-Tel burst section is a conventional color TV circuit. This circuit synchronizes the color detector with the station's color signal. The circuit's job is painting the right color over the right part of the picture. What the burst block really does is to choose picture hues. The burst circuit also allows you to adjust the picture hue to your liking. The burst block operates invisibly, between video lines. That's why the block connects to your TV's horizontal sync signal. This signal switches the block on at the end of a line. For a short time, the burst amplifier picks up color sync from the station. As the new scan line starts, the burst amplifier gates off.
Extra Parts. The processor has a few "extra parts." You won't find these parts in a regular color TV. These special parts serve three purposes...
- They select one of three colors coming in from the station.
- They keep the wheel in step with the station's vertical sync signal.
- They associate each color wedge with its picture field on the CRT.
Motor Control Block. The motor control block accepts pulses from the TV's vertical sync signal. Between video fields, the station transmits this sync signal. The signal is a train of short pulses. The motor control block relates these vertical pulses to disc wedge position. At every pulse, a new color wedge moves into place over the CRT. When this statement is true, the controller allows the motor to continue operating at medium speed. When the disc is slow, the controller speeds up the motor. When the disc is fast, the controller slows down the motor.
A sensor at the disc passes vertical pulses to an error amplifier. The "error" that I refer to is the difference between color wedge change and vertical sync events. The error amplifier produces a correction voltage. This error voltage drives a power amplifier. In the power amplifier, the error voltage becomes an error current. This varying error current excites a transformer primary. For our purposes, the transformer is the motor control element. The transformer secondary is in series with the color wheel drive motor and the line voltage. The error current induces a varying voltage in the transformer secondary. As the voltage varies, it aids or opposes line voltage to the motor. This varying transformer voltage is what controls motor speed. The effect is the same as turning a speed-control knob.
The power line and TV vertical sync signal run at slightly different frequencies. The controller also compensates for these differences.