|Hawes Mechanical Television Archive||by James T. Hawes, AA9DT|
Color Wheel FAQs
ANSWER. A color adapter is an electromechanical device. This device converts a standard, black-and-white TV set for color reception. In the device, some form of mechanical scanning adds hues to the monochrome picture. The most common scanning device is a spinning wheel. (Some adapters employ belts or drums.) The wheel contains wedges of transparent color filter material: Wratten filter numbers 26 for red, 58 for green and 47 for blue.
QUESTION. How does a field-sequential TV adapter work?
ANSWER. Behind the color disc is an ordinary, monochrome TV set. That is, the pictures are black and white. Viewers watch these TV pictures through filters on the disc. If the telecast is in color, then viewers see a color picture.
The mechanical disc alone can't create color pictures. Here's where the electronics come into play. The electronics modify the video signal on the display tube. The circuit sends to the tube one color field at a time. The CRT gun then paints the color information on the tube face as a varying intensity monochrome signal. Picture data travels to the tube in a repeating sequence: Red, and then blue, and then green. This sequence is the reason for the system name, "field-sequential television." The sequence substitutes for typical color CRT operation: That is, simultaneous color from three color guns.
Adapter electronics also keep the color wheel in step with the video signal. If the adapter works properly, the images now appear in color. The color quality varies with the construction quality and circuit design. The picture is 60% dimmer than black and white, but only slightly dimmer than color TV of the time. Remember, the shadow mask on a color picture tube loses 50% of the brightness. Some viewers prefer adapter color to CRT color.
QUESTION. Why do some viewers prefer adapter color to CRT color?
ANSWER. For many reasons...
QUESTION. Can Col-R-Tel make a black-and-white show appear in color?
ANSWER. No. Although Col-R-Tel operates on a black-and-white TV, Col-R-Tel requires a color telecast. Col-R-Tel electronics are a simplified form of the circuit in a conventional color TV set. Col-R-Tel can't do anything more than a regular color set can do.
In fact, Col-R-Tel can't do as much. On every video frame, Col-R-Tel must discard two-thirds of the color information. Col-R-Tel came out before automatic fleshtone correction and the Vertical Interval Reference signal. Just as with 1960s-vintage color TVs, you must periodically readjust the hue and saturation controls. Also, since Col-R-Tel doesn't simultaneously reproduce colors, it flickers slightly. The problem isn't very severe. Yet you might notice this flickering as a small rainbow during rapid action sequences.
Col-R-Tel also lacks a color killer circuit. In fact, Col-R-Tel doesn't need it. During black-and-white broadcasts, a color killer shuts off the color circuits. Instead of this active shutdown method, Col-R-Tel uses a clever passive system. Black-and-white telecasts include no color burst signal. Without a burst, the Col-R-Tel crystal quits ringing. Without a ringing crystal, the synchronous color demodulator has no subcarrier reference. The lack of this reference disables the color demodulator. The Col-R-Tel crystal ringer circuit is an example of very compact, efficient design. Many color TVs use this same type circuit.
QUESTION. Do you have a color adapter schematic?
ANSWER. No. I don't have Cliff Benham's schematic. He first built the color adapter in 2003. Since then, he says he uses the setup to test new circuits. Apparently even he doesn't have a schematic. He's always changing the circuit.
QUESTION. Where can I sell my Col-R-Tel (or other television equipment)?
ANSWER. You could sell your equipment on eBay (www.ebay.com). TV collectors search there for items. Opening a seller account is fairly easy, and takes about two hours. Of course, you'd have to arrange for shipping or pickup. To pick up the right artifact, some collectors would drive a long way. Also, eBay has a "local" designation. That option removes the shipping hassle by allowing you to sell to someone nearby.
If you need help with eBay selling, visit your local library. Check out a book about eBay. If you can't find what you need, Amazon and Barnes & Noble carry many helpful books. You can even buy a used how-to book at eBay or its fixed-price store Half.com.
QUESTION. What attracts hobbyists to the Col-R-Tel circuit?
ANSWER. Col-R-Tel's circuits are simple and flexible. Considering how basic these circuits are, they deliver acceptable color. Except for the color wheel, many of the parts are fairly common. The circuits allow for substitution, too. A hobbyist can justify his time and expenses on the basis of a reasonable return. Col-R-Tel remains an advanced-level project. This is more an invention process than an assembly process. When you start, the answers are unwritten. You must reach to grasp. You must chart the course before you can open the map. When you begin, your chances of finishing aren't very high. On the other hand, Col-R-Tel is like all mechanical TV projects: Col-R-Tel teaches lessons that you can't learn elsewhere. The manner of instruction involves both agony and glee.
QUESTION. Is Col-R-Tel another name for the CBS color system?
ANSWER. No. Peter Goldmark designed the field-sequential color system for CBS. That system debuted and disappeared from the airwaves in 1951. For several more years, medical systems continued to use CBS field-sequential technology. Col-R-Tel is a different field-sequential system that came on the market in 1955. Yet Col-R-Tel is incompatible with the CBS system. CBS compatibility would be no advantage. By 1955, commercial stations had ceased broadcasting in the CBS color standard.
Other manufacturers also produced NTSC-compatible, field-sequential systems. Two systems that made a splash are Colordaptor and Spectrac. Both systems are more elaborate than Col-R-Tel is. Colordaptor stayed on the market for many years. Spectrac never went into production.
QUESTION. What is field-sequential TV?
ANSWER. In field-sequential TV, the transmitter sends one color field at a time. A field is a complete picture, but only in one color. That is, the field has one-third of the normal color information. The field can represent red, green or blue information. Three fields make up one color picture or frame. (Some adapters use a different number of colors. Three colors is the most common number.)
Each color field scans alternate video lines. The first field conveys even lines, and the second field conveys odd lines. The third field repeats the even lines. By the end of the sixth field or second color frame, each color has painted both odd and even lines.
The transmitter sends each field in a fraction of a second, one after the other. The viewer's eye merges the three fields into a three-color picture. In Peter Goldmark's CBS field-sequential system, each field arrived on the screen in 1/144 second. A whole frame required 1/48 second. This is the same amount of time that a movie theater displays a film frame.
Interestingly, the transmitter sends the field as monochrome information. A black and white set could reproduce the field as an ordinary, black-and- white picture. In field-sequential TV sets, viewers watch the monochrome fields through a filter disc. Each field appears on screen when its color filter passes before the CRT. The right color filter must be in place for the corresponding color field. Otherwise, false colors appear to the viewer.
Field-sequential color can also work with a conventional color picture tube. Engineers have designed three methods of field-sequential color with three-color picture tubes...
QUESTION. What is compatible color TV?
ANSWER. Compatible color TV allows monochrome TV receivers to reproduce color pictures in black-and-white. Likewise, on a color TV receiver, monochrome pictures reproduce in black-and-white.
Another name for compatible color is dot-sequential color.
RCA engineers pioneered compatible color. NTSC compatible color debuted over US TV stations in December, 1953.
QUESTION. How does field-sequential TV differ from compatible color?
ANSWER. CBS engineer Peter Goldmark invented the original field-sequential TV system. Only CBS-designed TV receivers could reproduce CBS field-sequential pictures. CBS subsidiary Hytron manufactured such sets. Unfortunately, NTSC monochrome receivers couldn't reproduce CBS shows as black-and-white pictures. The line and field rates of the NTSC and CBS systems differed. Because of these differences, NTSC sets displayed tearing and flipping pictures. Clever tinkerers developed workarounds involving set modifications.
In compatible color TV, either color or monochrome TVs can reproduce the same picture. Compatible color systems transmit and receive all three colors simultaneously.
Field-sequential and simultaneous color need not be incompatible. Col-R-Tel proves that engineers can design a worthwhile and compatible field-sequential TV system. Such a system differs from compatible color only at the receiver. There, adapter electromechanics convert NTSC simultaneous color to field-sequential color.
At the transmitter, the Apollo color moon telecasts performed a similar conversion task. The Apollo color cameras were field-sequential devices. The earth receiving station converted the field-sequential pictures to simultaneous color pictures. Other converters translated the incoming NTSC to PAL and SECAM for worldwide distribution.
Aside from compatibility issues, field-sequential differs from compatible color in both transmitting and receiving.
Field-sequential color TV transmits one color at a time. The colors arrive at the set as full-bandwidth, monochrome fields. The main carrier brings the color fields to the receiver. Inside the receiver, the main picture demodulator recreates a standard, monochrome signal. This output signal drives the CRT electron gun. Meanwhile a color sync circuit operates a color disc. This disc paints the appropriate color across each TV field.
Compatible color TV transmits all three colors simultaneously. The colors arrive at the set as one-eighth-bandwidth, monochrome fields. Two subcarriers bring the color fields to the receiver. Inside the receiver, color demodulators recreate three color signals from the two chroma signals. A burst signal also arrives from the station. Burst functions similarly to the color sync signal in field-sequential TV. Burst keeps demodulators in phase with incoming chroma signals. The result is that the right colors appear in the right screen locations. The output signals drive the CRT color emitters.
QUESTION. Was there ever a Col-R-Tel camera?
ANSWER. In a way, yes. The color "moon cameras" include Col-R-Tel-like technology. Col-R-Tel adapts NTSC simultaneous color pictures to a field-sequential receiver. The Apollo moon cameras turn the same trick, but at the transmitting end.
The moon cameras sent field-sequential pictures to terrestrial downlink stations. On earth, converters manufactured simultaneous color fields from the field-sequential ones. The converters also produced a summed monochrome signal, plus burst and two chroma signals.
Lunar Col-R-Tel. Many Apollo histories mention that the color moon camera used the CBS color system. Yet the CBS system never went to the moon! Yes, the moon TV cameras transmitted field-sequential color, an invention of CBS. But the moon cameras transmitted pictures that only NTSC monochrome TVs could reproduce. Only after additional conversion could color NTSC TVs reproduce moon color. With or without picture conversion, CBS-standard TVs could never reproduce the moon shots.
If not CBS, then what system? The moon color signal was actually a Col-R-Tel signal, instead of a CBS signal! Consider that both Col-R-Tel and the moon cameras operate at the NTSC scanning rate. The CBS system operates at a different and incompatible rate. The moon camera and Col-R-Tel use very similar color wheels. (Here, the three systems converge. The moon wheel and the Col-R-Tel wheel are more or less CBS color wheels. The moon camera has opaque divisions between color wedges. Otherwise, it's a match for the CBS wheel.)
Exclusive! Find out more about Col-R-Tel on the moon.
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.|