June 1955 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Television, in 1955, was still a relatively new phenomenon to many - maybe even most - people. According to multiple sources, the portion of American households with a TV set went from under 20% in 1950 to nearly 90% ten years later in 1960. That was a meteoric rise, particularly considering the expense of even a minimal TV. The technology was not even available commercially when most people were born, so the rush to join in on the craze was akin to the mass adoption of cellphones in the 1990s. "Carl & Jerry" creator John Frye used his pair of electronics-savvy teenagers to help make the "magic" behind recreating a moving picture on a cathode ray tube (CRT) miles away from where it was created. Water flowing through a garden hose has often been employed as an analogy for current flowing through a wire to explain electricity to laymen and beginning students of the craft. Here, it is not water flowing through the hose but water leaving the hose and flowing through the air that serves to represent an electron stream travelling from the electron gun to the phosphor-coated glass front of a CRT. Frame rates, scan lines, deflection coils, and other relevant terms are introduced. Not mentioned, understandably, is that due to the relativistic speed of electrons in the beam, the magnetic field of the deflection coils needs to be increased to account for the increased effective mass of the electrons. Otherwise, the scan width and height would be less that that predicted by classical physics.
Carl & Jerry: TV Picture
By John T. Frye
Aided by a garden hose, Jerry explains how a TV picture is made to appear on the screen of a receiver.
It was hot, and Jerry certainly hurrying was not at his job of washing the family car. From time to time he looked wistfully across the back yard at the house of his friend, Carl Anderson; but he never presented himself - never, that is, until Jerry had given the gleaming hood a final flick of the chamois and collapsed on the ground to mop his sweating round face.
Then Carl ambled out the back door of his house and slowly strolled over to where Jerry was sprawled on the grass beside the garage. "Rather warm today, isn't it ?" he remarked politely as he stifled a bored yawn.
"I wouldn't know," Jerry grunted. "I've been too busy to notice. Of course, if one is too lazy to help one's buddy out, and if all one does is sit around the house like a lounge lizard, I suppose one might think it hot."
"Now don't get your nose hard," Carl said with a disarming grin. "I made up my mind that this was one time you were not going to rope me into helping you. For once," he boasted, tapping his temple with a forefinger, "I used the old bean and stayed in the house until you were finished. Anyway, I was doing some heavy thinking. I was trying to figure out exactly how a TV picture is made to appear on the screen of a receiver."
"Nothing hard about that," Jerry scoffed, as he pulled up a handful of grass and threw it at his chum.
... at this instant the kinked hose in Carl's hands suddenly gave way ...
"Okay," Carl challenged; "suppose you fill me in on the subject."
"In the first place," Jerry began, as he picked up the garden hose and opened the nozzle, "you have what is called an electron-gun structure at the very back of the picture tube neck. This gun emits a stream of electrons in the same way this hose shoots a stream of water at that garage wall. The electron beam is focused to produce the smallest possible round beam of electrons just as I adjust the nozzle here to produce a small round stream of water.
"When the stream of electrons strikes the fluorescent material coated on the inside of the picture tube face - the part you call the screen - the material glows and gives off light at the point of impact. Generally speaking, the more electrons in the beam, the brighter is this spot of light on the screen.
"Keep in mind that the electron beam current in a picture tube corresponds to the plate current in an ordinary radio tube. In the picture tube the fluorescent screen takes the place of the radio tube plate. In a vacuum tube you put a fixed negative bias voltage on the grid to make the resting plate current assume a certain value, but in a picture tube you establish a similar bias voltage level with the brightness control, so the beam current has a certain static no picture value. In a vacuum tube you then apply a signal to the control grid and that causes the plate current to move up and down in accordance with the amplitude and polarity of the signal voltage. In the picture tube, the beam current follows the picture signal voltage applied to the control grid in the same manner. The only difference is that in a radio tube circuit you have to use meters to observe the changes in plate current; but in a picture tube you can observe the variations in beam current as an increase or decrease in fluorescent brightness on the screen."
"I'm still with you," Carl drawled, as he kept his eyes closed behind his horn-rimmed glasses. "But all that does is produce a bright spot. What I want to know is how a picture is made."
"You've got to learn to crawl before you walk," Jerry admonished. "That little spot of light is our paint brush, and we must be able to move the beam producing it to any portion of the screen. What's more, the movement of this beam must be done in a uniform and systematic manner. Suppose this portion of the garage wall I'm marking off with water from the hose is our picture tube screen, and the stream of water represents the electron beam inside the tube. Now I'll start over here in the upper left-hand corner of our screen and move the beam across to the right. Then I jerk it back very quickly, move the stream down a little, and draw another line below the first. Then I draw another line below that, and so on until I reach the bottom of the screen. Next I go back up and draw another series of lines between those already drawn until I again reach the bottom. Because the screen material will continue to glow for a small fraction of a second after the electron beam has moved on and because of the persistence of human vision, the result of this rapid back-and-forth and slower up-and-down deflection of the electron beam results in a raster of a number of interlaced parallel horizontal lines on the face of the picture tube."
"How many lines ?" Carl wanted to know.
"The first trip down across the face of the tube the beam draws 262 1/2 lines to complete the first field as it is called. Then the beam goes back to the top and draws 262 1/2 more lines between those already drawn to make the second field. The total number of lines drawn in the two fields that are combined to make a single picture or frame as it is called, is 525."
"I'll count 'em sometime and see," Carl said skeptically.
"Well, don't expect to get exactly 525," Jerry warned. "That back and forth motion keeps right on going while the beam is being returned from the bottom of the picture to the top between fields, but you do not see these retrace lines because they are blanked out. About twenty -five lines per frame are lost in this manner."
"How is that beam moved back and forth and up and down ?" Carl quizzed.
... Jerry pulled up a handful of grass and threw it at his chum ...
"That's a little complicated to explain in simple terms, but I'll try," Jerry said manfully. "You know that voltmeter I have that has a zero-center scale. When we a current through the meter coil in one direction, the pointer moves in one direction; but if we reverse the direction of the current, the pointer is deflected to the opposite side of the scale. Reversing the direction of the current through the meter coil reversed the polarity of the magnetic field produced by that coil; and this field reacted with the fixed magnetic field of the field magnets in the meter to cause the pointer's action.
"A coil called a deflection yoke is divided into two parts and these two series-connected coils are arranged opposite one another along the neck of the picture tube at a point along the path of the electron beam on its way to the screen. Now a beam of electrons creates a magnetic field about it just as a stream of electrons, representing a direct current through a wire, creates a field about that wire that can be detected with a compass. You remember we did that experiment in physics class when we were studying the right-hand rule. If we put a current through our series-connected coils, they set up a magnetic field in the portion of the tube neck between them. The magnetic field of the electron beam and the magnetic field produced by these deflection yoke coils react with one another in such a way that the electron beam moves in a direction which will minimize this reaction. The direction and extent of the movement of this weightless, inertia-free electron pointer depends upon the strength and direction of the current through the deflection yoke coils.
"By making the current through the deflection yoke take the form of a saw-tooth - a current pulse that builds up gradually from zero to a certain value and then falls very quickly to zero again - we can make the beam move comparatively slowly from left to right across the tube face and then snap back quickly to the left side of the screen.
"A similar saw-tooth of current at a much lower frequency which is passed through another pair of deflection yoke coils mounted at right angles to the ones producing horizontal deflection makes the beam move comparatively slowly from the top of the tube to the bottom and then snap back to the top again. These two magnetic fields, exerting their combined influence on the electron beam simultaneously, cause it to describe the line-drawing process, or scanning, as it is called, that we were talking about."
"How many of these frames or pictures occur in a minute ?"
"It's easier to measure them by the second. Sixty fields, or thirty complete frames, occur every second. Breaking each picture up into two fields cuts down the possibility of flicker and also has some other advantages. Movie cameras in theatres use the same basic system to reduce flicker. The film speed in those machines is twenty-four frames per second, but each frame is projected twice to produce a total of forty-eight picture-showings per second."
"That little old spot of light on the picture tube must be hustling."
"You're not kidding. I figured out one time that on a twenty-one inch tube the spot of light must be traveling at a line-drawing speed of about 16,000 miles per hour. The speed during retrace action is about ten times that."
"Okay, let's get on with it," Carl prodded. "All you've produced so far is a raster of 500 parallel lines of brightness on our screen."
"Muy bien, Buddy, but you can help from here on in," Jerry said. "You can be the modulator for our TV transmitter. Throw a kink in that hose and when I say Vut, stop the water until I say Open. This fresh section of garage wall I'm marking off with the hose will be our screen. The picture we receive will be the simple one of a black telephone pole standing out in a snow storm. Remember the wet wall indicates white on our screen and the dry portion represents black.
"At the transmitter," Jerry went on, "there is a tube in the camera that is a sort of miniature version of the picture tube in the receiver. The image of the scene being photographed is focused on the screen of this tube, and this image is scanned by an electron beam just as we described. What's more, the scanning beam in the camera tube and the one in the receiver picture tube are kept locked exactly in step with even more precision than the movements of the June Taylor Dancers on the Jackie Gleason show."
"Now I'm beginning to get a picture," Carl murmured with his eyes still closed.
"When the scanning beam of the camera tube strikes a light portion of the picture, it causes the amplitude of the transmitter carrier to be reduced; when it moves to a dark picture element, the carrier amplitude increases. How much the carrier decreases or increases depends upon how light or dark that particular picture element is.
"In our TV receiver this increase or decrease of carrier strength is translated into increasing or decreasing negative signal voltage applied to the control grid of the picture tube. Keep in mind that a change in this voltage is immediately apparent as a change in brightness in the line or lines being traced on the screen at the instant the change takes place. Holding all this in mind, let's start scanning our garage-wall picture. First I start at the upper left-hand corner with the stream of water. As I get to about the middle of our screen, I say, 'Cut!' "
Obediently Carl kinked the hose sharply and the water stopped. Jerry moved the nozzle over a bit and commanded, "Open," and Carl released the pressure so that the drawing of the line could be completed. In this manner several interrupted wet lines were drawn across the garage wall inside the rough rectangle Jerry had marked off with the stream. Then the water was cut off while Jerry went back to the top of his "screen" and started drawing another set of interrupted lines between those already drawn. The result was a completely wet rectangle with a rather crude vertical dry stripe up the center. Between directions to Carl, Jerry continued to lecture.
"The directions I'm giving you are the ones given to the transmitter by the pickup tube in the television camera. These directions are passed on to the TV receiver through its antenna, and inside the receiver are passed right to the electron beam inside the picture tube. That means that when the scanning beam of the camera tube is moving across a light portion of the scene, the TV picture tube is showing a bright line. When the camera tube scanning beam is on a coal-black picture element, the beam of the picture tube is cut off and the screen is allowed to go black. In short, since these two beams are in exact synchronization, whatever is seen by the camera tube scanning beam is shown on the face of the picture tube by variations in the intensity of its beam. Gray shades are portrayed simply by reducing the intensity of the picture tube beam without actually cutting it off entirely. The nearer the voltage applied to the control grid of the picture tube approaches the cutoff voltage, the dimmer is the line drawn by the beam and the darker is the shade of gray. Well, here goes the last line of our picture. What do you think - ?"
He never got to finish his question. At this instant the kinked hose in Carl's hands suddenly gave way and threw a great spray of water over both boys and over the gleaming automobile against which they had been leaning.
"Hey, the modulator's busted!" Carl yelled as he scrambled for the valve to shut off the hose. It was too late. The freshly washed car was splattered all over from the shower it had received.
Jerry surveyed the damage ruefully for a moment and then picked up two pieces of chamois skin and held one out to Carl.
"Be my guest!" he invited.
Posted August 8, 2019
Carl & Jerry: Their Complete Adventures is now available. "From
1954 through 1964, Popular Electronics published 119 adventures of Carl Anderson and Jerry Bishop, two teen boys with
a passion for electronics and a knack for getting into and out of trouble with haywire lash-ups built
in Jerry's basement. Better still, the boys explained how it all worked, and in doing so, launched countless
young people into careers in science and technology. Now, for the first time ever, the full run of Carl
and Jerry yarns by John T. Frye are available again, in five authorized anthologies that include the
full text and all illustrations."
Carl & Jerry Episodes on RF Cafe
Bell Bull Session, December 1961
Cow-Cow Boogie, August 1958
Picture, June 1955
- Electronic Eraser,
- Electronic Trap, March
- Geniuses at Work, June
- Eeeeelectricity!, November
- Anchors Aweigh, July
- Bosco Has His Day,
- The Hand of Selene,
- Feedback, May 1956
- Abetting or Not?, October
- Electronic Beach
Buggy, September 1956
- Extra Sensory
Perception, December 1956
- Trapped in a Chimney,
- Command Performance,
- Treachery of Judas, July
- The Sucker, May 1963
- Stereotaped New
Year, January 1963
- The Snow Machine, December
Education, July 1963
- Slow Motion for
Quick Action, April 1963
- Sonar Sleuthing, August
- TV Antennas, August 1955
- Succoring a Soroban,
- "All's Fair --", September
- Operation Worm Warming,
Two Tough Customers - June 1960
Transistor Pocket Radio, TV Receivers and Yagi Antennas, May 1955
Stomping, March 1962
- The Blubber Banisher,
- The Sparkling Light, May
- Pure Research Rewarded,
- A Hot Idea, March 1960
- The Hot Dog Case, December
- A New Company is Launched,
- Under the Mistletoe,
- Electronic Eraser,
- "BBI", May 1959
- Ultrasonic Sound Waves,
- The River Sniffer, July
- Ham Radio, April 1955
- El Torero Electronico,
- Wired Wireless, January
- Electronic Shadow,
- Elementary Induction,
- He Went That-a-Way,
- Electronic Detective,
- Aiding an Instinct,
- Two Detectors, February
- Tussle with a Tachometer,
- Therry and the Pirates,
- The Crazy Clock Caper,