March 1953 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
If television was "a chinch," in 1953 as this Radio-Electronic article claims, the world would have had it long before then. Just like looking up the work-out solutions to a physics problem in the back of a textbook, a lot of things look simple and obvious once someone else has already done it. I guess that's not really a fair criticism of this piece since author Aisberg's goal is to assuage some of the doubts and misconceptions a lot of people had about the relatively new technology. 1953 is the year that the NTSC (National Television system Committee) formalized its color TV standard, which, BTW, was careful to accommodate B&W transmissions on the same channels - similar to how AM-FM stereo and stereo FM radio can coexist with monaural (mono) broadcasts. Television, in case you are not aware, began as an electromechanical system with picture frames and shutters, spinning discs, and other Rube Goldberg contraptions.
Here are a few article on TV's history: The Du Mont Television System (1938 Radio-Craft), Color Television? (1949 Radio & Television News), Color TV (1953 Radio & Television News)
Television? ... It's a Cinch!
By E. Aisberg
Second Conversation--The Nipkow Disc
From the original "La Télévision? ... Mais c'est très simple!" Translated from the French by Fred Shunaman. Ali North American rights reserved. No extract may be printed without the permission of Radio-Electronics and the author.
Our Hero's Dizzying Adventure
Ken - Don't bother telling me what you're trying to do, Will. Either you're practicing to become a whirling dervish, or TV has you going around in circles!
Will - Wrong on both counts, Ken! I'm just trying to read without having to jerk my eyes back from right to left at the end of each line.
Ken - I know I'll regret .asking, but why?.
Will - Because I've been thinking about the way a scene is scanned in television. You remember, the last time we talked you said it was like reading a book - line by line. But when you think of how fast the TV camera has to read, it seems there ought to be some way to save the waste of time getting back from the end of one line to the beginning of the next. So, when I finish one line, I spin rapidly round so my eyes fall on the beginning of the next one without having to snap back from the end of the last!
Ken - Bright idea, but I don't think you're going to save much time that way. You can get pretty dizzy, though! But you might be interested to know that your continuous-scanning method was the one they used in most of the early mechanical television systems.
A Little Geometry
Will - I'd like to hear a little more about some system that really was used! All you've told me about "scanning" and "image analysis" so far has been pretty much up in the air. But just how do you "explore successively the elements of the image" in real live television?
Ken - I hadn't figured on telling you how mechanical television worked, because it's been abandoned entirely in favor of electronic methods. But maybe you'll be able to understand the more advanced systems better if we start in with the simplest-and the oldest-system: the Nipkow disc!
Will - The Nipkow disc? I've heard about it somewhere. What is it?
Ken - We're going to make one right now! Take a look at this piece of thin Bristol board. We'll cut a circle about 16 inches across out of it. Now I'm going to layout a series of concentric circles on it. The first will be 13 inches in diameter, and each one will be an eighth of an inch bigger, till we have 16 circles. Then we proceed to divide up the circumference of our disc into 16 equal parts ...
Will - This is fine! We've been going through arithmetic and algebra - now we're getting a geometry exercise. When do we start integral calculus?
Ken - Never mind the calculus - I'll be satisfied if you learn television! Now let's get back to our disc. We have 16 radii, or arms, going to equally-spaced points on its circumference. I needed all these lines so I could layout a spiral. I just mark the point where the first radius crosses the first circle, another point where the second radius crosses the second circle, and so on, going around the circle clockwise.
Will - That gives you 16 points arranged in a spiral. What do you intend to do with them?
Pinhole View of Life
Ken - You'll see in a minute. First let's make - with a very small punch - a series of holes, one at each point on our spiral. And here is our Nipkow disc!
Will - And you really think you can use this for scanning television images?
Ken - I do, and what's more, I'm going to prove it! Let's make a little design - something very simple in black and white - about two by three inches. Fasten it on the bottom of the lampshade here. Now put the disc on" this knitting needle, hold it in front of the design, and spin it
Will - I see your design just as though the disc were transparent!
Ken - Now - just so we can see what's going on - let's turn the disc a little slower.
Will - I get it! This is just a big improvement on the piece of paper with the window in it we had last time. When the disc turns the first hole scans a line across the design. (Not exactly a straight line either, it's an arc of a circle, but that doesn't seem to make any difference.) Just as it finishes its line, the second hole starts across the picture and scans a line just below the first. And each hole follows (beginning at the outside of the circle or top of the design) and scans a line, till the whole design is covered.
Ken - And then the whole thing starts again with the second revolution of the disc. You see that if you turn the disc fast enough you apparently see the whole image, though really only one of its elements is visible at anyone instant through one of the holes in the disc.
Will - I see too that the disc reads in the whirling-dervish style, without having to make any backward movement to get to the beginning of each line. And I can see that it has to turn pretty fast before the eye blends all the elements into a single picture.
Reading - The Hard Way
Ken - Yes, and when I let the disc slow down just a little, the image shimmies as if light and dark waves were going across it. That's because the sensation produced in the eye by the light from each hole doesn't last very long.
Will - Just how fast does the disc have to turn to get rid of this flickering?
Ken - You know - to do a good job you need 30 complete images a second.
Will - Yes, that's our television standard. You told me before the Europeans get by with fewer. But is 30 really enough? Wouldn't it be a good idea to scan even faster?
Ken - Don't forget that your video frequency is proportional to the number of images you transmit a second. It's not a good idea to do anything that will increase that frequency too much. Fortunately, there's a way you can kill the flicker without increasing the band of frequencies you have to transmit. It's called interlacing.
Will - This TV business really has a language of its own! What's interlacing?
Ken - Instead of starting at line No.1 and transmitting all the lines of the image one after the other, you transmit all the odd-numbered lines first: 1, 3, 5, etc.; then go back and transmit all the even ones. The whole scanning time is 1/30 second. That means that half the lines, covering the whole surface of the image, are transmitted in 1/60 second, and the rest of the lines are transmitted during the next sixtieth.
Will - If I tried to read a book that way, I wouldn't get much out of it.
Ken - If it were an ordinary book, you wouldn't! But try this little sheet; you'll have to "interlace" to read it. Your eye will follow the exact course that would be followed by the scanning beam of a modern television camera.
To read this text correctly, you must
ner in scanning first the group (or
first peruse the odd lines, then the
field) of odd lines, then afterward
even ones. Interlaced sweep permits
the even ones. To sweep the image 30
"reading" the lines in the same man-
times a second, 60 fields are scanned.
Will - This is interesting, to say the least. Maybe it was turned out by a drunk compositor! But can you really scan that way in television? It sounds awfully complicated.
Ken - No problem at all! Suppose we make a Nipkow disc with two spirals, one on each half of the disc. We'll have lines 1, 3, 5, etc., on one spiral and lines 2, 4, 6, and the rest on the other.
Will - Of course! It can't help but work! But now that we've proved that we can scan a picture - interlaced or otherwise - with a Nipkow disc, where do we go from here? How does it help to transmit a television program?
Now a Little Chemistry
Ken - Do you know anything about photocells?
Will - Of course! I use an exposure meter when I take photographs. It's a photo-cell connected to a meter. The meter is calibrated to show how much light there is on the subject being photographed.
Ken - Then the photocell is a device for changing light energy into electric energy. The current from the cell is proportional to the amount of light that falls on it. The photocells (or rather phototubes) used in television are the photo-emissive type. The simplest phototubes of that kind are little glass vacuum tubes with one inside wall covered with photo-emissive material.
Will - Is that material that emits light?
Ken - On the contrary. It's material that emits electrons when struck by light rays.
Will - What kind of substances do that?
Ken - Most of the so-called alkaline metals: cesium, sodium, potassium, rubidium and lithium, as well as some of the rare earths, though they're not as commonly used.
Will - I've got an idea! If all these metals give out electrons when you turn a light on them, you could use them for vacuum-tube cathodes! Then you could get along without filament supplies. In the daytime, just keep the tubes in the light. And at night, put your radio near a lamp!
Ken - Believe it or not, the idea isn't absurd! Unfortunately, the number of electrons emitted wouldn't give you enough current to be of much practical value. But to get back to our television - if we are going to have current in our photo-tube, we need one thing more. The photoactive surface is the cathode ...
Will - I see! We need an anode. We'll have to put a plate in our tube and put a positive voltage on it to attract the electrons.
Ken - That's the idea, but a "plate" would block off the light. So our anode will be a wire ring or a fine grid.
The Image Is Scanned
Will - Now I think I see how to make a TV transmitter. I'll take my camera, but in place of the ground glass I'll put the outer part of our Nipkow disc. Then it'll be right where the lens forms its image. And behind the disc, I'll put a phototube. What do you think? Will it work?
Ken - Absolutely! You're practically reinventing television! Your phototube is now receiving - from instant to instant - the light from each successive element of the picture being scanned, and translating it into an electric current of proportional intensity. That gives us a video-frequency signal that can be amplified easily and used to modulate the v.h.f. or u.h.f. carrier that takes it out into space.
The Image Is Reproduced
Will - How about the receiver?
Ken-It has to have a Nipkow disc like the one at the transmitter, and moving in exact step with it.
Will - Is that what they call synchronization?
Ken - Right! And that's another word for your technical vocabulary.
Will - But how do we get the variations in current back into light again?
Ken - Very simply - with a neon lamp. You understand them, of course?
Will - Oh, yes! I even engineered an accident to the one on the restaurant across the street when it began putting out more static than light.
Ken - I'm not interested in your criminal record. The lamps most commonly used in television in this country had two plates about the size and shape of the image to be reproduced. When you put enough voltage between the two electrodes, one of the plates glows over its whole surface. A large d.c, voltage makes a bright glow ...
Will - And less d.c. means a weaker one, I suppose. But how ... ?
Ken - Let me finish! If we add the varying voltage of the video signal onto the d.c. we started with, the brightness of the plate varies with the instantaneous signal voltages.
Will - Yes, but how do we manage to light each point of the plate to the brightness of that exact spot in the televised scene?
Ken - You don't have to! Your Nipkow disc in front of the neon lamp will show you each point on the plate at the instant it has the right brightness.
Will - Of course! At any instant the disc lets us see just one element of the surface of the plate. And at that same instant, the brightness is just right for that spot in the televised scene. For instance, when the first element of the picture is transmitted, the whole neon lamp is lighted to the brightness of that point. But we can see only that one spot through the hole in the disc. And when the hole passes to the next element, the whole plate is just as bright as that spot ought to be, and so on. So we see all points of the scene in their proper places and with their proper brightnesses, and the whole image is reproduced!
Ken - Bravo! You have described exactly the system of television first outlined about the end of the 19th century and put into practice around 1924 by Jenkins, Baird, and others.
Mechanics vs. Electronics
Will - It looks like a very simple and practical system to me, and I doubt if it would be easy to improve it!
Ken - Pull in your hat-band, chum! They gave up that idea years ago! They couldn't get enough detail with it - 180 lines was about the most you could get in a single image.
Will - Couldn't they get more lines by using bigger discs with more holes?
Ken - No. At the speed the discs would have to turn, centrifugal force would tear them apart.
Will - Couldn't you make the holes smaller?
Ken - Not very much smaller. You would cut down the amount of light that could get through, and after a certain point you'd be up against the very disagreeable phenomenon of diffraction.
Will - It seems I don't have any good ideas today!
Ken - No matter how good they might be, you wouldn't be able to save mechanical television. It had other bad faults. For example, the photo tube at the transmitting end received the light from each point of the image for such a short time you had to use very high illumination on the subject to get enough photoelectric current to use. And the efficiency at the receiving end was very low, because you can see only a very small part of the neon lamp's plate at any instant. And finally, we're living in the age of electronics now!
Will - Then why did you take time off to explain a system that belongs in a museum?
Ken - Because once you understand a simple system of television, your brain cells will find it easier to absorb the more complex details of the electronic systems.
Will - I've got a feeling I'm letting myself in for something awfully complicated!
To Be Continued
Posted March 19, 2019