October 1930 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
It really wasn't all that long ago when most people worked on
computers with Color Graphics Adapter
that had just 16 colors (4-bit pixels).
In the late 1980s (wow, maybe it really
was a long time ago), the luxury of a 256-color
(8-bit pixels) Video Graphics
monitor and video card would cost you around $300 each. I recall
seeing ads for "16 million color" displays by ViewSonic that
ran north of a kilobuck. My first 'real' monitor was bought
in 1987 and was 4-bit monochrome. Televisions, as you know,
began as black and white (actually a
infinite number of gray levels between black and white).
When TVs first arrived in people's homes, they were glad for
any kind of display, but it wasn't long before marketing gurus
convinced the masses that anything less than color would be
unacceptable. This article from a 1930 edition of Radio-Craft
describes one of the early attempts to satisfy consumer demands.
It was long before the
NTSC standard was adopted for a purely television electronic
system. Hence, here you will find a combination of mechanical
and electronic devices.
Television in Twelve Colors
Ingenious novelties developed by European experimenters in
the popularization of "sight-at-a-distance"
By Dr. Fritz Noack (Berlin, Germany)
The success of colored motion pictures, for which special
films are prepared, suggests the application of somewhat similar
methods to television. However, colored television necessitates
either a special wavelength for each color employed, or else
modulating the transmissions at twice or three times the normal
image frequency. Either widens the waveband to an extent which
At the left, the experimental transmission
set-up of the Ahronheim apparatus, by which it is planned to
pick up television images in their natural colors, as determined
by the dispersion of white light into its various frequencies.
At the right, the receiver of the Ahronheim apparatus, with
a loud speaker above and the image-screen below.
In addition to this, the superposition of two or three pictures,
each in a single color, does not give an absolutely natural
effect; because the spectrum of visible light is much more complex.
Then, too, the methods of colored television heretofore published
are in principle exactly like those used in black-and-white
reproduction. The different image points, varying in illumination,
are converted into corresponding electrical impulses at the
transmitter; they must be faithfully and exactly reproduced
at the receiver, to have a perfect picture. But there are many
causes of faulty reception, the most unpleasant of which is
fading; not only does this cause interruptions of reception,
but it also suppresses parts of the transmission, corresponding
to certain frequencies. In a loud speaker, this means that sounds
are lost or changed in timbre; in a television, that details
may disappear entirely.
The effect of fading may be overcome in telegraph work by
employing such a modulation that the signal swings back and
forth between zero and a fixed value. This, for instance, is
done in the transoceanic short-wave work of the Telefunken Co.;
a receiver of amplification so high that the signals never disappear
entirely is connected to recording apparatus through an automatic
volume control. This, of course, is impossible in telephony,
where variation of loudness as well as frequency is part of
the signal; and it is also unsuited to television, where we
must reproduce different light-values. If a process of scanning
should be adopted, in which pictorial points of the same intensity
are always selected, then fading could readily be overcome.
Dr. Schroeter, of the Telefunken Co., has made the suggestion
that the image points be distinguished not by reproducing them
at equal intervals with varying intensity, but by giving them
the same brilliancy for varying periods of time; just as in
telegraphy dots and dashes of the same strength are sent out,
instead of dots of varying strength. In order to make this practical,
it will of course be necessary to invent a method of converting
the image from points of varying brilliancy into dots and dashes
representing the same light values in terms of length.
However, a television system has been announced, which overcomes
these difficulties and makes color television possible without
widening the waveband, without fading and with the greatest
fidelity to nature. It is that of a Berlin engineer named Ahronheim,
who has lately acquainted me with its details.
It is based upon the assumption that the colored image points
show, not different intensities of light, but different color
tones that dark red and light red are not merely reds of different
intensity but, actually different colors. His methods of scanning
the image are the same as in previous systems; the novelty lies
in filtering the light before it enters the photo-cell, according
to its place in the spectrum. The visible spectrum contains
many gradations of color, but Ahronheim undertakes to reproduce
it with twelve.
Then, if a revolving disc is arranged with colored glass
sectors (Fig. 1) through which the light must pass from the
scanning apparatus to the photo-cell, the ray of light can penetrate
only the filters of appropriate color. The scanning mechanism
operates as in other systems; and the filter disc only must
revolve at higher speed, to make up for the fact that the light
ray penetrates only one of its sectors at each revolution.
Fig. 1 - Fundamental principle of the Ahronheim
system: light from the image 1 is concentrated by lenses 2 on
the scanning disc 3; but reaches the photo-cell 7 only when
the proper filter is presented by the disc 5, which is geared
(6) to revolve faster than the scanner.
At the receiving end, a suitable system of scanning is used
to build up the image; but here also a glass filter with twelve
sectors is placed between the source of light and the eye of
the observer. If the two discs are synchronized, a picture corresponding
most exactly to the image at the transmitter will be seen.
Since the photo-cell receives just as many impulses as with
black-and-white telegraphy, the modulating frequency required
is no higher for colored television; and if the transparency
of the filter is properly regulated, the image impulses in the
input of the transmitter, and the signals it sends out are of
Use of a Prism
However, Ahronheim proposes, not to build a mechanical filter
system of the kind described above, but to utilize a prism which,
as we well know, decomposes white light into its constituent
colors. The entire spectrum will come out only when light has
been directed into the prism; light of a single color will emerge
unchanged. Since the light rays are dispersed at different angles,
according to their wavelengths, we could arrange twelve photoelectric
cells side by side behind a prism; so that one would receive
all the dark-red light which entered the prism, another all
the light blue, etc. In practice, only one cell will be used,
however; and, by the use of a scanning device, only the color
corresponding to the image point reproduced at the instant will
be conveyed to the cell.
The model at present completed is arranged for but a few
color tones; but it demonstrates the fundamental characteristics
of the invention. It is especially well adapted for televising
colored motion-picture film, and it is possible to transmit
simultaneously from the sound track. It is understood that an
international moving-picture organization is interested in the
The Fries universal scanner: it will be seen,
at the left, that A and B combined as at C give a spiral of
square holes. Four discs, two spiral (A-B) and two slotted (C-D),
at the right give any desired "grain" to the image; and revolving
them on two shafts as shown below gives any desired scanning
Universal Television Receiver
At the present time, the English and German television transmissions
are extremely interesting to radio enthusiasts. However, the
systems used differ in details; the German transmission are
made at the rate of 750 thirty-line frames per minute, framed
at the top of the disc; the image is a third wider than it is
high. In the English (Baird) system, while the number of lines
and rate of speed are the same, the image is viewed at the side
of the disc, and it is higher than it is wide. Since this presents
complications to the set owner, a method of adapting a radiovisor
to different systems will be valuable.
In America, where several types of transmission are in use,
it has been proposed to pierce the scanning disc with several
spiral rows of holes, at different distances from the center;
this also makes it necessary to shift the glow lamp and "window"
for each reception. In addition to this, the images received
nearest the center of the disc must be smaller.
The use of separate discs entail unpleasant labor in interchanging
them, which would be too much of an annoyance for the set owner
who purchases his equipment.
However, the new Fries system overcomes all these difficulties.
Radiovisors using this method have already been produced commercially.
The simplest, intended for the present transmissions by English
and German broadcasters, have two picture windows and two rows
of holes in the disc; the regular glow lamp is used. The holes
for the English transmissions are on radii midway between those
for the German; since there are thirty of each; but the pitch
of the spirals is different. Only one glow lamp is required.
To prevent the two sets of holes from causing optical interference,
a special disc is used to cover one of them. It has a set of
30 slits, and may be turned on the axis of the main scanning
disc to uncover one or the other spiral; it is then clamped
tightly to the main disc. The English image, received sideways
at the top of the disc, is made to appear right-side-up by the
use of mirrors.
Four Discs Required
However, the inventor has carried his idea to the point of
making it possible to receive transmissions on other systems,
with holes of different numbers and different sizes. He accomplishes
this, as shown in Fig. 2, by the use of four discs - two pairs
of which are alike. Two of these have spiral slits and two have
radial slits; the latter pair 20 each, presumably the least
number of pictorial lines which will afford an image.
One disc of each pair is keyed to the axle, while the other
may be turned, as before, to open or close the slits, before
clamping it to its mate. The result is, that the holes apparently
produced by the passing of a radial slit over a spiral slit
may be made of any size suited to the image being received.
The spiral discs turn once while each frame of the image is
being scanned the radial discs are rotated by a separate shaft,
at a speed sufficient to provide the necessary number of lines
in each frame.
For instance, in the case of the German and English transmissions,
the spiral discs revolve 12 1/2 times a second; but since the
number of lines in the received images is 375, the 20-line radial
discs must turn at the rate of 18 3/4 times a second, or 1125
R.P.M. The width of the slots is, necessarily, related to the
number of lines in the image: the more lines, the finer the
slots must be.
Fig. C - A commercial German televisor, on
the Mihaly system, as produced by the Telehor Company.
In the Fries system, it is necessary to synchronize the two
parts of the scanning system with each other, as well as with
the transmitter. The simplest method of coupling the pairs of
discs is by gears; but this would necessitate frequent gear
shifting. For that reason, two electric motors are connected
to vacuum-tube generators which modulate each other. As will
be seen, a good deal of complexity is bound up in the operation
of this device which, however, demonstrates a solution of the
problem of universal television.
In the endeavor to overcome the difficulties caused by the
low available light intensities, in practical television, Col.
Schildenfeld, an Austrian inventor, has patented the idea of
applying regeneration to a photocell system. It is well known
that the amount of light required for successful television
is often rather trying, to the subjects and, outside of laboratories,
it is seldom available. By this invention, the light impulses
are caused to modulate a light source which shines directly
back into the photoelectric cell.
Fig. 3 - Optical regeneration is produced
by flashing the impulses of the photo-cell back into it from
Many practical difficulties, of course, are yet to be solved,
including those of time-lag and possible optical "oscillation."
The idea is especially well suited to oscillograph work; but
it is being tested for its possibilities in television.
Color Television Articles
Posted September 16, 2015