Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Velocity modulation, aka deflection modulation, of
electronic images was evidently considered by some engineers to be potentially disruptive technology
when this article was published in a 1951 issue of Radio & Television News. You can see
from the pictures that the result is an image that today's digital software would render with an 'emboss'
technique. More vertical relief seems to be generated with the analog velocity modulation technique
compared to what my graphics program does when embossing the original photo. At the bottom of the page
is a velocity modulation video demonstration found on YouTube.
By merely adding a single switch and a few wires any electrostatic deflection type set can be converted.
Although the operating principles of television system are now well established, in the early stages
of the art a television system was developed which operated quite differently from those in use today.
Prior to the perfection of the modern high-vacuum kinescope, the early gas-filled version of the cathode-ray
tube was incapable of adequate grid control. In view of this fundamental limitation, the idea was conceived
of operating the cathode-ray tube with constant beam current and applying the video voltage to the horizontal
deflection circuit so that the picture information was supplied by changes in the horizontal scanning
velocity of the fluorescent spot on the screen of the cathode-ray tube.
Fig. 1. Live subject reproduced by (left) velocity modulation and (top) by conventional
This deflection-modulation principle was first proposed by Boris Rosing, a Russian teacher, in 1911,
and a satisfactory working system was demonstrated by the German scientist, Von Ardenne, in 1931. Later,
in 1933. the British workers Bedford and Puckle perfected an ingenious system which utilized a combination
of deflection-modulation and intensity modulation.1 Following this period the field of deflection-modulation
lay dormant; beam-intensity modulation had proved more advantageous and was developed to its present
state of perfection. However, this field was recently reopened when research by the authors revealed
that standard television programs transmitted by commercial stations could be reproduced with unusual
results by use of deflection-modulation. A description of this work and a basic analysis of the underlying
principles was presented to the 1950 National Convention of the Institute of Radio Engineers. In this
present article, the circuitry is described and receiver modifications are shown so that the amateur
experimenter can demonstrate this deflection-modulation reproduction on his own television set.
This discussion is confined to the consideration of an electrostatic-deflection receiver, although
deflection-modulation has also been demonstrated in the laboratory using magnetic deflection. However,
since the high-voltage is usually obtained by fly-back pulse rectification, the conversion of a magnetic-deflection
receiver is not recommended. In a receiver employing this type of power supply, any video signal inserted
in the deflection circuit interacts with the anode voltage and causes distortion of the picture raster.
Fig. 2. An example of printed material as reproduced by (left) velocity modulation
as described in the text and (right) conventional intensity modulation techniques.
The conversion of a National type NC-TV7M electrostatic-deflection receiver will now be considered
in detail. The diagram shown in Fig. 3 illustrates the modifications which provide deflection-modulation
reproduction. When the two-position switch S1 is in position 1, the circuit operates in the
usual manner, in which case the video signal is coupled from the video amplifier V8 through
coupling condenser C34 to the cathode of the kinescope. It is to be noted that in this receiver
the kinescope is cathode-driven.
When the switch is in position 2, the receiver is converted to velocity modulation reproduction.
In this mode of operation, the video signal is removed from the kinescope circuit by opening the video
signal lead immediately to the right of C34, and is then connected to the horizontal-deflection
amplifier V14 through a small condenser Ca. This adds the video signal to the
saw-tooth sweep voltage so that it is amplified along with the normal horizontal signal. Due to the
amplification provided by the sweep amplifier, a value of Ca on the order of 3 μμfd.
provides ample coupling between the video and deflection circuits. This value is not critical and a
capacitance of 1 to 10 μμfd. will prove satisfactory. This capacitance may be provided by twisting
together several inches of insulated wire.
Since the kinescope beam current is 1:0 longer modulated by the video signal some other reasons must
be provided for blanking the kinescope spot during the time required for retrace. During the vertical
retrace period the spot is readily blanked by coupling the vertical sweep signal from V12
through Cb into the kinescope cathode circuit. A value of Cb = 1000 μμfd.
was found satisfactory although the optimum value depends upon the receiver and can best be determined
experimentally. The horizontal retrace lines were not objectionable so blanking was not provided for
them. If the capacitance Cb is omitted, the velocity-modulated image will still be produced
although the vertical retrace lines will be visible.
It will now be instructive
to consider the magnitude of video signal required to produce a deflection-modulated picture. Let us
assume that the 7JP4 kinescope requires a peak-to-peak voltage of 800 volts for full horizontal deflection.
Since a velocity-modulated image is produced by spot excursions on the order of one spot width, which
corresponds to approximately 1/1000 of the raster width, a video signal voltage on the order of (800)
/ (1/1000) = 0.8 volt on the deflection plates will produce the desired image. If the deflection amplifier
has a gain of 18, the desired voltage on the deflection plates is supplied by a voltage of 0.8/18 =
.044 volt on the grid circuit of the deflection amplifier.
By following the suggestions in this article, the experimenter will find that deflection modulation
is capable of reproducing printed material and line drawings with acceptable legibility, although it
does not achieve the fine contrast gradation produced by conventional television systems. A consideration
of possible applications indicates that deflection modulation may offer some advantages for specific
industrial and military applications due to its unique presentation of the subject matter. Furthermore,
the possibility of circuit economy should be considered, since the numerical example worked out in a
previous section indicates that the required video signal is of such a small magnitude that it could
easily be obtained directly from the video detector, thus eliminating the video amplifier in the receiver.
Fig. 3. - Circuit diagram showing modifications of an electrostatic deflection receiver
required to provide deflection modulation reproduction. When switch S1 is in position 1,
the receiver operates in the normal manner. When S1 is in position 2, receiver is converted
to deflection modulation reproduction. The only changes involved in converting the receiver are those
connected directly with the switch.
The preceding discussion naturally suggests the possibility of combining conventional beam modulation
with deflection modulation in a television receiver. This type of presentation has been investigated
in the laboratory and can easily be demonstrated on a receiver of the type shown in Fig. 3. This is
accomplished by leaving the video signal lead connected to the kinescope circuit as in normal operation
and at the same time coupling the video signal through Ca to the horizontal deflection amplifier
to produce the velocity modulation. The images produced by this modified circuit exhibit an outlining
edge on one side of bright objects and adds a crispness to the picture which seems to improve the apparent
Deflection modulation is of further importance from a standpoint of standard receiver performance
due to the fact that stray capacitances may exist in a conventional television receiver and produce
some degree of spurious deflection-modulation in combination with the conventional presentation. In
other words, due to faulty receiver design or failure of component parts, an undesired deflection-modulation
image may be super-imposed on the regular television picture; thus causing some positional distortion
and loss of contrast. This type of distortion may result in a halo effect similar to that caused by
overshoot in the video section, or by ghost images caused by reflections and may therefore be improperly
Research in the laboratories and by the experimenter may reveal additional points of interest since
the full implications of the combination of deflection modulation with beam modulation are not completely
apparent at this date.
The writers are grateful to the Engineering Experiment Station at Georgia Tech for its financial
support of the investigation of deflection-modulation television systems.
Wilson" J. C.; "Television Engineering," Pitman and Sons, Ltd., London, p. 102, 1937.
Zworykin, V. K. and Morton, G. A.; "Television" Wiley & Sons, Inc., New York, p. 238, 1940.
1 Deflection-modulation is customarily referred to as "velocity modulation" since the lateral velocity
of the scanning spot is modulated in the horizontal direction. Unfortunately, however, the term "velocity
modulation" has also gained acceptance in an entirely different sense, as the "velocity-modulated" klystron
tube. When used with this latter meaning, it applies to the change in velocity of the electrons in the
electron beam itself, such as would be produced by modulating the second-anode voltage in a cathode-ray