is the best way to go?
The subject is motors, the prime mover
for scanning disks (and a bit of history too). Two general types were used and
both could do the job, but each has its share of advantages and disadvantages.
But before we discuss that, let's consider the problem.
that you have a camera with some sort of motor driven scanning device. It's important
that the motor operate at a constant speed, both in the short term ( less than
one revolution) and long term, measured in hundreds of revolutions. This will
assure that each line of video information generated by this camera will have
the same time period
The receiver in turn
must reproduce each line as it arrives from the camera,
placing each image element in the appropriate position, just as it was at the
camera. The only way this can happen is if the receiver disk is in the exact same
relative position as the camera disk (the same phase) and the RPM of the two motors
is also the same and remains so. When this happens, it is said that the motors
In their earliest
work of some of the first experimenters used a line shaft some 4 to 6 feet long
with a Nipkow disk mounted on each end. This shaft
was driven by a single motor. They then placed a light barrier, usually a curtain
or a wall between the disks so that strong lighting could be placed in the vicinity
of the camera disk without interfering with the viewing disk. Set up in this manner,
the disks were absolutely synchronized. A bit later, wanting to separate the camera
from the receiver, J. L. Baird used variable speed motors with a small AC generator
connected to the motor shaft. He used this arrangement at both the camera and
receiver disks, placed a room apart. By the adjustment of variable resistors and
rheostats in both motor circuits, he had both disks rotating at approximately
the desired speed. He then connected the two alternators together with a switch
and the two disks would immediately begin running at the same speed and phase
relation. This was the method of synchronization he used in his color television
demonstration of 1928.
A variation on this
method was to use a variable speed motor on the camera disk and have this motor
also coupled to a small AC generator rated at 20 to 30 watts. The output of the
AC generator would in turn, power a synchronous motor on the receiver disk. The
result of this hookup was that the camera and receiver disks would always be synchronized
and the camera motor speed could be varied at will , knowing that the receiver
motor would follow.
In the time frame of
1928, there were two ways to accomplish synchronization in the early mechanical
systems. The first is to have a synchronous system, where all of the motors were
of the sort that would run synchronous to the AC line frequency.
in 1928 the typical electrical power station supplied relatively few customers.
As a result, there were numerous comparatively small power stations spread throughout
the United States. This severely limited the number of potential viewers, because
if a viewer were located in a different electrical power grid, there was no way to hold synchronization
with the television station. Instead, a variable speed motor was used with a speed
controlling rheostat on the end of a cable long enough to reach to the viewer.
Using a "course control" rheostat and setting it to cause the motor
to operate at the approximate correct speed, the rheostat given to the viewer
then provided a "fine control" and final adjustment for speed. The fact
was that final-final adjustments would be required every few seconds. This 1928
photo shows Hugo Gernsback watching television. Note the wire to the rheostat
in his left hand. If viewers could see a recognizable image for more than a few
seconds, they would be delighted.
way to achieve synchronizim was to drive the disk with two motors, one much more
powerful that the other. The larger, or "main " motor was an AC or DC
variable speed, rheostat controlled motor. These motors were usually of a variety
that used brushes and had relatively poor speed regulation characteristics, but
induction motors were also used. Coupled to the same shaft was a secondary or
"synchronizer" motor. This motor's winding was supplied by either the
AC line or by an amplified form of the line scanning frequency derived from the
television stations signal. In the latter case, the smaller motor was known as
a "phonic" motor. It was easy to tell the difference. The AC operated
synchronizer was a true synchronous motor and had 6 or 8 "teeth" or
poles on its rotor, depending if it ran at 900 or 1200 RPM. The one shown here
on the right was used in an English "Major" receiver and it has 8 poles.
Because the receiver and the synchronizer operated from 50 Hz power, the synchronous
speed of this 30 line disk was 750 RPM. This provided 12.5 image frames per second.
The company also provided a 30 tooth phonic rotor to those who needed it.
"phonic motor" on the other hand had a rotor with 24, 30, 48 or 60 "teeth"
or poles, depending if there were 24, 30 48 or 60 lines in the picture. The one pictured here
was used on the Baird "Televisor". The rotor has 30 teeth and the disk
speed was approximately 750 RPM and synchronous to the signal. The coil drive
signal for the phonic motor was derived by filtering it out from the picture signal
and then sometimes amplified to a higher level. This signal usually took the shape
of square or rectangular waves.
In use, the
main motor supplied about 95% to 105% of the power to rotate the disk at its normal
speed, ( yes, it might be going to fast). The phonic motor, on the other hand
could supply about 10% (plus or minus) the power needed to operate the disk. So
with the disk speed close to where it should be, the smaller motor could add or
subtract the necessary power to control the final disk speed within very close
limits, thereby holding sync with the camera disk. The level of synchronization
was generally acceptable, but large black or white areas in the scene, would sometimes
cause a temporary instability. Any loss of picture signal, as when there is a
switch from one camera to another, might also cause an instability.
should also be noted that there were still many areas where electrical power was
being supplied in the form of direct current (DC). Only Baird's method with the
AC generators and the phonic motor method would operate in those circumstances.
Television of Chicago used synchronous motors in their television station cameras
and in the television receivers sold to the public. There were no synchronizing
signals transmitted with the picture signals. Each Western Television receiver
was equipped with a single knob, a phasing control able to adjust the both the
horizontal and vertical phasing. This knob actually rotated the entire body of
the motor. More often than not, a single adjustment of the control took care of
an entire evening of television. In those locations supplied with 50 Hz power,
suitable motors were supplied.
is not 1928 anymore, we need to consider other possibilities. One that seems to
fit very well is the "closed loop servo". In practice, it usually consists
of a DC motor controlled by an amplifier that includes a phase sensitive detector
(PSD). The circuit works by comparing synchronizing (sync) pulses that originate
at the camera with similar pulses generated in the receiver, usually at the receiver
scanning disk. A common way to do this is to have an extra circle of holes in
the scanning disk with an optical fork positioned so as to detect and respond
to these holes passing by as the disk rotates. The PSD then compares the incoming
pulses to those from the disk and outputs a current to the motor circuit that
tends to correct any difference in frequency and phase between the sync pulses
and those from the disk. This method is able to provide good synchronization.
is the best way to go? Synchronous or synchronized? Let's look at them one more
Starting with the synchronous motors,
they are certainly the easiest way to achieve really good synchronization...but,
these motors have always been more costly and are not readily available as the
other types. Unless equipped with multiple windings, they are limited to a single
speed and usually are somewhat larger physically. They also tend to run hotter.
Some synchronous motors also have an unusual problematic characteristic, in that
its torque reduces to its lowest value just before it jumps into its synchronous
speed. The inertia of the disk and its windage losses may be high enough to prevent
the jump into sync on a motor with much more power than needed, had the motor
been able to reach its sync speed. This problem may beovercome
by using a spring loaded coupling to the scanning disk. A common method is shown
here. In this type, the motor shaft drives one end of a spring through a collar
fastened to the motor shaft. The torque is then coupled through the spring to
the scanning disk which is free to rotate a limited amount on the shaft.
60 Hz systems, the common shaft speeds available from synchronous motors are 450,
600, 900, 1200, 1800 or 3600 RPM. These speeds correspond to 7.5, 10, 15, 20,
30 and 60 revolutions per second. The most useful of these for television are
the 900, 1200, and 1800 RPM variety, because with a Nipkow disk attached, these
shaft speeds will provide 15, 20 or 30 pictures per second. With appropriate gearing,
the other motors can provide the same results.
On 50 Hz power systems, These
same motors listed before will operate at these slightly lower speeds; 375, 750,
1000,1500 and 3000 RPM or 6.25, 12.5, 16.66, 25 and 50 pictures per second with
the most useful speeds being the 750, 1000 and 1500 RPM models.
long as each synchronous motor is operating from the same AC power source, even
though hundreds of miles apart, all of the motors in the system will run "in
sync." However, the phase may may be different, but it will remain a constant
difference unless purposely changed. A simple adjustment can make the correction.
The main transmitting station of the Western Television Company was located in
Chicago. Because of the transmitting frequencies used, in the evenings, their
signals would carry far beyond the power grid that they were located in. Therefore,
those with receivers equipped with synchronous motors would not hold proper sync,
even though the signal strengths were quite often more than adequate.
1931, the Western Television Company and it engineers made an effort to have all
of the power companies in the United States synchronize their 60 Hz generators
to each other and in so doing, correct the synchronizing problem for television.
They went so far as to propose having a radio station, with a carrier powerful
enough to be received all over the country and have only 60 Hz as its modulation.
Then all of the power companies could synchronize to it.
Television was not successful in this effort because the power companies saw this
simply as an added cost to their operation, for which they would receive nothing
in return. As it happens, in later years and by the end of WWII, the power companies
did connect their systems together because they found it be to their benefit to
be able supply or receive power from other grids and so equalize their loads.
you have access to synchronous motors and/or you are willing to pay the additional
cost for the motor and possibly a spring coupled disk hub, it is the best way
An unusual source of synchronous motors
are certain types of bicycle generators, the sort that are clamped to the wheel
fork and rub on the side of a tire. Many of these are 6 volt, 8 pole AC generators
and will operate on 6 volts AC as a synchronous motor. On 60 Hz power they run
at 900 RPM, however, they are not self starting. So, some external means must
be provided to get them up to synchronous speed. As a motor, they will operate an 8 to 10
inch diameter disk. This photo shows an example of one I built some years ago
for a 45 line camera, based on the Sanabria triple interlace format. The 10 inch
scanning disk operates at 900 RPM. In the photo, I'm holding the assembly by the
generator, on the shaft of which is part of the hub that supports the scanning
disk.The hub has a rubber tire mounted on its largest diameter, which is actually
an "O" ring. Mounted on a platform above the generator, is a 12 volt
permanent magnet DC motor, with a wheel and tire on its shaft. The original purpose
of this motor was to propel a "HO" scale model train. The DC motor is
mounted on a hinged support at its rear . At the front of the motor, a spring
is used to keep the tire on motor wheel separated a short distance from the tire
on the the hub. A small amount of pressure on the top of the DC motor will compress
the spring and allow the two rubber tires to contact. Pressing on the DC motor
also operates a micro switch located below the motor.
power supply for this assembly consists of a 12 volt center tapped, 1 ampere filament
transformer, with a full wave bridge rectifier connected across the 12 volt terminals.
The AC generator is connected to the center tap and either of the 12 volt transformer
terminals. The DC output of the bridge rectifier connects to the DC motor with
the micro switch mentioned earlier, connected in series with either of the motor
With power applied to the transformer
, the AC generator may hum, but doesn't run. Applying a downward pressure near
the front of the DC motor puts the two tires in contact and closes the micro switch
that turns on the DC motor. The DC motor brings the AC generator up to its operating
speed of 900 RPM. Since the generator has 60 Hz power applied, it tends to run
only at 900 RPM. When the pressure is removed from the DC motor, the tires separate
and it slows to a stop while the AC generator now running as a motor continues
to rotate at 900 RPM.
The second approach
for driving receiver scanning disks was the one with two motors, with the second
being a synchronizer or a phonic motor. All of these models used a variable speed
motor and a secondary
one to provide sync. Some manufacturers gave you a choice of the two types, such
as the "Major" mentioned earlier and also the Jenkins Kits, which were
sold in the United States. Jenkins kits, such as the one shown here to the right,
provided your choice of one or the other. Some were shipped with both. The Hollis
Baird kits (from Boston Mass.) were supplied with a phonic motor only (but they
also offered a synchronous motor model too) and the Globe kit had a six pole synchronizer
only. The See-All kits appeared to be in many respects, copies of the Jenkins
kits and came with a 6 pole synchronizer only. ICA produced a kit with an induction
main motor and a synchronizer motor. All of the sets mentioned here used a rheostat
type of speed control for the main motor.
pulses were not required with phonic motors but synchronization could be further
improved by causing the beginning of each scan line to be black for a short period
of time. Mr. Baird used this method in his broadcasts from England. It worked
by increasing the output of the filter that drove the phonic motor coils.
the performance between using a synchronizer or a phonic motor, usually comes
out in favor of the synchronizer, partly because you have sync at all times, even
without signal. In these times, with power companies all tied together into a
common grid, it is just the easiest way to go. Given a choice, the synchronizer
if properly sized, will be the more effective of the two.
closed loop servo type of motor control can be very successful and usually affords
the lowest cost. Smaller DC motors are plentiful and generally cost very little.
The same is true for the necessary electronics. The NBTVA
has published a number of proven circuits that are relatively simple and
do in fact work quite well. These circuits add very little to the total cost.
But as in all things, much will depend on how much you are able to do yourself.
Unlike the phonic motor systems, the closed loop system will require a signal
source that includes sync pulses in its output. Unless the pulses are available
separately, any interruption in the picture signal will probably result in a loss