Synchronous or Synchronized,
which is the best way to go?
The subject is motors, the prime mover for scanning disks
(and maybe 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.
Say 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
are "synchronized."
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 this 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.
However, 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 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.
The second 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.
The "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.
It 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.
Western 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.
Since this 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 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 can provide good synchronization.
So...which is the best way to go? Synchronous or synchronized?
Let's look at them one more time.
Starting with the synchronous motors, they are certainly the
most simple way to achieve really good synchronization...but,
these motors have always been more costly and not readily available
as the other types. Unless equipped with multiple windings, they
are limited to a single speed and usually a somewhat larger physically
and quite often, they do run hotter. Synchronous motors also have
an unusual problematic characteristic, in that its torque reduces
to its lowest value just before it jumps into its sync
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 be
overcome 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.
For 60 Hz systems, the usual 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.
As 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.
In 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.
Western 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.
So...if 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 to go.
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.
The 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 leads.
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.
Sync 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.
Comparing 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.
The 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
on a separate cable, any interruption in the picture signal will
probably result in a loss of sync.
Peter Yanczer