MIHALY-TRAUB SCANNER
Another
innovative method of scanning was developed to overcome two of the major problems
of mirror drums. The first was that with the higher line rates, so many mirrors
were required that the drum tended to become very large and heavy. This made the
drum difficult to keep in synchronization with the picture signal. The second
problem was that the tilt adjustments on each mirror required special tools and
could only be made with the drum stationary. With the higher line rates, the adjustments
became so critical that many attempts needed to be made before a satisfactory
adjustment could be achieved. This problem was compounded by the condition that
with the drum operating at speed, centri
fugal
forces on the mirrors caused them to shift out of their preset positions, but
only while running.
The Mihaly-Traub scanner was basically mirror drum
turned inside out. It consists of a ring of stationary mirrors (one for each line),
all facing inward. Each mirror has tilt adjustments, just as those on a mirror
drum. In the center of the ring, there is a motor driven mirror, silvered on both
sides. It rotates at half the picture rate. A beam of light, modulated with the
picture signal is directed under the mirror ring to the lower part of the rotating
mirror. From there, the light reflects to one of the mirrors in the ring, back
to the upper part of the rotating mirror and then over the top of the ring and
on to the screen. The rotation of the center mirror throws the beam of light on
one 
stationary
mirror after another. As the beam sweeps across each mirror in the ring, a corresponding
scan line is focused on the screen. Because each mirror has a slightly different
tilt from the one before and the one after, the scan lines are traced on the screen,
just , below the previous one. Each rotation of the center mirror forms two complete
pictures on the screen. The double reflection from the rotating mirror is an ingenious
feature of this scanner allowing the scanner to be smaller than it might otherwise
be. Another feature is that the stationary mirrors can be adjusted while the scanner
is operating.
With the original Mihaly-Traub scanner a stationary mirror
was required for each line scanned. At some point, it was determined that the
number of lines scanned was actually a product of the number of stationary mirrors
times the number of rotating mirrors. Changing the rotating mirror to a six sided
one, would allow the stationary mirrors to be reduced by a factor of three. Furthermore,
they would remain the same size as before, but only occupy one third of the circle
(120 degrees), resulting in a further reduction in the size of the scanner. The
central mirror, now rotating at picture speed had its six sides set at slightly
different vertical angles. As one of these mirrors swept around, it scanned a
group of lines, the same as the number of stationary mirrors. The next central
mirror with a slightly different tilt, began its group of lines where the previous
ones left off and so on with the remaining mirrors.
A
typical scanner would have the stationary mirrors located on a 4" to 6"
radius around the rotating mirror. In the center would be a rotating mirror assembly
with four to ten sides. A scanner for 120 line transmissions might have a six
sided rotating mirror working with 20 fixed mirrors in a 120 degree arc. Another
design could use an eight sided rotating mirror with 15 fixed mirrors in a 90
degree arc, There are additional possibilities with center mirrors of 4, 5 or
10 sides, which you might work out as an exercise.
Because the image is projected
on a screen, picture size is limited only by the amount of modulated light available.
The advantages of the Mihaly-Traub scanner are similar to those of the mirror
screw, but with the additional advantage of requiring a very small, easy to synchronize
motor for the rotating mirror. A scanner of this sort was successfully operated
on the British 405 line systems.
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Systems