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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
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, centrifugal forces on
the mirrors caused them to shift out of their preset positions.

The Mihaly-Traub scanner was basically a 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.