MIRROR DRUMS
A very early improvement over
the Nipkow disk was the Mirror drum. For its size, it was a very light efficient
mechanical scanner. In its original form, there were as many mirrors as there
were lines in the picture and
each mirror was tilted at a different angle compared to the axis of the drum.
For horizontal scanning, the mirror drum axle was supported in a vertical plane.
Therefore for vertical scanning, the axle was horizontal. As it rotated, each
mirror caused a line to be scanned below (horizontal scan) or beside (vertical
scan) the previous one. The drawing shown below is of a basic vertically scanned
mirror drum system.
In
this drawing, the modulated light originates at a small aperture at the lower
left, then passes through a projection lens. This lens is adjusted so that the
the light at the aperture itself is focused on the screen. Of course, in order
for the light to get there, it must be reflected off of mirrors on the drum. The
mirrors are thereby able to control the position of the light spot on the screen
as it rotates. Since each mirror is carefully set at a different but proper angle,
as the drum rotates, the focused light spot takes on the appearance of a set of
parallel horizontal or vertical lines, commonly referred to as a raster. With
proper modulation of the light, you have a picture. In some cases, two drums were
used. One would the provide only the horizontal shift necessary and a second drum
mounted at a right angle to the first, would provide the necessary vertical shift.
The light source used in large screen theater systems might have been
a modulated arc or a standard arc lamp, the latter requiring a Kerr cell along
with its associated components. In later years, the arc light would be replaced
with powerful incandescent lamps. For receivers used in the home, crater arc lamps
were able to provide sufficient image brightness.
On
the left is a picture of a mirror drum receiver manufactured by John Logie Baird
in 1932. It provided a 30 line image of 9 inches by 4 inches. A larger version
of this receiver produced a 14 inch by 6 inch picture.This receiver used a
100
watt incandescent lamp as a white light source, that was in turn modulated with
the video signal by a Baird "Grid Cell". This was Baird's version of
the Kerr cell.
Beyond 60 lines with mirrors of a reasonable size, the
drum became tended to be large. The mirrors were also difficult to adjust. Shown
in the two photos below are three examples of recently made 32 line mirror drums.
The mirrors are of the very efficient first surface variety and measure approximately
one inch square. 
The
drum itself is nine inches in diameter. Each mirror has three adjustment screws.
The drum is driven by its shaft through a tension spring. This allows the synchronous
driving motor to uncouple from the heavy mass of the drum, during start up. Without
this feature, some synchronous motors would not be able to reach their synchronous
speed.
There was constant pressure to increase the number of lines in
the image. However, this tended to increase the size of the scanning assembly
as well the signal bandwidth requirements. Commercial television started with
24 line pictures, mostly because of the bandwidth limitations of transmitters
operating in or just above the Broadcast Band. As the transmissions were allowed
to move up in frequency, more lines could be added to the image, thereby improving
image resolution. With pictures up to 60 lines, scanning disks/drums with holes,
lenses or mirrors could provide acceptable pictures with disks of reasonable size.
Beyond 120 lines, holes, lenses or mirrors became too small or the disk/drum too
large and the requirement for manufacturing precision too great for scanning disks
to see any further use in receivers. By 1932, the Nipkow disk and mirror drum
had both pretty much had their day. This was also the end of the "low definition"
era of television and the beginning of the "High Definition" era.
The
Mihaly-Traub scanner overcame the disadvantages of
the mirror drums in a very clever way. From this time on, there was a major effort
to develop small, low cost scanners that would produce large, bright pictures,
with improved resolution. The scanner shown here on the right is an example of
a drum able to produce 120 line images. At 120 lines and more, television can
take on movie like qualities, with "long shots" as well as close-ups,
where the scenery adds to the impact of the actors. This was the beginning of
high definition television (HDTV), and some innovative ideas were tried to make
it happen. Continuing efforts to develop small light efficient mechanical scanning
assemblies, tended to fall into two general categories; vibratory
scanners and rotating mirror scanners.