Re: Mechanical television

[Dan Strassberg <dan.strassberg@worldnet.att.net>]

The CBS mechanical color TV system was what is called a field-sequential
system. The scene was scanned once through a red filter, again through a
green filter, and a third time through a blue filter. The home TV set
recreated the image by reversing the process. A monochrome tube would paint
the red-filtered image on its screen in white while a red filter was in
front of the tube, and would repeat the process for each of the other two
colors. Because the eye's response is relatively slow, the result appeared
to be in color. The problem with compatibility with monochrome TV was that
the frame rate had to be increased by a factor of three to accommodate the
triple scanning of each frame. I think because of bandwidth limitations, the
number of lines in a frame had to be reduced to 440 from the 525 lines that
were standard in black-and-white TV and are standard in the current NTSC
(National Television Standards Committee) compatible color TV that we use in
North America.

Although the CBS system was related to the early mechanical TV of which
Donna spoke, it was not the same. In the CBS system, the actual scanning of
the image was accomplished electronically using the same type of tube found
in the TV cameras of the day. In mechanical TV, the scanning itself was
accomplished mechanically--I believe by a large spinning disc in front of a
photocell. The disk had a spiral of small square openings etched or milled
into it. Each opening exposed the photocoell to a different portion of the
camera's field of view. I believe that the picture comprised 48 lines (in
other words, the resolution--measured in lines, not pixels--was less than
1/10 that of current TV). I don't know what the resolution was in the other
direction (horizontal).

Current TV has a horizontal resolution--in the direction parallel to the
scan lines--of, at best, about 400 lines. This horizontal resolution is
limited by the bandwidth of the TV channel, which establishes the bandwidth
of the transmitted signal. The bandwidth of several components in the
receiver also limits the horizontal resolution. A signal taped on a VCR can
have a horizontal resolution of as few as 250 lines. Even so, this
resolution corresponds to nearly 100 times the number of pixels displayed on
a 1928 mechanical TV. Moreover, high-definition TVs and computer monitors
offer considerably better resolution than do current TVs.

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Dan Strassberg (Note: Address is CASE SENSITIVE!)
ALL _LOWER_ CASE!!!--> dan.strassberg@worldnet.att.net
(617) 558-4205; Fax (617) 928-4205

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