of photographs, drawings, maps, and written or printed
words by electric signals. Light waves reflected from
an image are converted into electric signals, transmitted
by wire or radio to a distant receiver, and reconstituted
on paper or film into a copy of the original.
is used by news services to send news and photos to
newspapers and television stations, by banks, airlines,
and railroads to transmit the content of documents,
and by many other businesses as an aid in data handling
and record keeping.
systems involve optical scanning, signal encoding,
modulation, signal transmission, demodulation, decoding,
and copy making.
is done in a manner similar to that used in television.
An original, a photo for example, is illuminated and
systematically examined in small adjacent areas called
pixels (picture elements). Light reflected from each
pixel is converted into electric current by an electronic
device, a photocell, photodiode, or charge-coupled
such device may be used to cover one pixel after another
in a row, row after row from top to bottom until the
entire image has been translated into electric impulses.
This is rectilinear scanning. Scanning may also be
done a row at a time by a battery of devices; this
is array scanning.
scanning, a vertical array of photodevices moves across
the image, examining the pixels column by column.
As the array passes down the copy, it produces a set
of current pulses from each photodevice. The separate
currents, however produced, are then transmitted successively
over a single circuit to the distant receiver.
fine detail in the reproduced image it is necessary
to use very small pixels. In one standard, Group 3
of the International Telegraph and Telephone Consultative
Committee ( CCITT ), each pixel is a rectangle 0.12
by 0.13 mm ( 1 inch=25.4 mm ). On this standard, subject
copy measuring 8 by 11 inches ( 20 x 28 cm ) is divided
into 3.6 million pixels.
with about 200,000 pixels for televised images. The
pixels used in high-resolution facsimile systems have
dimensions one-fifth those of the CCITT standard mentioned
above, whereas in low-definition systems the dimensions
may be twice as great.
may be illuminated as in rectilinear scanning, or
a relatively large area of the image may be illuminated,
the photodevice viewing the image through a lens aperture
that restricts its field to a single pixel at a time.
In a commonly
used facsimile scanning system ( invented by Frederick
Bakewell in 1848 and based on Alexander Bain's work
of 1842 ) the subject copy is wrapped around a drum.
A finely focused spot of light falls on the copy and
the light reflected from that pixel is picked up by
the photodevice. The drum is rotated so that the light
spot traces a line across the copy, examining each
pixel in turn.
drum rotates, the light source is moved slowly on
a carriage parallel to the drum axis, tracing out
a spiral of adjacent lines until the entire area of
the copy has been scanned. At least once in each rotation
of the drum a signal transmitted to the recorder keeps
the scanner and the recorder in step.
scanning, the copy may also be illuminated broadly
and examined by a photodevice fitted with a lens aperture.
always be conveniently wrapped around a drum. In such
cases, flat copy may be scanned by a spot of light
directed across its surface by a moving mirror. Mirror
scanning may also be used when the copy is wrapped
on a drum, or while it is being pulled from a roller.
Laser light produces a very fine beam that travels
across the copy, row by row, as the copy moves vertically.
arrangement the mirror is rocked back and forth, moving
the beam across the copy. In another, a rotating polygonal
mirror is used. This mirror typically has 18 flat
mirror surfaces on its periphery, each capable of
scanning a row of pixels.
scanning can be achieved by rapid rotation of the
mirror and corresponding vertical motion of the copy.
The beam is reflected from each pixel into a photodevice
that converts successive light values into corresponding
currents. Electronic scanning of flat copy may also
be done by arrays of photodiodes or charge-coupled
rates higher than about 6 rows per second laser beams
with polygonal mirrors and arrays of photodevices
of the Facsimile We owe development of fax to a Scottish
inventor, Alexander Bain, who was granted a patent
for his creation back in 1843. Bain's original concept
is still the basis for modern facsimile machines.
& SSTV History
(Fax) a method of encoding data, transmitting it over
the telephone lines or radio broadcast, and receiving
hard (text) copy, line drawings, or photographs.
Bain invented a machine capable of receiving signals
from a telegraph wire and translating them into images
in 1842 by Alexander Bain, a Scottish clockmaker,
who used clock mechanisms to transfer an image from
one sheet of electrically conductive paper to another.
fax transmitter was designed to scan a two-dimensional
surface (Bain proposed metal type as the surface)
by means of a stylus mounted on a pendulum.
Telephone Facsimile - Digital Facsimile
1920 and 1923 the American Telephone & Telegraph
Company (AT&T) worked on telephone facsimile technology,
and in 1924 the telephotography machine was used to
send pictures from political conventions in Cleveland,
Ohio, and Chicago to New York City for publication