Each time that a key on a teletypewriter keyboard is depressed, the keyboard automatically produces a series of
measured pulses (marks) and pauses (spaces) with a different sequence for each letter of the alphabet. This coded
series of pulses may be used to key a radio transmitter in a number of different ways, depending on the type of
transmitter, receiver, and teletypewriter equipment.
(1) In one method, the mark is made by turning on the carrier, and the space is made by turning off the
carrier. It is characteristic of this on-off system that background noise may increase during the time
the spacing impulse (no signal) is being sent.
(2) A very widely used radio communication system for single-channel operation is known as frequency-
shift-keying (FSK), a system in which the carrier frequency is shifted back and forth between two
distinct frequencies. The mark is sent at one frequency and the space is normally 850 Hz from the
mark frequency. The superiority of this system over the on-off system is that the continuous presence
of a radio signal on either marks or spaces reduces noise buildup.
e. Radio Facsimile. Radio facsimile (fax) is a method of transmitting pictures, maps, sketches, and written
or printed material by means of radio signals.
(1) In the transmitting system, the subject matter, or copy, is wrapped around a drum which revolves at a
constant speed and progresses a uniform amount on a drive shaft with each revolution. By this
movement, different parts of the copy are exposed to a sharply focused beam of light. This light
beam is reflected from the copy and passes through another focusing lens to a photoelectric tube,
where varying amounts of light from the corresponding white and black portions of the copy are
changed to electrical signals. This process is called scanning. Electrical signals produced by fax
equipment may be transmitted over any radio or wire channel suitable for voice transmission.
(2) The receiving system also uses a similar scanning arrangement, but the process is reversed. The
received signals of varying intensity control the intensity of a light beam playing on a photographic
negative wrapped around a drum. This receiving drum has the same physical dimensions, rate of
speed, rotation, and axial movement as the one used at the transmitter. In addition, the receiving
drum rotation is synchronized with the rotation of the transmitting drum.
f. Television. This method of communication instantaneously reproduces at a distance, by means of an
electrical communication channel, visible images of actual or recorded scenes. The scanning process is similar to
that of fax, except that the scanning occurs at a very much higher rate. This results in a channel that is 2,000 or
more times wider than the bandwidth of a fax channel. This restricts the use of television to the very high radio
frequencies where the available channel bandwidth exists. To transmit a picture by television, the optical image
of the subject is divided into a series of extremely small light and dark areas. These areas are represented in the
transmitted signal by individual elements that activate particles in a