(2) Transmission of weather maps and military maps is especially important to military services.
(3) Facsimile signals can be successfully transmitted and received over any normal voice circuit.
f. Disadvantages of Facsimile Transmission.
(1) Transmission of visual images by facsimile is a relatively slow process. The speed of transmission is
limited by the bandwidth capabilities of the interconnecting facilities. A normal telephone circuit
passes a voice band from approximately 300 to 3,500 Hz, or, in other words, a 3,200-Hz bandwidth.
This bandwidth limits facsimile transmission to a rate that allows the transmission of a weather map
or large photograph in about 20 minutes. Insufficient bandwidth results in lack of contrast between
light and dark shading.
(2) Facsimile images are subject to distortion by circuit noise and by interfering signals from adjacent
lines. Noise causes spots and streaks to appear in the received image.
(3) Speed control of transmitting and receiving facsimile reproducers is a critical item. A synchronizing
system helps to minimize this problem by including phasing pulses in the facsimile signal.
(4) The principal cause of "ghosts" or shadows in a received facsimile picture is phase delay of signal
elements. Serious ghost interference of picture quality by the interconnecting lines requires that the
lines be corrected by phase delay devices that counteract the delay characteristics.
g. Telegraph Transmission. Telegraph systems are utilized to transfer information in written form. A
telegraph system is normally composed of a sending device which converts mechanical motion to dc pulses, a
transmission path, and a receiving device which converts de pulses to mechanical motion.
h. Advantages of Telegraph Transmission.
(1) Teletypewriter communication produces a printed or "hard" copy. The most important characteristic
of a hard copy is the printed record, which gives far more flexibility of use than a telephone voice
message.
(a) A copy can be filed, and thus serve as a future source of reference if needed. Thus, dependence
on human memory is minimized.
(b) Responsibility for the message and its contents can be positively identified by identifying the
originator.
(c) Received messages can be acted upon at the appropriate time, or action may be delayed, as the
situation warrants.
(2) By means of automatic sending devices, the messages can be sent at uniform speed to realize
maximum efficiency of circuit operation.
(3) Messages can be routed through alternate channels.
6
Previous Page
