Facsimile

FACSIMILE (fax) is a method of transmitting still images over an electrical communications system. The images, called "pictures" or "copy" in fax terminology, may be weather maps, photographs, sketches, typewritten or printed text, or handwriting. Figure 3-32 shows a facsimile transceiver. You must realize that the still image serving as the fax copy or picture cannot be transmitted instantly in its entirety. Three distinct operations are performed. These are (1) scanning, (2) transmitting, and (3) recording or receiving.

Figure 3-32. - Facsimile transceiver.

Scanning consists of subdividing the picture in an orderly manner into a large number of segments. This process is accomplished in the fax transmitter by a scanning drum and phototube arrangement.

The picture you want to transmit is mounted on a cylindrical scanning drum. This drum rotates at a constant speed and at the same time moves longitudinally along a shaft. Light from an exciter lamp illuminates a small segment of the moving picture and is reflected by the picture through an aperture to a phototube. During picture transmission, the light crosses every segment of the picture as the drum slowly spirals past the fixed lighted area.

The amount of light reflected back to the phototube is a measure of the lightness or darkness of the segment of the picture being scanned. The phototube changes the varying amounts of light into electrical signals. These are used to amplitude modulate the constant frequency output of a local oscillator. The modulated signal is then amplified and sent to the radio circuits.

Signals received by the fax receiver are amplified and actuate a recording mechanism. This recorder makes a permanent recording (segment by segment) on paper. The paper is attached to a receiver drum similar to the one in the fax transmitter. The receiver drum rotates synchronously with the transmitter drum. Synchronization of the receiver and transmitter is done to reduce distortion. Synchronization is obtained by driving both receiver and transmitter drums with synchronous motors operating at the same speed. Drum rotation continues until the original picture is reproduced. The recording mechanism may reproduce the picture photographically by using a modulated light source shining on photographic paper or film. It may also reproduce directly by burning a white protective coating from specially prepared black recording paper.

The receiver drum is FRAMED with respect to the transmitter drum by a series of phasing pulses that are transmitted just before transmission. The pulses operate a clutch mechanism that starts the scanning drum in the receiver. This ensures proper phasing with respect to the starting position of the scanning drum in the transmitter.

Figure 3-33 is a block diagram of the equipment necessary for radio facsimile operation. View A shows the receiving system. This system consists of a standard radio receiver, a frequency-shift converter, and a facsimile recorder. View B shows two systems for transmitting TIF signals. The upper row of blocks is for carrier-frequency shift transmission. This system consists of a facsimile transceiver, a keyer adapter, a frequency shift keyer and a transmitter capable of fsk emission. The lower row of blocks is for audio-frequency shift transmission and uses a fax transceiver, a radio modulator, and an AM transmitter.

Figure 3-33. - Radio facsimile systems.

SECURITY, QUALITY MONITORING, AND SAFETY

Security, quality monitoring, and safety are important areas that you must be aware of. If the fundamentals are followed, you will see higher quality communications. You will also help meet the communications goals of the Navy. Let's find out what these fundamentals are and what they will do for you.

TEMPEST

Compromising emanations (ce) are, generally referred to as TEMPEST. These signals may be unintentional, data-related, or intelligence-bearing. If intercepted or analyzed, these signals could disclose classified information. TEMPEST problems are associated with material transmitted, received, handled, or otherwise processed by electrical information processing equipment or systems. Any electrical information processing device may cause problems. Even your electric typewriter or a large, complex data processor may emit interceptable compromising emanations. Some countermeasures taken to ensure against TEMPEST problems are listed below:

• Design of equipment in which ce is suppressed
• Approved installation criteria that limits interaction between classified and unclassified signal lines, power lines, grounds, equipment, and systems
• Low level keying and signaling
• Shielded enclosures for equipment installations
• Proper shipboard grounding of equipment, including ground straps

TRANSMISSION SECURITY

Transmission security includes all measures designed to protect transmission from interception, traffic analysis, and imitative deception. Every means of transmission is subject to interception. In radio transmission, it should be assumed that all transmissions are intercepted.

Speed Versus Security

Three fundamental requirements of a military communications system are reliability, security, and speed. Reliability is always first. Security and speed are next in importance and, depending on the stage of an operation, are interchangeable. During the planning phase, security is more important than speed. During the execution phase, speed sometimes passes security in importance.

Radio Transmission Security

When a message is transmitted by radio, the originator may know some of those who are receiving it, but will never know all of those who are receiving the message. You must assume that an enemy receives every transmission. Property prepared messages using modern cryptographic systems may prevent an enemy from understanding a message. However, they can still learn a lot. For example, as time for a planned operation approaches, the number of messages transmitted increases. An enemy then knows that something will occur soon, and their forces are alerted. Strict radio silence is the main defense against radio intelligence.

The amount of radio traffic is not the only indicator used by an enemy. Statistical studies of message headings, receipts, acknowledgments, relays, routing instructions, and services are also used by an enemy. Communications experts can often learn much about an opponent from these studies. Direction finders are another aid the enemy can use to determine where messages originate.

Radiotelephone Security

Radiotelephone networks are operated so frequently that many operators tend to be careless. There are too many instances of interception of vhf and uhf transmissions at distances of many thousands of miles. You may have occasion to work on or around this type of equipment. If you are ever required to bring any transmitter on the air for any purpose, you must be familiar with and use all the correct procedures.

Q.31 The transmission of still images over an electrical communications system is known as what?
Q.32 The term TEMPEST refers to what?
Q.33 What are the three fundamental requirements of a military communications system?
Q.34 Which of the above requirements is most important?