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Friday, 3 September 2021

Facsimile Communication System

Facsimile Communication Systems


In a communications system, it is necessary to send visual signals in addition to fundamental signals such as speech, music, or telegraph codes. The receiving end of a facsimile transmission receives an identical replica of a document, image, or still photograph. A television system varies from a facsimile in that the scene can be ‘live' (i.e., include movement). Television transmission needs a greater bandwidth, whereas facsimile transmission is achievable over telephone lines.


Uses


i) Photographic transmission ( For example, in the press)

ii) Transmission of papers, weather maps, and so forth.

iii) Transmission of linguistic texts for which the teleprinter is insufficient (example: Chinese)


Facsimile sender


The scannable message can take one of three forms:


i)  A single sheet, which is often wrapped around a cylindrical drum in the sender to allow scanning.

ii) Tape that is too narrow to be continuous.

iii) Continuous sheet paper, often known as wide tape.


Scanning is accomplished in two ways.


1. Optical scanning, in which a laser spot moves through the message.

2. Resistance scanning, in which the message's letters have variable resistance and are brought into the circuit by touching and sliding a stylus over them.


Cylindrical scanning


The message is initially clipped around the drum in this technique. The drum is then simultaneously rotated around its axis and traversed along with it under a fixed scanning point. The light reflected from the scanning region is focused on a photocell. The photocell's electrical output represents the signal.

Cylindrical Scanning

This system's configuration is represented in Figure. The signal is converted into a modulated wave by the chopper disc, and the carrier frequency is determined by the speed of the disc. The modulated signal is simpler to amplify than the straight photocell signal.


The message lighting area is rather big, and a mask with a tiny aperture produces the spot that illuminates the photocell. In most scanning configurations, the spot takes a spiraling route around the drum. Scanning in a succession of closed rings is an alternative configuration. As the fixing clips pass beneath it, the spot moves from one ring to the next. However, this approach is not widely utilized.


Facsimile receiver


The mechanical characteristics of scanning in the receiver are similar to those in the transmitter, and identical equipment is usually used on both ends. Scanning in the receiver produces an optical output from the electrical input. This is the inverse of what happens in the transmitter. For the received signal to have the right link to the broadcast signal, the signals must be synchronized, passed appropriately, and have the same height/breath ratio.


Synchronization


If the information is documented, it is sufficient to utilize synchronous motors for both transmitter and receiver and to work on frequency control supply mains. A synchronization signal with a frequency of 1020 Hz must be delivered when the image is sent (international standards). The transmitter speed has a known connection to this, and the receiver speed is changed via a stroboscope to match the relationship with an accuracy of 1 in about 105.


When the signal is modulated onto a carrier, the carrier must be sent together with the sideband. Since the carrier is present, the precise 1020Hz synchronizing signals may be retrieved. For signal recovery, a local oscillator at the receiver is enough. The consequence of improper phasing is seen in the figure.

Figure:  A – Input, B – Effect of incorrect Synchronization, C – Effect of Incorrect Phasing  Phasing

To ensure that the image of the clips holding the paper to the drum does not overlap the transmitted picture, proper phasing is required. The following procedure is used to modify the pulley phasing for each transmitted image.


The receiver operator first adjusts the speed to the proper value using the synchronizing signal and then places the drum in the correct starting position. A switch secures this position. A pulsed signal is sent from the sender to mark the commencement of transmission, and the pulse releases the switch that holds the receiver drum. The figure depicts the result of improper phasing.


Index of co-operation


The height/breadth ratio must be the same for both transmitted and received images, which is determined by the scanning pitch and the diameters of the drums used in the transmitter and receiver. The product of total line length and the number of lines per unit length divided by π is the Index of Cooperation.

Figure: Sender and Receiver Correlation in FAX.

Let

D→ diameter of sending drum

d→ diameter of receiving drum

P→ scanning pitch of sender

p→ scanning pitch of receiver

n→ number of lines scanned


The transmitted picture has a breadth of nP, while the received picture has a breadth of np. The broadcast image's height is proportional to D, whereas the received image's height is proportional to 'd.' As a result, the proper height/breadth ratio must be maintained.

Figure:  Effect of Index of Cooperation

IOC has a fixed value of 352 as per CCITT. The figure represents the consequences of receiving facsimile images with varying IOC levels.


Direct recording reception


This method uses a highly absorbent chemically treated paper. When a voltage is given to the paper using a metal stylus, the electrolyte dissociates, and one of the dissociation products reacts with the stylus to produce a metallic salt.


This, in turn, interacts with a color chemical in the paper, resulting in a mark on the paper. The intensity of the mark is determined by the quantity of dissociation, i.e. the signal voltage. Since it generates black coloration, a steel stylus is frequently utilized. The paper must be maintained in sealed containers. It has a shelf life of around one month after opening.


A resistant paper marketed commercially as Teledeltos paper is used in another form of direct recording reception. This comprises a metalized backing with a carbon black-like material placed on top and a very thin layer of insulation on top of it. The paper is pressed down with the help of a stylus. The burning happens when the signal voltage is applied, resulting in the blackening of the paper.

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