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FAX (facsimile)

Facsimile or fax machine is an

electronic system.

It is used for transmitting graphical

information through wires or

through free space(i.e. with the helpof electro-magnetic waves).

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To send a fax means to send graphical

information on a paper.

When we want to send a fax, the

graphical information on paper is

SCANNED by strong light beam(itsworking is fairly similar to Xerox

machine).

While scanning, reflected light varies

depending on the details in graphical

information.

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The variations in light are converted

into equivalent electrical signals.

This conversion is done with the

help ofCharge Coupled Device (CCD)

and electronic memory circuits.

Then the signals are transmittedover telephone line or through free

space.

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Fax machine

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By opposite process, the transmitted

graphical information is reproduced inprinted form (on paper) at receiving end of

fax machine.

Fax machine is used to send letters,

photographs, maps etc. In short, any

information in printed form on paper can

be sent with the help of fax machine.

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How a document is scanned in

fax machine?

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. The light beam reflects from paper surface and through mirror assembly, it is incident on CCD (Charge Coupled Devices).

The process of scanning is done

electronically.

The paper is fed into therollers. Upper rollers rotate clockwise and

lower rollers rotate anticlockwise. So the

paper is pulled into the machine.

The light source focuses a strong light

beam on the paper. The light beam reflectsfrom paper surface and through mirror

assembly, it is incident on CCD (Charge

Coupled Devices).

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The CCD is a light sensitive

semiconductor device.

It has a very large number of tinycapacitor like devices.

These are reverse biased silicon

photodiodes arranged in matrix form on a

silicon chip.

Working of CCD

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They convert variations in light

signals into proportional electricalsignals.

The complete information onpaper is NOT stored on CCD.

The beam oflight source scans the

paper horizontally(i.e. along width of

paper, row-by-row).

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When it goes from one end to

another, the CCD becomes occupied.

Then charged information on CCD

is given out to electronic memory and

next scanning process starts, until

complete paper is scanned.

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Fax machine individual block

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Transmitter block When paper as graphical information is

inserted into fax machine, it is scanned row byrow.

The CCD converts this information intoproportional analog signals. This output is fed to

A/D converter circuit. Its output is in digital

form.

This digital data becomes extremely huge

due to scanning details in the document. So it is

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This circuit is made up of VLSI (Very

Large Scale Integration) technology. Hence,

the size of data in bits (binary digits) isreduced.

The compressed data output is fed tomodulator.

It is the modem, which can modulate

& demodulate digital data. In modem, a

carrier wave is modulated using the data and

transmitted over tele hone line.

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Receiver block When fax signal reaches receiver block

through telephone line, it is demodulated

using demodulator within modem.

Thus, at its output we get original dataonly in compressed form.

To expand the data it is fed to digital

data expansion block. The data is recovered by removing its

data compression into ORIGINAL SIZE.

The signals are fed to thermal printer.

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Control blocks

This is very complex circuit used forHANDSHAKING between two fax machines,

during communication.

During this process, different audio

tones and beeps are exchanged and

produced.

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This process takes place as follows:

When a fax machine is dialed, calledmachine responds to it by producing an

audio tone.

The calling machine sends synchronous

signals, so both machines start at the same time

The called machine compares this signal as per

its own standards and acknowledges to sync.

signals. Then the printing begins.

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Operator controlsblock provides

user-friendly controls like start, stop,number to be dialed and a number of other

functions.

Motor control circuit controls speed

of motor and paper rollers. It is a constant

speed controller circuit called as governorcircuit.