FACSIMILE(commonly called fax) means an exact reproduction and fax transmission provides an exact reproduction of document at the receiving end. Television system is used to reproduce any scene at the receiving end but it differs from facsimile in that the scene may be live. Information is transmitted at a much faster rate in TV. Television transmission requires a much larger bandwidth and wideband circuits. FAX requires much smaller bandwidth and hence it can be used for transmission through normal telephone channels.
Document Photo electric conversion ModulationTransmission To receiver SynchronizationScanning BASIC BLOCK DIAGRAM OF FAX MACHINE
CCD A/D converter Digital Data Compression Modulator Telephone Line Interfaces Demodulator Data expansion Thermal printer Control Logic Microcontroller circuits Roller drive motors
Dissects the source document into matrix of picture elements(pixels). Senses the image density for each pixel one line at a time(raster scanning). Signal information is modulated onto an analog transmission channel or encoded on a digital data channel. Digital information undergoes data compaction Digital data may be stored in memory for transmission at a later time.
Raster scanning is used in fax systems. Image to be scanned is divided vertically into several horizontal segments called scan lines. Each scan line is divided into equal segments called pixels. Each pixel may be assigned either black/white. Beginning and end of scan marks are inserted with picture data to provide synchronization between transmitter and receiver.
CCD scanners are of flat bed type. Document is placed face down on a fixed glass plate and cover is used to close it. Rest of scanning mechanism is mounted on a carriage on rails. Carriage drive speed is controlled so that it can move one line width during the time of one electronic scan cycle in CCD chip. Lamp is used to illuminate the document. Image of one scan line is focussed through a slit and lenses into the slit window of the CCD chip.
Optical unit used in fax machine for scanning. Light sensitive semiconductor that converts varying light amplitudes into an electrical signal. It is an integrated circuit that has 2048 image sensors. Works similar to photodiode. When light falls on it, a voltage is created. Level of voltage is proportional to the intensities of light. It acts like an capacitor and stores this voltage and transfers it through rest of circuit.
(…contd) Higher the intensities of light, higher the voltages produced. Light reflected off white paper is high & the voltage would be high. Light reflected off black paper would be low and the voltage would be low. It also have an optical unit made of light source to reflect off light from the original, mirrors and lenses to direct or focus the image into the CCD unit.
Similar to flatbed design. It incorporates the light source, optical unit & CCD device into one compact unit. Design allows fax machine to reduce the physical size but still maintaining the quality.
After the original document is broken down into picture elements and are converted to varying voltage levels, the CCD units outputs an analog signal for each line of data. Before transmission over phone line, it must be converted to digital signal. Binary signals indicate whether the pixel will be black or white. Reference voltage(Slice Level) is used to divide the analog signal into 2 groups. V>slice level = White pixel(high level) V
Slice Level Waveform after A/D conversion CCD image sensor o/p waveform Low level=black High level=white
Data to be transmitted undergoes compression. It reduces the amount of data to be transmitted and hence it reduces the time required for transmission. Before transmission through telephone lines, it should be modulated. Analog fax systems uses AM or FM. Digital fax systems uses PSK or QAM.
Performs the reverse process of facsimile transmitter. It receives the modulated or encoded signal from the communication channel and demodulates or decodes it to recover original serial pixel information and record them on the target document. It can operate in two basic modes: Synchronous operation: scanning, transmission and recording in real time, in synchronous manner. Asynchronous operation: encoding and storage of data for transmission at a later time and it will be at a different rate.
In order for the received image to have the correct relationship to the transmitted image, the three factors need to be considered: Synchronization Phasing Index of Cooperation(IOC)
In a synchronous system, the receiver scanner and transmitter scanner must run at exactly the same speed. If the receiver runs at slightly faster speed, then each of the scan line will overlap into the following scan line, causing the image to skew diagonally to right. If the receiver is slower, the image will skew diagonally to the left. AC motors running on the common grid was used to maintain synchronization. Now a days, separate local oscillators are used at both transmitter & receiver.
Width-height ratio of the document reproduced at receiver must be same as original document to avoid distortion. IOC is a number derived from width-height ratio. Transmitter & receiver must have the same IOC for proper reproduction
IOC = S.W S=Scan density(lines/mm) W=width of source document(mm) Scan density, S S=n/L n = total no. of lines in document L = document length(mm) IOC = nW/L In a drum scanner, IOC = D/P P=scanning pitch(mm/line) D=drum diameter(mm) Scan density, S S=1/P IOC=SW/
After recovering the pixel information from modulation at the receiver & placed in the proper sequence, it must be recorded to create image facsimile. Different used in order to recover the information back: Duddell mirror oscillograph(a small mirror attached to galvanometer moving coil) is used as a modulator in early form of receivers. Crater lamp is used as the light source. Lasers are used for modulating and focussing a light beam.
Image is transferred directly to the copy paper and no further processing is required. It can be done in several ways Using a electrolytic paper chemically treated with damp electrolyte. Electro resistance recording system- it uses Teldeltos paper Electro thermal recording Transfer Xerography
It has a rotating drum covered with a thin layer of Selenium. Se is a semiconductor that under dark conditions is a good insulator, but becomes a moderately a good conductor when illuminated(bcoz of creation of electron-hole pairs). Se is exposed to high electrostatic field and places a uniform layer of trapped charges on surface. As drum rotates, it is exposed to modulated signal light source which is a laser/rotating mirror system.
Light areas correspond to white & where Se is illuminates, photocurrent drains the surface charge Turning drum is then dusted with a dry powder black toner that adheres where charge remains. Drum turns against a pressure roller that transfers the image from drum to copy paper Passed through a heater that fuses the toner image to the paper surface to complete the printing process. The image undergoes a mirror reversal during transfer, it is necessary to scan in reverse direction during exposure.