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G3 facsimile equipment

World-wide there are approaching 20 million (1991) compatible facsimile transceivers capable of interoperation using the Public Switched Telephone Network. Designs and implementations vary but the most popular scanning method is by CCD (charge coupled device) and printing by thermal head on heat sensitive paper.

Figure 60.1 shows a block diagram of a G3 machine. Discrete LSI is often used for online signal processing, to ensure a manageable load for the main CPU which takes care of the user interface, dialling functions, mechanism control housekeeping and reporting.

The CCITT G3 recommendations include a number of options, for example fine resolution, MR coding, V29 modem, ECM. For these to be used they must be available in both send and receive machines and are usually negotiated automatically.

In a typical CCD scanner a section of the original is illuminated by a fluorescent or cold cathode tube. The facsimile line to be scanned is focused onto a 1728 (A4 width) or 2048 (B4) element CCD single line image sensor. The independent elements are discharged by radiated light from the original which is focused on to them. After a suitable time (lms) the elements are recharged sequentially. By monitoring the current for this recharge the video signal amplitude is obtained. The signal is then processed to allow for background and level variations, and to preserve character size, then converted to binary levels for normal transmission, as a black/white image. It may be converted to amplitude bits for processing for pseudo half tone/grey scale transmission. The original is then advanced l/3.85mm or l/7.7mm for the next line to be scanned. If the line transmission buffers are full the scanning may then wait until further data is required.

A second popular method of scanning uses CIS (Contact Image Sensor). This is a single line sensor assembly fitted close to the paper at the document scanning point. It requires an array the full width of the paper, but it saves the space taken up by the CCD optical path. The copy is illuminated by LEDs (Light Emitting Diodes) within the unit and the radiated light from the area being scanned is collected by a multiple lens assembly and focused on to the image sensor array. If the sensors are brought al most into contact with the paper, the multiple lens is not required and in this case there are holes in the sensor array to allow the LED illumination from behind.

The detection of black or white at a particular picture element depends on the density and the position of the edge of the image with reference to the sensor element. The resolution in G3 is adequate to preserve character shape but there is almost always a difference between the raster images of identical characters even when adjacent.

To provide facsimile copy with a similar appearance to computer generated printer output, the resolution needs to be greater by about 30%. This has led to resolution incompatibility problems for fac­simile and computer interoperation.

Дата добавления: 2015-03-31