Thông tin thiết kế mạch P10

THE FACSIMILE MACHINEAlthough the facsimile machine was invented in the 1840s, it remained largely a device used in the newspaper industry for the transmission of pictures until the mid1980s. There were several reasons for this; some were technical and the others commercial. The technical problems which held up the development of the fax machine are illustrated in Figure 10.1. For simplicity we use the letter H and assume that scanning is carried out horizontally from the top left side to the right. ...
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Thông tin thiết kế mạch P10 Telecommunication Circuit Design, Second Edition. Patrick D. van der Puije Copyright # 2002 John Wiley & Sons, Inc. ISBNs: 0-471-41542-1 (Hardback); 0-471-22153-8 (Electronic) 10 THE FACSIMILE MACHINE10.1 INTRODUCTIONAlthough the facsimile machine was invented in the 1840s, it remained largely adevice used in the newspaper industry for the transmission of pictures until the mid-1980s. There were several reasons for this; some were technical and the otherscommercial. The technical problems which held up the development of the fax machine areillustrated in Figure 10.1. For simplicity we use the letter H and assume thatscanning is carried out horizontally from the top left side to the right. The scanninghead then moves to the second line and the process is repeated. Again, for simplicity,the scanned field is divided into a matrix 20 by 20. Thus 400 pieces of informationhave to be sent to the receiver in order to reconstruct the H. The first task is tomeasure the level of light reflected or produced by each square and to assign a valueof 1 or 0; we assume here that a 1 is assigned when a square is white and a 0 when itis black (the opposite would work just as well). Figure 10.1(b) shows the result. Eachone of these pieces of information is called a pel (which is a pixel with its gray scaleor color information placed in two categories, black or white, depending on itsrelative brightness). For the transmission to be successful, the transmitter has to‘‘tell’’ the receiver precisely which squares are to be left white and which are to bemade black. In other words, the ‘‘read’’ head in the transmitter and the ‘‘write’’ headof the receiver must be exactly on their corresponding squares at the same time, thatis, they must be in synchronism and in phase. To obtain synchronism, twopendulums of the same length (with a mechanism for keeping them in phase)were used [1]. The pendulum was not very practical because it had to be made quitelarge to store enough energy so that the losses during the scanning and printingprocesses would be negligible. Improved synchronization was obtained when thetuning fork replaced the pendulum but this new technique did not become accurateenough for the purpose until the 1940s. The problem of synchronization was never 305306 Figure 10.1. (a) The letter H showing white and black pels. (b) Binary representation of the pels. 10.2 SYSTEMS DESIGN 307satisfactorily solved; indeed, the problem disappeared eventually when digitaltechniques were applied to fax machine development. Another technical problem which accounts for the slow development of the faxmachine was the speed at which the information could be transmitted. We recallfrom Chapter 1 that the initial attempts to construct fax machines took place beforethe telephone was invented. The telegraph lines at the time used single wires withground returns. These were subject to electrical noise mostly generated by electricstreet vehicles, which were very popular at the time. To compound the problem, thetelegraph lines used relays to extend their reach (Morse’s relay) and these wereinherently too slow to convey the volume of information required to make the faxmachine a success. Note that at the minimum rate for scanning a 8.5 Â 11 inch(21.6 Â 28 cm) page (200 lines per inch) the number of pels generated is 2.86 Â 106.Even with modern coding schemes the telegraph lines could not have handled thesheer volume of information in a reasonable time to make this a success. The next technical problem that had to be solved was the adoption of a suitablemethod of coding the information so as to reduce the high level of redundancy. Weobserve from Figure 10.1(b) that row 1 is completely white and hence it isrepresented by a row of twenty 1s. A shorter code made up of a few 1s and 0scould be used to signal to the receiver to insert twenty 1s in row 1. Similarly, in row2, there are two transitions from 1 to 0 and two transitions from 0 to 1. To conveythis information it is possible to devise a code word with less than twenty bits to tellthe receiver where the transitions occur and whether they are from 1 to 0 or viceversa. It can also be seen from Figure 10.1(b) that, in our example, row 3 is the sameas row 2. A further reduction in the bits required can be achieved by sending arelatively short signal to the receiver to repeat row 2. At least in the United States, commercial rivalry discouraged cooperationbetween the engineers who were working on the dev ...