Wireless World, June 1974 183 Laser wirephoto system A report on the design and development of a wirephoto transceiver to be used by Associated Press news agency As part of the trend towards more efficient usage of telephone links, telephone companies are encouraging a changeover from high quality analogue lines to digital links. Professor W. Schreiber of Massachusetts Institute of Technology has developed a wirephoto transmitter- receiver and digital picture processor which meets this requirement. A method of transmitting colour pictures Of the types of photosensitive papers modulation. Processing to a digital form is over telephone lines has been recently available, dry silver paper seemed to be accomplished by a separate analogue-to- described in Wireless World.' However, most suited since it was capable of pro- digital converter contained within a signal although the processing of colour pictures ducing pictures of the desired quality and processing computer to be described later. was novel, the basic mechanism of the uses a very simple heat treatment for Better resolution and higher scanning transmitter; receiver and the continuing use picture development. The scanner/writer speeds are possible where other appli- of analogue signals over the telephone selected was a low -power helium -neon cations are considered. link were fairly conventional. The limita- laser, for the basic reason that it was Since the tone scale of the output pic- tions of such techniques are becoming reliable, cheap and, since it could be ture was considered to be the most impor- more obvious with time since the necessary directly modulated, eliminated the need tant single aspect of picture quality, control high quality audio links (800Hz to 18kHz for a separate modulator. Feedback control of the tone scale was provided by ensuring for the colour transmitter) are expensive of the laser brightness also achieves a con- stable processing of the exposed paper. In to establish and maintain. sistent control of the paper exposure. From addition a locally generated step wedge It would seem that the new digital links this starting point, several subsequent is printed at the beginning of every picture being offered by telephone companies are development decisions are possible con- and a non -linear amplifier operates on the a solution to some of these problems, cerning the method of scanning. Those receiver input signal to produce the since no special alignment is necessary to selected were a flat -bed scanner offering a required photo transfer characteristic. accommodate extreme bandwidth, making simple, low -cost paper drive, a galvano- it possible to provide temporary service meter- driven mirror for horizontal scan- The optical system over the ordinary local telephone distri- ning, continuous paper motion for vertical The optical system is based on a 1mW bution network, quickly and at low cost. scanning and oven processing of the modulatable He -Ne laser. The power out- With the increasing' availability of digital paper. put is more than adequate to expose the wired links, the digital encoding of picture The final system designed for use by 3M type 7771 dry silver paper, the only information makes economi; sense. Quite newspaper agencies handles both the in- disadvantage being that the spectrum of recently the American news agency put photographs and output paper as a light is such that a panchromatic paper is Associated Press commissioned a develop- continuous web, I lin wide and of any required, resulting in the need to expose ment project for a new telephoto trans- length. The scanning density is 1001ines per and process in darkness. Colour picture mitter /receiver and digital picture processor inch, per minute and to meet existing transmission would require an alternative which would meet the needs 'of the future. telephoto transmission standards the trans- light source. The laser selected is cheap This project was assigned to the Electronics mission signal is double-sideband amplitude and already in commercial production. A Research Laboratory of the Massachusetts Institute of Technology and was directed by Professor William F. Schreiber. The results of his work was the laser scanning, dry silver facsimile system and a separate beam digital picture described below expansion processor lens and in two papers published by MIT. 2'3 Selection of scanning/writing system galvanometer In developing a new facsimile machine advantage could be taken of the opportun- scanned ity to seek ways of improving the picture line quality over existing systems. Since the transmitter more often than not scans a horizontal continuous tone photograph, there is a deflection natural to a mirror tendency choose photo- focusing sensitive writing process. An additional lens advantage of such a choice is that the scanning and writing systems may be made to perform the dual function. Fig. 1. Mechanical layout ofthe optical scanner. .