Patent GB-1454441-A (GB)
Title: Electro-optical communication apparatus
IPC: H04N1/113
Filed: 1974-01-15
Abstract: 1454441 Facsimile transmission MASSACHUSETTS INSTITUTE OF TECHNOLOGY 15 Jan 1974 [15 Jan 1973] 01851/74 Heading H4F A fascimile transmitter or receiver comprises an optical scanning system wherein a light beam, e.g. from a He-Ne laser 18, is scanned by a mirror 20, Figs. 2 and 3, rotatable about an axis 54 tilted at an acute angle α, with respect to its reflecting surface towards the scanning surface defined by the sweep of the reflected beam, which itself forms an acute angle α with the incident beam, whereby to produce a substantially planar scanning surface, and hence a substantially linear scan across a document or photographic film 10. The scanning field is defined by an aperture 14, Fig. 1. The document 10 is moved vertically by rollers 12; slow drive (1 inch/min.), for scanning, fast drive for initial insertion and final removal of the document, which conditions being detected by paper sensors (not shown). The 10 kHz modulated light beam from laser 18 exits therefrom, horizontally parallel to the plane of aperture 14, and is then horizontally reflected twice at 90 degrees by mirrors 40, 42. The beam then passes horizontally through beam expansion lens 44 to mirror 46 and is reflected downwardly to mirror 48, from which it is reflected obliquely up at an actue angle α (e.g. 15 degrees) through horizontal focusing lens 50 whereby it is collimated for incidence on scanning mirror 20, which is tilted so that the reflected beam is horizontal, to be reflected by folding mirror 52 to aperture 14. The beam scans the document horizontally, since the scanning axis 54 on which mirror 20 is rotated by galvanometer 21 is also at an angle α to the mirror reflecting surface. The light reflected from the document is picked up by photodetectors 22. The 10 kHz modulation on the light beam is thus modulated by the variation of reflectivity across the document and is  amplified, and band limited in processor 24 where it is further heterodyned with an 8 kHz signal to produce a 2 kHz double side band modulated signal which is coupled to a telephone line by transformer 26. A digital processor 30 uses the output of a 1.44 MHz crystal oscillator 28 to provide a 100 pulse per minute to the galvanometer 21,-32Db blanking pulses associated with each scan flyback, and start line up and stop tones. In the receiver, in which the optical system is identical (Fig. 4, not shown), the signal is first passed to an analogue processor (60) which provides initial equalization and synchronizing pulses to drive the scanning mirror. The signal is passed through a non- linear amplifier with a characteristic for matching the signal to the dry silver photosensitive paper on which the transmitted image is reproduced. Some of the light (e.g. 6%), from the scanning system is removed by means of a beam splitter (76) the intensity of which is used in a feedback system to improve the accuracy. Before actual reception of the picture and during the line-up period, a step wedge pattern is generated to judge the correctness of the laser modulation and the heat processing of the paper.