GB1342555A - Facsimile signal system - Google Patents

Abstract

1342555 Facsimile transmission; digital transmission systems; transistor pulse circuits VICTOR GRAPHIC SYSTEMS Inc 10 March 1972 11359/72 Headings H4F H4P and H3T In an information system, particularly a facsimile transmission system, in which two-level information-representative signal pulses are converted for transmission into three-level signals of half the frequency of the two-level signals and with the central level corresponding to one level (mark level) of the two-level signal and the outer levels alternately corresponding to the other level (background level) of the twolevel signal, the two-level signal pulses are reduced in width prior to conversion by an amount equal to the width of the shortest mark pulse which the system can transmit and reproduce. Two examples A and B of two level pulse trains from scanner and amplifier 1 are shown in Fig. 1. These pulse trains are squared and given a uniform amplitude by Schmitt trigger circuit 2. The pulses then pass through a controlled rise time circuit 3, which gives each pulse a sloping leading edge, and applied to a second Schmitt trigger circuit 4 set to trigger at the broken line shown on pulse trains E and F. Mark pulses below the limit of resolution of the system are thus eliminated and the remaining mark pulses are shortened. The pulses are converted to three-level form as described in Patent Specification 1,213,359. The controlled rise time circuit (Fig. 2) may comprise a transistor 9, a field effect transistor 10, a current determining resistor 11 and a capacitor 12. A resistor 13 may be provided to swamp the input resistance of Schmitt trigger circuit 4. The capacitor gradually charges when a pulse is received and when the pulse is removed the capacitor discharges through field effect transistor 10 and the emitter resistor of transistor 9.