Patent GB-1431292-A (GB)
Title: Facsimile communication system
IPC: H04N1/42
Filed: 1973-05-08
Abstract: 1431292 Facsimile system XEROX CORP 8 May 1973 [16 May 1972] 21868/73 Heading H4F In a method of transmitting facsimile video signals over a standard telephone line the video signal is initially reduced in bandwidth, then frequency modulated on a carrier signal and the frequency modulated carrier signal passed through a vestigial sideband transmission filter. The system uses identical transmitter-receiver sets at either end of a standard telephone link whereby all audio messages can be transmitted over the telephone link in the normal manner and the transmitter-receiver sets connected into the link when desired. Each set is normally in a standby printer mode and upon instruction can operate in one of three possible modes. Principle mode-at the local set the original document is manually loaded into the scanning position and the remote set operates in an automatic unattended print-out mode. Auto feed mode-a stack of original documents are put into the local set and are sequentially automatically loaded into the scanning position. Remote operation mode-the local set transmits a signal to the remote set instructing it to start transmitting an original, the local set then operating as the printer. A forward control signal is produced by means 58, Fig. 2, and after frequency modulation on carrier FS at 38 is transmitted over the telephone line 21 to the remote set to there produce after demodulation at 40 and detection at 59 an enquiry as to whether the remote set is ready to receive and print. The remote set replies from 50 with an acknowledgement signal which is received back at the local set at 62. A start button is now pushed and a carrier at FS is transmitted to the remote set where it is received at 59 and indicates that the remote set is coupled to a similar transmit receiver set and also clears away any echo suppressions in the telephone line. A series of equalization pulses 80, Fig. 3, are transmitted by the local station and utilized at the remote station by an automatic equalizer to compensate for distortion in the transmission line. Sync. pulses follow from the local station and enable scan and framing synchronism at the remote station. The width of the synchronizing pulses is varied to indicate whether or not video information is to follow being narrow for novideo and wide for video to follow. Pairs of video pulses indicating end of transmission, and both local and remote stations return to a standby mode if a new video signal is not received within a set time. Two of the sync pulses determine and signal the scan line pitch by means of their amplitudes (86, 88). Video signals from scanner 90, Fig. 5A, after peaking and clipping at 91, 92, are encoded at 93 to reduce bandwidth. One method of bandwidth reduction is the phase inversion of alternate lines of video. In the frequency modulator 38 the bandwidth reduced video signal controls a variable oscillator 95 the output of which is fed via an output driver 98 to a vestigial side band filter 100 whereby further bandwidth reduced video is fed to the telephone line 12. When operating in the remote mode or as a printer the received video signal, after passing through preamp 103 and equalization in network 104 using the already referred to equalization pulses, is frequency demodulated at 60 using limiter 108 and zero crossing detector 110. The frequency demodulated signal is now passed through a full wave rectifier which removes the alternate phase reversal produced by encoder 93. The video signal is passed to a half-tone printer 236. The various widths of the received synchronizing pulses are detected by means 128, 130, 126. The signal from the remote set indicating ready to receive is received and detected by means 145, 143. An emergency stop signal can also be received and detected by means 153 and 155. The scanning and printing means both use a laser 150, Fig. 9. In the scanning mode the original 164 is scanned through a glass plate 170 by the laser beam 172 via mirrors 184, 186, 180, 194 and 196, a cylindrical lens 188 giving an elliptical cross-section to the beam, mirror 180 being a galvanometer mirror vibrated under the control of a line scanning sawtooth wave, and mirror 194 being in the position shown in full lines for scanning, but in the dotted position for printing. Light reflected from the original is detected by photodetector 174 and subjected to D.C. restoration at 176. A variable attenuator filter 182 is used to compensate when more than one scanning speed is used. In the printing mode a xero graphic system is used. Such system is described in U.S.A. Specification 2,297,691. To produce half tone signals from the continuous video signal 243, Fig. 15, the video signal is added to a triangular wave 242 and the sum compared with a threshold 245, the intercepts of the summation wave with the threshold being turned into a variable pulse width half tone actuator signal 246. Flow diagrams for all of the modes described and the variations thereof are fully given Figs. 18, 19, 20, 21, 22, 24, 25, 28 and 29, all not shown.