Patent GB-1169913-A (GB)
Title: Facsimile System
IPC: H04N1/17
Filed: 1966-09-27
Abstract: 1,169,913. Facsimile transmission systems. XEROX CORP. 27 Sept., 1966 [1 Oct., 1965], No. 43079/66. Heading H4F. In a facsimile transmission system in which non-information bearing lines of the original to be transmitted are scanned at a greater rate than information bearing lines selectively operable means are provided for skipping alternate scan lines on the information bearing lines. This arrangement allows for still further reduction in transmission time and yet provides intelligible, although less pleasing, facsimile copies. The invention described in connection with a transmission system utilizing a normal telephone speech channel to connect transmitting and receiving stations each of which is provided with a transceiver to allow of two-way working as depicted by the arrangement of Fig. 2 (not shown). The transmitting means comprises a photo-electric scanner utilizing an oscillatable mirror. 184 (Fig. 8), also Fig. 4 (not shown), and a photo-electric device 186 for scanning the original which is moved in over a slotted platen 302 by means of linked rollers 177 driven by step motor 178, the original, which is illuminated by fluorescent lamps 181 causing, when in position a switch 306 to produce via inverting amplifier 313 a " transmit control" voltage T whilst if an original is not in position a " receiver control " voltage R is produced via inverting amplifier 323. As the original moves across the platen it operates a further switch 307 which operates to hold the relay K1 controlled by switch 306 in its operated position so that the T voltage continues to be produced until the original has been completely scanned. The T or R voltages which are representative of a logical " 1 " level of zero volts are only produced when a switch 308 is closed, this condition indicating that the transmitting and receiving devices are correctly coupled to the telephone hand-set in box 127 (see also Fig. 1). If switch 308 is open the voltages at the T and R outputs are representative of a logical " 0 " level of minus six volts. Assuming an original comprising a white sheet carrying information in black is in position for scanning and switch 308 closed the operation is as follows. The mirror 184 is oscillated at a high speed by a pre-scan generator 319 which scans from left-to-right and from right-to-left and provided that no information bearing lines (black) are encountered, this being determined by the level of the video signal output of photoelectric device 186 (see also Fig. 9), the original is stepped on at the end of each scan by motor 178 and a signal, termed an advance signal, is transmitted to step-on the pen carriage in a co-operating receiver. If however information is detected the operation of step motor 178 and the transmission of the advance signal are prevented and the pre-scan generator 319 is replaced by a comparatively slow-scan generator 320. This repeats the scanning of the information bearing line, during which video information is transmitted and at the end of such scan motor 178 operates one step, an advance signal is transmitted and the slow-scan generator 320 produces a rapid retrace during which video signals are not transmitted. If information is detected during such retrace (along the next line) a further slow scan cycle is effected whilst if no information is detected an advance is effected and further rapid scans and advances are made until information is again detected. To still further reduce the transmission time the operation of the circuit of Fig. 9 may be modified by closure of switch 421 to allow two consecutive advance pulses to be transmitted and applied to stepping motor 178 between each slow scan. Thus, alternate scan lines on information bearing regions are omitted but, should information occupying only a narrow horizontal region be detected during the pre-scan, the double advance mode of operation is terminated and the operation reverts to the single advance mode. The operation of the gates of Fig. 9 is controlled by the various pulse trains of Fig. 7 (not shown), which are produced by the timing circuit of Fig. 6 (not shown), such that, in addition to video signals from photo-electric device 186, gate 406 also supplies modulator 314 (Fig. 8), which feeds a loudspeaker 309 acoustically coupled to the telephone microphone, with additional signals including signals concerned with synchronization, the advance signals and signals termed pre-video signals which alert the co-operating receiver prior to the actual transmission of video signals. Also, should the transmit control signal T be lost for any reason a relay K4 (Fig. 8) operates to disconnect modulator 314 and to connect an alarm oscillator 315 to the transmission channel. At a co-operating receiver the output of the telephone derived from the leakage flux of the earpiece via coil 311 (Fig. 8) is supplied via demodulator 316 to the receiver and synchronizing circuitry of Fig. 15 and via alarm demodulator 318 to alarm circuitry in the printer position of the receiver (Fig. 12). The drum drive motor 150 of the latter is supplied with drive pulses A from its own timing circuit (Fig. 6) and the synchronizing action of Fig. 15 operates by comparing locally produced signals with received signals and in accordance with the discrepancy resets the counter (203) which, at each transceiver, is controlled by a free-running stable oscillator (201), before it has completed its full count so that it rapidly " catches-up " with the operation at the transmitter. The receiving circuitry (733, Fig. 65) after reception of a pre-video signal, then operates to supply video signals to the marking element 154 (Fig. 12) via amplifier 310 and additionally, via gate 736 (Fig. 15) and amplifier 611 (Fig. 12), supplies signals which withdraw element 154 from the drum during the passage of the clamping bar, which secures the record sheet, thereunder. The signals from alarm oscillator 315 (Fig. 8) which is switched to the transmission channel when the T signals fail to be produced, e.g. due to opening of switches 306, 307 when the original to be scanned is either not in position or has been completely scanned, or due to the opening of switch 308 when the hand set is incorrectly positioned, operate an audible warning device 624 at both transmitting and receiving positions which is also operated by limit switch 613 when the pen carriage 154 reaches the end of its travel along lead screw 159. Device 624 continues to sound until a re-set switch 130 is operated at which the reverse step motor 161 is switched to replace the forward step motor 160 to return the carriage to the start end of its travel. Instead of synchronizing the scan generators 319, 320 (Fig. 8) by the pulse trains H and J (also Figs. 6 and 7, not shown) the scanning mirror may be controlled directly by the outputs of potentiometers driven by the printer mechanism (Fig. 13, not shown). However, the scanning mirror may be replaced by a cathode-ray tube scanner and the printing mechanism described may be replaced by any other suitable facsimile printer.