Patent GB-1112858-A (GB)
Title: Improvements relating to facsimile transmission systems
IPC: H04N1/32
Filed: 1966-09-07
Abstract: 1,112,858. Facsimile systems. MAGNAVOX CO. 7 Sept., 1966 [20 Sept., 1965; 24 March, 1966], No. 40039/66. Heading H4F. The invention relates to a facsimile system in which transceiver equipments may communicate over a telephone circuit 14 with the aid of acoustic coupling elements 18 operated in conjunction with the standard telephone handsets 16, 16<SP>1</SP> and from one aspect comprises the provision of a readiness monitor circuit 40 which suppresses the supply 24 to the scanning motor 22 and cuts off the transmission amplifier 30 when the document 10 or recording sheet 12 is not correctly positioned in the equipment and the telephone handset 16, 16<SP>1</SP> is not placed on the acoustic coupling element. The detection of the positioning is effected by switches. From a second aspect the invention provides for the transmission of an audio signal from the equipment which is set up to receive to the transmitting equipment to indicate that it is ready to record. The equipment is set to receive or transmit by operation of a switch 42. In the Figure the upper equipment is set to transmit and the lower equipment to receive. Transmission is by frequency modulation in the band 1500-2500 c/s. using a voltage-controlled oscillator 25 driven by the pick-up transducer 20. In operation, the communication is first established by normal telephone conversation. At the receiving station switch 42 is operated appropriately and the record sheet 12 is placed in position. The power supply 24 and amplifier 30 are initially blanked off, but as soon as the telephone handset 16<SP>1</SP> is placed on the coupler 18 the amplifier 30 is unblocked for one second to send an audio signal from the voltage-controlled oscillator 25 to indicate to the transmitting station that the receiver is ready. The power supply remains blocked. At the transmitting station when the operator hears the ready signal he places his handset 16 on the coupler 18 and, providing the document 10 has been positioned correctly, power supply 24 and amplifier 20 are unblocked and signal transmission commences. At the receiving station the readiness monitor also responds to a signal derived from signals incoming over the telephone line. As soon as a normal facsimile signal is received (thereby indicating that the transmitter is transmitting) the power supply 24 is unblanked and recording commences. The recording continues so long as a facsimile signal is received, the readiness monitor blanking off the power supply when the signal ceases. From a third aspect of the invention, on noisy telephone circuits provision may be made to transmit a stop signal of a frequency, e.g. 1100 c/s., which is outside the normal facsimile transmission band and which furnishes the receiver with a positive indication that normal transmission has ceased. The stop signal is produced at the transmitting station by utilizing the switch which detects the document 10 to apply a signal to the voltage-controlled oscillator so as to produce a frequency of 1100 c/s. in the absence of a document. A detailed circuit for the invention is described with difference to Fig. 4 (not shown). The readiness monitor comprises two diode logic circuits, one for the power supply 24 and one for the amplifier 30. The logic circuits receive inputs from the paper switch, the acoustic coupling switch and the send/receive switch. In addition the logic circuit for controlling the power supply receives an input from the detective responsive to the received signals. The power supply receives an input from a standard frequency source, e.g. a tuning fork 26 or crystal-controlled oscillator, and drives the scanning motor through a multivibrator. The logic circuit controls a transistor in the connection between the standard frequency source and the multivibrator. The voltage-controlled oscillator comprises an astable transistor multivibrator whose frequency is controlled by a voltage applied to the bases from the pick-up transducer. The stop signal is produced by blanking the input from the pick-up transducer and changing the potentials on the collectors so that the frequency is changed from the normal range (1500-2500 c/s.) to 1100 c/s.