Patent GB-1298770-A (GB)
Title: Telephonic transmission of data in graphic form
IPC: H04N1/327
Filed: 1970-02-24
Abstract: 1298770 Facsimile system GRAPHIC SCIENCES Inc 24 Feb 1970 [24 Feb 1969] 8953/70 Heading H4F A fascimile transmission system includes an automatic signalling arrangement in which the unit, Fig. 1, at one end of the line transmits an initiating signal and the other unit, Fig. 2, responds with a reply signal. After a predetermined interval following the end of the reply signal, the transmitter sends a start signal which begins document scanning at the transmitting station and facsimile printing at the receiving station. The start signal is used by both units as a reference for synchronizing their respective operations. During transmission of the document, the operator at the receiving unit can interrupt the process by resuming transmission of the reply signal, the transmitter responding to this signal, by turning off. Similarly, the receiver responds to an interruption in the received signal by turning off, thereby providing the operators at both ends of the line with a means for rapidly signalling each other if the transmission is to end prematurely. The transmitting unit, Fig. 1, includes an oscillator 28 connected (switch 36) to operate at a carrier frequency   f 0 , a multivibrator 6 periodically turning said oscillator on and off when the transmitting unit is turned on. A carrier detecting circuit 68 &c. in each of the units, detecting the transmission of a signal on said carrier frequency #o by said transmitting unit, includes a first filter 68 selectively passing signals at said carrier frequency, a fast carrier-frequency accumulator 72 connected to accumulate the output of said first filter 68 and develop an output corresponding to a relatively short term average value of the output of said first filter, a slow carrier-frequency accumulator 74 connected to accumulate the output of said first filter and develop an output corresponding to a relatively long term average value of the output of said first filter, coincidence circuits 92, 94 providing an output corresponding to the outputs of the carrier frequency accumulators when enabled by an enabling signal (from 80), and a trigger circuit 96 connected to provide an internal start signal when the output of the coincidence circuits increases above a predetermined level. In the receiving unit, Fig. 2, the oscillator 28 is arranged to operate at a second continuous frequency f 1 , and each unit includes a reply frequency detector (circuit 86 &c.) connected to detect the transmission of a signal on said second frequency by said receiving unit, and comprising a second filter 86 selectively passing signals at the reply frequency f 1 , and reply frequency accumulator 58 connected to accumulate the output of the second filter and develop a signal corresponding to the average of the second filter output. In each unit a flip-flop 80 is set by the output of the reply frequency accumulator 58 via a first gate 76 when not inhibited by the output of said slow accumulator 74. In the receiving unit a second gate 78 turns on the oscillator 28 on the coincidence of the output of said slow carrier-frequency accumulator 74 and the reset condition of the flip-flop 80, whereby the receiving unit transmits the reply signal upon detection of the initial carrier frequency signal from the transmitting unit. In the transmitting unit gate 76 inhibits the transmission of signal at the carrier frequency in response to the output of the reply frequency accumulator 58, whereby the outputs of the slow carrier frequency accumulators 74 in both units decrease so as to enable the first gate 76 and thereby set flip-flop maintaining the said transmitting unit oscillator on in the absence of the inhibiting of said oscillator output. In each unit the set condition of flip-flop 80 provides the coincidence circuit (96) enabling signal, whereby in response to the reply signal there is a decrease in the output of said carrier frequency accumulators in both units, thereby inhibiting the transmission of said reply frequency signal by said receiving unit, and in turn providing a decrease in the output of the reply frequency accumulator 58 in both units and enabling the transmission on the carrier frequency by the transmitting unit. Shortly after the resumption of said carrier frequency transmission the fast carrier frequency accumulator 72 in each unit provides an output causing the trigger 96 to emit said internal start signals and bring about synchronous starting and running of the scanning drum motors 20.