Patent GB-1194870-A (GB)
Title: Facsimile System
IPC: H04N1/36
Filed: 1967-09-28
Abstract: 1,194,870. Facsimile transmission system. XEROX CORP. 28 Sept., 1967 [30 Sept., 1966], No. 44134/67. Heading H4F. In an asynchronous facsimile transmission system i.e. one in which analogue video signals are transmitted, synchronism is effected by time division multiplexing a digital sync. word on the same channel as the video signals. The facsimile receiver comprises a 250-kilocycle crystal-controlled clock oscillator 10, Fig. 2, the output of which is fed through a squaring amplifier 12 which provides pulses of - 12 volts for logical #0 " and zero volts for logical #1". These pulses are applied as shifting pulses to a five-stage shift register 14 and are counted in a ten-stage counter 18. The " 0"  outputs of the sixth and tenth stage are connected to a logic circuit 20 which derives a window signal extending from count 0 to count 31 since the sixth to tenth binary digits of numbers 0 to 31 are all zero. The received facsimile input signal passes through an amplifier 24 which converts it to the same 0, - 12 volt levels. The signal from amplifier 24 is passed through shift register 14 which is connected to a decoding diode matrix 16 which recognizes a particular five-bit digital word generated by the transmitter as a sync. signal. When this word is fully loaded into register 14, matrix 16 generates an output pulse which presets counter 18 to the proper count corresponding to the receipt of the sync. word. For example, if the transmitter periodically transmits a 32-bit digital sequence of which the sync. word constitutes bits 18 to 22, detection of the sync. word sets counter 18 to a count of 22. At the next reception of the sync. word, the window signal from logic 20 gates the entire 32 bit digital signal and the sync. word is detected at a count of 22 so that the signal generated by matrix 16 does not change the state of counter 18. Thus an approximate synchronization is achieved between the transmitter and receiver, but to ensure that the receiver clock is in phase with the transmitter clock oscillator 10 includes a control element such as a voltage variable capacitor to permit the oscillation frequency to be adjusted over a small range in response to a control signal from oscillator control circuit 22 (Fig. 3, not shown). The non- digital signals are applied through a gate 26, controlled by the window signal from logic 20 to a facsimile printer 28. Gate 26 prevents the digital sync. signal from being recorded and a sweep generator 30 for the recorder is also actuated by the window signal. An error detection circuit 32 (Fig. 4, not shown) connected to a signal light 34 operates to produce an error signal after four consecutive failures to detect the sync. word, and this signal is used to alert the operators of the facsimile receiver and transmitter. By having linked transmitters and receivers at two locations it is possible for a receiver to send control signals back to its associated transmitter only using the video transmission circuits between the transmitters and receivers and the local links (Fig. 5, not shown). A receiver which includes an arrangement to prevent false detection of a sync. word is described with reference to Fig. 7 (not shown).