Patent GB-1449597-A (GB)
Title: Encoding and decoding information for transmission by facsimile  apparatus
IPC: H04K1/02
Filed: 1960-05-06
Abstract: 1449597 Facsimile apparatus R HELL 6 May 1960 [8 May 1959] 16208/60 Heading H4F A method of coding and decoding information, such as line drawings which are free from half tones, for transmission by facsimile apparatus, the coding or decoding being effected by mixing the image pulses or secret pulses with irregularly produced cypher pulses, comprises generating a time scale the frequency of which is at least equal to the highest image point frequency (about 2000 cycles per second) which occurs during the recording, and quantising the cypher pulses and the image pulses by shortening or lenthening each pulse up to an integral multiple of the timing period disposed next to it. Fig. 1 shows the transmitter which is started by pressing key 1 to generate a pulse which passes through "either" unit 3, mixer unit 5 and quantising device 6 to open gate 7 and allow A.C. of about 2000 cycles/sec to pass from the timing generator 8 to the telegraph line 10. When switch 1 is released differentiating circuit 11 produces a pulse which switches over a monostable switch 12 which changes back again after 10 milliseconds and then produces a pulse in differentiating circuit 14 which changes over switch 15 for a few milliseconds. The pulse from switch 15 opens gate 7, via units 3, 5 and 6, and allows an after signal to pass down the line 10. When switch 15 reverts, a pulse produced by differentiating circuit 18 switches on the stable switch 19 which opens gate 20 and allows timing pulses from unit 8, differentiated in unit 21, to pass into the cypher pulse device (shown below the dotted line). The first pulse fed to the cypher pulse device changes over switch 24 and starts multivibrator 26 which operates at a much higher frequency than timing generator 8, for example 50 kcs/sec. The first pulse also changes over switch 27 thus closing gates 30, 31 and 33 and opening gates 34 and 35. The first pulse from multi-vibrator 26 changes over switch 37 which, through differentiator 39 and the open gate 34 brings switch 41 to the rest position if it is not already there. After a short delay gate 43 is opened by switch 37 to allow the second and all subsequent pulses from the multivibrator to pass into the cypher pulse generator 44. The cypher pulse generator 44 may be as described in German Specification 1,012,635 and produces pulses irregularly, the first pulse within each timing period being used to produce an output which is either positive or zero. The output is fed alternately to bistable switches 41, 48 through gates 35, 31 which are opened and closed alternately by switch 27. No corresponding output passes from the switch 41 or 48 during the current timing period and the corresponding gate 33 or 29 is closed, but during the next period the gate is opened by switch 27 and the output passes to the mixing circuit 5. When a cypher pulse is produced by unit 44, an output pulse is sent through line 44 to reset switches 24 and 37 to their rest positions. After cypher pulse production has started the transmission of the facsimile signal may, but need not, be started. Facsimile signals from unit 52 are rectified in unit 54 and fed to the mixer 5 which produces an output if the current signal and the current cypher pulse are opposite but not if they are the same. The output of the mixer is fed to the "quantising" unit 6 which ensures that changes occur only at the same point in each time period; thus the signal pulses are shortened or lengthened to make them an integral multiple of the time period. At the receiver, Fig. 2 (not shown), the initial signal caused by key 1 of the transmitter starts a timing generator and synchronises it with that of the transmitter; the after signal sent by the transmitter 10 milliseconds later starts a cypher pulse device exactly the same as that in the transmitter so that a similar cypher pulse sequence is produced. The cypher pulses'are mixed with the rectified received signals in a mixer of the same type as mixer 5, and the output of the mixer controls a gate allowing signals from the -timing generator to pass to the facsimile receiver.