GB1144831A - Arrangement for synchronizing a communication link - Google Patents

Abstract

1,144,831. Television and facsimile transmission; data transmission systems. CSF-COMPAGNIE GENERALE DE TELEGRAPHIE SANS FIL. 12 May, 1966 [21 May, 1965], No. 21233/66. Headings H4F and H4P. A ciphering arrangement for transmitting facsimile or television signals comprises means for pulse code modulating the video and synchronizing information and means for ciphering the video information, the synchronizing information being transmitted in clear, i.e. unciphered. Facsimile transmission, Figs. 1 and 3.-A document 2 on a rotating drum 1 is helically scanned by a linearly moving photo-electric cell 4 and the video signals are sampled and quantized in device EC to produce binary digital signals, ciphered in device CHT by adding signals from a key generator C1 (disclosed in Specification 954,313) controlled by a clock HD1, and applied to terminal 2 of a switch C. The line synchronizing signal, produced by contact 6 touching a projection 5 on the drum, starts clock HD1 which causes key generator C1 to generate N successive signals which correspond to the number of points to be transmitted per line, and also triggers a characteristic pulse sequence generator GSC, the output from which is applied to a second terminal 1 of switch C. Switch C is actuated by clock HD1 so that the video and synchronizing signals are transmitted alternately, the video signals only being ciphered. At the receiver, Fig. 4, the characteristic digital pulse sequence is extracted by device ESC and the video signals are deciphered in device DECHT. Extractor ESC produces a reference frequency signal which by means of a phase discriminator DIS controls an oscillator OSC having the frequency of the reference signal. A clock HD2, which synchronizes a key generator C2, the output of which deciphers the video signal, is controlled via a preferential circuit P by the reference signal directly if this signal appears at the end of the clock cycle, or by the signal from oscillator OSC in all other cases. Television transmission and reception, Fig. 7.- The signal to be ciphered (i.e. a binary digital signal) is applied directly to contact 2 of a switch C, via a ciphering device CHT to contact 1 of switch C and to a device SLS which extracts the frame and line synchronizing signals and produces line and frame pulses. For reception device CHT comprises a deciphering device. The ciphering device is connected to a key generator C1 which is controlled by a clock HD1 triggered by line synchronizing signals from output L of device SLS. The frame synchronizing signals at output I of device SLS are connected to a counter CN which counts the line signals and after N lines (one frame) stops clock HD1. On arrival of a frame synchronizing signal the counter CN is reset to zero and clock HD1 starts. The counter and clock are connected to an OR-gate OU whose output controls switch C. Two embodiments of device SLS which extracts the line and frame synchronizing pulses are described with reference to Figs. 8-11, not shown.