GB1280152A - Improvements in or relating to information transmission systems - Google Patents

Abstract

1280152 Facsimile apparatus SMITHS INDUSTRIES Ltd 27 June 1969 [29 March 1968] 15151/68 Heading H4F To reduce the bandwidth of binary signals obtained by scanning an information-field e.g. a transparency, the field (preferably square) is divided into square regions e.g. 64 x 64 regions, which are separately raster scanned e.g. by a flying spot scanner, and the resulting binary signals from each region which comprises for example 16Î16 elemented areas are stored in a memory unit having 16 x 16 locations for storing either binary digit "0" or "1". The memory unit is interrogated following each region-scanning cycle by first detecting whether the digit "1" is stored in any of the 256 storage locations (digit "1" corresponds to an item of information e.g. a line on a map). If the digit "1" is not present i.e. there is no item of information in the scanned region (transparency opaque) then a signal "0" is supplied for transmission and the interrogation process for this region is completed. If the digit "1" is present in one or more of the locations, the digit "1" is transmitted and the interrogation process continues with interrogation of the storage locations in groups by dividing the 256 storage locations into four equal groups which in turn are each divided into four sub-groups with four blocks of four storage locations in each. A digit "0" is transmitted if none of the locations in the interrogated group contain the digit "1" and a digit "1" is transmitted otherwise. Only if "1" is transmitted is the interrogation of this group continued by interrogating in a similar manner the sub-groups, and whenever a digit " 1" is transmitted the four blocks of the sub-group are similarly interrogated in turn. If any one of the four locations of each block stores digit "1" then signals representation of the actual digits stored in the four locations are supplied in sequence for transmission. At completion of the interrogation the memory unit is cleared ready to receive the next region scanning signals. Each complete scanning field is preceded by a synchronizing pulse, and at the receiver these are used to generate suitable timebase waveforms for a display CRT, the digital signals being applied to the tube via a digital-toanalogue converter. The display screen of the receiver is photographed on a continuous film strip and when the 64Î64 separately-scanned regions have been displayed the exposed area of the film strip is advanced via a processing unit to an optical projector. If excessive noise is present on the transmission (e.g. telephone) line some of the digital signals from a scanned region may not be detected at the receiver, and to ensure that this does not result in errors in the reconstruction of subsequently scanned regions at the receiver, unique synchronizing signals are transmitted before the digital signals for each region. Each such synchronizing signals comprises thirteen digital "0" signals, twelve being the maximum for any region. If on receipt of a synchronizing signal, the receiver has not completed reconstruction of a region because it has failed to detect at least one of the digital signals, a command may be issued to the transmitter to repeat the information content of that region before proceeding to scan the next region.