Patent GB-790180-A (GB)
Title: Improvements in or relating to image transmission systems
IPC: H04N7/14
Filed: 1956-05-04
Abstract: 790,180. Intercommunication systems. WESTERN ELECTRIC CO., Inc. May 4, 1956 [May 5, 1955], No. 13855/56. Class 40 (4). [Also in Group XL (b)] In a combined television and telephone system the bandwidth requirement of the television signals is reduced to that which the telephone channel can handle by transmitting, during the period of n frames, brief samples of the signals occurring during one only of these frames the selected frame being stored and then "read" at the required rate for this purpose and, at the receiver, storing the received samples and then supplying them to the picture reproduced n times in succession during the period of n frames. The general arrangement is illustrated in Fig. 1, which shows only the transmitting apparatus of station A and only the receiving apparatus at station B. At both stations, the scanning of the pick-up tube 4 and the picture reproducing tube 5 is effected under the control of induction generators 16 driven at 20 revs. per second by synchronous motors 12 and supplying a 2,520 c.p.s. signal which after frequency division in 18 and 20 produce a scanning roster having 40 interlaced fields, each of 31¢ lines per second. At the transmitter (station A), the picture signals produced by tube 4 are supplied to a relay 26 which is operated by a <SP>1</SP>/ 2o  see. pulse (derived from generator 30 and divider 32 from the output of divider 20 and supplied to the relay winding via a <SP>1</SP>/ 40  sec. delay device 28) so that during the first revolution of the drum the picture signals due to one complete frame scan are supplied directly to a sampler 38 and, at the same time, are recorded, via a transducer 36, on a magnetically sensitive (or other recordsensitive element) drum 14 which is driven at 20 revs. per sec. by motor 12. At the end of the first drum revolution the pulse from 30 ceases thus releasing relay 26 and for the next 19 revolutions of the drum the recorded signals are supplied, via amplifier 40 and 'the back contact of the relay, to the sampler 38 the complete cycle commencing again <SP>1</SP>/ 40  sec. after '(due to delay device 28) on the occurrence of the next pulse from 30 which also operates (via relay 46) an erase head 42 positioned diametrically opposite to transducer 36 (i.e. positioned <SP>1</SP>/ 4o  sec. in advance of transducer 36). Sampling of the signals is effected by means of pulses of short duration derived from the output (2,520 c.p.s.) of the induction generator 16 via a rotary phase shifter 50 which is driven at 1 c.p.s. by means of a motor 52 so that, depending on the direction in which 50 is driven, the sampling pulses (which are produced by clipping in 54, differentiation in 56 and further clipping in 58 of the output from 50) have a frequency of either 2,519 or 2,521 h.p.s. With this arrangement the phase of the drum rotation " creeps " slowly with respect to that of the sampling pulses applied to sampler 38 and the latter is arranged to hold each sample until the next sampling instant, thus producing a train of samples which are " stretched " in time (in the present instance, assuming a sampling frequency of 2,519 h.p.s., the bandwidth requirement is reduced by a factor of 20 : 1) and the order of which differs from that (the natural order) in which the corresponding signal elements occur on the drum 14. The output of 38 is then modulated (in 60) on to a carrier of 2,400 c.p.s. (the approximate centre of the telephone channel 66) and, at the second station B after demodulation in 601 the samples are supplied to a sampler 381 (controlled by sampling pulses derived in similar manner to the sampling pulses enployed at station A) which " passes on " brief pulses of the samples to a relay 26<SP>1</SP> operated by the output of a multivibrator 33<SP>1</SP> to " change over " once per second. The samples from 38<SP>1</SP> are thus recorded on one of two tracks on drum 14<SP>1</SP> via transducer heads 361 or 371 depending on the position of relay 26<SP>1</SP>, the order in which the samples are supplied to the heads being the same as that in which they are received but, due to the rotation of the drum between the occurrence of the sampling pulses the actual order in which the samples are recorded is the same as that in which they occur on drum 14 at station A (i.e. the natural order). At the end of one complete recording cycle (1 second) relay 26<SP>1</SP> "changes over" and the recorded samples are then supplied to the picture reproducing tube 5<SP>1</SP> repeatedly for the duration of the next cycle whilst the samples being received are recorded on the other track. In this manner a succession of 20 identical frames is displayed on tube 51 every second, the picture content of which changes only once per second. There is thus produced a picture comprising 63 lines each having 40 picture elements (i.e. a total of 2,520 elements) and by restricting this in size, for which purpose the dimensions of the picture are made 1# by <SP>7</SP>/ 8  inch (a miniature tube 51 being employed), a reproduction quality comparable with standard (newspaper) halftone reproduction is obtained. For normal two-way operation the apparatus shown at stations A and B respectively is duplicated at stations B and A respectively, a phase-shifter 70 being employed to bring about the correct phase relationship between the motors 12 and 12<SP>1</SP>. An adjustable eye-guide 7, 7<SP>1</SP> is provided at each station which ensures that when the reproduced image can be seen through the guide by the viewer the latter is centred in the field of view of cameras 4, 4<SP>1</SP> and, additionally allows the viewer to remain outside the field of view of the cameras, if desired, whilst still seeing the reproduced image by viewing the latter from around the guide. Specification 775,832 is referred to.