Patent GB-1080226-A (GB)
Title: Improvements relating to binary picture transmission method and system
IPC: H04N7/12
Filed: 1964-08-13
Abstract: 1,080,226. Television. B. LIPPEL. Aug. 13, 1964, No. 33050/64. Heading H4F. The invention relates to a digital television video signal transmission system, which conserves bandwidth or transmission-channel capacity, in which the picture is scanned to provide digital signals corresponding to its brightness, the different digital positions or ranks in the digital numbering system corresponding to different levels of brightness, and the digital signals are transmitted in a sequence of pulses in which digits representing less significant brightness levels occur less frequently than digits representing more significant brightness levels. In Fig. 1 an object 11, shown as a sphere, is in the field of a television camera tube 13 and produces an inverted image 15 therein to be scanned. The intensities of brightness zones defined by the zone outline circles are indicated by numerals 0 to 6 and also marked in binary rotation both natural code (N) and Gray Code (G) separated by commas, the 0 and 1 levels being alike. As the beam 17 scans the image a video signal is generated at the camera output 23 which, after amplification, is digitized in the analogue to digital converter 27. In this embodiment the converter 27 provides a Gray Code representative of the output of amplifier 25 on the parallel output leads 28, each lead providing one binary digit of the Gray Code, the most significant bit being furnished to input unit 43A of scan converter 33A, the next most significant being applied to scan converter 33B, and so on. Deflection means 19 of the camera and the input deflection means 39A, 39B, 39C, 39D, of the four scan converters are connected in parallel to the same scan generator 21, so that the camera beam 17 and the converter input beams 37A, 37B, 37C, 37D, are all at corresponding points of their storage surfaces 35A, 35B, 35C (35D not shown) at the same time. Each of the storage surfaces thus receives a particular bit map A, B, C, D, such that their combination represents the image 15 in the camera by means of the Gray Code. Gray Code is used because it is a unit-distance numbering code characterized by no more than one digit change between successive number values and thus artificial contour lines along equal-brightness contours are avoided inside shaded areas even when some bit maps are unsharp, whereas with natural binary code shading is not reproduced properly unless all pattern boundaries are in exact register and are equally sharp. Fig. 1 shows on the three storage surfaces 35A, 35B, 35C the Gray Code bit maps corresponding to the image 15, the intensity of shading symbolizing different significance of each bit map, equal electrical signals being actually stored. The bit map for scan converter 33D is not shown since it would correspond to a fourth bit, requiring a larger number of circles on the image 16. The deflection and output circuits of the four scan converters are arranged to transmit each separate bit map by use of an appropriate channel, illustrated by a common output, using half the total channel capacity for map A, one-fourth for map B, and one-eighth each for maps C and D. This is accomplished by providing the greatest spatial resolution, e.g. 525 lines, as well as the highest, frame rate for map A; by reducing both horizontal and vertical resolution by a factor of approximately the square root of two, e.g. to 370 lines, for the B, C and D bit maps, and by further halving the frame rates for C and D. The four read-out signals from the four bit maps, as well as a short synchronizing signal which precedes map C, appear one at a time at the combined output, map A being scanned for 20 milliseconds, followed by maps B for 10 milliseconds which is followed by map C on all odd-number cycles and D on even-numbered cycles each for 10 milliseconds. Copies of these bit maps are subsequently stored in the receiver, Fig. 2, not shown, which is substantially the complement of Fig. 1, and there recombined to provide the picture by scanning the four bit maps simultaneously, at least one bit map being reproduced with essentially full spatial resolution, the other bit maps being " unsharp " since they are transmitted with less channel capacity. For the system of this example only about 6-7 megacycles/sec. is required whereas for the prior art PCM system a bandwidth of about 13-3 megacycles/sec. is required. In arrangements where each bit map, except the last, requires half the channel capacity of a previous bit map, a total of only twice the analogue bandwidth is required. Instead of frame-by-frame multiplexing, bit maps may be multiplexed on a lineby-line basis thereby minimizing moving image break-up, with line-interlace of bit maps to reduce stroboscopic effects. Further, bit maps may be scanned in various directions, not necessarily horizontally. To reduce spatial resolutions for successive bit maps it is preferable to reduce the number of scanning lines, e.g. in the ratio of the square-root of two than to successively halve the frame rates. For writing and reading images of less resolution the scanning apertures are increased or spot wobble utilized. Three times as much bandwidth for all bit maps collectively as for the best-definition map alone may be utilized by (a) successively applying a factor of <SP>2</SP>/ 3  in geometric progression for the more significant bit maps, (b) using equal capacities for the two most significant bits, then successively halving the others, or (c) using the same capacity as before for the most significant bit, but equal capacities for successive pairs, halving after each pair of bits instead of each bit. The binary output signals may be recoded before transmission, e.g. into a code which transmits signals indicating only the outlines of bit-map areas. In two further embodiments, Figs. 3 and 5, not shown, the transmitters operate directly upon the natural binary code output from an analogue to digital converter, sample the 2- valued code signals appearing on leads (28A- 28G), and transmit the samples in time-multiplex fashion, that is, the various digit outputs are sampled one at a time but the rate and/or manner of sampling of each bit signal on the converter leads (28A-28G) is different for each lead. In these embodiments storage devices, e.g. scan converters of Fig. 1, are unnecessary. The binary coded signal transmitted is decoded by means of a receiver, Fig. 6, not shown, including a complementary de-sampler synchronized with the transmitter, and a digital to analogue converter. Such a system is analogous to the receivers of prior art PCM systems modified by different sampling rate for each digit, e.g. the first bit map may be assigned weight ¢, the next weight “, the next weight # &c., so that a different matrix of dots for each bit map is produced, Fig. 4, not shown. Random sampling for all but the most significant bit map is carried out in the embodiment of Fig. 5, the average number of pulses per second being controlled to be proportional to the bit weight. Natural binary code is used in the embodiments of Figs. 3 and 5 since all bit maps are scanned simultaneously with the same beam and only the average spacing between accurately positioned dots varies with the bit weight, and thus the problem of registration in shaded picture areas does not occur. Two-channel transmission may be employed for the embodiments of Figs. 3 and 5 using two bits per resolution element with 4 mc./sec. bandwidth.