Patent GB-1025967-A (GB)
Title: Improvements in and relating to image scanning systems
IPC: H04N11/02
Filed: 1963-08-15
Abstract: 1,025,967. Television. PHILCO CORPORATION. Aug. 15, 1963 [Aug. 15, 1962], No. 32288/63. Heading H4F. [Also in Division G3] In a television bandwidth reduction system the image is scanned at a normal speed until an area is encountered containing image detail (i.e. brightness changes), the scanning means is then caused to retrace and rescan the area at a lower speed and more finely. The principle of the invention is illustrated in Fig. 2 where the image is regarded as divided by vertical and horizontal index lines 34, 38 into blocks each comprised of nine elemental picture areas. The scanning element 62 defines three areas R, G and B corresponding to elemental picture areas and is scanned in a conventional raster formed by horizontal movements constrained within successive pairs of horizontal index lines. Where no picture detail occurs within a block, scanning proceeds at a normal speed and only the signal due to area R of the scanning element is transmitted, the signals due to areas G and B being redundant since they are identical with the signal from area A. Where, however, detail occurs within a block, the scanning element is caused to retrace and rescan the block at a speed of one-third of the normal speed. The scan is repeated three times during which the areas R, G and B are brought into use successively so that each of the nine elemental areas forming the block is analysed and a corresponding signal transmitted. Fig. 6 illustrates a typical sequence of operations where A shows the image detail of a line of blocks and B shows the transmitted signal. Only one video impulse, 306, 308, 310 &c. is transmitted for each of blocks 306<SP>1</SP>, 308<SP>1</SP>, 3101, 316<SP>1</SP> and 320<SP>1</SP> since no detail occurs in these blocks, whereas nine signals 312a- 312i, 314a-314i and 318a-318i are transmitted for each of blocks 312<SP>1</SP>, 314<SP>1</SP> and 318<SP>1</SP> which contain detail. 302 is a line synchronizing signal, 312, 314 and 318 are " block " signals which are used to signal to a receiver the reduction in speed and repetition of scan which takes place in a detailed block. Fig. 1 shows details of transmitting apparatus. The scanning element is provided by a C.R.T. flying-spot scanner 24 which may be a multibeam tube providing red, green and blue coloured light in areas R, G and B arranged as shown in Fig. 2, or a single beam tube the light from which is coloured appropriately over the three areas by the use of prism 26. The video signals are derived by three photo-electric cells 50, 52, 54 provided with red, green and blue optical filters 56, 58, 60. According to an alternative arrangement the three parts of the light beam are pulsed at different frequencies and the photo-electric cells operate in conjunction with electric filters. The video signals pass through a quantizer 66 and delay circuits 72-74 to gates 82-84. During normal scanning only gate 82 is transmissive, whereas during the repeat scanning of a detailed image block, the three gates are operated in succession. The scanning is controlled by a main clock source 130, the horizontal time base comprising a counter 134 and ladder network 136, and the vertical time base comprising a counter 166 and ladder network 168. Counter 134 is driven from the clock directly, whilst counter 166 is driven by horizontal reset pulses obtained from counter 134 via matrix 158. Vertical reset pulses are also obtained from counter 166 by matrix 172, and the horizontal and vertical reset pulses are used to control circuits 93 and 94 to introduce scanning synchronizing pulses in the transmitted signal. By means of partially reflecting mirrors 28 and 30 the flying-spot scanner is also caused to scan masks 32 and 36 which, by means of appropriately positioned apertures, define respectively the vertical and horizontal index lines 34, 38. Two photo-electric cells 42, 42, associated respectively with red and blue filters 46, 48 serve to detect whether the scanning light spot is correctly located between a pair of lines 38. The cells feed a subtractor circuit 173 and any departure up or down results in the application of correcting signal through filter 174 to an adder included in the connection from the vertical time-base 166, 168 to the scanning yoke 154. A single photoelectric cell 40 located behind mask 32 serves to detect whether the light spot occupies the correct horizontal position. The cell triggers a time-base 184, 186 similar to the horizontal time-base 134, 136 and the two time-base signals are compared in a subtractor 187 to derive a connection signal which is added to the horizontal time-base in adder 150. The presence of detail in an image block is detected by a first circuit 78 which responds to high-frequency signal components which occur when there is detail in the horizontal direction, and a second circuit 76 which compares the signals derived from adjacent areas R, G and B to detect detail in the vertical direction. The circuits feed on " OR " circuit 214 which is triggered by a pulse from clock 130 and functions (1) to stop the main scanning operation by operating gates 132 and 182; (2) to trigger circuit 92 to add a "block" synchronizing pulse to the transmitted signal (see, for example, pulse 312 in Fig. 6); and (3) to trigger a flip-flop circuit 228 which resets (via elements 264, 266) a sevenpoint counter 238 to count state a. This counter controls the retrace and rescan of the image block which is effected by a time-base formed of a reversible three-stage counter 250 and ladder network 270. The counter 250 is driven in the forward direction by clock source 246, which operates at the same frequency as clock 130 and thus results in a scan speed equal to one-third normal and is driven in the reverse direction (i.e. for the retrace or flyback) by a fast clock 260. The counter drives are completed via gates 244 and 258 which are opened respectively by outputs b, d, f and a, c, e of counter 238, by way of " OR " circuits 242, 262. Counter 238 is itself controlled to step by the signal from photo-electric cell 40 which detects when the scanning spot reaches the vertical lines which define the edges of the detailed block and applies a signal to the counter via elements 230, 232, 236. In response to the detection of detail by circuits 76 and 78 the scanning spot is therefore caused to retrace the block containing the detail and then to rescan it three times at one-third normal speed. During the three scans, gates 82, 83, 84 are opened in succession by outputs b, d and f of counter 238 so that the signals from areas R, G and B are transmitted in sequence. At the completion of the third scan counter 238 assumes state g, the output from which resets flip-flop 228 and causes normal scanning to be resumed. In the video signal channels delay networks 72, 73, 74 introduce delays equal to the normal scan time for one block. The signals for transmission are held in a store 88 and released by a pulse from photo-electric cell 40 applied via delay circuit 176. A receiver for the system, Fig. 9, uses a display 360 (which may be photographed or have storage characteristics) in the form of a three-beam C.R. tube which scans over three aligned areas (as at R, G and B in Fig. 2) but does not reproduce in different colours. The received signals at 350 are applied to control the beams through gates 335, 336 and 337 which, for normal scanning, are all made transmissive so that the one received signal for a non-detailed image block is reproduced in each of the three areas. When however a " block " signal is received an output at 368 controls a gate generator 370 whereby the gates are made transmissive in sequence. The "block" signal is blanked from the video signal channel by a blanking amplifier 354 controlled by the " block " signal via pulse shaper 374. The scanning is controlled by circuits 364 (which may be similar to these shown in Fig. 1) which are controlled by horizontal and vertical synchronizing pulses applied from 366, "block" signals from 368 and clock signals from 376. The video channel includes a quantizer 352 driven by clock 376. The operation of a modified system, which permits a greater reduction in bandwidth, is illustrated in Figs. 9 and 10. For non-detailed blocks 306<SP>1</SP>, 308<SP>1</SP>, 310<SP>1</SP>, 320<SP>1</SP> the operation proceeds as described above in connection with Fig. 6, but for detailed blocks the operation distinguishes between those having detail in only one direction, i.e. 312<SP>1</SP> and 314<SP>1</SP> and those having detail in two directions, i.e. 318<SP>1</SP>. In the latter case the operation proceeds as described for Fig. 6 with the exception of the use of a three-unit "block" pulse 388 to denote the operation. For block 312<SP>1</SP> having detail in only the horizontal direction a oneunit " block " pulse 380 is sent and only a single scan at one third normal speed is made to produce signals 312a, 312b and 312c. This provides a complete analysis of the block detail which may be reproduced at the receiver (Fig. 9) with all of gates 335, 336 and 337 transmissive. For block 314<SP>1</SP> having detail in only the vertical discretion a two-unit " block " signal 384 is sent and three scans of the block are made at normal speed to produce signals 386a, 386b and 386c. Alternatively, it is sufficient merely to stop the main scanning and to open gates 82, 83 and 84 (Fig. 1) in sequence.