Patent GB-1488863-A (GB)
Title: Television transmission
IPC: H04N7/12
Filed: 1974-01-23
Abstract: 1488863 Television MARCONI CO Ltd 24 Oct 1974 [23 Jan 1974] 03101/74 Heading H4F The transmission bandwidth of a television system is reduced by dividing the picture into three or more sets of elemental areas and transmitting one set only for each field scan of the camera 14, Fig. 2, each set being distributed over the picture in an apparently random manner as in Fig. 1 (not shown). In the preferred embodiment the camera 14, Fig. 2 scans two interlaced fields each of 200,000 elements at a rate of 50 fields/sec. The picture rate is reduced from 12 Hz to 4 Hz by dividing the picture into 12 sets of 33,333 elements, six sets, in each field, and transmitting one set only during each field scan of 20 ms. duration. Each field is divided into consecutive groups of six elements, one from each set in that field, a different element from each group being sampled in any six consecutive scans of that field ensuring that each element is transmitted once only during one complete picture scan. A clock generator 17 runs synchronously with the camera, its output is connected to an element counter 18 the output of which is at any instant representative of the element being scanned. A sample counter 19 also receives the clock output and is advanced by six after each sample, its output varying from 0 to 400,000 in steps of 6 for each picture scan, i.e. each pair of field scans. The sample counter output is added to the output of a Pseudo Random Number Generator (P.R.N.G.) 21 which comprises a sequence of six numbers 0 to 5 randomly arranged in a cyclic shift register, not shown (e.g. 532014). Each time the counter 19 advances, the sequence is advanced to the next number for adding thereto (e.g. 453201) until each number has been output to the adder 20 when the cycle repeats. The output of the adder 20 is thus representative of the number of the next element to be sampled and is input to comparator 34 together with that of element counter 18. When the comparator inputs are equal the video signal from camera 14 is sampled by a sample/hold circuit 16 then converted to a digital signal by analogue to digital converter 22 prior to transmission via line 23. At the end of each picture scan the counters are reset to zero in response to a signal from the camera over lines 24 and 25. In order to ensure a different element is sampled from each group (i.e. one from each of the six sets represented therein) in any six consecutive scans of the field it belongs to, the total number of elements is a non-integer multiple of the number of elements per group, however, in an alternative embodiment wherein the total number of elements is an integer multiple thereof the P.R.N.G. 21 is advanced by one step in the cycle prior to each picture scan. The received signal is decoded and read into a buffer store 27, Fig. 3. The individual elemental video signals are stored at addresses, corresponding to their location in the original picture, derived from a third counter 29 and a second P.R.N.G. 30 running synchronously with the counter 19, Fig. 2, and P.R.N.G. 21 in the transmitter. The complete picture is displayed line sequentially via line 28, Fig. 3. A random scan reproduction can be achieved by storing the incoming video signals sequentially and using components 29, 30 and 31 as above but controlling the store read phase rather than the store write phase of the receiver. The counter 29 and P.R.N.G. 30 receive via line 32 sync. signals derived from the coded elemental signal to cause each unit to advance one step in the sampling sequence in synchronism with the corresponding transmitter units 19 and 21, Fig. 2. In addition both units 29, Fig. 3, and 30 receive over line 33 a picture start signal to reset them to a common starting position.