Patent GB-1393406-A (GB)
Title: Redundancy reduction apparatus
IPC: H04N7/36
Filed: 1972-06-27
Abstract: 1393406 Digital television WESTERN ELECTRIC CO Inc 27 June 1972 [1 July 1971] 30041/72 Heading H4F In transmission apparatus for reducing the redundancy of a video signal comprising framed lines of successive elements, digitized element to element differences for an entire frame of picture elements are stored in a frame memory 115, Fig. 1, instead of the individual element amplitudes. The three bit stored difference signals are reconverted to element amplitude by a decoder 110 for passage to one input 0 of a two-way switch 113. The second input 1 of the two-way switch 113 is fed with the eight bit digital values of the real time sampled elements from an analogue to digital encoder 101. Each real time element amplitude from the encoder 101 is compared in a digital subtractor 106 with the corresponding element amplitude from decoder 110 indicating the amplitude of the corresponding element in a previous frame. When a significant frame to frame difference is detected using a threshold detector 107, a switch over signal is produced on line 108 which causes switch 113 to change to its upper 1 position whereby the real time element amplitude is fed to an inter frame coder 114 for conversion to an element to element three bit word and storage in the frame memory 115 as a replacement for the old equivalent element to element value. If a significant frame to frame change is not detected by subtractor 106 the switch 113 stays in its shown position and the element to element values, after decoding at 110, are merely recirculated through the loop 110, 113, 114, 115, 116. As inter frame coder 114 only produces an element to element word when a frame to frame change has been detected, redundancy reduction is achieved and said words are passed to a digital transmitter 119 via the gate 118 enabled via an OR gate 120 via the output of threshold detector 107. The address of the gated through element to element word is given by a generator 121 synchronized by the horizontal and vertical sync signals as produced by sync separator 102. The eight bit address signal is fed via gate 126 also enabled by a signal from OR gate 120. Such that this address need only be the position of the element within a line the start of a line is forcibly transmitted by a detector 127, providing enablement signal for the OR gate 120 and the beginning of a frame is also forcibly transmitted by a start of frame generator 122 likewise producing enablement signal for the OR gate 120. The transmission on line 131 therefore comprises a three bit signal representing element to element differences and an eight bit signal representing the address of such an element to element difference. At the receiver, Fig. 2, the element to element bits and the address bits are separated out by a receiver 200 and respectively fed to a switch 214 and an address comparator 212. The receiver comprises an address generator 207 and a starter frame generator 208 synchronized by use of the received horizontal and vertical sync signals the corresponding generators 121 and 122 at the transmitter whereby an element address signal is produced from an OR gate 211 for comparison in the comparator 212 with the received address signals. Thus when a received address signal is equalized by an address signal from the OR gate 211 the comparator produces a signal causing the two-way switch 214 to its upper or one position. The element to element signal then received is then fed via the gate to a decoder 217 for conversion to an element amplitude and passage via a second two-way switch 215 to an amplitude to element to element difference intra-frame coder 220 whereby the frame of element to element difference signals in frame memory 221 may be updated. The switch 215 enables the output signal to be supplied from the frame memory 221 after decoding into element form when no element to element signal has been received from the transmitter and from the decoder 217 when such an element to element difference signal has been received. The output of switch 215 is therefore   supplied via a digital to analogue converter to one input of a mixer 221 the other input of which is connected to the sync generator 204. The full frame analogue video signal is thereby produced for display.