Patent GB-1402609-A (GB)
Title: Noise reduction system for video signals
IPC: H04N9/78
Filed: 1972-12-19
Abstract: 1402609 Colour television ELLANIN INVESTMENTS Ltd 19 Dec 1972 [20 Dec 1971] 58545/72 Heading H4F To reduce the noise and improve luminancechroma separation and thus improve the quality of video pictures a number of frames of the video signals are compared to determine which portions of the picture are in motion and which are stationary. The moving portions are processed in a conventional manner without any noise reduction while the stationary portions are combined on a frame-by-frame basis to produce a noise-free averaged signal with improved luminance-chroma separation. In Fig. 5, odd fields 1, 5, 3 and 7 (alternating with even fields 2, 6, 4 and 8) are simultaneously supplied by means of three one frame period delays to inputs 22, 24, 26 and 28 respectively. Inputs 22 and 24 are compared 34 and any difference therebetween which would be due to moving portions of the compared frames acts as a control signal for gate 36. Gate 36 gradually changes from a closed-circuit condition to an open-circuit condition as the control signal increases from a level representing 2% difference between inputs 22 and 24 to a level representing 20% difference. The D.C. clamped (42 and 44) A.C. input to amplifier 46 thus comprises an average of inputs 22 and 24 when gate 36 is in the closed-circuit condition (i.e. stationary portions of the image), and the whole signal 24 alone when gate is in the open-circuit condition (i.e. moving portions of the image) with a gradual transition between these extremes. The same operation is effected on signals 26 and 28 so that the outputs of amplifiers 46 and 56 represent a noise reduction of 50% in the stationary portions of the picture, i.e. where the average combination is produced. The output of amplifier 46 is fed to a conventional comb filter 58 (includes a one line period delay) which separates the luminance from the chroma components which pass via D.C. clamping amplifiers and trimming delays to outputs 30 and 32. In addition the output of amplifier 46 is fed via one line delay 74 (to compensate for the comb filter 58) and added to the similarly delayed output of amplifier 56 at junction 78 to produce pure luminance during stationary portions of the picture since the chroma components (NTSC type signal) are 180 degrees out of phase. Similarly an operational amplifier 96 subtracts the outputs of amplifiers 46 and 56 to give pure chroma during stationary portions of the picture. Further the outputs of amplifiers 46 and 56 are fed via a colour phase inverter 100 and via a compensating delay 102 to a motion detector 104 which produces a control signal which switches gates 90 and 98 from their normal closed-circuit condition to their open-circuit condition if there is any difference between the two signals, i.e. during moving portions of the image. When gates 90 and 98 are in their closed-circuit condition the output amplifiers 64 and 70 only see the combined signals from amplifiers 88 and 96. To avoid false difference signals being produced due to luminance transients being effected by the phase inverter 100 and thus erroneous actuation of gates 90 and 98, the control signals from motion detectors 34 and 52 are applied through isolation diodes 106 and 108 to control grid of gate 116 which shorts out the inputs to motion detector 104 whenever they are absent. With this arrangement up to 75% of all noise is removed from the fully stationary areas of the image. Instead of comparing four frames an even better noise reduction may be achieved by comparing eight frames (Fig. 3, not shown). Fig. 6 shows an improved luminance-chroma separator, which takes into account horizontal interfaces in the image, for inclusion between points A, B and C of Fig. 5. The comb filter 130 is a conventional comb filter containing an interface detector circuit (similar to circuit 34 of Fig. 5) connected to detect dissimilarities between the two lines which comb filter adds and subtracts to produce its luminance and chroma outputs. If the interface detector detects a disparity between the lines being compared control signals are produced to unblock gates 132 and 134 so that the luminance and chroma outputs are fed respectively via band-pass filter 152 which filters out all signal components lying in the colour sub-carrier frequency band, and band-pass filter 156 which passes only signal components lying in the colour sub-carrier frequency band.