Noise reduction system for television signals
This invention concerns reducing noise that disturbs a television signal so that the signal may produce pictures that are not annoyingly disturbed by &#34;snow&#34; or other noise effects. 
     Systems for noise reduction in television signals are known in which the television signal is delayed by a full picture interval and then, after being weighted with a correction factor, are added to the undelayed signals. The operation of this known system depends on the fact that differences from one full television picture to the next are averaged out. This averaging produces not only the desired noise reduction, however, but also makes movement fuzzy, by reducing the sharpness of moving edges. It is therefore necessary to reduce the effect of averaging of successive pictures when there is movement in the pictures. Means are also provided to make it possible to distinguish between differences in picture values resulting from noise and those resulting from movement. 
     In this connection it is known from U.S. Pat. No. 4,058,836, to provide on a current basis the difference between the undelayed and the delayed television signal and to furnish the result to a threshold circuit. This type of processing goes on the basis that noise in a television signal is of relatively small amplitude, whereas in the case of movement, greater local changes in luminance are produced, which will pass through the threshold circuit and cause the noise reduction effect to operate only weakly. 
     These systems have the disadvantage that movement in low contrast scenes is not taken account of and are thus subjected, like noise, to contrast reduction. In consequence, moving edges lose sharpness and appear fuzzy or out of focus. 
     In another known system disclosed in DE-OS 29 37 284, processing is based on the fact that small picture details cannot be distinguished well by the eye during movement. A low-pass filter for the movement signal in the horizontal direction and/or the vertical direction suppresses the corresponding noise components in the movement signal. This known system has the disadvantage that with relatively little low-pass filtering noise peaks have an effect on the amount of noise suppression, because they are &#34;mistakenly&#34; treated as originating from picture movement. In this case, the noise reduction is controlled by the noise itself and produces a coarse-grained picture structure disturbance that occasionally distorts the picture more than the original noise. With heavy low-pass filtering, however, detail aspects of movement are suppressed, and a loss of sharpness in the portions of a picture that moves becomes quite visible. 
     THE INVENTION 
     It is an object of the present invention to provide noise reduction on the basis of comparison of successive television pictures in a manner that produces negligible impairment on the sharpness of moving portions of a picture. 
     Briefly, a circuit that continuously determines a variable noise reduction correction factor is controlled by a circuit of variable transmission characteristics equipped with manual or automatic control of the transmission characteristic. The circuit with controllable transmission characteristics is supplied with a signal from a stage that forms the difference, and in fact the absolute value of the difference, between the delayed and undelayed circuit. 
     The method and apparatus of the system has the advantage of providing an optimum fit of the noise reduction effect to the signal-to-noise ratio in the input signal. This provides complete picture sharpness without disturbing effects in the case of average noise content of the input signal. Only in the case of a high content must a loss of detail resolution in picture portions that move be taken as the price of noise reduction. Strong disturbing effects resulting from control of noise reduction by the noise itself are avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further described by way of illustrative examples with reference to the annexed drawings, in which: 
     FIG. 1 is a circuit block diagram of a first embodiment of a system according to the invention; 
     FIG. 2 is a block circuit diagram of a second embodiment of a system according to the invention, and 
     FIG. 3 is a graph showing different values of characteristic curves in the threshold circuits of FIGS. 1 and 2. 
    
    
     BRIEF DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     At the terminal 1, at the upper left of FIG. 1, the television signal is introduced which is to have its noise content reduced. It proceeds directly to the addition circuit 4, where after addition to a quantity to be described presently, a sum output is passed on to a delay unit 5 which delays the television picture by the interval corresponding to a full picture. The symbol &#34;2V&#34; is applied in FIG. 1 to the delay unit 5 to indicate that this interval is twice the period of the vertical deflection signal of the television receiver in the usual 2-field interlaced presentation. Delay units, such as the unit 5, are sufficiently well known that they need not be described further here, except to say that they are advantageously provided by the use of a digital full-picture storage unit with suitably timed write-in and read-out functions. 
     The delayed signal provided at the output of the delay unit 5 is supplied to a subtraction circuit 2 by means of which the undelayed signal is subtracted from the delayed signal. The output of the subtraction circuit 2 is then processed by a multiplication circuit 3 and thereafter, as so processed, furnished to a second input of the addition circuit 4. Thus, the system composed of the circuits 2,3,4 and 5 operate as a so-called recursive filter, which averages and thereby reduces the signal amplitudes which do not repetitively return from full-picture to full-picture. 
     The factor k supplied to the multiplication circuit 3 determines the degree of the averaging which reduces the signals corresponding to features of the picture that do not reappear in successive frames. The more the factor k approaches the value 1, the stronger is the effect of the filter and hence the greater is the time over which the averaging (integration) is performed. The television signal with reduced noise can be taken from the circuit point 6 or the circuit point 7. For motionless pictures, a practically unlimited noise reduction is possible when a sufficiently long integration takes place. 
     With pictures that move, however, the integration does impair the sharpness of edges, so that it is to be reduced in effect whenever motion takes place in the picture. For this purpose, a so-called motion detector 8 is provided. This consists essentially of an absolute difference formation circuit 9, by means of which the difference between the undelayed and the delayed signal is continuously found. Since the noise-reducing effect should be independent of the sign of changes of picture content, the absolute value of the difference is formed in the circuit 9. According to the magnitude of the changes, a correction factor k for the degree of noise reduction is produced in the circuit 10. Since, in addition to movement, noise also produces an output signal in the circuit 9, which however should not be used for control of the integration process, there are interposed between these circuits 9 and 10 both a threshold circuit 11 and a low-pass filter 12. As described in the introduction, the threshold circuit 11 and the low-pass filter 12 are controlled by a control voltage applied at 13 and 14 for changing the transmission characteristic. 
     This control voltage corresponds to the average amplitude of the noise contained in the input signal. In order to measure the latter, a subtraction circuit 15 is provided by which the signal freed from noise is subtracted from the input signal. The difference of the two signals then provides an indication of the noise voltage present in the input signal whenever there is no movement or only negligible movement in the picture. In accordance with this significance, the difference signal is put through a gate circuit 15&#39;, which is controlled by the signal k in such a manner, that the difference signal is provided to a rectifier circuit 16 only if the value K exceeds a certain predetermined value. The latter is the case if no movement or only slight movement is occurring in the television picture. 
     The difference signal that is allowed to proceed out of the gate circuit 15&#39; is then used for the derivation of a control voltage by means of a rectifying and integrating circuit 16, in order to provide this derived voltage as the respective control voltages 13 and 14 of the circuits 11 and 12. The integration time constant of the circuit 16 is so chosen that the control voltage will hold its value for such a time, as on occasion motion takes place in the picture, that in a following movement-free period that may occur, a correction can still be produced. The time constant of the circuit 16 can be chosen quite large, since a change of the signal-to-noise margin of the input signal is rarely to be expected, for example only in the case of switchover between different picture sources, as for example television cameras and film scanners. If now television signals with a stronger noise content are supplied to the system of FIG. 1, the limit frequency of the low-pass filter can be lower, and the threshold of the circuit 10 raised. In this case the desired improvement of the margin of signal over noise is still obtained, but when there is movement this is at the cost of a certain loss of sharpness at the edges. 
     The factor k determines--as already mentioned above--the degree of the integration of the recursive filter and thereby the degree of noise reduction, which follows the formula ##EQU1## Thus for k=0.6 a noise reduction of 6 dB results. The noise amplitude in the input signal is then twice as great as the amplitude of the noise content of the output signal. When movement is present in the picture, the factor k is made smaller by means of the movement detector 8, but when there is no movement this factor k becomes approximately equal to the value corresponding to the degree of desired noise reduction. This can, for example, be k=0.6, if the apparatus is set for a 6 dB noise reduction. 
     Since commonly the noise content in television signals does not change often, it is practical, within the scope of the present invention, to provide also a manual setting of the threshold value and also of the filter cut-off frequency, for which manual controls (not shown in the drawing) are then provided. It is likewise possible for particular applications to provide to advantage for individual adjustments respectively of the threshold value and of the cut-off frequency. 
     FIG. 2 shows a further illustrative embodiment according to the invention in which simply the recursive filter has been provided in form deviating from that illustrated in FIG. 1. The same components are provided with the same reference numerals in these two figures. The difference from the system of FIG. 1 consists essentially in that the delayed television signal is multiplied in the multiplication circuit 18 by the factor k before the addition operation in the addition circuit 17, whereas the undelayed input signal is multiplied by the factor (1-k) in a multiplier circuit 19 and as thus modified is supplied to the addition circuit 17. Furthermore, in the system of FIG. 2, two further delay units 20 and 21 are shown, which equalize signal propagation times which occur in different function blocks of the system of FIG. 2. Such precautions are familiar to those practiced in the art, and in this connection should be regarded merely as an indication that such details should be taken account of in providing a practical embodiment of either FIG. 1 or of FIG. 2. 
     FIG. 3 shows on a graph the characteristic of the threshold circuit 11. The output signal u 2  is plotted as a function of the input signal u 1 . The characteristic line 22 is provided for input signals with small noise content, and the characteristic 23 for input signals with high noise content, thus showing the effect of change of the control voltage applied through the connection 13 from the rectifying and integrating circuit 16. 
     Although the invention has been described with reference to particular illustrative embodiments, variations and modifications are possible within the scope of the inventive concept. Thus, for example, the sequence of the circuits 11 an 12 is not at all fixed and, instead of being as shown in FIGS. 1 and 2, can also be selected differently in providing a practical embodiment of the invention.