METHOD AND APPARATUS FOR GENERATING AN ADAPTIVE PEAKING SIGNAL INCREASING THE SHARPNESS OF A VIDEO SIGNAL

Abstract: A conventional peaking signal derived by delaying the incoming video of luminance signal first by one horizontal line delay (12) and then by a second horizontal line delay (14) in modified by multiplication (32) with an adaptive factor. The adaptive factor is derived by first taking the difference (24) between the incoming signal and the twice delayed signal, then deriving the magnitude (26) or the square of this difference and multiplying (28) this by a factor K empirically derived for the particular displayed device. The resultant signal is then subtracted (30) from unity. This yields the factor by which the conventional peaking signal is multiplied (32). The conventional signal multiplied by this factor constitutes the final peaking signal which is added to the 1H delayed luminance signal to yield the desired peaked luminance signal.

Description: The present invention relates to apparatus for generating an adaptive peaking signal, comprising first means for receiving an incoming video signal having an amplitude varying as a function of time, and generating a first peaking signal (D) varying at least in part in dependence upon said amplitude of said incoming video signal at a predetermined first and second time instant. The invention also relates to a method for generating an adaptive peaking signal. Apparatus of the above mentioned kind is known from US patent US-A-3.743.766. In such known television apparatus, peaking signals are added to the video signals to improve the sharpness of either vertical and/or horizontal transitions. Generally, the user sets the sharpness control to achieve a pleasing picture for mid range transitions. The adjustment is generally made so that low to medium level detail areas of a scene, such as leaves on a tree, fabric texture, facial lines, etc., are peaked for pleasing viewing. However, at this setting of the peaking control the areas of the scene having large amplitude video transitions, such as the edges of text characters, are over-peaked. Spot blooming can also occur on large white going overshoots. It is an object of the present invention to eliminate or substantially decrease the over-peaking and spot blooming mentioned above without, however, substantially affecting the detail in the low to medium transition areas of the scene. The invention is based on the realization that the above-mentioned undesired effects are due to the conventional method of generating the peaking signal, i.e. conventionally this signal is dependent on both the rate of change and the amplitude of the video transition. A peaking signal less dependent on transition amplitude would tend to decrease over-peaking and spot blooming. To accomplish this end, the apparatus according to the invention is characterized by second means connected to said first peaking signal generating means for generating a peaking factor signal varying at least partly in accordance with the difference (CA) between said amplitudes at said first and second time instants; and third means for multiplying said first peaking signal and said peaking factor signal, thereby creating said adaptive peaking signal (P) with decreased overpeaking at the large signal transitions. Consequently the conventional (first) peaking signal is modified by multiplying it by an adaptive peaking factor signal. Preferably in deriving the conventional peaking signal, the incoming video signal is delayed by a first time delay and a second time delay. Since both the incoming and the second delayed signal are readily available, the present invention teaches the subtraction of one from the other of the signals. The magnitude of the so-derived difference signal is multiplied by a constant, the product is subtracted from a second constant, preferably unity, and the resultant factor is used to multiply the conventional peaking signal. Alternatively, the square of the difference signal can be substituted for the magnitude of the difference signal. The present invention, as well as other objects and implementations thereof, will be more readily understood upon reference to the following description taken in conjunction with the accompanying drawing. FIGURE 1 is a block diagram illustrating the present invention as interconnected with a conventional vertical peaking circuit; FIGURE 2 is a plot of peaking signal as a function of transition amplitude at various values of K; and FIGURE 3 is a graphic representation of the method of the present invention. The circuit for generating the conventional peaking signal, i.e. signal to be added to the video signal to increase its sharpness is denoted by reference numeral 10 in FIGURE 1 and enclosed in dashed lines. Block 10 generates a peaking signal for vertical peaking. The incoming video signal, e.g. the luminance signal in a colour television receiver, is received at a terminal A. Terminal A is connected through a 1H delay 12 to a terminal B. Terminal B in turn is connected through a 1H, i.e. a one line, delay 14 to a terminal C. Further, terminal A is connected through a divider circuit 16 which divides its amplitude in two, to one terminal of a summing circuit 18. The other terminal of summing circuit 18 is connected to terminal C through a second divider circuit 20. The output of summing circuit 18 is connected to one input of a subtraction circuit 22 whose other input is connected to terminal B. The conventional peaking component signal is furnished at output terminal D of subtraction circuit 22. It should be noted that the conventional circuit and the present invention can be implemented in either analog and digital form. Thus, dividers may be implemented as voltage dividers or operational amplifiers, summing circuits may be operational amplifiers, and the delay circuits may be standard 1H delay lines for an analog implementation. For digital implementations, divider circuits which divide by a half can be shift circuit. The same is true for delay circuits. Addition and subtraction circuits are standard digital circuits well known to anyone skilled in the art. To implement the present invention, terminals A and C are connected to respective input terminals of a subtraction circuit 24. The output of subtraction circuit 24 is connected to a circuit generating the magnitude of this signal, i.e. the amplitude independent of sign. This circuit is denoted by reference numeral 26. The output of block 26 is multiplied by a constant factor K in a multiplier circuit 28. The output of multiplier circuit 28 is subtracted from unity in a subtraction circuit 30. The output of subtraction circuit 30 is applied to one input of a multiplying circuit 32. The other input of multiplying circuit 32 receives the conventional peaking signal. The output of multiplying circuit 32 is the adaptive peaking signal of the present invention. The operation of the above described circuit will now be described with reference also to FIGURE 3. In FIGURE 3, the incoming video signal at terminal A is illustrated in the correspondingly marked line A. Similarly, the signals at terminals B and C are shown on lines B and C of FIGURE 3. It is assumed that the signal received at terminal A is zero or black for the time intervals corresponding to lines 1 and 2. At line 3, a full transition to white occurs. The full white value remains until line 7 where it changes back to full back after expiration of another 1H time interval. In line B, same signal is shown, but delayed by one line interval. Similarly, the signal on line C is the same signal as that on line A, but delayed by two line intervals. The variation of A+C/2 is illustrated in the fourth line. The resulting conventional peaking signal is illustrated in the fifth line of FIGURE 3. It should be noted that this conventional peaking signal is added to the signal at terminal B to form the peaked luminance signal. This is also true of the peaking component signal as modified by the adaptive circuit of the present invention. Returning now to FIGURE 3, it is noted that the addition of the conventional peaking signal as shown in line 5 to the signal of line B would, for a positive-going transition as illustrated in line A, result first in the addition of a component tending to drive the signal of the line B in the opposite direction, i.e. towards a blacker level. In the subsequent H line, the peaking signal reverses and results in an addition to the signal on line B which drives in the same direction as the transition, i.e. the signal on line B is driven to an even whiter level. Since the signal was already at full white, this additional peaking component could cause it to be over-peaked or to bloom. The increase in the spot size resulting from the over-peaking would cause a definite deterioration of picture quality. Similarly, when a transition from white to black takes place in horizontal lines 8-9, the full white luminance signal of line B is augmented by a peaking signal having an amplitude of one half in H line 8, while the black signal in H line 9 is driven further in the black direction. While an improvement in sharpness would result by the increased difference between the luminance signal amplitude in H line 8 and 9, the blooming caused by the substantial over-peaking in line 8 is undesirable. It should be noted that the effect of a fully black (i.e. tube cut-off) luminance signal is not changed by the addition of a peaking signal. The value of the delay luminance signal in H line 9 should thus be thought of as somewhat less than full black. As can be seen with reference to the last two pairs of lines in FIGURE 3, the over-peaking is substantially decreased or may be eliminated by use of the adaptive component of the present invention. The bottom pair of lines in FIGURE 3 indicate the multiplication factor (adaptive peaking factor signal) which is to be applied to the conventional peaking signal present at terminal D of FIGURE 1 and illustrated in the correspondingly marked line in FIGURE 3. The multiplication factor is between 0 and 1, depending on the value of K. For K = 0 the multiplication factor would be 1, i.e. the conventional peaking signal would be added to the signal of line B. If the value of the constant K is taken, for example, to be .5, the peaking signal in line 5 would be multiplied by one half. Signal B would still be decreased prior to a positive-going transition and increased upon occurrence of the transition, thereby increasing the difference in luminance signal amplitude from one H line to the next and thus the vertical sharpness of the picture; but the over-peaking at the large transition would be substantially decreased. For K = 1, no peaking signal would result when |C-A| = 1, i.e. no over-peaking would occur for large luminance signal amplitude changes. It should be noted that the above discussion holds true as well if the magnitude of C-A is replaced by (C-A)². This is indicated by dashed lines in FIGURE 1. The effect of changes in the value of K of different values of C-A is illustrated in FIGURE 2. The solid lines in FIGURE 2 represent the variation of the peaking signal as a function of C-A when the magnitude of C-A is used in deriving the adaptive component. The dashed lines are the same plots when the value of (C-A)² is used. It will be noted that the adapted peaking signal P actually goes to zero for a differential signal representing the maximum change (black to white or white to black), but approaches the normal peaking signal at low- to mid-level changes. The value chosen for K may be chosen empirically and will generally fall somewhere between K = 1 and K = 0.5. It is desirable to provide some peaking for the maximum change of (C-A), but to keep this peaking below that which would cause spot blooming of the particular display device. It is thus seen that with the present invention the problems of over-peaking and spot blooming at high level transitions can be avoided without sacrificing the benefits of increased sharpness at low and mid level transitions.

Claims: Apparatus for generating an adaptive peaking signal, comprising first means (10) for receiving an incoming video signal having an amplitude varying as a function of time, and generating a first peaking signal (D) varying at least in part in dependence upon said amplitude of said incoming video signal at a predetermined first and second time instant, characterized by second means (24, 26, 28, 30) connected to said first peaking signal generating means (10) for generating a peaking factor signal varying at least partly in accordance with the difference (C-A) between said amplitudes at said first and second time instants; and third means (32) for multiplying said first peaking signal and said peaking factor signal, thereby creating said adaptive peaking signal (P) with decreased overpeaking at the large signal transitions. Apparatus as set forth in claim 1, wherein said first means comprises fourth means (12, 14) for delaying said incoming video signal (A) by a first time delay and by a second time delay exceeding said first time delay, thereby generating a first time delayed signal (B) and a second time delayed signal (C); and wherein said peaking factor signal varies in accordance with the difference in amplitude between said incoming video signal (A) and said second time delayed signal (C). Apparatus as set forth in claim 2, wherein said second means comprises means (24) for furnishing a subtraction signal corresponding to the difference in amplitude between said second time delayed signal and said incoming video signal, means for furnishing a predetermined reference value signal (1), and means (30) for subtracting said subtraction signal from said predetermined reference value signal. Apparatus as set forth in claim 3, further comprising means (26) connected to said subtraction signal furnishing means (24) for generating a magnitude signal corresponding to the magnitude of the difference in amplitude between said second time delayed signal and said incoming video signal. Apparatus as set forth in claim 4, further comprising means (28) connected to said magnitude signal generating means (26) for multiplying said magnitude signal by a constant (K). Apparatus as set forth in claim 1, wherein said means for combining said first peaking signal and said peaking factor signal comprises a multiplier circuit (32). Method for generating an adaptive peaking signal, comprising the steps of receiving an incoming video signal (A) having an amplitude varying as a function of time; and generating a first peaking signal in response thereto; characterized by generating a peaking factor signal varying at least in part in accordance with the difference in amplitude of said received video signal at a first and second predetermined time instant; and multiplying said peaking factor signal and said first peaking signal, thereby generating said adaptive peaking signal (P) with decreased over-peaking at the large signal transitions. Method as set forth in claim 7, wherein said step of generating said first peaking signal comprises the step of receiving said incoming video signal (A) and delaying said incoming video signal by a first time delay and a second time delay exceeding said first time delay, thereby generating a first (B) and second (C) time delayed signal and the step of subtracting said incoming video signal and said second time delayed signal from said first time delayed signal, characterized in that said step of generating said peaking factor signal comprises generating a subtraction signal varying in accordance with the difference in amplitude between said second time delayed signal (C) and said incoming video signal (A).

Inventors: HITCHCOCK, JAMES EDWARD, SKINNER, KENNETH ROMAYN
Applicants: NORTH AMERICAN PHILIPS CORPORATION
Application year: 1987
Main classification: H04N5
IPC codes: H04N5