The present invention relates to the field of television signal processing, and in particular to an apparatus and method for improving the sharpness of a television picture that is formed from at least one television signal.
A conventional television picture is formed from a luminance signal that contains brightness information, and two chrominance signals that contain color information. These signals are referred to hereinafter as picture signals.
The transmission bandwidth of the luminance signal and the chrominance signals is fixed, and the bandwidth of the chrominance signals is narrower than the bandwidth of the luminance signal. This bandwidth limitation reduces the sharpness of the television picture. The color and brightness information is present line-by-line in each picture signal. That is, chronologically successive values for the picture signals contain the brightness or color information of the successive pixels in a line of the television picture. A change in brightness or color between adjacent pixels of a line results from an increase or decrease over time in the amplitude of the given picture signal. The band width limitation in the transmission of the pictures signals limits the steepness of the slopes of the picture signals so that random color or brightness differences between adjacent pixels cannot be displayed. This bandwidth limitation is especially evident in color edges or brightness edges in a picture in which the brightness transition or color transition proceeds continuously, and not abruptly as desired. Due to the narrower bandwidth of the chrominance signals, the transition at the color edges is slower. That is, it extends over a greater number of adjacent pixels than does the transition for brightness edges.
A known approach to enhancing the slopes of the picture signals is to filter the picture signals with a peaking filter. The peaking filter amplifies the amplitude of high-frequency components. This process can be accomplished, for example, by filtering the picture signal with a high-pass filter or a bandpass filter, and subsequently adding the picture signal and filter output signal.
One problem with slope enhancement of the signal slopes by the peaking method is that overshoots and undershoots occur before and after the slopes of the output signal that distort the brightness or color of the pixels before and after the brightness or color edges.
European Patent Application 0 224 302 A1 describes a circuit for increasing the sharpness of color edges, the circuit determining the slope of the chrominance signal and driving a slope enhancement circuit when the specified slope value is exceeded, the circuit generates a chrominance signal with a steepened slope.
European Patent Application 0 457 931 A1 describes a device and method for color edge improvement. Here the turning points in a chrominance signal are determined. When a turning point occurs, one slope in the region of the turning point is replaced by a steeper slope according to the known method.
The reference Ohara, Kunzmann: “Video Processing Technique for Multimedia HDTV with Digital Micro-Mirror Array”, IEEE Transactions on Consumer Electronics, 8–99, Vol. 45, No. 3, pp. 604 et seq. discloses a method for digitally processing a picture signal in which initially a time-discrete picture signal and a signal filter corresponding to the time-discrete picture signal obtained by a digital peaking are determined from the analog picture signal. The following are taken into account with each signal value of the picture signal: a “left” signal value lying a specified number of places before the signal value and a “right” signal value lying a specified number of places after the signal value, and the peaking signal value corresponding to the signal value. If the peaking signal value lies within an interval defined by the left signal value and the right signal value, the peaking signal value is output as the picture signal value. If the peaking signal value lies outside this interval, the left signal value or the right signal value are output as the picture signal value. This method in which the positions of the limits of the interval are fixed for the comparison with the peaking signal value may result in problems, especially for signal curves in which the signal has an oscillating pattern as shown in FIGS. 1A–1C. FIG. 11A is a plot of a time-discrete picture signal s1[k] which was obtained by sampling a continuous signal; FIG. 1B is the curve of an associated peaking signal s2[k]; and FIG. 1C shows a picture signal s3[k] resulting from the known method.
According to this prior art technique, to form a modified picture signal value s3[k0], the associated peaking signal value s2[k0] and a left picture signal value s1[k0−2], and aright signal value s1[k0+2] are compared. The peaking signal value s2[k0] lies in the signal curve in the example outside an interval specified by the left and right picture signal values s1[k0−2] and s1[k0+2] so that the interval limit s1[k0−2] or s1[k0+2] that lies closer to the peaking signal value s2[k0] is output as the modified picture signal value s3[k0]. In the previous picture signal value s1[k0−1] and the following picture signal value s1[k0+1], the associated peaking signal value s2[k0−1] or s2[k0+1] lie within the interval specified by the limits s1[k0−3] and s1[k0+1] or s1[k0−1] and s1[k0+3] so that the peaking signal value s2[k0−1] and s2[k0+1] are selected as the associated picture signal value s3[k0−1] and s3[k0+1]. The selection of the peaking signal values s2[k0−1] and s2[k0+1] as picture signal values s3[k0−1] and s3[k0+1], and the signal value s1[k0−2] or s1[k0−2] as the signal value s3[k0] results in an unwanted “collapse” of the picture signal value s3[k0] relative to adjacent values.
Therefore, there is a need for an apparatus and method for increasing the contrast of a television picture that is formed from at least one picture signal.