Digital Video Black Expander

This invention relates in general to digital television systems and in particular to a black expander for the digital video in such a television system.

In many reproduced televised scencs, the contrast may not be optimum and changing the setting of the receiver "contrast control" setting may not produce any improvement. While the optimum contrast of a particular scene is a matter of subjective judgment, circuits are being constructed to produce scenes of perceived optimum contrast. One prior art television receiver has a circuit for automatically expanding the portion of the video luminance signal that corresponds to grey areas of the picture, to make the picture elements in these areas darker. The resulting improvement in contrast, by expanding the grey areas, is considered to yield a beneficial result. That circuit is an analog implementation and operates by seeking out the blackest portion of the video signal and moving it toward, or fixing it at, the receiver black level. Other video signals between the blackest portion and an arbitrary "breakpoint" are proportionately expanded. Video signals above the breakpoint are not affected.

With the advent of digital television receivers, such analog techniques are not applicable. The present invention is directed to a black expander for a digital television receiver and utilizes moving average filters and comparators to alter the video input/output translation characteristic to enhance or expand signals representative of grey areas to move them toward black level and hence make them appear darker.

A principal object of the invention is to provide an improved digital television video system.

Another object of the invention is to provide a digital video system having enhanced contrast characteristics.

These and other objects and advantages of the invention will be apparent upon reading the following description in conjunction with the drawings, in which:

FIG. 1 is a partial block diagram of a black expander constructed in accordance with the invention; and

FIG. 2 illustrates the video input and output characteristic for the circuit of FIG. 1.

Referring to FIG. 1, a source of digital video (luminance) 10 supplies a digitized Y (or luminance) signal. In accordance with standard digital practice, the Y signal is represented on an eight line bus and is supplied to a moving average filter 12 wherein a fixed number of pixels, during each horizontal line of the input Y signal, are sampled and averaged. The number of pixels may be 30 (out of approximately 900 pixels in a line), although the actual number should not be considered limiting of the invention, because of the subjective nature of the effect, as mentioned above. An input P (programmable control) to moving average filter 12 indicates that the number of sampled pixels is programmable by the user or manufacturer. The output of moving average filter 12 is supplied to the negative input of a comparator 14 and to a latch 16. Latch 16 has an output that is supplied to the positive input of comparator 14 and is, in turn, enabled by the output of comparator 14. As indicated by the connections to the comparator 14, latch 16 is only enabled when the output of moving average filter 12 is more negative than the value that is presently in latch 16. Consequently, latch 16 operates to select the most negative, i.e., the portions of the average signal output produced by moving average filter 12 that are nearest black. Latch 16 is reset to a level corresponding to peak white each field by virtue of the connection of its reset terminal to a source of vertical signal, not shown. The output signal of latch 16 is supplied to another moving average filter 18 which operates on a field basis to develop a moving average of a number of fields of the output of latch 16. This output is called Xav and represents an average of the sampled video level and is supplied to a block 20 where an arbitrary breakpoint value, referred to as Xo, is calculated. The breakpoint, as indicated, is calculated, for example, by taking Xav and adding a constant C to it. Constant C may be negative or positive and, of course, like control input P for filter 12, may be software controlled. The value of Xo should be limited to a certain maximum value. The value of Xav may also be used to obtain C from a look up table, or the like, and the calculation as described should not be considered limiting of the invention.

The output of the breakpoint calculator block 20 is supplied to another comparator 22 and as a negative input to a summation circuit 26. The digital Y signal is supplied to the positive input of comparator 22 where it is compared with the Xo input to develop an output control signal for application to the S (select) input of a multiplexer circuit 28. The digital Y signal is also directly supplied to multiplexer circuit 28 as an A input and to a multiplication circuit 24. Multiplication circuit 24 is indicated as "shift left," which in digital processing results in a multiplication by a factor of two. Its output is therefore indicated as 2Y. The 2Y output is supplied as a positive input to summation circuit 26 where it is algebraically combined with -Xo. The output of summation circuit 26 (2Y-Xo) is applied to multiplexer 28 as a B input.

As indicated by the table shown on multiplexer 28, a high select (S) input results in an output of signal A from multiplexer 28 and a low select input results in an output of signal B. It is thus seen that for signals above the breakpoint Xo, the Y signal is passed by multiplexer 28 without change. For signals below the breakpoint Xo, the output of multiplexer 28 is 10 defined by the equation 2Y-Xo.

Referring to FIG. 2, which displays the translation or transfer characteristic of the black expander of the invention, it will be seen that the straight line 30 defines a slope of unity (1) for processing the Y signal. That is, Y signals above the breakpoint are translated with unity gain. As indicated by the dashed line 32, which has a slope of 2, Y input signals below the breakpoint are translated with a gain of 2. Since the origin 0 defines the black level of the Y signal, signals near black, that is grey, but below the breakpoint Xo, are expanded in the direction of black to make them appear blacker. The selection of the number of samples and the rate of updating the moving average are based upon subjective evaluations to a great extent and no fixed criteria are contemplated. Changes or updates in black expansion Xo calculations should not be made so often that noticeable "jerkiness" in the video display results.

It is recognized that numerous modifications and changes in the described embodiment of the invention will be apparent to those skilled in the art without departing from its true spirit and scope. The invention is to be limited only as defined in the claims.