Gamma correction circuit approximating non-linear digital conversion

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gamma (.gamma.) correction circuit to be used, for example, with video camera or the like.

2. Related Background Art

Conventionally, a gamma correction curve used by a gamma correction circuit of a video camera is approximated by using a polygonal line which is realized by a diode circuit such as that shown in FIG. 1A. In this circuit, crossover points are determined by voltages E1 to E3, and the slopes of lines are determined by resistors R.sub.0 to R.sub.3. FIG. 1B shows the gamma correction characteristics realized by the circuit shown in FIG. 1A. I.sub.1 represents a current source, and D.sub.1 to D.sub.3 represent diodes.

The advantage is that this circuit is simple. However, it has a problem that unnecessary distortion is imparted to gamma-corrected signals because the frequency characteristics of the input level changes due to junction capacitance when the diode turns off.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a gamma correction circuit solving the above problem.

In order to achieve the above object, the embodiments of the present invention provide a gamma correction circuit for converting N-bit input data into M-bit output data by using a gamma correction curve approximated by a polygonal line having a plurality of straight line sections each having a different slope, said gamma correction circuit comprising: calculation means for executing conversion calculations for each said straight line section; and control means for controlling said calculation means in accordance with a value of said input data so that said calculation means executes said conversion calculations for each said straight line section corresponding to said value.

According to the present invention, an input signal is processed in a digital manner to precisely perform gamma correction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a circuit diagram showing a conventional gamma correction circuit;

FIG. 1B shows a gamma correction curve used by the circuit shown in FIG. 1A;

FIG. 2A shows a gamma correction curve according to the present invention; and

FIG. 2B is a block diagram showing an embodiment of the gamma correction circuit according to the present invention; and

FIG. 3 is a block diagram showing a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described with reference to the accompanying drawings.

The first embodiment of the present invention will now be described. In this embodiment, the number of input bits is eleven, and the number of output bits is eight. Therefore, the input maximum value is 2047 (2.sup.11 -1), and the output maximum value is 255 (2.sup.8 -1). Representing these input and output maximum values as 300% and 120% respectively, the values at 100% are 767 and 213, respectively. The coordinate (767, 213) is therefore used as a knee point (reference point). Next, the section between the origin (0, 0) and the knee point (767, 213) is approximated by a polygonal line having first to fourth straight lines. The slope of the first straight line is set to four times the slope 213/767 of a straight line passing through the origin and the knee point, i.e., 852/767.about.1.1. In order to simplify the circuit, the slope of the first straight line is set to 1. The slopes of the second to fourth straight lines are set to 1/2, 1/4, and 1/8, respectively. The slope of a fifth straight line passing through the knee point (767,213) and the input/output maximum point (2047, 253) is set to 1/32.

Each cross point is set to a coordinate represented by a powers of 2 or by a combination of power of numbers, so as also to simplify the circuit. In this manner, a gamma correction curve is substantially and definitely approximated by a polygonal line such as shown in FIG. 2A. Specifically, the first straight line (1) is from a coordinate (0, 0) to (15, 15), the second straight line (2) is from a coordinate (15, 15) to (207, 111), the third straight line is from a coordinate (207, 111) to (463, 175), the fourth straight line is from a coordinate (463, 175) to the knee point (767, 213), and the fifth straight line is from the knee point (767, 213) to (2047, 253). The relation between input x and output y of each straight line is expressed by the following equations:

1st straight line: y=x (1)

2nd straight line: y=(x-15)/2+15 (2)

3rd straight line: y=(x-207)/4+111 (3)

4th straight line: y=(x-463)/8+175 (4)

5th straight line: y=(x-767)/32+213 (5)

FIG. 2B shows a gamma correction circuit having the gamma correction characteristics described above. In FIG. 2B, character a represents a memory for storing a coordinate of each crossover point in the input/output space (numbers in FIG. 2B correspond to coordinate values), character b represents four subtracters, character c represents four constant-multiple circuits (numbers in FIG. 2B correspond to constants in the equations (1) to (5)), character d represents four adders, characters e1 to e5 represent gate circuits (numbers in FIG. 2B correspond to respective first to fifth straight lines), and character f represents a gate switching circuit for turning on one of the five gate circuits in accordance with an input video signal value.

The operation of the circuit will be described below. If an input video signal level is from 0 to 15, only the gate circuit e1 turns on in response to an output from the gate switching circuit f. In this case, the input video signal itself is outputted. This corresponds to the equation (1). Next, if an input video signal level is from 16 to 207, only the gate circuit e2 turns on in response to an output from the gate switching circuit f. In this case, the input video signal is calculated by the equation (2), using data corresponding to the number 15 in the memory a, one subtracter, one 1/2 constant-multiple circuit, and one adder. The calculated result is outputted from the gate circuit e2. In this case, the input/output relation corresponds to the equation (2). Similarly, if an input video signal is from 208 to 463, from 464 to 767, or from 768 to 2047, the corresponding gate circuit e3, e4, or e5 is turned on to calculate an output value in accordance with the equation (3), (4), or (5). With the above operations, gamma correction using a polygonal line can be executed.

FIG. 3 shows the second embodiment of the present invention. The gamma correction characteristics of this embodiment are the same as the first embodiment. In FIG. 3, reference numeral 1 represents a 2's complement display converter for inverting the most significant bit of the 11-bit input data. Reference numeral 2 represents an adder. Reference numeral 3 represents a table for storing a 2's complement of a coordinate of each crossover point on the input axis (hereinafter called a crossover point input coordinate). Reference numeral 4 represents a constant-multiple circuit, reference numeral 5 represents a table for storing a gain of the constant-multiple circuit 4, and reference numeral 6 represents an adder. Reference numeral 7 represents a table for storing a coordinate of each crossover point shown in FIG. 1A on an output axis (hereinafter called a crossover point output coordinate). Reference numeral 8 represents a 2's complement display converter for inverting the most significant bit of an 8-bit output data. Reference numeral 9 represents a table control circuit for selecting the values in the tables 3, 5, and 7 in accordance with the input data level, and sets the selected values as the addition data of the adders 2 and 6 and as the gain of the constant-multiple circuit 4.

The operation of the second embodiment of the present invention will be described below. An 11-bit input binary data is converted into 2's complement display data by the 2's complement display converter 1. Next, the adder 2 executes a subtraction operation of values within the parentheses of the first left side terms of the equations (1) to (5) for the first to fifth straight lines. The table 3 stores five types of 2's complements of the crossover point input coordinates which are selected in accordance with the input data level and added to the input data (actually they are subtracted from the input data because they are 2's complements). Selecting the five types of crossover point input coordinates is carried out by the table control circuit 9. If an input data level is from 0 to 15, the table control circuit 9 selects 0 from the table 3 and outputs it to the adder 2. If an input data level is from 16 to 207, the table control circuit selects 1037 (2's complement of 15) from the table 3, and outputs it to the adder 2. Similarly, if the input data level is from 208 to 463, from 464 to 767, or from 768 to 2047, the table control circuit 9 correspondingly selects 721 (2's complement of 207), 369 (2's complement of 463), or 897 (2's complement of 767), and outputs it to the adder 2. The data obtained by subtracting the crossover input coordinate from the input data is multiplied by the slopes of the first to fifth straight lines at the constant-multiple circuit 4. The values of the slopes are being stored in the table 5. If the input data level is from 0 to 15, from 16 to 207, from 208 to 463, from 464 to 767, or from 768 to 2047, the table control circuit 9 correspondingly selects 1, 1/2, 1/4, 1/8, or 1/32, and uses it as the gain of the constant-multiple circuit 4. The constant-multiplied data is then supplied to the adder 6 to be added to the crossover output coordinates which are stored in the table 6. If the input data level is from 0 to 15, from 16 to 207, from 208 to 463, from 464 to 767, or from 768 to 2047, the table control circuit 9 correspondingly selects 0, 15, 111, 175, or 213 and outputs it to the adder 6. Next, the 2's complement data converted by the 2's complement display converter 8 is converted into original binary data, and the lower eight bits only are outputted as the gamma corrected 8-bit video signal.

As described above, according to the present embodiments, it is possible to suppress a change in the gamma correction characteristics influenced by the performance of circuit elements, which might otherwise be associated with a conventional gamma correction circuit made of diodes, resistors and the like. Furthermore, setting a knee point to 100% point within the input/output dynamic ranges of 300% and 120%, a gamma correction curve is approximated by a polygonal line including four straight lines between the origin and the knee point whose slopes are about four-, two-, one-, and one second-folds of the slope of a straight line passing through the origin and the knee point, and one straight line after the knee point whose slope is about one eighth-fold, thereby realizing a good gamma correction with a simple circuit structure.

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