Video signal processing apparatus and video signal processing method

According to one embodiment, a video signal processing apparatus includes an acquisition unit to obtain the frequency of each luminance level from the input luminance signal worth of one frame, a frequency conversion unit that logarithmically converts the frequency of each luminance level obtained and adds a preset offset value, a preparation unit that prepares a nonlinear correction processing table to cumulatively add the frequency-converted data and provide nonlinear correction processing for the input luminance signal, and a processor that provides nonlinear correction processing to the input luminance signal in accordance with the prepared nonlinear correction processing table.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Applications No. 2005-278060, filed Sep. 26, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND

One embodiment of the invention relates to improvement of a video signal processing apparatus and video signal processing method, in which tone correction processing is provided to luminance signals based on the frequency obtained for each luminance level.

2. Description of the Related Art

As everyone knows, in recent years, flat-panel type large screen displays are developed and are put into practical use for color TV broadcast receivers etc. Now, in this kind of large-screen displays, in order to make displayed video images look clear, it is common practice to carry out tone correction processing for luminance components of video signals.

As this kind of tone correction processing for luminance components, a technique to correct tones in accordance with the frequency distribution of the luminance level of input video signals is known. The basic concept of this technique is to increase the gradient of the tone correction characteristic curve for the luminance level with large frequency and to decrease the gradient of the tone correction characteristic curve for the luminance level with small frequency.

By doing this, a dynamic range of a luminance level region occupying most of the input video signal is enlarged. As a result, the contrast feeling of video images is improved or correction is made to effectively express subtle tone differences.

Now, in the current basic tone correction processing means using the frequency of each luminance level, by cumulatively adding the frequency obtained at each luminance level from the low-order luminance level, luminance input/output conversion parameters, that is, a tone correction characteristic curve is prepared.

However, in this kind of tone correction processing means, in the event that information is locally concentrated to a specific luminance level, the luminance gradient of the concentrated portion becomes excessively steep, and conversely, in the portion with no information, the luminance gradient scarcely exists.

On the other hand, presently, it is considered to handle the frequency obtained at each luminance level by establishing limit values for the upper limit and the lower limit, respectively, but since this is a simple omission padding processing, the effects tend to be reduced for the original information.

In Jpn. Pat. Appln. KOKAI Publication No. 2005-86772, there is disclosed a configuration to automatically set a limit value of correction amount of each copy, prepare tone correction characteristics, and carry out tone correction in accordance with the luminance distribution of video image data read, but the correction amount limiting processing or the way to give the setting is complicated.

DETAILED DESCRIPTION

Various embodiments according to the invention will be described hereinafter with reference to the accompanying drawings. In general, according to one embodiment of the invention, a video signal processing apparatus includes an acquisition unit to obtain the frequency of each luminance level from the input luminance signal worth of one frame, a frequency conversion unit that logarithmically converts the frequency of each luminance level obtained and adds a preset offset value, a preparation unit that prepares a nonlinear correction processing table to cumulatively add the frequency-converted data and provide nonlinear correction processing for the input luminance signal, and a processor that provides nonlinear correction processing to the input luminance signal in accordance with the prepared nonlinear correction processing table.

FIG. 1schematically shows a video signal processing system of a TV broadcast receiver11to be explained in this embodiment.

That is, digital TV broadcast signals received by an antenna12for digital TV broadcast reception is supplied to a channel select demodulation unit14via an input terminal13. This channel select demodulation unit14tunes in on a station of broadcast signals of a desired channel from the inputted digital TV signals, and demodulates and outputs this tuned-in-on signal to a decoder15.

This decoder15generates digital luminance signal Y and color signal Cb/Cr, respectively, by providing decode processing to signals inputted from the channel select demodulation unit14and output them to a selector16.

In addition, analog TV broadcast signals received by an antenna17for analog TV broadcast reception are supplied to a channel select demodulation unit19via an input terminal18. This channel select demodulation unit19tunes in on a station of broadcast signals of a desired channel from the input analog TV signals, and this tuned-in-on signal is demodulated and analog luminance signal Y and color signal Cb/Cr are generated, respectively.

The analog luminance signal Y and color signal Cb/Cr generated at this channel select demodulation unit19are supplied to an A/D (analog/digital) converter20and converted into digital luminance signal Y and color signal Cb/Cr; then, they are outputted to the selector16.

In addition, analog luminance signal Y and color signal Cb/Cr supplied to an external output terminal21for analog video signals are supplied to an A/D converter22and converted into digital luminance signal Y and color signal Cb/Cr; then, they are outputted to the selector16. Furthermore, digital luminance signal Y and color signal Cb/Cr supplied to an external input terminal23for digital video signals are supplied to the selector16directly.

Now, this selector16selects one from digital luminance signals Y and color signals Cb/Cr supplied from the decoder15, the A/D converters20,22and the external input terminals21and23, respectively, and supplies the signal to a video signal processor24.

This video signal processor24, whose detail will be later discussed, generates R (red), G (green), and B (blue) signals by providing specified signal processing to the inputted digital luminance signals Y and color signals Cb/Cr.

The R, G, and B signals generated at this video signal processor24are supplied to a video display unit25and used for video display. By the way, for this video display unit25, a flat panel display comprising, for example, a surface-conduction electron-emitter display, liquid-crystal display, plasma display, etc. is adopted.

Now, this TV broadcast receiver11has a variety of operations including various above-mentioned receiving operations integrally controlled by a control unit26. This control unit26is a microprocessor with a built-in central processing unit (CPU), etc. and receives manipulation information from an operating unit27including not-illustrated remote controller and controls each unit, respectively, so that the manipulation content is reflected.

In such event, the control unit26primarily utilizes read only memory (ROM)28which stores a control program executed by the CPU in the control unit26, random access memory (RAM)29that provides the CPU with a work area, and nonvolatile memory30in which various kinds of setting information, control information, etc. are stored.

FIG. 2shows one example of the video signal processor24. That is, the digital luminance signal Y and color signal Cb/Cr chosen by the selector16are supplied to an interlace progressive (IP) conversion/scaling processor32via input terminals31aand31b.

This IP conversion/scaling processor32provides progressive conversion processing and scaling processing to the inputted luminance signal Y and color signal Cb/Cr in order to display at the video display unit25(a flat panel display comprising surface-conduction electron-emitter display, liquid-crystal display, plasma display, etc.) and outputs the signals to an enhancer processor33.

This enhancer processor33provides enhancer processing to the inputted luminance signal Y and color signal Cb/Cr to make rising edges in the vertical and the horizontal directions steep or to vary sharpness and outputs the signal to a signal correction unit34.

This signal correction unit34provides nonlinear correction processing for tone correction to the inputted luminance signal Y and at the same time provides amplitude control processing to the color signal Cb/Cr associated with the nonlinear correction processing, and outputs the signals to a color space converter35.

This color space converter35converts the inputted luminance signal Y and color signal Cb/Cr into R, G, and B signals and outputs the converted signals to an RGB gamma correction unit36. This RGB gamma correction unit36provides white balance adjustment to the inputted R, G, and B signals and at the same time provides gamma correction processing to the video display unit25, and output the processed signals to a dither processor37.

This dither processor37provides the inputted R, G, and B signals with compression processing to convert high-tone bit expression with bit-number extended to enhance expression into a low-tone bit number corresponding to the video display unit25, and then, outputs the signals to the video display unit25via output terminals38,39, and40.

FIG. 3shows one example of the signal correction unit34. That is, the luminance signal Y outputted from the enhancer processor33is supplied to a luminance nonlinear correction processor42via an input terminal41, and, after undergoing nonlinear correction processing for tone correction, is outputted to the color space converter35via an output terminal43.

Now, the luminance nonlinear correction processor42, whose detail will be later discussed, prepares an LUT (look-up table) for luminance nonlinear correction processing in accordance with the control data supplied from the control unit26via a control terminal44, and carries out nonlinear correction processing to the luminance signal Y based on this LUT.

In addition, the color signal Cb/Cr outputted from the enhancer processor33is supplied to a multiplier46via an input terminal45and provided with amplitude control processing by multiplying the color signal by a color correction signal outputted from a color signal correction unit47. Then, it is outputted to the color space converter35via an output terminal48.

This color signal correction unit47searches for color correction signals which serve as color gains to carry out amplitude control for color signals Cb/Cr on the basis of the level of the luminance signal Y supplied from the color correction processing LUT supplied from the control unit26via the control terminal49to the input terminal41and outputs the signals to the multiplier46.

FIG. 4shows the details of the luminance nonlinear correction processor42. That is, the luminance signal Y supplied to the input terminal41is supplied to the nonlinear correction processor42bafter going through the input terminal42aand at the same time, supplied to a frequency acquisition unit42c. Among these elements, the frequency acquisition unit42cobtains the frequency for every luminance level to the luminance signal worth of 1 frame inputted.

Then, the frequency acquired at this frequency acquisition unit42cis supplied to a frequency conversion processor42d. This frequency conversion processor42d, whose details will be later discussed, provides the frequency of each inputted luminance level with frequency conversion processing on the basis of the control data supplied from the control unit26via control terminals44and42e, and outputs the frequency to an LUT preparation unit42f.

This LUT preparation unit42fprepares an LUT for luminance nonlinear correction processing on the basis of the data after frequency conversion processing outputted from the frequency conversion processor42d, and outputs the LUT to the nonlinear correction processor42b. Then, this nonlinear correction processor42bprovides nonlinear correction processing to the inputted luminance signal Y in accordance with the LUT and outputs the signal to the color space converter35via output terminals42gand43.

FIG. 5shows a flow chart which summarizes a series of nonlinear correction processing operations which the luminance nonlinear correction processor42provides to the luminance signal Y. That is, when processing is started (Block S1), the frequency acquisition unit42cobtains the frequency for each luminance level, respectively, in Block S2.

This frequency can be obtained by dividing a dynamic range of the luminance level into n levels and counting the pixel count that corresponds to each luminance level1through n for the video signal worth of one frame. In such event, the resolution of the luminance levels1through n should be set thoroughly finely. For example, in the event that the input video signal is 8 bits, the resolution of the luminance level when the frequency is obtained should be 8 bits.

FIG. 6shows one example of the frequency for each luminance level obtained from one frame of the video signal of 720 pixels in the horizontal direction and 480 pixels in the vertical direction. In this case, the resolution of the luminance level is set to 8 bits (0-255). That is, the number of pixels which correspond to each of 256 luminance levels from 0 to 255, respectively, are obtained. Consequently, to add all the frequencies at each luminance level, the total becomes same as the number of pixels (720×480=345,600) worth of one frame which the input video signal possesses.

Thereafter, the frequency conversion processor42dexecutes frequency conversion processing to the frequency of each luminance level obtained on the basis of the control data supplied from the control unit26. First of all, the frequency conversion processor42dconverts the frequency of each luminance level to logarithms with 2 used as a base, respectively, at Block S3.

This logarithmic conversion processing with 2 used as a base means, to be concrete, to carry out right bit shift arithmetic processing to the frequency of each luminance level until zero is reached, respectively, and to output the number of divisions carried out until zero is reached as the logarithmically converted frequency. As described above, the frequency shown inFIG. 6is frequency-converted as shown inFIG. 7by providing logarithmic conversion processing with 2 used as a base.

In this case, the bit shift calculation can achieve remarkably high-speed processing, and to think of the case in which the frequency of 8-bit tone obtained from a video signal comprising the total pixel count of 345,600 (720×480) as described above is processed, the number of bit shift calculations is about 3000 times at maximum, and this could be sufficiently executable processing when it is carried out as picture quality correction by software, too.

Thereafter, the frequency conversion processor42dadds a specified offset value to the frequency after the logarithmic conversion processing at Block S4. This offset value is the value (about 10.4) obtained by providing logarithmic conversion processing with 2 used as a base to the value (1350) obtained by dividing the total pixel count (345600) by the luminance tone number (256). In this manner, the logarithmically converted frequency shown inFIG. 7is shifted by as much as the offset value as shown inFIG. 8.

By the way, for this offset value, values pre-operated in accordance with pixel counts of input video signals are stored in the above-described nonvolatile memory30. That is, the offset value (about 10.4) that corresponds to the pixel count (standard picture quality) comprising 720×480, the offset value (about 13) that corresponds to the pixel count (high picture quality) comprising 1920 pixels in the horizontal direction×1080 pixels in the vertical direction (=2073600), etc. are prepared in the nonvolatile memory30in advance. Then, the offset value that corresponds to the pixel count of the input video signal is read by the control unit26and is supplied to the frequency conversion processor42dvia control terminals44,42eas control data.

When frequency conversion processing is carried out in this way, then, the LUT preparation unit42fsuccessively cumulatively adds the data after frequency conversion outputted from the frequency conversion processor42dat Block S5in the direction to increase the luminance tone from the low-order luminance level. In this manner, the cumulative additional value of each luminance level as shown inFIG. 9can be obtained.

Thereafter, the LUT preparation unit42fprepares luminance input/output conversion parameters, that is, an LUT for luminance nonlinear correction processing as shown inFIG. 10, by normalizing the cumulative additional value with the 8-bit tone level at Block S6. Then, the nonlinear correction processor42bprovides nonlinear corrosion processing to the luminance signal Y on the basis of the LUT prepared by the LUT preparation unit42fat Block S7and ends processing (Block S8).

The LUT for luminance nonlinear correction processing shown inFIG. 10has a characteristic to increase the gradient in the vicinity of the intermediate luminance level with a large number of frequency ofFIG. 6, and by carrying out nonlinear correction processing for the luminance signal Y using this characteristic, contrast of video image in the intermediate luminance level portion can be improved.

According to the above-mentioned embodiment, only carrying out frequency conversion processing by remarkably simple arithmetic processing with a small calculation amount, that is, logarithmic conversion processing with 2 used as a base to the frequency of every luminance level and adding a specified offset value to this converted value for one frame of input video signals, it is possible to provide optimum tone correction processing to the input video signals.

In addition, by multiplying the offset value to be added to the logarithmically converted frequency by a specified coefficient, it is possible to make the luminance linear correction characteristics variable. For example, in the event that the value obtained by multiplying coefficients 0.5, 0.25, and 0 by the value (about 10.4) obtained by providing logarithmic conversion processing with 2 used as the base to the value (1350) obtained by dividing the total pixel count (345600) by the luminance tone number (256) is added to the logarithmically converted frequency as an offset value, the LUT for luminance nonlinear correction processing has the characteristics made variable as shown inFIGS. 11,12, and13.

As clear from comparison of the characteristics shown inFIGS. 11,12, and13, the magnitude of correction effects is varied in accordance with coefficients to be multiplied. In this way, by simple arithmetic processing only by multiplying the offset value by a coefficient, it becomes possible to easily vary the strength of luminance nonlinear correction processing, and it can be used, for example, for selection of picture quality mode by user manipulation.

Now, in the embodiment described above, logarithmic conversion processing with 2 used as a base was provided for the frequency of every luminance level obtained from the input video signal, but if there is any allowance in the performance of the arithmetic processing or comprehensive arithmetic processing, it is possible to carry out general logarithmic conversion processing of cases other than that with 2 used as the base, too.

In addition, with respect to the offset value to be added to the logarithmically converted frequency, the total pixel count was divided by the luminance tone number to obtain the divided value by logarithmic conversion, but it is needless to say that the present invention should not be limited to this.

Furthermore, the offset value to be added to the logarithmically converted frequency may not be constant but may be varied in accordance with the luminance level. In addition, when the frequency is obtained, it is possible to include a process to remove the frequency equivalent to the specific luminance level or others, too.