Television – Image signal processing circuitry specific to television – Gray scale transformation
Reexamination Certificate
1999-06-18
2001-08-07
Peng, John K. (Department: 2714)
Television
Image signal processing circuitry specific to television
Gray scale transformation
C348S674000, C348S675000, C348S676000, C348S677000, C348S254000, C348S255000, C358S519000, C382S274000
Reexamination Certificate
active
06271891
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present Invention relates to a video signal processing circuit having an inverse gamma correcting circuit for effecting an inverse gamma correction to three primary color input video signals or three primary color digital video signals, respectively.
2. Description of Related Art
Gamma correction is commonly effected to a video signal. When processing such a video signal by a signal processing circuit and displaying the processed signal on a display apparatus, it is general to use an inverse gamma correcting circuit for performing a correction opposite to the gamma correction provided in the signal processing circuit.
FIG. 1
shows the structure of a video signal processing circuit conventionally used for displaying video signals onto a plasma display panel (hereinafter, referred to as PDP). An analog luminance signal Y and first and second analog color difference signals (B-Y) and (R-Y) indicative of the video signals are supplied to a matrix circuit
1
by which analog three primary color video signals of red (R), green (G), and blue (B) are generated. The analog three primary color video signals RGB are supplied to analog/digital converters (hereinafter, referred to as A/D converters)
3
r
,
3
g
, and
3
b
and are converted to digital signals having a predetermined number of bits, respectively. The three primary color digital video signals are supplied to inverse gamma correcting circuits
4
r
,
4
g
, and
4
b
wherein the input signals are processed by an inverse gamma correction operation, thereby substantially linear three primary color video signals Rout, Gout, and Bout are produced. The three primary color video signals are then supplied to the PDP and an image corresponding to the video signals is displayed.
In the conventional structure described above, characteristics of the output signals are as shown in
FIG. 2
with respect to the three primary color digital video signals RGB supplied to the inverse gamma correcting circuits
4
r
,
4
g
, and
4
b
. That is, the output signal characteristics with respect to the input three primary color digital video signals are in the form of characteristics of an exponential function or characteristics which are approximate to them.
In this conventional circuit, since input/output characteristics of the inverse gamma correcting circuits
4
r
,
4
g
, and
4
b
are of exponential function characteristics or characteristics approximate to them as described above, if a maximum dynamic range of the A/D converters
3
r
,
3
g
, and
3
b
to which the input signals of the inverse gamma correcting circuits are supplied is not completely used, the contrast of the image which is displayed on the PDP deteriorates in the manner of an exponential function. More specifically, when each of the A/D converters
3
r
,
3
g
, and
3
b
produces, for example, 8 bit data, the dynamic range will be a range from “0” to “255”. If, however, the maximum signal level of the input video signals to be displayed, which are output from the A/D converter
3
r
,
3
g
, and
3
b
is, for example, at a level “Da” in
FIG. 2
, the dynamic range will be a range from “0” to “Da” which is smaller than 100% of the broadest range. In this way, a reduction of dynamic range occurs. Consequently, it becomes difficult to obtain desired contrast. In such a case, the luminance of the screen will be reduced to render the whole screen dark, so that it also makes difficult to obtain desired brightness characteristics.
OBJECTS AND SUMMARY OF THE INVENTION
The invention has been made in view of the above problems and an object of the invention is to provide a video signal processing circuit which can produce a signal having a dynamic range extended nearly to 100% of its capability, so that a desired contrast can be obtained, and desired brightness characteristics can be obtained by an increase in the maximum luminance of the screen.
In order to solve the above problems, according to a first aspect of the invention, there is provided a video signal processing circuit having inverse gamma correcting circuits for effecting an inverse gamma correction to three primary color input video signals according to three primary color video signals of red, green, and blue carrying video information, comprising: peak level detecting means for detecting a peak level of each of the three primary color video signals; and a level control circuit for controlling a signal level of each of the three primary color input video signals which are supplied to the inverse gamma correcting circuits in accordance with the detected peak levels, wherein the level control circuit performs a control operation in which the level of each of the three primary color input video signals which are supplied to the inverse gamma correcting circuits equals a peak value when the peak levels are equal to or larger than a predetermined value.
According to a second aspect of the invention, in the video signal processing circuit according to the first aspect of the invention, the level control circuit performs a control operation in which, when the peak levels are equal to or less than the predetermined value, the three primary color input video signals which are supplied to the inverse gamma correcting circuits are set to signal levels obtained by multiplying the three primary color video signals of red, green, and blue carrying the video information by a predetermined coefficient of “1” or more.
According to a third aspect of the invention, there is provided a video signal processing circuit comprising: a gain control amplifier which amplifies analog three primary color video signals of red, green, and blue indicative of video information by a predetermined gain and in which the gain is controlled by a gain control signal; analog/digital converter for converting the analog three primary color video signals which are generated from the gain control amplifier into digital signals; inverse gamma correcting circuits for effecting an inverse gamma correction to the three primary color digital video signals; a peak level detecting circuit for detecting a peak level of each of the analog three primary color video signals which are supplied to the analog/digital converters; and a gain control signal generator for generating the gain control signal in accordance with the detected peak levels, wherein the gain control signal generator generates the gain control signal to control the gain control amplifier so that when the peak level is equal to or larger than a predetermined value, a level of each of the three primary color digital video signals which are generated from the analog/digital converters is set to the peak value.
According to the fourth aspect of the invention, in the video signal processing circuit according to the third aspect of the invention, when the peak level is equal to or less than the predetermined value, the gain control signal generator generates the gain control signal so that the gain control amplifier amplifies the analog three primary color video signals by a predetermined gain of “1” or more.
According to the fifth aspect of the invention, there is provided a video signal processing circuit comprising: a matrix circuit to which an analog luminance signal and first and second analog color difference signals indicative of video information are supplied and which generates analog three primary color video signals of red, green, and blue; analog/digital converters for converting the analog three primary color video signals into digital signals; inverse gamma correcting circuits for effecting an inverse gamma correction to the three primary color digital video signals; a peak level detecting circuit for detecting a peak level of each of the analog three primary color video signals; and a level control circuit for controlling a signal level of the analog luminance signal in accordance with the detected peak level, wherein the level control circuit performs a control operation in which, when the peak level is equal to a predetermined value or more,
Kawaguchi Hirofumi
Kikuchi Nozomu
Ogawa Shuji
Takeda Masayuki
Usui Junichi
Morgan & Lewis & Bockius, LLP
Natnael Paulos
Peng John K.
Pioneer Electronic Corporation
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