JPH0584981B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an imaging signal compression circuit used in imaging devices such as television cameras and video cameras.

[Summary of the invention]

In the present invention, an input terminal to which an imaging signal is supplied is connected to an output terminal via a first resistor, a series circuit of a diode and a second resistor is connected to the output terminal, and the second resistor In a compression circuit for an image signal in which a compression start level is determined by a reference voltage supplied to the image signal, the compression circuit is improved by providing a level limiter for the image signal in a control system that controls the reference voltage according to the level of the image signal. This improves compression characteristics.

[Conventional technology]

Conventionally, imaging devices use a compression circuit 31 called a knee circuit shown in FIG. 3, for example, in order to compress and reproduce signals with a wide dynamic range within a certain level. There is. The compression circuit 31 has a resistor R1 arranged between an input terminal 32 and an output terminal 33, and a series circuit of a diode D1 and a resistor R2 is connected to the output terminal 33 to a power source E. When the input signal supplied to the input terminal 32 is below the voltage of the power source E, the compression circuit 31 has linear input/output characteristics and no signal compression is performed. In a range higher than the voltage, the diode D1 conducts and the output voltage is compressed according to the characteristics determined by the resistors R1 and R2. That is, in the compression circuit 31, an input signal having a level higher than the voltage of the power supply E is compressed. In this case, the voltage of the power supply E becomes the compression start level of the input image signal.

By the way, if an excessive input signal is supplied to this compression circuit 31, the output signal will be compressed, but since this voltage will become high, other circuits will
For example, it is clipped by a limiter. When the image is clipped in this way, a phenomenon called whitewashing occurs on the screen.

In order to solve this problem, the applicant of the present invention has previously proposed a compression circuit for image pickup signals that controls the above-mentioned compression start level in accordance with the level of the input signal (Japanese Patent Application No. 167604/1982).

[Problem that the invention seeks to solve]

When controlling the above-mentioned compression start level according to the level of the input signal, the voltage of the above-mentioned compression start level is controlled to be high when the input level is low, and the voltage of the above-mentioned compression start level is controlled to be low when the input level is high. do. Therefore, when an excessively large input signal is supplied, the voltage at the compression start level is lowered, so that the output signal of the compression circuit is not clipped, and the phenomenon of so-called whitewashing does not occur.

However, if the voltage at the compression start level falls too much below a predetermined voltage, the image will be adversely affected. For example, in the case of a color television camera, the reproducibility of desired colors such as skin tones deteriorates.

In view of this, the present invention provides an image signal compression circuit that prevents the voltage at the compression start level from dropping too much when an excessive input occurs, and that can obtain a wide dynamic range without adversely affecting the image. be.

[Means for solving problems]

In order to achieve the above object, in the present invention, an input terminal to which an imaging signal is supplied is connected to an output terminal via a first resistor, and the output terminal is connected to a series circuit of a diode and a second resistor. The second
In an imaging signal compression circuit in which a compression start level is determined by a reference voltage supplied to a resistor, the imaging signal level detecting means and the imaging signal level signal obtained by the level detecting means are set to a predetermined level or higher. An imaging signal compression circuit includes a limiter that limits the reference voltage, and a control circuit that controls the reference voltage according to the level signal that passes through the limiter.

[Effect]

In the image signal compression circuit according to the present invention, when an excessive image signal is supplied to the compression circuit, the limiter is activated and the reference voltage is suppressed to a predetermined lower limit value.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram of the compression circuit of this embodiment. This embodiment is a case in which the present invention is applied to, for example, a color video camera.

In the figure, red, green, and blue color signals are supplied to input terminals 1R, 1G, and 1B, respectively. In addition, these input terminals 1R, 1G, and 1B are each connected to a resistor R.
Output terminals 2R, 2 through 1, R2, and R3, respectively.
Connected to G and 2B. A series circuit of a diode D1 and a variable resistor VR1 is connected to the output terminal 2R, and a reference voltage PR for the red color signal is supplied from the control circuit 1 to the variable resistor VR1. In addition, a diode D2 is connected to the output terminal 2G.
A series circuit of a variable resistor VR2 is connected to the variable resistor VR2, and a reference voltage for the green color signal is connected to the variable resistor VR2.
PG is supplied from the control circuit 1, a series circuit of a diode D3 and a variable resistor VR3 is connected to the output terminal 2B, and a reference voltage PB for the blue color signal is connected to the variable resistor VR3.
Supplied by

Next, the control circuit 1 that controls these reference voltages PR, PG, and PB will be explained. The control circuit 1
is a NAM that detects the peak signal from each color signal supplied from the input terminals 1R, 1G, and 1B.
(Non Adder Mixer) circuit 2, a limiter 3 that clips the output signal of the NAM circuit 2 above a certain value, a peak rectifier circuit 4 that rectifies the output signal of the NAM circuit 2 and outputs it as a level signal, and the peak rectifier circuit 4 a DC amplifier 5 for amplifying the level signal output from the switch 6, the reference voltage PR,
It is composed of DC amplifiers 7R, 7G, 7B, etc., which supply PB and PG to the variable resistors VR1, VR2, and VR3, respectively. The switch 6 is a switch for changing between a mode in which the reference voltages PR, PB, and PG are manually adjusted and a mode in which they are automatically adjusted. Further, a switch 8 is provided on the input side of the DC amplifiers 7R, 7G, 7B, and when adjusting the white balance, the DC amplifiers 7R, 7G, 7B
By grounding the input signal line of
Stop automatic adjustment of PR, PB, and PG.

This control circuit 1 lowers the reference voltages PR, PG, PB, which are signal compression start levels, when the output signal level of the NAM circuit 2 is high, and lowers the reference voltages PR, PG, and PB when the output signal level is low. ，
Operates to increase PG and PB. Further, the voltage of the compression start level of each color signal is determined by the voltage of the compression start level of each color signal.
R, 7G, 7B variable resistance VR10, VR
9. By adjusting VR8, it can be set to a desired value.

Next, the characteristics of this compression circuit will be explained with reference to the diagram shown in FIG. In the figure, the slope of the straight line l is determined by, for example, the ratio between the resistor R1 and the variable resistor VR1, and indicates the signal compression ratio. This straight line l and the straight line a of the waveform of each incident light amount,
The level of the color signal corresponding to the amount of incident light at the intersections Pa and Pb with b, etc. is adjusted so that the level of the color signal corresponds to the compression picture tube. For this adjustment, use the variable resistor of NAM circuit 2 above.
VR4, variable resistance VR6 of the DC amplifier 5, variable resistance of each of the DC amplifiers 7R, 7G, 7B
VR10, VR9, VR8, etc. are used.

When the amount of incident light changes like the above straight line a,
Compression begins at point Pa. Further, when the amount of incident light changes as shown in the above straight line b which shows a larger change than the above straight line a, the compression start point Pb is located below the above compression start point Pa on the above straight line l. This is because the level signal output from the peak rectifier circuit 4 increases, and the reference voltage PR,
This is due to the decline in PG and PB.

As shown by straight line c, if the amount of incident light changes even more excessively, the output of the NAM circuit 2 is clipped by the limiter 3, and the level signal becomes a constant value. The above reference voltage at this time
PR, PG, and PB correspond to the straight line m. Therefore, the starting point of the pressure line will not fall below the above straight line m, and the point
Pc is the starting point for compression.

The straight line m is the variable resistance VR of the limiter 3.
5 or by changing the gain of the DC amplifier 5 using a variable resistor VR6. That is, the lower limit of the compression start point can be adjusted.

In this way, in this compression circuit, the lower limit of the compression start point can be set, so that when an excessive image signal is input, for example, the point Pc-
The reproducibility of the color corresponding to the portion Pc' is not impaired.

In addition, in manual mode, the above variable resistor
By adjusting VR7, the voltage at the compression start point, that is, the compression start level can be adjusted.

Further, when performing automatic white balance adjustment in a color video camera using the compression circuit, the switch 8 is controlled and the automatic adjustment mode is stopped. The white balance is then adjusted while the white subject is displayed on the screen. After this adjustment is completed, the switch 8 is controlled to enter the automatic adjustment mode. At this time, even if an excessive image signal is supplied to the compression circuit, the compression start level is suppressed to a constant value by the limiter. The imaging signal will not be extremely compressed and the image will not disappear.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to realize a compression circuit that can obtain a wide dynamic range without impairing desired color reproducibility even for excessively large imaging signals.

Further, as shown in the embodiment, by making it possible to adjust the reference voltages of each color signal, it is possible to match the compression start levels for each color signal.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of an imaging signal compression circuit according to the present invention, and FIG. 2 is a diagram for explaining the characteristics of the compression circuit. FIG. 3 is a circuit diagram showing a general compression circuit. 1... Control circuit, 1R, 1G, 1B... Input terminal, 2R, 2G, 2B... Output terminal, D1, D
2, D3...Diode, R1, R2, R3...
Resistor, VR1, VR2,..., VR10...Variable resistor, 2...NAM circuit, 3...Limiter, 4...
Peak rectifier circuit, 5, 7R, 7G, 7B...DC amplifier, 6, 8...Switch.

Claims (1)

[Claims] 1 An input terminal to which an imaging signal is supplied is connected to an output terminal via a first resistor, a series circuit of a diode and a second resistor is connected to the output terminal, and the input terminal is supplied to the second resistor. A compression circuit for an imaging signal in which a compression start level is determined by a reference voltage, comprising: a level detection means for the imaging signal; and a limiter for limiting the level signal of the imaging signal obtained by the level detection means from exceeding a predetermined level. and a control circuit that controls the reference voltage according to the level signal passed through the limiter, and the limiter determines a lower limit value of the reference voltage, and when the level of the imaging signal is high, the reference voltage is adjusted. An imaging signal compression circuit characterized in that the reference voltage is lowered, and when the level of the imaging signal is low, the reference voltage is increased.