JPS5910076A - Gamma compensating circuit - Google Patents

Abstract

PURPOSE:To make the best use of the advantages of a gamma compensating circuit utilizing the nonlinearity of a diode and to ensure a stable and accurate operation at all times, by detecting the black level of a video signal obtained after gamma compensation and performing feedback control to the working point of the diode in the direction where the black level of the video signal is converged to a prescribed value. CONSTITUTION:Amplitude is detected for a clamp pulse (b) which is contained in a video signal (e) emerging at the output out after gamma compensation, and the result of detection is fed back to the working point of a diode 6 for gamma compensation. Then the amplitude of the pulse (b) contained in the signal (e) is converged to a prescribed value decided by the clamping voltage E81 and the comparing voltage E82. As a result, the diode 6 always works at a fixed level of terminal voltage. This process ensures the operation with optimum compensating characteristics at all times and with no change of gamma compensating characteristics which is due to a temperature change, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to gamma correction circuits used in television cameras.

A gamma correction circuit is a type of nonlinear circuit that adjusts the amplitude characteristics of the entire television transmission system, including the photoelectric conversion system, to predetermined characteristics. Conventionally, a circuit as shown in FIG. 2 based on a polygonal line approximation using the switching characteristics of a diode has been mainly used.

Among these, the circuit using the nonlinearity of the diode shown in Figure 1 can obtain a nearly ideal correction curve, but since the transfer characteristic of the diode is highly dependent on temperature, the correction characteristic is The disadvantage is that it is difficult to always perform accurate correction.

On the other hand, the circuit shown in FIG. 2 has the drawback that it is difficult to make the correction curve sufficiently close to the ideal shape due to the broken line approximation, and it is difficult to obtain sufficient color reproducibility in a color television camera or the like.

SUMMARY OF THE INVENTION An object of the present invention is to provide a gamma correction circuit that eliminates the drawbacks of the prior art described above, utilizes the nonlinearity of diodes, has good temperature characteristics, and always provides accurate gamma correction.

In order to achieve this object, the present invention detects the black level of the video signal without gamma compensation t2, and performs feedback control on the operating point of the diode in the direction of converging to a constant value D. shall be.

Embodiments of the gamma correction circuit according to the present invention will be explained below with reference to the drawings.

FIG. 3 shows an embodiment of the present invention, in which 1.2 is a resistor that constitutes an adder circuit, 3 to 5 are transistors, 6 is a diode (more than one is often used) for providing gamma correction characteristics,
7 is a clamp circuit, 8 is a clamp circuit and pressure source that provides a clamp voltage E111, 9 is a sample and hold circuit, 10
11 is a reference ring and pressure source that provides a comparison voltage EBB. In addition, in is the input to which video No. 48 (a) to be gamma corrected is supplied from the DC component regeneration circuit, O)) is the input to which the clamp pulse (b) is supplied, and out is the gamma-corrected video signal (home). ), and sp is the input to which the sampling pulse (to) is supplied, and the clamp pulse (b) is generated at every predetermined timing within the horizontal retrace period, and the sampling pulse (to) is generated at a predetermined timing during the horizontal retrace period. It is designed to occur at a predetermined timing within the line periods V and BL.

Resistors 1 and 2 connect the video signal (a) from input 1n and input cp
Add the clamp pulse (b) from
It functions to create a signal (c) in which a clamp pulse (b) is inserted within the horizontal retrace period.

The diode 6 constitutes a non-linear amplification circuit for gamma correction together with the transistors 3 to 5, and the gamma-corrected image 41
function to obtain ).

Clamp circuit 7 has input cp or clamp pulse (b)
It operates at the timing when is supplied, and serves to clamp the video signal (E) appearing at the output OUT to the clamp voltage El11.

The sample hold circuit 9 operates by the sampling pulse (to) supplied from the input sp, and the output out
K samples the voltage level of a predetermined portion within the vertical retrace period of the video signal (e) that appears, and uses it for the period until the next sampling pulse (e) is supplied, that is, 1
It functions to output the sample and hold voltage SH over the vertical scanning period.

The operational amplifier 10 compares the voltage SH with the comparison voltage Ess and generates a control signal as according to the relationship (OS=SH-E8!).

Next, the operation will be explained.

When the video signal (a) is supplied to the input in, the diode 6
A gamma-corrected video signal 5 is obtained at the output due to the non-linear characteristics given by As shown in Figure 4, the clamp circuit 7 is activated by the clamp circuit 7, so the video signal (E) appearing at this output is as shown in Figure 4. The clamp voltages Ks and Ic are fixed. Note that since the clamp pulse (b) inserted into this video signal (e) exists during the horizontal blanking period, it goes without saying that it will be removed by subsequent signal processing.

In this way, the tip A of the clamp pulse (b) becomes (t, t
The video signal (E) clamped to EK as is output ou
The voltage 8) is taken out from t and input to the subsequent processing circuit, and is also supplied to the sample and hold circuit 9, and is sampled by the sampling pulse at the timing shown in FIG. 4, so that the voltage 8) appearing at its output 1
is a voltage representing the black level B of the video signal 0), and as a result, the voltage difference between the potential at point A and the output voltage SH of the sample and hold circuit 9 is the video signal (E) gamma-corrected by the characteristics of the diode 6. represents the amplitude value of the current clamp pulse (b), and the potential at point A is the voltage El1.
By being fixed at 1, the output voltage 8H represents the amplitude value of the clamp pulse (b) after gamma correction.

Therefore, the comparison voltage Es from the reference voltage source 11 is now the voltage S that appears at the output of the sample and hold circuit 9 when the diode 6 is at the operating point that provides the optimal correction characteristics.
Suppose that it is set to a value equal to H. Note that at this time, it goes without saying that the amplitude of the clamp pulse (b) supplied from the input cp must always be kept at a constant value.

Then, while the operating point of diode 6 is maintained in the correct state, the input of operational amplifier 100 is (S H = E
s! Since the state of 1) is maintained, the output voltage O8 becomes a constant value, and the emitter voltage of transistor 3 and the collector voltage of transistor 4 are both kept as they are.
As a result, the bias state of diode 6 does not change.

Next, suppose that the characteristics of diode 6 change due to some reason, such as temperature change, and the voltage between its terminals decreases. The amplitude of the middle clamp pulse (b) also decreases, which causes the level at point B to rise, and the output voltage SH of the sample and hold circuit 9 increases in accordance with the level at point B, so that the output voltage and voltage of the operational amplifier 10 increase. aS is (3) (-Kam)
becomes a positive voltage in proportion to , which increases the emitter voltage of transistor 3, so that the operating point of diode 6 is moved in the cut-off direction, and as a result, its terminal voltage increases towards its original level and appears at the output OUT. It operates to compensate for the decrease in the amplitude of the clamp pulse (b).

Suppose that the characteristics of the diode 6 change in the direction of increasing its terminal voltage, then the video signal becomes 0.
The amplitude of the clamp pulse (B) in the circuit increases and the level at point B decreases, so the output voltage SH of the sample-and-hold circuit 9 becomes lower than the comparison voltage E8Q. OS becomes a negative voltage.

As a result, the emitter voltage of the transistor 3 is lowered to 1, the operating point of the diode 6 moves in the conduction direction, and the terminal voltage decreases, thereby causing the video signal 6'
The amplitude increase of the clamp pulse (b) in l→ is expressed as sleeve (Xi
It works like that.

In other words, in this embodiment, the gamma-corrected output
Detects the amplitude of the clamp pulse (B) contained in the video signal 0 → that is received at the station, and applies gamma correction 1 to the result.
It is operated so that the amplitude of the clamp pulse (B) included in the video signal (P) converges to a predetermined value determined by the clamp voltage Eel and the comparison voltage E811. As a result,
The diode 6 for gamma correction always operates under a constant terminal voltage, so that the gamma correction characteristic does not change due to temperature changes, etc., and it always operates under the optimum correction characteristic. This is possible.

Furthermore, as a characteristic of such a feedback control system,
The higher the gain of Mebeamb 10, the more accurate the control (1+i value) can be obtained, but on the other hand, if the gain is increased too much, the operation becomes unstable, so in practice, the gain should be adjusted based on these compromises. Just set it.

As explained above, according to the present invention, the characteristic change of the diode during gamma correction operation can be detected and compensated for by the reactor 1. By taking advantage of the advantages of the comma correction circuit used in Example 4, it is possible to easily provide a gamma correction circuit that always operates stably and accurately and has a wide dynamic range.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional example of a gamma correction circuit using the nonlinearity of a diode, and Figure 2 is a circuit diagram showing a conventional example of a gamma correction circuit using a broken line approximation method using the switching characteristics of a dimate, W73. Figure 1.2... Resistors, 3 to 5... Transistors, configuring the adder circuit,
6... Gamma correction diode, 7...
・Clamp circuit, 9... Sample bold circuit, 10... Operational amplifier.

Claims (1)

[Claims] In a gamma correction circuit using a diode to provide non-linear transmission characteristics, there is provided a means for inserting a reference pulse having a predetermined constant level into a horizontal retrace period of a video signal to be input to the diode; Means for clamping the tip of the reference pulse present in the video M month to a predetermined potential, and sampling means for extracting the black level of the video signal clamped by the means are provided, and the extracted black level and the reference black are provided. @
Gamma correction circuit.