DE2428489B2 - GAMMA CORRECTION - Google Patents

Description

40

The invention relates to a gamma correction circuit with an image signal amplifier at its output terminal a semiconductor circuit with non-linear characteristics is connected to the at least contains a controllable diode circuit, the anode of which is connected to the output terminal of the amplifier is.

A gamma correction circuit is also referred to as a gradation equalization circuit. Such Circuitry is used, for example, in a color television camera related so that these gamma characteristics, namely non-linear input-output characteristics, shows. If in a color picture tube the brightness or the luminance is directly proportional could be a signal supplied to the grating, then the color picture tube would have linear input-output characteristics and would reproduce an ideal image. In fact, however, the brightness is proportional roughly the 2.2th power of the grid input signal, resulting in non-linear input-output chafo characteristics of the color picture tube leads. Therefore, when an output signal from a color television camera, the corresponds to an object in the foreground of the camera, delivered unchanged to a color television tube then not only the brightness, but also the color gradation and the hue on the The screen of the color picture tube is very different from what is directly from the object in the foreground Camera has the above significant, non-linear input-output characteristics of the color picture tube are therefore made by counteracting the non-linear input-output characteristics a gamma correction circuit, the input amplitude of which is almost This is proportional to a power of the reciprocal of about 2.2 of the amplitude of the input signal the brightness of a color picture tube is almost directly proportional to an input signal of a color television camera, thereby achieving a linear relationship therebetween and an accurate representation of the image of an object in the foreground of the camera on the screen of a picture tube becomes possible.

The one mentioned at the beginning, known from "Introduction to Television Technology", Volume 2 by W. D i 11 e η b u r g e r Gamma correction circuit has an image signal amplifier and first and second diodes. their anodes are connected in parallel to the output terminal of the amplifier via appropriate resistors and whose cathode control signals are supplied. When the output voltage from the amplifier increases to make the diodes conductive, provide the resistors between the amplifier and the diodes are connected, a resistor parallel to the amplifier.

If the control voltage supplied to the cathode of the second diode is chosen so that you Level is above that of the control voltage that is supplied to the first cathode, then the first and the second diode is made conductive one after the other with the increase in the output voltage from the amplifier, whereby the output terminal of the amplifier is given an increasing resistance and the gain of the amplifier is changed. Hence the Input / output characteristics of the amplifier changed when the first and second diodes are conductive will. Since the switch-on voltage of the diodes corresponds to the control voltage of their cathodes, the result Input-output characteristics of the known gamma correction circuit a polygonal curve, at which the points at which the characteristic changes correspond to the switch-on voltages of the diodes. A conventional gamma correction circuit. which is structured as described above thus provides a polygonal gamma characteristic. that of a counteracting, non-linear input-output characteristic that compensates for the essential, non-linear input-output characteristic of the picture tube.

However, the known gamma correction circuit has the disadvantage that by changing the Ambient temperature is influenced considerably. The blocking circuit in the forward direction falls with increasing Ambient temperature and increases as the ambient temperature decreases. That changes the switch-on voltage of the diode with the ambient temperature. If the switch-on voltage becomes smaller as a result of a rising temperature, it falls the resistance in the output circuit of the amplifier, which leads to a decrease in the output voltage of the gamma correction circuit leads. As the ambient temperature rises, the points of the polygo nal gamma characteristics, at which the characteristic changes, to lower output voltages shifted the gamma correction circuit, which leads to a change in the gamma characteristic.

The change in the gamma characteristic influences

; in picture on the screen of a picture tube, so that an accurate reproduction of the picture of an object in the Foreground becomes impossible. In order to overcome these difficulties, one should try once, the diode keep in a thermostat, and secondly to increase the amplitude of the amplifier to the video signal to increase and thereby the change dt r reverse voltage in the forward direction of the diode relative to belittle.

The first method, however, requires time until the gamma correction circuit starts its normal operation, and on the other hand the circuit very extensive due to their additional energy requirement to maintain a constant temperature, so that their practical use is defeated. The second method suffers from the disadvantage a maximum power consumption and provides difficulties in integrating the gamma correction circuit. Another disadvantage of the conventional gamma correction circuit is that that it uses a polygonal characteristic that approximates a non-linear gamma characteristic, which compensates for the essential, non-linear input-output characteristics of the picture tube. the end the DT-OS 15 49 620 is a correction circuit with non-linear characteristics known that several to a load resistor blows up diodes connected in parallel and several associated emitter follower transistors and changes in ambient temperature are slightly compensated.

It is the object of the invention to provide a gamma correction circuit of the type mentioned at the beginning Kind to train in such a way that with a simple structure there is a change in the ambient temperature stable gamma characteristic results.

The semiconductor circuit in the gamma correction circuit according to the invention is used to achieve this object from a number of diodes connected in series, and the output terminal of the semiconductor circuit is available with the output terminal an emitter follower transistor circuit with several in cascade Emitter transistors in connection, the input terminal of which the control signal is fed to.

A circuit designed in this way shows a steady course of the gamma characteristic and is easy to integrate.

In the following a preferred one is based on the drawing Embodiment of the invention explained in more detail.

F i g. 1 shows the circuit diagram of a gamma Korrekiur- w circuit;

F i g. 2 shows the gamma characteristics of the in FIG. 1 g. 1 circuit shown;

F i g. 3 shows the voltage characteristics of a Diode according to the temperature,

Fig. 4 shows the characteristics of the base-emitter voltage an NPN transistor according to the temperature;

F i g. Fig. 5 shows the circuit diagram of the embodiment of the gamma correction circuit according to the invention;

F i g. 6A and 6B give comparisons of the time course of the voltage of an input signal and the Voltage of an output signal of the in FIG. 5 illustrated embodiment;

F i g. 7 shows the input-output characteristic of a diode for explaining the functions in FIG. 5 shown Embodiment.

In Fi g. 1 shows a gamma correction circuit formed in the manner known from DT-OS 15 49 620. An input image signal is converted into an input signal £ "with the aid of a black level display in a clamping circuit (not shown) an NPN transistor 1. The output terminal of the amplifier or the collector of the transistor 1 is connected to a positive voltage source Eb via a load resistor 2 and to the anodes of the diodes 5 and 6 via the corresponding resistors 3 and 4.

The cathodes of the diodes 5 and 6 are connected to the emitter output terminals of the transistors 7 and 8, each of which forms an emitter follower circuit. The emitters of the transistors 7 and 8 are grounded through the resistors 9 and 10, respectively, and the collectors of the transistors 7 and 8 are commonly connected to the positive source Eb . Control signals £ 1 and £ 2 are supplied to the bases of transistors 7 and 8, respectively. The collector output signal of the transistor 1 forms the output signal Eo of the gamma correction circuit.

In the following, on the basis of FIG. 2 to 4 the mode of operation the above-described gamma correction circuit explained in more detail.

When the control signal £ 1 has a higher voltage level than the control signal Ei , the gamma characteristic shows a straight line O- £ i in FIG. 2, before the anode voltage of the diode 5 reaches a value equal to the sum of the emitter voltage of the transistor 7 and the reverse voltage in the forward direction of the diode 5. If the anode voltage exceeds this sum, the diode 5 becomes conductive and the resistor 3 is connected in parallel to the transistor 1, whereby the gain of the transistor 1 is reduced. Then the gamma characteristic runs along a line £ i- £ 2 in Fig. 2. When the output voltage of the transistor 1 increases further and the anode voltage of the diode 6 has a value equal to the sum of the reverse voltage in the forward direction of the diode 6 and the emitter voltage of the transistor 8 exceeds, the diode 6 is conductive. At this point in time, the resistor 4 is also connected in parallel with the transistor 1, which brings about a further reduction in the gain. Then the gamma characteristic is along a line Ei-P in FIG. 2. The output voltage Eo at the point where the gamma characteristic changes is a function of the control signals £ 1 and £ 2 supplied to the bases of the transistors 7 and 8, respectively. Therefore, the points at which the characteristic changes are in F 1 g. 2 marked with h and egg . The in F i g. The gamma correction circuit shown in FIG. 1 shows a polygonal characteristic as shown in FIG. 2, and this polygonal characteristic approximates the desired gamma characteristic. A certain gamma characteristic can be achieved in that the Stevir stresses £ 1. £ 2 or the resistance value of resistors 3 and 4 can be selected appropriately.

The temperature compensation in the gamma correction circuit of F 1 g. 1 follows on from the following Way.

According to the voltage characteristics of the diodes 5 and 6, the reverse voltage drops in the forward direction progressively with the increase in ambient temperatures 71, Ti and Ti in the order mentioned. As shown in FIG. 4, the base-emitter voltage Vbe of transistors 7 and I also falls

with increasing ambient temperature, since the voltage Vt of the transistors 7 and 8 corresponds to the reverse voltage in the forward direction of the diodes 5, 6. If, therefore, the control voltages £ i and Ei are kept constant, the emission voltage rises and approaches the base voltage as the temperature rises. As the temperature falls, the emitter voltage also falls, so that it differs more from the base voltage.

In the gamma correction circuit, the output voltages of the emitter follower transistors 7, 8 are supplied to the cathodes of the diodes 5, 6, as described above. While a rising ambient temperature causes the reverse voltage of the diodes 5 and 6 to drop in the forward direction and thus reduces their switch-on voltage, the rising temperature causes the base-emitter voltage Vbe of the transistors 7 and 8 to drop and their emitter voltage and the cathode voltage of the diodes 5 and to rise 6, whereby the switch-on voltage of the diodes also increases. The decrease in the switching voltage as a result of the reduction in the reverse voltage in the forward direction of the diodes 5 and 6 is therefore counteracted by the increase in their cathode voltage. js

When the ambient temperature drops, the Gamma correction circuit works in reverse and the resulting effect counteracts the diodes.

The switch-on voltage of the diodes 5 and 6 is kept constant regardless of a changing ambient temperature, in that changes in the reverse voltage in the forward direction of the diodes 5 and 6 are compensated for by changes in the base-emitter voltage of the transistors 7, 8. Therefore, even when the ambient temperature changes, the points E \ and Ei become the one in FIG. 2 shown polygonal characteristics, at which this changes. not shifted, so that the gamma characteristic of FIG. 1 shown gamma correction circuit remains stable. The use of an emitter follower circuit with a very low output impedance does not affect the desired gamma characteristic.

In the case of the in FIG. 1 are two circuits Diodes are connected in parallel to the load resistor, thereby approximating the desired gamma characteristic by adding two points £ 1, £ 2. at which the characteristic changes takes place. It is obvious that the number of diodes is not limited to two and at least one diode can be connected in parallel to the amplifier. A larger number of diodes connected in parallel leads to a larger number of points to be applied. at which the characteristic changes. Instead of the 5S Diodes 5 and 6 can be used as transistors in a diode circuit.

F i g. 5 shows an embodiment of the circuit according to the invention, in which an image signal with positive polarity is supplied from a terminal 11 via a capacitor 12 to the base of an N PN transistor 13. A positive clamp circuit pulse is provided from terminal 14 through capacitor 15 to the base of transistor 16. This transistor 16 is a switching transistor for level holding and is made conductive only when it is supplied with a clamping circuit pulse, so that the base voltage of the transistor 13 is fixed at a prescribed value. Transistor 16 generally switches once during the blanking interval of the video signal. By this switching, an input video signal is subjected to black level reproduction. An input signal E, which is reproduced in this way in the form of a direct current, is amplified by a differential amplifier with NPN transistors 13, 17. The emitters of these NPN transistors 13, 17 are grounded via a constant current source 18. The positive voltage source E is connected to the collector of the transistor coupled across the load resistor 19 17. the base of transistor 17 is supplied with a bias voltage having a certain level. the positive voltage source Eb is also connected to the collector of the transistor 13 in conjunction.

A non-linear semiconductor circuit comprises a series connection of three NPN transistors 21 to 23 in diode connection, in which the collector and the base of each transistor are connected to one another so that the transistor operates as a diode. The collector of the transistor 21 is via a resistor 20 with d <* m output of the differential amplifier. ie connected to the collector of transistor 17. The emitter follower circuit comprises three NPN emitter follower transistors 24 to 26 which are connected in cascade and which correspond to the three transistors 21 to 23 of the diode circuit. Their emitters are grounded via the corresponding resistors 27 to 29. The emitter of transistor 24, which forms the output terminal of the emitter follower circuit, is connected to the emitter of transistor 23 of the series circuit, and the base of transistor 26 of the emitter follower circuit is supplied with the control signal Ei. The collectors of the transistors 24 to 26 are jointly connected to the positive voltage source Eb.

As shown in Figs. 6a and 6B, the output signal Ev increases in that of FIG. 5 in a non-linear form. when the input signal E, decreases linearly. If the in F i g. 5, the gamma correction circuit shown is applied with an increasing input voltage £, the transistors 21 to 23 become conductive. Then, the resistor 20 and a load series circuit comprising the resistor components of the transistors 21 to 23 in diode connection are connected in parallel to the differential amplifier, thereby causing the gain to decrease in accordance with the change in the resistance value of the load series circuit.

The gamma characteristic of the in FIG. 5 will be explained later. In Fig. 7 the Stromspannupgscharakteristik a diode with 1 is designated. The current-voltage characteristic of three diodes, labeled II, shows an enlarged non-linear range with respect to a prescribed voltage amplitude of the input signal. In the non-linear range of the voltage characteristic of the diode, the non-linear resistance component of the diode decreases the higher the voltage of an input signal. In the case of the in FIG. 5, the increasing voltage of an input signal leads to a non-linear decrease in the load resistance, which contains the total resistance of the resistance components of the series-connected three transistors 21 to 23, whereby a non-linear change in the output signal Eo of the gamma correction circuit in Depending on the change in load resistance, as shown in FIG. 6A and 6B

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is shown. The gamma characteristic of the gamma correction circuit of Fig. 5 is derived from the impedance characteristic of the resistance components of the transistors 21 to 23 in diode connection with increasing voltage of the input signal Eo when the transistors 21 to 23 are conducting. The gamma characteristic of the gamma correction circuit of FIG. 5 therefore shows a more continuous course than the polygonal characteristic of that in FIG. 1 gamma correction circuit shown. Therefore, in FIG. 5 shows a gamma characteristic which corrects the input-output characteristic of a color picture tube even more effectively.

When the gamma characteristic is to be changed, it is recommended to change the resistance value of the resistor 20 or the control voltage Ej, thereby changing the operating voltage of the transistors 21-23. The F i g. 6A and 6B show changes in the curve of the gamma characteristic when the control voltage Ei has different values. The F i g. 6A and 6B each show the characteristic for a control voltage Ei of 5.8 V or 5.6 V. This results in a gamma value T '= 0.7 or 0.45. The gamma value is the exponent in the function that represents the gamma curve.

In the gamma correction circuit shown in FIG. 5, in which the series connection of the transistors 21 to 23 in diode connection and the circuit of the cascaded emitter follower transistors 24 to 26 are formed from the same number of transistors, there is a change in the base-emitter voltage of the transistors 21 to 23 due to a change in the ambient temperature by a change in the base-emitter voltage of the transistors 24 to 26 compensated. A rising ambient temperature leads to a decreasing base-emitter voltage of the transistors 21 to 23, whereby the switch-on voltage of the transistors 21 to 23 is reduced to a lower level. However, since the base-emitter voltage of the transistors 24 to 26 increases, the emitter output voltage of the transistor 24 increases with respect to the same control voltage Ei . thereby causing the emitter terminal voltage of the transistor 23 to rise. As a result, the turn-on voltage of the transistors 21 to 23 is raised to a higher level. Regardless of a change in the ambient temperature, therefore, neither the collector core voltage of the transistor 21 nor, consequently, the output voltage E or the gamma correction circuit change. The gamma characteristic of the in FIG. The gamma correction circuit shown in FIG. 5 is therefore not influenced by the ambient temperature. Since the emitters of the transistors 25, 26 are grounded via the resistors 28, 29, the collector-emitter current of the transistors is relatively high. As a result, the input impedance of transistor 24, and therefore its emitter impedance and emitter voltage, are low. Changes in the emitter voltage of the transistor 24 thus have only a slight influence on the output signal Eo, so that the gamma correction circuit operates stably.

If the in F i g. 5 shown circuit in integrated Form is carried out, the characteristics of the transistors 21 to 23 can be the same as those of the Transistors 24-26. This is the in F i g. 5 shown circuit like that in F i g. 1 circuit shown integrable.

In the case of the in FIG. In the embodiment shown in FIG. 5, the three transistors 21 to 23 can be connected in a diode circuit be replaced by three diodes.

The number of transistors that make up the cascaded emitter follower transistor circuit used is not limited to three. The gamma correction circuit according to the invention can also contain a circuit with cascaded emitter follower transistors consisting of any other number of transistors are formed. It is also possible to have a number of stages parallel to the, To switch amplifiers, each of which is a series connection of transistors and a circuit of Contains emitter follower transistors connected in cascade.

In the embodiments described, NPN transistors were used. According to the invention can with the same effect also PNP- Trar.si ^ toren use Find.

For this purpose 4 sheets of drawings

Claims (4)

Patent claims: 1. Gamma correction circuit with an image signal amplifier, at the output terminal of which a semiconductor circuit with non-linear characteristics is connected, which contains at least one controllable diode circuit, the anode of which is connected to the output terminal of the amplifier, characterized in that the semiconductor circuit is made up of a number of series-connected Diodes (21, 22, 23) and the output terminal of the semiconductor circuit is connected to the output terminal of an emitter follower transistor circuit with several emitter follower transistors (24, 25, 26) connected in cascade, the input terminal of which is supplied with the control signal (Ei). 2. Gamma correction circuit according to claim 1, characterized in that at least one of the Diodes (21, 22, 23) is a transistor whose base and collector electrodes are connected to one another are. 3. Gamma correction circuit according to claim 2, characterized in that the number of diodes (21, 22, 23) is equal to the number of those in cascade switched emitter follower transistors (24, 25, 26). 4. Gamma correction circuit according to claim 1, characterized in that the amplifier has a Clamping circuit (15, 16) whose input terminal is supplied with the image signal and whose output terminal provides a direct current output signal, and a differential amplifier (13,17), which with the output terminal of the clamp circuit (15, 16) and the direct current output signals amplified by the clamping circuit (15,16).