DE956587C - Arrangement for gradation equalization in television broadcasting systems - Google Patents

Description

In television broadcasts, efforts are made to reproduce the character of the image as faithfully as possible in terms of contrast reproduction. In some image recording methods, however, this is made more difficult by the fact that the amplitude transfer characteristic of the image recording tube or the amplifier causes a distortion of the signal amplitude. The amplitude transfer function is characterized in a similar way to the ίο blackening function of photographic layers by approximating it with a power function and using the exponent as a measure. In technical parlance, this figure is called gamma and is between 0.6 and 0.8 in size for an iconoscope, around 2 for a picture display tube and approximately 1 for an orthicon, although the exact value of the latter tube is not at all certain can be defined.

For a good and lifelike transmission it is necessary that the gamma of the amplitude characteristic on the sending side multiplied by the one on the receiving side results in the value 1. An arrangement is therefore required in television broadcasting systems in order to ensure this condition for gamma correction. So far one has in one

Correction circuit for the anode resistance of an amplifier tube via several connected in parallel biased diode resistors connected in parallel and thereby an approximation the required transfer function is achieved. However, it has been shown that the Film scanning required the amount of characteristic distortion a rather cumbersome circuit makes necessary and that with the low external resistance required in wide-screen amplifiers If the forward resistance of the diodes were not small enough, with a few diodes the required To make curvature of the amplitude characteristic possible.

In the inventive arrangement for gradation equalization in television broadcasting systems, especially for film scanning purposes, image signals of positive polarity are fed to the control grid of a cathode amplifier stage, the black level of which is held by a four-diode circuit at a value - Ug , which is selected so that the voltage between the cathode, the multiple subdivided cathode resistance is at a negative voltage, and a fixed reference value, preferably earth, becomes zero when the black level is transmitted and that between each of the connection points of the partial resistances and this fixed reference value there is a diode, preferably a germanium diode, such that when the signal becomes more positive the diodes become conductive one after the other. As a result, the output voltage, which is tapped at the connection point of the partial resistors furthest away from the cathode, experiences a strongly amplitude-dependent voltage division such that the voltage values assigned to the light areas of the object are significantly less amplified than the voltage values assigned to the dark areas.

It has already been proposed to use a germanium diode to regulate the gamma to be carried out, whereby the quadratic part of the characteristic curve of germanium diodes is used. Man is however forced to choose a suitable diode from a large number. In the In contrast, the circuit according to the invention, the characteristic of the diode plays a subordinate role, since the diodes are practically only effective as a switch with an internal resistance equal to the directional resistance are.

The invention is illustrated by means of an exemplary embodiment, which is illustrated in more detail by the accompanying drawing is explained, described in detail.

In the circuit shown, a television signal is supplied at 1, which in the present case is polarized in such a way that the black level is directed in the negative direction and the white level in the positive direction. This signal arrives at the control grid of a strongly negative feedback triode 2, from whose cathode resistor the converted image signal can be picked up in the same polarity. The resistor /? /, Serves as the working resistance for the decrease in the signal voltage. The potential of the control grid is kept at a fixed potential - U _s , which z. B. is generated by dividing the voltage between the battery - E ₁ and earth.

In order to achieve the desired characteristic for gamma control, the cathode resistance of the tube 2 is divided into several resistances R ₁ ... R _s and between each of the connection points of the partial resistances and a fixed reference value, e.g. B. Earth, switched on a diode. This circuit works as follows:

If, as the signal becomes more positive, which corresponds to an increasing object brightness when scanning a positive film, the diodes 4 become conductive one after the other, the signal voltage dropping at the cathode resistor is divided in a strongly amplitude-dependent manner, so that the voltage values assigned to the bright areas of the object are significantly less amplified \ verden as the voltage values assigned to the dark areas. In the case of a small image signal, the diodes 4 are all negatively biased and therefore not very conductive. The voltage appearing across the resistor Rj ₁ is therefore divided proportionally

R ₁ + R ₂ + R ₃ + R _k "

As the signal becomes more positive, the currents flowing in the cathode resistor are caused by the conductive diodes are increasingly derived, so that now the one occurring at the tube Voltage divided by the chain conductor formed from the diodes and the series resistors will. This results in a multiplicative effect of the diodes, so that a signal that z. B. at ι has the shape of the curve shape shown there, at 5 in a strongly changed curve shape is reproduced. The amplitudes corresponding to the black areas in the image are opposite the amplitudes corresponding to the bright areas are raised.

In order to maintain a fixed reference value for the video signal voltage in the case of the amplitude distortion in the circuit described, a keyed four-diode circuit known per se is connected to the grid of the tube 2. The voltage - Ug is calculated with the help of the voltage division i? ₄ - R _{5 selected} so that the voltage between the cathode and a fixed reference value, preferably earth, is zero when the black value is transmitted.

Claims (1)

Claim: Arrangement for gradation equalization in television broadcasting systems, especially for film scanning, characterized in that it consists of a cathode amplifier stage with a multiple subdivided cathode resistance lying at a negative voltage, whose Control grid the image signals with positive po- larity are supplied, as well as a four-diode circuit that holds the black level of the signals at a value - U _g , which is selected so that the voltage between the cathode and a fixed reference point, preferably earth, is practically zero when the black level is transmitted, and that between each of the connection points of the partial resistances and the fixed reference point there is a diode, preferably a germanium diode, in such a way that when the signal becomes more positive, the diodes become conductive one after the other, and that the output voltage is tapped at the connection point of the partial resistances furthest away from the cathode is made. Considered publications:
German patent application E 2360 VIII a / 21 a ¹ ; British Patent No. 535413;
French Patent No. 825,445;
"Funk und Ton", 1951, No. 10, pp. 505 to 513. 1 sheet of drawings 1 609 772 1.57