GB2226926A - Maintaining colour balance for video sensors with variable integration times - Google Patents

Description

-I- PROCESS FOR COMPENSATING NON-LINEARITIES IN A COLOUR VIDEO SIGNAL

APPARATUS This invention relates to a process for compensating non-linearities between the output signal voltage and the integration time of semiconductor sensors in a colour video signal apparatus.

Semiconductor sensors, e.g. CCD line sensors used as opto-electrical converters, operate in such a way that the photosensitive area is illuminated for a specific time called the integration time and at the end of this integration time the collected charge is transferred into a shift register. Following the transfer of the charges into the shift register a new integration time begins. The magnitude of the collected charge (charge quantity) is dependent on the illuminance and the integration time. The signal amplitude at the sensor output is directly dependent on this charge quantity.

In the case of the semiconductor line sensors used in television film scanners, the integration time was until recently fixed by the film format and the television standard, and it was only possible to change the signal amplitude at the'sensor output by varying the illuminance. However, in sensors which have recently become available it is also possible to shorten the integration time, although the maximum integration time is still predetermined by the film format and the television standard. As a result of the new control, it is now possible to reduce the signal amplitude from a maximum value to zero without modifying the illuminance. In order to be able to use this control in an appropriate manner, it is necessary for there to be a linear relationship betwee n the integration time and the output signal voltage.

However, the hitherto known CCD sensors have a nonlinear behaviour, the non-linearity not being dependent on the illuminance and generally differing between individual sensors. On varying the integration time, an uncorrected use of such semiconductor sensors would lead to colour purity errors and is consequently not appropriate.

According to the present invention there is provided a correction process for compensating non-linearities between the output signal voltage and the integration time of at least two semiconductor sensors in an apparatus for producing colour video signals, wherein first a white balance is performed by measuring the output signal voltage of each of the semiconductor sensors at a specific illuminance and maximum integration time and then reducing the integration times of the semiconductor sensors with other than the lowest output signal voltage such that their output signal voltages are reduced to the value of the semiconductor sensor with the lowest output signal voltage, so that for each semiconductor sensor the same output signal voltage, corresponding to the 100% level, is obtained for different integration times, and wherein then the integration time or the output signal voltage of each semiconductor sensor is reduced in steps to zero and the associated output signal voltage or integration time is measured and stored, and wherein in the case of normal operation of the apparatus the semiconductor sensors are controlled by the values of the integration times corresponding to a chosen output signal voltage.

The invention has the advantage that the output signal voltage is adjustable on each semiconductor sensor. so that chromatic distortions or colour fringing in the 10 reproduced image can be avoided.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, wherein:

is Figure 1 is a circuit for performing the process according to the invention, and Figure 2 represents the output voltage/integration time characteristics of three semiconductor line sensors.

The block circuit diagram shown in Figure 1 only includes those parts of a television film scanner necessary for illustrating the embodiment of the invention. In no.rmal operation of the scanner a film 1 is constantly illuminated by a white light source 2 and the light E.: modulated by the film images is split up into the three colour components E R, E G and E B with a beam splitter 3 and these components are projected onto the semiconductor sensors 4 for red, 5 for green and 6 for blue. The semiconductor sensors 4, 5 and 6 are preferably CCD line sensors. Video signal preamplifiers 7, 8, 9 are connected to the output of the line sensors 4, 5, 6 and amplify the respective colour signal R, G and B. Video processor circuits 11, 12, 13 are connected to the preamplifiers 7, 8 and 9 and in them the colour signals R, G and B are further processed and amplified and are then available at the output terminals 14!i 15 and 16. A circuit 17 supplies the semiconductor sensors 4, 5 and 6 with the necessary operating voltage and clock signals.

According to the embodiment of the present invention there is also a measuring circuit 18, whose inputs are connected to the outputs of the preamplifiers 7, 8 and 9. The outputs of the measuring circuit 18 are connected to corresponding inputs of a microprocessor 19, which contains a data store and a control signal generator. The outputs of the microprocessor 19 are connected across an integration time control circuit 21 to the integration time control inputs 22, 23 and 24 of the sensors 4, Sand 6. The microprocessor 19 can be controlled from the outside via inputs 26, 27 and 28, a measuring operation being started by a signal on input 26. Normal operation of the semiconductor sensors, thdt is normal film scanning as previously described, is started by a signal on input 27. Input 28 is used for presetting a desired value for the output signal voltages of the sensors to be achieved, that is all of them, through integration time control.

1 il For performing the compensation process a white balance is first performed. For this purpose a specific -intensity of the light source 2 is chosen and the photosensitive areas of the semiconductor sensors 4, 5 and 6 5 are continuously illuminated with the film 1 absent. The output signal voltages of the semiconductor sensors 4, 5 and 6 are measured in the measuring circuit 18 at maximum integration time and after amplification by the preamplifiers 7, 8 and 9. It is then established by the microprocessor 19 which signal channel supplies the lowest output voltage, i.e. which sensor has the lowest sensitivity. This smallest output signal voltage is now taken as the 100% level and the integration times of the two other semiconductor sensors are reduced, by means of the control voltages supplied to inputs 22, 23 and 24, until their output signal voltages are also at the 100% level. Thus, an automatic white balance is performed.

The integration times ti R' t'G and t'B corresponding to the 100% output signal level obtained through this automatic white balance are used as a basis for determining correction values. Each integration time ti W ti G and ti B is either reduced in steps to zero and at each step the output signal of the corresponding sensor is measured and stored in the microprocessor 19, or the output signal voltage of each sensor is reduced in steps to zero and at each step the corresponding value for the integration time is measured and stored.

In each case the data is stored in such a way that the values of the integration times ti R' tiG and t'B are associated with the corresponding values of the respective output signal voltages U a B9 U a G and U a R. Thus in normal operation ( in the case of control via terminal 27) the circuit is in a position to output the appropriate integration times ti R' t'G and t'B for any desired amplitude value of the output signals preset via the input 28 and to correspondingly control the sensors 4, 5 and 6 via the integration time control circuit 21.

On the basis of the values stored in the form of a table in the memory of the microprocessor 19, and whose characteristics are e.g. shown in Figure 2, it is possible to linearly modify the output signal voltages U a B, U a G and U a R by changing the integration times ti B' t'G and t'R without colour purity errors occurring.

The correction process can be adapted to any sensor, a change of sensor,merely making it necessary to perform the measuring process again. Figure 2 gives the characteristics R, G and B of the output signal voltage of the semiconductor sensors 4, 5 and 6 as a function of the integration time. On presetting a desired output signal voltage for U a R, U a G and U a B via the input 28, e.g. for a desired value at 80% or 45% of the maximum output signal voltage as shown, the corresponding integration time value trios are given as ti RP ti G and ti B

Claims (1)

1. A process for compensating non-linearities between the output signal voltage and the integration time of at least two semiconductor sensors in an apparatus for producing colour video signals, wherein first a white balance is performed by measuring the output signal voltage of each of the semiconductor sensors at a specific illuminance and maximum integration time and then reducing the integration times of the semiconductor sensors with other than the lowest output signal voltage such that their output signal voltages are reduced t9 the value of the semiconductor sensor with the lowest output signal voltage, so that for each semiconductor sensor the same output signal voltage, corresponding to the 100% level, is obtained for different integration times, and wherein then the integration time or the output signal voltage of each semiconductor sensor is reduced in steps to zero and the associated output signal voltage or integration time is measured and stored, and wherein in the case of normal operation of the apparatus the semiconductor sensors are controlled by the values of the integration times corresponding to a chosen output signal voltage.

2. A process according to claim 1, wherein the measured values of the output signal voltage and the associated integration time of each semiconductor sensor are stored in the form of a table.

4 3. A process according to claim 1 or claim 2. wherein the semiconductor sensors are semiconductor line sensors in a colour television film scanner.

4. A process as claimed in claim 1, substantially as described herein with reference to the accompanying drawings.

Published 1990 at The Patent Office. StateHouse.66 71 Highliolborn. London WC1R4TP-Furthercopiesma,.,beobtainedfrom The Patent Office Sales Branch. St Mary Cray. Orpington. Kent BR5 3RD. Printed Iky Mliltlpiex techniques ltd. St Mary Cray. Kent. Con 187,