GB2251352A - An automatic gain control circuit for a video camera - Google Patents

Abstract

The automatic gain control circuit is arranged to maintain the output video signal level constant even when the gain control circuit is reacting to changes in incident light. In this respect, the lens 1 of the camera focuses the incident light on an image sensor 12 which converts the light into electrical signals, the level of which is controlled by an AGC circuit 4. The gain of the AGC circuit is controlled by an AGC detector 8 responsive to the output of the AGC circuit 4. An iris driver 9 is also responsive to the output level to adjust the iris of the lens 1. Control means 13, 14, 16 are provided to detect the operation of the iris driver 9 and to control or prevent adjustment of the gain of said AGC circuit 4 in dependence upon the operation of said iris driver. The control means comprise an analog switching stage 16 which controls the connection of the AGC detector 8 to the AGC circuit 4. A micom 14 may momentarily supply the appropriate level for the AGC circuit whilst the iris is changing state. <IMAGE>

Description

AN AUTOMATIC GAIN CONTROL CIRCUIT FOR A VIDEO CAMERA The present invention relates to an automatic gain control circuit for a camera and to a video camera system incorporating such a circuit.

Currently video camera systems, especially camcorders and video systems for business use, have the disadvantage that there is degradation of the video image if the subject image is suddenly changed from bright to dull or vice versa. The problems arise because the motor operating the iris of the lens operates much more slowly then the electronic circuits react to a change in signal level.

Thus, the change is electronically detected and the iris driver is controlled to change the state of the iris, but the gain of the AGC circuit is also altered. Because the gain of the AGC circuit alters much faster than the position of the iris, the level of the video signal is momentarily increased or decreased beyond the standard level.

The present invention seeks to minimise the disadvantages of conventional systems.

According to the present invention there is provided an automatic gain control circuit for a camera comprising an AGC circuit, and an AGC detector coupled to receive the output of said AGC circuit and to adjust the gain of said AGC circuit in dependence upon the level of said AGC circuit output, the automatic gain control circuit further comprising an iris driver arranged to control an iris for a camera lens in dependence upon the level of said AGC circuit output, wherein control means are provided to detect operation of said iris driver and to prevent or control the adjustment of the gain of said AGC circuit in dependence upon the operation of said iris driver.

An automatic gain control circuit of the invention is able to maintain the output level of the AGC circuit constant. In this respect, the AGC level which satisfies the status of the iris driver is calculated from the operation speed and state of the iris driver.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically a conventional solidstate video camera, Figure 2 shows schematically a known automatic iris control circuit for a video camera, Figure 3A is a graph showing the video signal level and the ideal standard gain level for a video camera, Figure 3B is a graph showing the video signal level and the gain level of an AGC circuit of a conventional video camera showing the opening of the iris, Figure 3C is a graph showing the video signal level and the gain level of an AGC circuit of a conventional video camera showing the closing of the iris, Figure 4 shows schematically a video camera including an automatic gain control circuit of the present invention, Figures 5A, 5B and 5C are graphs illustrating compensation for the video signal level of an automatic gain control circuit of the invention, and Figure 6 is a flow chart of a software routine of a controller or microcomputer of the circuit of Figure 4.

Figure 1 shows schematically a solid-state video camera which is substantially conventional. As can be seen, the camera comprises a lens 1 arranged behind an adjustable circular aperture or iris. Adjustment of the iris enables the amount of light incident on the lens 1, and hence the exposure, to be controlled. The light is focused by the lens 1 on to a charge coupled device 12 forming the camera image sensor. The electrical signals generated by the image sensor 12 in response to the incident light are sampled by a first sample and hold circuit 2 and by a second sample and hold circuit 3 and are then fed by way of an automatic gain control (AGC) circuit 4 to a processor 5. The output of the processor 5 is fed by way of a matrix 6 to an encoder 7. The output of the encoder 7 is transmitted to the VTR, and is the video output signal.

The output of the AGC circuit 4 is also fed, by way of an AGC detector 8, back to an input of the AGC circuit 4.

This enables the gain of the AGC circuit to be adjusted to keep its output signal at a substantially constant standard level. Thus, if the input signal applied to the AGC circuit 4 falls, the AGC detector 8 will increase the gain of the circuit 4 to bring the output level back towards the standard. Similarly, if the input signal to the AGC circuit increases, the gain is lowered, again to keep the output substantially constant. Thus, it will be appreciated that by applying the electrical signal to one input of the AGC circuit 4 and the output of the AGC detector 8 to the other input of the AGC circuit 4, the level of the signal applied to the processor 5 can be kept substantially constant.

It will also be appreciated that the photo image received by the image sensor 12 is changed photoelectrically into an electrical signal, and that this electrical signal is sampled by the circuits 2 and 3 before being applied to the AGC circuit 4 and to the processor 5.

In this respect, the first sample and hold circuit 2 is arranged to sample the applied electrical signal by SHP (sampling and holding pulse for PG) at 9.54 MHz. The second sample and hold circuit 3 samples the electrical signal by SHD (sampling and holding pulse for DATA). Thus, the output signal of the AGC circuit 4 is a correlated double sampled signal, Spl, Sp2, at 4.77 MHz.

The sampled signal is applied to the processor 5 which separates the chrominance and luminance signals. The chrominance signal is separated into R, G, and B signals by way of the matrix 6 and an encoder 7 forms the signals Y, R-Y, and B-Y which form the video output signal.

It will be seen that the output of the AGC circuit 4 is also applied to an iris driver 9. In this way, the driver can be caused to open the iris to increase the light level when the output level of the AGC falls, and can alternatively cause the driver 9 to close the iris to decrease the light level when the AGC output level increases. The AGC detector 8 is continuously operated to change the gain of the AGC circuit 4 whilst the iris is driven in direct response to the output level of the AGC circuit 4. In this way, the AGC circuit 4 responds directly to the intensity of radiation of the light incident through the iris on to the lens 1 and hence on to the image sensor 12. A driver circuit 10 is provided for driving the image sensor 12 and is connected to a timing synchroniser 11 which also controls the encoder 7.

As stated above, in the circuit of Figure 1, the output level of the AGC circuit 4 directly controls the iris of the lens 1 by way of the driver 9. An alternative approach is to use an automatic iris system to control the incident intensity of radiation so that the output level of the AGC circuit is maintained constant even if the radiation intensity is changed. An automatic iris control system is shown in Figure 2 in which the iris driver 9 is shown to be a differential amplifier receiving a reference value at one input and the output level of the video signal at its other input. Such a circuit can be operated with two cases. One is the NAM (Non Additive Mix signal) which combines the R, G, and B signals with the equivalent level thereby driving out the peak level of which side among R, G, B.The other is the luminance (Y signal) which takes note that the mixing rate of each colour signal is to be 0.6G + 0.6R + O.lB, and controls the iris by the DC levelling for the peak value corresponding to the high band element.

However, with a camera system having the conventional systems disclosed, it is almost impossible to maintain the ideal video signal level and the AGC standard gain level as shown in Figure 3A.

Actually, the level of the video signal is changed in accordance with level changes of the incident light. If the intensity of the light changes, for example decreases, the AGC detector circuit outputs a control value which corresponds to the difference between the output level to be maintained and the actual value of the video signal level. The control value inevitably controls the AGC circuit to revert to the required output.

In practice, when the incident light levels change, the iris must open or close, and the level must be maintained constant by operation of the AGC circuit 4.

Thus, where the camera is suddenly focused on a dark image, the iris is controlled to open as indicated in Figure 3B.

However, the screen is momentarily darkened because the operational speed of the iris driver 9 is slow and can involve a hysteresis reaction.

When photographing a dark image and then moving the camera to a bright image, the iris has to be closed as indicated in Figure 3C. In this case, the screen should revert to normal brightness, but may be momentarily brightened because the AGC gain is momentarily increased as the iris driver is slow to react.

In general, this delay in obtaining the right level is more noticeable when focusing on bright subjects or portions thereof.

Accordingly, conventional camcorders and video cameras for business use do suffer from degradation of the quality of the video signal because of the oscillation of the AGC gain which is generated because of the difference in speed between the AGC circuit and the mechanical operational speed of the iris driver.

The circuit of Figure 4 shows a video camera including an automatic gain control circuit of the present invention which does not have the disadvantages described above. In a circuit of the invention, when the AGC circuit 4 is operated the AGC level is calculated until the iris has obtained its steady state by detecting the operation speed and state of the iris driver. Thereafter, the AGC level is maintained constant by applying the AGC level to the AGC circuit.

In the circuit shown in Figure 4 elements which are similar to those of the circuit of Figure 1 have been given the same reference numerals. It will immediately be seen that the circuit of the video camera shown in Figure 4 comprises the lens 1, the first sample and hold circuit 2, the second sample and hold circuit 3, the AGC circuit 4, the processor 5, the matrix 6, the encoder 7, the AGC detector 8, the iris driver 9, the CCD driver 10, the timing synchroniser 11, and the CCD image sensor 12.

In addition, the circuit of Figure 4 includes a micom or microcomputer 14 arranged to receive the output of an iris detector 13 which is connected to the output of the AGC circuit 4. A damper 15 is interposed between the output of the AGC circuit 4 and the processor 5. An analog switch stage 16 is controlled by said micom 14 and is arranged to control the connection of the AGC detector 8 to the AGC circuit 4.

Figures 5A to 5C show how the automatic gain control circuit of the camera circuit of Figure 4 is able to compensate the level of the video signal. Figure 6 is a flow chart of a software routine performed by the micom 14.

The micom 14 receives information as to the operational speed of the iris driver 9 and the level change, and is able to minimise the fault of the AGC reaction by way of the damper 15 and appropriate operation of the analog switch stage 16. The change of AGC level which caused deterioration or distortion of the image in the conventional video camera is minimised by control of the micom 14.

At a first function block S1 of the routine shown in Figure 6, the micom 14 detects the level of the video signal output from the AGC circuit 4. At a decision block S2 this level is compared with the standard level. If the result of the decision is that the video signal is at the standard level, the routine moves on to function block S3 at which the micom 14 controls the switch stage 16 to connect the switching contact points a and c, whereby the AGC detector 8 is connected to control the AGC circuit 4.

The fact that the circuit is operating in normal AGC mode is noted at function block S4 and the routine then goes to the end from where it can be recycled.

If the decision block S2 finds that the level of the video signal differs from the standard, the routine passes on to a decision block S5 where the error between the video signal level and the standard level is determined. If this error is within a predetermined range, then the routine passes on to function block S3 and hence the AGC circuit 4 is operated in the normal mode.

If the difference between the video signal level and the standard is not within the allowed range, the routine moves on to decision block S6 where it is determined if the level of the video signal is the upper limit of the allowance error. If the signal is at the upper limit, the routine moves on to function block S7 where a calculation of the necessary AGC gain down value is made.

At a next decision block S8, the decision is made as to whether the calculation is complete. If it is not complete, the AGC gain down value is continually calculated by a loop involving the function block S7 and the decision block S8. When the calculation is complete, the routine moves on to the function block S9 where the switch contact points a and c are connected to control the gain of the AGC circuit 4. The gain value of the AGC circuit is then output at the function block S10. The routine then moves on to decision block Sll which checks again if the level of the video signal is within the allowance error range. If the level of the video signal is within the range then the function block S3 connects the contact points a and b and then moves on to the normal AGC mode by way of function block S4. If the video signal is not within the allowance error range then decision block S5 moves the routine on again to decision block S6.

If the decision block S6 shows that the video signal level is not at the upper limit, the routine moves to decision block S12 and if this shows that the level of the video signal is at the lowest limit, the micom calculates the up value of the AGC gain at function block S13. Again, a loop of function block S13 and decision S8 establishes if the calculation has been completed, and when it has been completed the routine moves to the function block S9 to control the analog switch 16.

If it is found by decision block S6 that the video signal is not at the upper limit of the allowance error, and by decision block S12 that it is not at the lowest limit of allowance error, then the routine returns to decision block S5. If the signal is still not within the allowance error range then the routine may perform the functions and decisions of blocks S6 to S11 and return to the fifth decision block S5, or sequentially move from decision block S5 to decision block S12 and by way of function block S13 to the blocks S8 to S11.

The circuit shown in Figure 4 is, as will be appreciated, momentarily able to supply the AGC level until the iris is in a normal state. In this respect the operational speed and state of the iris driver 9 is provided to the micom 14 by way of the iris detector 13.

Whilst the AGC detector is not controlling the AGC circuits, the level of the video signal can be maintained by way of the damper 15.

It will be appreciated that modifications in and variations of the present invention as defined above may be made within the scope of the present invention.

Claims (9)

1. An automatic gain control circuit for a camera comprising an AGC circuit, and an AGC detector coupled to receive the output of said AGC circuit and to adjust the gain of said AGC circuit in dependence upon the level of said AGC circuit output, the automatic gain control circuit further comprising an iris driver arranged to control an iris for a camera lens in dependence upon the level of said AGC circuit output, wherein control means are provided to detect operation of said iris driver and to prevent or control the adjustment of the gain of said AGC circuit in dependence upon the operation of said iris driver.

2. An automatic gain control circuit as claimed in Claim 1, wherein said control means comprise analog switching means for controlling the connection of the AGC detector to the AGC circuit.

3. An automatic gain control circuit as claimed in Claim 2, wherein said control means comprise a controller coupled to receive the output of said AGC circuit, and to receive information as to the operation of said iris driver, and arranged to control said analog switching means.

4. An automatic gain control circuit as claimed in Claim 3, further comprising a damper coupled to the output of said AGC circuit.

5. An automatic gain control circuit as claimed in Claim 3 or 4, further comprising an iris detector connected to said iris driver.

6. An automatic gain control circuit for a camera comprising; a lens, a first sample and hold circuit, a second sample and hold circuit, an AGC circuit, a processor, a matrix, an encoder, an AGC detector, an iris driver, a CCD driver, a timing synchronizer and a CCD image sensor, wherein a micom is connected to an iris detector and is also connected to an output stage of said the AGC circuit, and wherein a damper is connected between the AGC circuit and the processor, and an analog switching stage is connected between said AGC circuit and the AGC detector, the level of said AGC circuit being arranged to be maintained constant by control of the analog switching stage by way of said micom.

7. A video camera system incorporating an automatic gain control circuit as claimed in any preceding claim, wherein said camera system comprises a lens arranged to focus incident radiation onto an image sensor, and wherein an iris is arranged to control the amount of radiation impinging on said lens, said iris being controlled by said iris driver.

8. An automatic gain control circuit substantially as hereinbefore described with reference to the accompanying drawings.

9. A video camera system substantially as hereinbefore described with reference to the accompanying drawings.