JPH07105958B2 - Video camera signal processing circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal processing system for a video camera that captures images even under low illuminance.

[Background of the Invention]

Home video cameras are often used in dark places such as indoors. When shooting under such low illuminance, the level of the image signal obtained by the video camera becomes small, so the image reproduced by the receiver has a very poor SN ratio (signal-to-noise ratio). . Therefore,
It is desired that a high-quality reproduced image can be obtained even when shooting is performed under low illuminance such as indoors.

FIG. 4 and FIG. 5 are a block diagram showing a signal processing system of a conventional video camera and an input / output characteristic diagram of a gamma correction circuit thereof, and the conventional technique will be described with reference thereto.

FIG. 4 is a block diagram showing an example of a signal processing system of a conventional frequency separation type single tube color video camera. 1 is an image pickup tube, 2 is a preamplifier, and 3 is an automatic gain control circuit (hereinafter referred to as an AGC circuit). ), 4,5 are low-pass filters, 6 is a bandpass filter, 7 is a gamma correction circuit, 8 is an aperture correction circuit, 9 is an encoder, 10 is a color separation circuit, 11, 12, 14 are gamma correction circuits, and 13 is a subtraction circuit. An arithmetic circuit, 15 is an output terminal.

In FIG. 4, the image signal obtained from the image pickup tube is amplified by the preamplifier 2 to a level suitable for signal processing, and then the AGC is performed.
It is supplied to the circuit 3. In the AGC circuit 3, level adjustment is performed so that a constant level NTSC signal can be obtained even if the level of the supplied image signal changes due to being photographed in a dark place or a bright place. The level-adjusted image signal is supplied to the low-pass filters 4 and 5 and the band-pass filter 6, respectively.

The low pass filter 4 separates the luminance signal. This luminance signal has a gamma value of about 2.2 on a television receiver. Therefore, in order to correct this and reproduce an image having faithful gradation characteristics, the gamma value as shown in FIG. But
It is processed by the gamma correction circuit 7 having an input / output characteristic of 0.45 and supplied to the aperture correction circuit 8.

The aperture correction circuit 8 corrects the aperture effect caused by the limited scanning beam diameter of the image pickup tube and the image receiver, and after the processing for reproducing an image with a high resolution is performed, it is supplied to the encoder 9.

On the other hand, the bandpass filter 6 separates the chrominance signal modulated at a high frequency. This color signal is separated by a color separation circuit 10 into a baseband red signal and a blue signal. These color signals are guided to the gamma correction circuits 11 and 12, respectively, and are supplied to the gamma-corrected subtraction circuit 13. And the cutoff frequency is 0.6M
The low-pass luminance signal separated from the image signal from the AGC circuit 3 by the Hz low-pass filter 5 is converted into a predetermined color difference signal between the red signal and the blue signal by the subtraction circuit 13 via the gamma correction circuit 14. After that, the encoder 9 encodes it into the NTSC signal and outputs it from the terminal 15.

By the way, the image signal obtained from the image pickup tube 1 depends on the amount of received light, and since the amount of received light differs between a dark place and a bright place, a difference occurs in the SN ratio of the image obtained by the receiver. Since the image pickup tube 1 does not generate noise, the SN ratio of the video camera is determined by the level of noise generated by the preamplifier 2 itself and the level of the image signal obtained from the image pickup tube 1.

However, as described above, in the AGC circuit 3, the amplification factor is changed according to the signal level from the preamplifier 2 in order to obtain the NTSC signal of a constant level. Therefore, even if the image looks the same on the receiver, the SN ratio when shooting in a dark place is
Compared to when shooting in a bright place, the image quality deteriorates significantly.

Furthermore, since the aperture correction signal is created by the luminance signal that has been gamma-corrected and the low-level portion is emphasized, the aperture correction signal in the low-level portion with a poor SN ratio, that is, in the dark confidential body portion is excessive. However, there is a disadvantage that the SN ratio is further deteriorated because the brightness signal is aperture-corrected by the aperture correction signal.

[Object of the Invention]

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a signal processing system of a video camera capable of reproducing an image having a good SN ratio even in shooting under low illuminance.

[Outline of Invention]

To achieve this object, the present invention reproduces an image with an apparently good SN ratio by changing the characteristics of gamma correction particularly in a portion close to a black level with a poor SN ratio when shooting under low illuminance. It is characterized in that it has made it possible.

Example of Invention

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of a signal processing system of a video camera according to the present invention, in which 1 is an image pickup tube, 2 is a preamplifier, 3 is an AGC circuit, 4 and 5 are low-pass filters, and 6 is a band. A filter, 7 is a gamma correction circuit, 8 is an aperture correction circuit, 9 is an encoder, 10 is a color separation circuit, 11, 12, and 14 are gamma correction circuits, 13 is a subtraction circuit, 15 is an output terminal, and 16 is a control signal. is there.

In the figure, the image signal obtained from the image pickup tube 1 passes through the preamplifier 2 and the NTSC of a constant level is always provided by the AGC circuit 3.
Processing for obtaining the signal is performed and supplied to the low-pass filters 4 and 5 and the band-pass filter 6. The low pass filter 4 separates the luminance signal and supplies it to the encoder 9 through the gamma correction circuit 7 and the aperture correction circuit 8 which are the main parts of the present invention.

On the other hand, the low-band luminance signal separated by the low-pass filter 5 and the baseband red signal and blue signal separated by the color separation circuit 10 through the low-pass filter 6 are gamma correction circuits which are the main parts of the present invention. After passing through 11, 12, and 14, the subtraction circuit 13 converts the color difference signal into a predetermined color difference signal, and the encoder 9 converts the color difference signal into an NTSC signal, which is output from the terminal 15.

The gamma correction circuits 7, 11, 12, and 14 change the gamma characteristic of the black level portion by the control signal 16 from the AGC circuit 3 as shown in FIG.

The control signal 16 controls so that the inclination of the black level portion becomes smaller as the level of the image signal supplied from the preamplifier 2 to the AGC circuit 3 becomes smaller. For this reason,
When the image signal level is low, that is, when the image is taken in a dark place, the amplification factor of the black level portion having a poor SN ratio becomes small, so that the SN ratio of the image signal passed through the gamma correction circuit is improved. When shooting in a darker place, the control signal 16
Since the slope of the gamma characteristic is further reduced, the apparent SN
The deterioration of the ratio can be suppressed to a small level.

Figure 3 is a circuit diagram showing an embodiment of a gamma correction circuit constituting a main part of the present invention shown in FIG. 1, the transistor TR1 constitutes a resistor R ₁ and an emitter follower, subsequent resistor R
Through ₂ drives a circuit consisting of a transistor TR 2 and a resistor R ₇ which form an emitter follower having a high input impedance. The black level applied to the input terminal 17 of the gamma correction circuit is fixed at E ₀ , and if the image signal is a triangular wave as shown in the figure, the control signal 16 will be lower than E ₁ when shooting in a bright place. , Don't make FETTR ₃ conductive. In addition,
The voltage of each constant voltage source has a relationship of E ₁ <E 2 <E ₃ <E ₄ . As for the image signal applied to the input terminal 17, when the image level becomes larger than E ₂ , the diode D ₂ becomes conductive, and the image level of E ₂ or more of the image signal supplied from the emitter of TR 1 is the resistance R ₂
And the resistor R ₄ divides the output level appearing at the emitter terminal 18 of TR 2 becomes smaller. When the image level becomes higher than E ₃ , the diode D ₃ becomes conductive, and the image level of E ₃ or higher of the image signal supplied from the emitter of TR1 is divided by the resistor R ₂ and the parallel resistors of resistors R ₄ and R _5. And the image level above E ₃ becomes smaller. When the input image level becomes higher than E ₄ , the diode D ₄ conducts, so that the image level above E ₄ is divided by the resistor R ₂ and the parallel resistors of the resistors R ₄ , R ₅ and R ₆ .
As a result, the image signal at the output terminal 18 becomes a signal obtained by subjecting the image signal applied to the input terminal 17 to gamma correction having a gamma characteristic as shown in FIG.

By the way, when shooting in a dark place, the control signal
16 becomes a little larger than E ₁ and makes TR 3 conductive. As a result, for image level signals greater than E _{0 and} less than E ₁ , diode D ₁ conducts and resistor R ₂ and resistor
R ₃ and TR 3 are divided by the sum of the resistance shown according to the potential of the control signal 16 applied to the gate. As a result, the slope of the gamma characteristic becomes small as in the part where the input is 0.2 or less in FIG. When shooting in a dark place, the control signal
Since the potential of 16 rises and makes the conduction of TR3 better, TR3
The resistance value indicated by becomes smaller, and the inclination of the gamma characteristic of the above portion becomes smaller as shown in FIG. That is, the gamma correction circuit in the present invention is a gamma correction circuit having a normal gamma characteristic of 0.45 for shooting in a bright place, but when shooting in a dark place, that is, under low illuminance,
The potential of the control signal 16 of the C circuit 3 rises to make TR3 conductive, and the gamma value of the gamma characteristic can be changed as shown in FIG. 2 according to the darkness.

〔The invention's effect〕

As described above, according to the present invention, when shooting in a bright place, normal normal gamma correction is performed, and SN
When shooting in a dark place where the ratio is poor, the slope of the black level is reduced according to the darkness, so
The amplification factor of the black level part that makes the ratio look bad is reduced, the visual deterioration of the SN ratio is avoided, and if it is applied to a home video camera that is often used in dark places such as indoors, it can be used even in low illumination. It is possible to provide a signal processing system of a video camera which has a sufficiently good SN ratio and has an excellent function excluding the above-mentioned drawbacks of the prior art.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a signal processing system of a video camera according to the present invention, FIG. 2 is an input / output characteristic diagram of a gamma correction circuit according to the present invention, and FIG. 3 is an example of a gamma correction circuit according to the present invention. FIG. 4 is a circuit diagram showing an embodiment, FIG. 4 is a block diagram for explaining a signal processing system of a video camera according to the related art, and FIG. 5 is an input / output characteristic diagram of a gamma correction circuit according to the related art. 1 …… Sensor tube 2 …… Preamplifier 3 …… AGC circuit 4,5 …… Low-pass filter 6 …… Band filter 7,11,12,14 …… Gamma correction circuit 8 …… Aperture correction circuit 9 …… Encoder 10 ...... Color separation circuit 13 …… Subtraction circuit

Claims (1)

[Claims] 1. An image pickup apparatus for outputting an image signal, which generates a control signal indicating a change in brightness of a photographing place according to a change in level of the image signal and has a constant level regardless of the brightness of the photographing place. Gain control circuit whose gain is controlled by the control signal so that the output signal of the automatic gain control circuit is output, means for separating a luminance signal and a plurality of color signals from the output signal of the automatic gain control circuit, the automatic gain control circuit In accordance with the control signal of 1., a gamma characteristic that is controlled so that the inclination of the black level portion becomes small in a place photographed in a dark place, and gamma correction by the gamma characteristic is performed on the luminance signal and the plurality of color signals. A plurality of gamma correction circuits for performing the above, and means for generating a predetermined video signal from the luminance signal and the plurality of color signals that have been gamma corrected by the gamma correction circuit. Signal processing circuit of the video camera to be.