JPH0153547B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a level detection circuit used in an automatic amplitude control circuit for video signals.

Video camera or VTR (video tape recorder)
In devices that handle video signals, such as, it is often necessary to stabilize the amplitude of the video signal. For example, in the case of a video camera, when the intensity (illuminance) of illumination light that illuminates a subject changes, the amplitude of the video output signal of the video camera changes, resulting in insufficient amplitude or excessive amplitude. Normally, in such cases, the amplitude of the output signal is kept constant at an appropriate value by controlling the aperture of the lens or the gain of the video signal amplification circuit, but the above control is particularly difficult for home video cameras. It is desirable that the process be automated to simplify operations.

The most common method to automatically control the video signal amplitude is to detect the output signal level, compare its magnitude with a specified value, generate an error signal based on the comparison result, and use this as a control signal to control the lens aperture. This method provides feedback to the gain control terminal of the device or video signal amplification circuit.

The concept of such an automatic control system can be roughly divided into a peak value control type and an average value control type, depending on the characteristics of the level detection circuit used therein.

These two ideas will be explained using FIG. In FIG. 1, A and B are representative examples of subjects, with the shaded areas being dark areas and the blank areas (non-hatched areas) being bright areas. C to F are output signal waveforms (horizontal scanning waveforms) when the respective subjects are imaged, C and D are peak value control type, and E and F are average value control type.

The peak value control type is a method of detecting the peak value of the video output signal and controlling the gain so that it reaches a specified level (l ₁ ). In such a case, the output signal waveform will be as shown in (c). In this case, the shaded area (dark area) that occupies most of the screen is reproduced as a darkened image, and even if this area contains the image content that you want to photograph, it will not be visible at all. For example, when photographing a person, just a small bright light shining on a part of the background can cause the person's image to appear dark. In order to alleviate such a phenomenon, if the specified output force level of the peak value is set high in advance, then in a bright area where the duty ratio is large as shown in (b), and in this bright area, If there is image content to be photographed, the output signal level in this bright area becomes excessive, resulting in an unsightly image.

On the other hand, the average value control type is a method of integrating the video output signal waveform and controlling the gain so that the integrated value (that is, the average value) becomes a specified level (l ₂ ). The output signal waveform of is as shown in E. In this case, the shaded areas (dark areas) are reproduced brighter than necessary, and the blank areas (bright areas) have excessive amplitude. For example, if you take a long distance shot of a person being bathed in a spotlight on stage,
This is a phenomenon in which the dark parts of the background are reproduced more prominently than necessary, and the most important part of the person's face is washed out in white due to excessive amplitude. In order to alleviate this phenomenon, if the standard level of the average value is set low in advance, the output signal amplitude will tend to be insufficient when photographing a subject with a large duty ratio in bright areas, such as in B. This results in another poor image.

As described above, both the peak value control type and the average value control type have their own drawbacks, and it is not appropriate to use them alone. Therefore, it is common practice to use a combination of the two characteristics, taking advantage of the fact that the characteristics of the two tend to contradict each other.

FIG. 2 shows a conventional example of a video signal level detection circuit having characteristics such as a combination of both. In the figure, 1 is an input terminal of the level detection circuit, and 2 is an output terminal. In this circuit, the combination of peak value type and average value type is determined by the resistance
Determined by constant settings of R ₁ and R ₂ . For example, _R2
When is made infinite, capacitor _C1 is charged to the bright peak value of the video signal applied to input terminal 1 (we will explain that the forward offset voltage of diode _D1 is not present for simplicity), and this The voltage, that is, the peak value appears at the output terminal 2 while being held. Also, conversely, by decreasing R ₂ ,
When R ₂ ≪ R ₁ is set, the charge charged in C ₁ is R ₂
To compensate for this, the charge charged to C ₁ through D ₁ and R ₁ increases,
In other words, since the period during which D ₁ is conducting increases, C ₁ is charged to approximately the average DC voltage of the signal obtained by dividing the input signal by R ₁ and R ₂ . R ₁
By appropriately selecting the constant relationship between and R ₂ , it is possible to obtain a voltage at the output terminal 2 that is intermediate between the peak value and the average value. However, in such a conventional circuit, the degree of freedom in combining the two is insufficient.
For example, when trying to strengthen the mean component, R ₂
If is made small, the average value obtained as described above will only be obtained by dividing the average value of the input signal by R ₁ and R ₂ , and a sufficient voltage will not be obtained.

An object of the present invention is to eliminate the drawbacks of the prior art as described above, and to enable the setting of the peak value regulation level and the average value regulation level to be performed independently, and also to freely set the mixing ratio of the peak value element and the average value element. Therefore, it is an object of the present invention to provide an amplitude detection circuit with a high degree of freedom in combinations of peak value type and average value type.

The key point of the present invention is that the input signal to the level detection circuit (that is, the signal whose signal level should be constant and is a positive polarity video signal) is capacitor-coupled.
The voltage is guided to a first voltage comparator that detects the average value element and a second voltage comparator that detects the peak value element, and the outputs of both are appropriately added to form the output signal of the level detection circuit.

More specifically, the first voltage comparator is configured such that the bias potential of the capacitor-coupled circuit is
The first voltage whose potential is lower by the specified average level that you want to set.
The second voltage comparator has a comparison reference potential of ,
It has a second comparison reference potential that is higher by a voltage related to the specified peak value level that you want to set, and a comparison output signal is generated at a portion of the signal waveform higher than this second reference potential, and this is used as the peak value element. The average value element and the peak value element detected in this way are added at an appropriate mixing ratio, and this is used as the output signal of the level detection circuit.

Hereinafter, one embodiment of the present invention will be explained in detail with reference to FIG. In the figure, 1 is an input terminal of a level detection circuit, 2 is an output terminal, 3 is a coupling capacitor, 4 is a bias supply resistor, 5 is a bias power supply for the capacitor coupling circuit, 6 is a first voltage comparator 61 7 is its input terminal, 7 is the input terminal of the second voltage comparator 71, 8 is the power supply for the first comparison reference potential, 9 is the power supply for the second comparison reference potential, 10 is the mixing circuit, and 11 is the smoothing capacitor. It is.

The case where a video signal as shown in FIGS. 4a and 4b is applied to the input terminal 1 will be explained. The portion A of the waveform in FIG. 4 corresponds to the black level during the blanking period, and the portion B is the white peak portion. The input terminals 61 and 7 of the first and second voltage comparators are
The video signal waveform obtained in 1 is superimposed on the voltage V0 of the bias power supply 5, as shown in the figure.
The average level of the waveform matches the bias voltage _V0 . Since the portion A corresponds to the black level, the voltage between A and C represents the average value of this video signal. The first voltage comparator generates an output signal when the voltage applied to its input terminal 61 is less than the first comparison reference potential (V ₀ -V ₁ ), (level 2 in FIG. 4). For example, as in the case of the waveform in Figure 4a, when part A reaches the level D, that is, when the average value of the input signal waveform (voltage between A and C) reaches V ₁ . A first voltage comparator generates an output signal. This is an average value element.

On the other hand, the second voltage comparator 7 generates an output signal when the voltage applied to its input terminal 71 is equal to or higher than the second comparison reference potential (V ₀ +V ₂ ) (level E in FIG. 4). do. For example, as in the case of the waveform in Figure 4b, when the part B (white peak part) reaches the level E, that is, when the peak value (voltage between E and E) of the input signal waveform changes from the average value ( The second voltage comparator generates an output signal when the voltage minus the I-H voltage reaches _V2 . In other words, an output signal is generated when the difference between the peak value and the average value is large, or in other words, when the average value is low but the peak value is high (a signal with a small bright duty ratio). This is the peak value element.

The output signals of the first and second voltage comparators are mixed by a mixing circuit 10, and a smoothing capacitor 1
1 and used as a signal to be fed back to the lens aperture device of the video camera or the gain control terminal of the video signal amplification circuit. When the duty ratio of the bright part is relatively large and the average value is large, as shown in the waveform in Figure 4a, the first voltage comparator that detects the average value element starts operating first, and the effect of the feedback described above is detected. This works to keep the average value of the signal constant. Also, when the bright duty ratio is small, only the peak value is large, and the average value is small, as shown in the waveform in Figure 4b, The second voltage comparator, which detects the peak value component, starts operating first and also works to keep the difference between the peak value and the average value constant due to the feedback effect. In this case, since the average value is originally low, it approximately corresponds to the peak value being kept almost constant.

In this way, the video signal amplitude can be changed so that it acts as an average value type for signals with a relatively large average value, and as a peak value type for signals with a small average value and a large peak value. An automatic control system is obtained. Then, the first and second comparison reference potentials are appropriately set, and the mixing circuit 10
By appropriately setting the mixing ratio of the average value element and the peak value element, both elements can be freely combined and desired characteristics can be obtained.

As explained above, according to the present invention, the average value is detected for a video signal with a large average value and a small peak value, and the average value is detected for a video signal with a large peak value and a small average value. A video signal level detection circuit that can detect the peak value and freely set the combination of both can be obtained. If this level detection circuit is used to configure an automatic control system for the video signal amplitude of a video camera, etc.,
It is possible to always obtain a relatively good image without being greatly influenced by the magnitude of the bright duty ratio of the signal waveform or the illumination conditions of the subject. In addition, this circuit is
It is suitable for IC implementation, and in particular, if the comparison reference potential of the voltage comparator is kept stable, it is sufficient to set the amplitude of automatic amplitude control without adjustment, and it is also effective in reducing costs.

[Brief explanation of drawings]

FIG. 1 is a waveform diagram showing an example of an object and its corresponding video signal, FIG. 2 is a circuit diagram of a conventional example of a video signal level detection circuit, and FIG. 3 is an embodiment of a video signal level detection circuit according to the present invention. FIG. 4 is a video signal waveform diagram for explaining the operation of the embodiment shown in FIG. 1...Input terminal, 2...Output terminal, 3...Coupling capacitor, 4...Bias supply resistor, 5...
...bias power supply, 6...first voltage comparator, 7...second voltage comparator, 8...first
power source for comparison reference potential, 9...power source for second comparison reference potential, 10...mixing circuit, 11...smoothing capacitor.

Claims (1)

[Claims] 1. A first comparator that generates an output voltage of one polarity when the voltage at the first input terminal is lower than the voltage at the second input terminal; a second comparator that generates an output signal of the same polarity as the voltage when the voltage is higher than the reference voltage; a reference voltage source that generates a DC voltage; bias means that supplies a reference DC voltage to the first input terminals of the first and second comparators; a first comparison voltage supply means for supplying the two input terminals;
The second comparison voltage supply means supplies a second comparison voltage higher than the reference DC voltage to the second input terminal of the second comparator, and the mixing means mixes the output signals of the first and second comparators. Features video signal level detection circuit. 2 The difference between the reference DC voltage and the first comparison voltage is the specified average level of the video signal whose amplitude is controlled by the output of the mixing means, and the difference between the reference DC voltage and the second comparison voltage is the specified average level of the video signal whose amplitude is controlled by the output of the mixing means. Claim 1, wherein the difference is a difference between a specified peak value level and a specified average value level of the video signal.
The video signal level detection circuit described in .