JPH0710093B2 - Automatic black level setting circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic black level setting circuit for a video portion when an analog video signal is digitally converted.

2. Description of the Related Art In recent years, a technique for converting an analog signal into a digital signal and digitally processing the signal has been actively studied. This has become particularly remarkable in the field of image processing, and is indispensable when displaying an image on a new display element such as a liquid crystal. However, in order to reach the image quality level of a conventional image displayed on a cathode ray tube by processing an analog signal, a large number of circuit components are required, and at the present time, it is costly to increase overall. Unavoidable. Therefore, various attempts have been made to reduce the cost by reducing the number of circuit components as much as possible. This is also the reason for reducing the number of digitized bits in the case of A / D conversion. In order to obtain a better image with the limited number of bits, there is a problem in which range of the analog video signal is digitized, that is, how to set the white level and the black level.

An example of the above-described conventional level setting circuit will be described below with reference to the drawings.

FIG. 6 shows the function of a conventional A / D converter with level setting. FIG. 7 is a waveform model of a video signal in the horizontal period. FIG. 8 is an actual circuit diagram of a conventional level setting circuit.

In FIG. 6, 7 is a white level voltage supply unit, 8 is a black level voltage supply unit, and 9 is an A / D converter.

The operation of the level setting circuit configured as described above will be described below.

First, in FIG. 6, when an analog video signal is input to the U _A terminal of the A / D converter 9, it is digitally converted,
U _D terminal (the number of terminals is determined according to the number of bits) is the digital output from. At this time, the highest level of the digital output is determined by the voltage of the a terminal (the voltage of the white level voltage supply section 7), and the lowest level is set by the voltage of the b terminal (the voltage of the black level voltage supply section 8).

Here, the white level voltage supply unit 7 and the black level voltage supply unit 8
Are fixed, and therefore, the black level of the video signal is adjusted so that the video signal input to the A / D converter 9 becomes the voltage level by the black level voltage supply section 8. The white level of the video signal is adjusted so that the voltage level by the level voltage supply unit 7 is reached. They are,
In each case, the video signal is pedestal clamped (the pedestal level is fixed to a certain DC voltage), the pedestal voltage is adjusted, and then the video signal is amplitude-adjusted.

Now, considering the video signal waveform of FIG. 7, assuming that the pedestal part is set to the black level and the peak part is set to the white level, the highest level of the digital output of the A / D converter 9 is The video component of the peak portion of the signal, the lowest level, represents the video component of the pedestal portion.

FIG. 8 shows a concrete circuit of FIG. The white level and black level setting voltages are determined by the voltage division ratio of the resistors R11, R9, R8. The paired transistors of Q8 and Q9 and Q6 and Q7 are for supplying the above divided voltage with low impedance to the white level setting terminal V _H and the black level setting terminal V _L of the A / D converter IC1. . In this circuit, HA19203 of Hitachi, Ltd. IC is used as the A / D converter IC1.

However, in the above-mentioned configuration, when the white level voltage and the black level voltage are set as shown in FIG. 7, the video signal within the voltage range V is A / D changed. Although it is digitally output, the maximum black level of the video signal is higher than the black level voltage threshold by ΔV, and considering that the portion required as an actual video signal is in the range of V−ΔV, It cannot be said that the digital output is efficient. That is, as the gradation level of the signal, (V-ΔV)
It means that the contrast is reduced at the ratio of / V.

In view of the above problems, the present invention provides an automatic black level setting circuit that automatically sets a black level voltage to a level according to the blackest level of an image signal in order to efficiently convert an analog image signal into a digital signal. Is.

Means for Solving the Problems In order to solve the above problems, the automatic black level setting circuit of the present invention includes a video portion extracting portion for extracting only a signal in a video information section from a video signal to be digitally converted, and this video portion. A black level detector that detects the highest black level of the signal from the sampling unit, a black level voltage stabilizer that holds the detected highest black level voltage for a certain period of time, and the voltage of this black level voltage stabilizer is A / D converted. It is configured by means for giving the black level setting terminal of the container.

The present invention has the above-described configuration to detect the blackest level of the video portion of the video signal, and the A / D
This is to automatically set the black level of the converter, and the black level setting voltage changes in response to the black level of the video portion of the video signal, so the A / D conversion of the video signal is performed efficiently. be able to.

Example Hereinafter, an automatic black level setting circuit according to an example of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of an automatic black level setting circuit according to an embodiment of the present invention. In FIG. 1, 1 is a video signal buffer unit, 2 is a waveform shaping unit, 3 is a video part extracting unit, 4 is a black level detecting unit, 5 is a black level voltage stabilizing unit, 6 is a black level voltage buffer unit, and 7 is A white level voltage supply unit, 8 is a black level voltage supply unit, and 9 is an A / D converter.

The operation of the automatic black level setting circuit configured as described above will be described below with reference to FIGS. 1 and 2.

First, FIG. 2 shows the state of the signal by the video signal in each part of FIG. 1, and the video signal buffer unit 1 receives the video signal A (A ′). Here, A and A'are the same video signal, and A mainly indicates the horizontal period,
A'denotes a vertical equivalent pulse period. This relationship is the same for B and B '. On the other hand, a signal for removing a signal other than the video information section, for example, a blanking pulse is input to the waveform shaping section 2, and this is shaped by the waveform shaping section 2 into a waveform of B (B ′). At this time, it should be noted that the waveform of B (B ') is always higher than the voltage of the pedestal portion except the image portion of the image signal. In the video part extracting section 3, a logical sum of the waveforms of A and B is obtained and becomes a waveform of C. Black level detector 4
Then, the blackest level of the C waveform (ΔV from the pedestal level)
(There is only a high voltage position), the voltage level is input to the black level voltage stabilizing unit 5, the black level voltage is held for a certain period, and the stabilized voltage of D is output from the black level voltage buffer unit 6. Output.

This voltage is applied to the black level setting terminal of the A / D converter 9 via the black level voltage supply unit 8 to effectively expand the digital video output dynamic range of the A / D converter 9.

Here, in order for the automatic black level setting circuit of this embodiment to operate properly, the voltage of the blackest level of the video signal input to the A / D converter 9 is the black level voltage buffer unit 6 of FIG. It is best to match the output D of the above, and therefore the video signal applied to the A / D converter 9 is the same as the input video signal of the video signal buffer unit 6.

Further, the signal level of the output waveform B of the waveform shaping section 2 can be arbitrarily changed by selecting a circuit constant (this will be clarified in a concrete example of FIG. 3 described later), and by this,
A voltage level higher than the pedestal level of the output waveform C of the image part extracting unit 3 by V _P is determined, but this is still the upper limit of the blackest level detected by the black level detecting unit 4. That is, when ΔV becomes larger than V _P in the waveform C, the black level detection unit 4 detects the level of V _P. When the upper limit is not set, the signal level of the output waveform B of the waveform shaping section 2 may be set so that V _P becomes larger than V. on the other hand,
It is possible to prevent noise from mixing in the video signal, and this level may become lower than the pedestal level, causing the circuit to malfunction and lowering the voltage of the black level setting pin of the A / D converter from falling below the pedestal voltage. Therefore, the lower limit can be set at the output portion of the black level voltage buffer unit 6. This will be clarified in a concrete example of FIG. 3 described later.

FIG. 3 shows a concrete embodiment of the automatic black level setting circuit of the present invention. 4 and 5 show an embodiment in which a part of FIG. 3 is modified. In Figure 3, IC1 is an A / D converter,
I am using IC HA19203 manufactured by Hitachi, Ltd.

In FIG. 3, the video signal is applied to the base of the transistor Q1 and output to the emitter. On the other hand, a pulse for removing a portion other than the image, for example, a blanking pulse is applied to the base of the transistor Q2 via the diode D1 and output to the collector. When transistor Q2 is on, diode D2 does not conduct and the video signal
Added to the base of Q3. When the transistor Q2 is off, the voltage level of the video signal changes the power supply voltage V _CC to the resistance R1,
When the voltage level is lower than the voltage level divided by the diode D2 and the resistor R4, the diode D2 becomes conductive, and the divided voltage of the resistor R1, the diode D2, and the resistor R4 is applied to the base of the transistor Q2.
Therefore, the upper limit of the maximum black level detected by the video signal is determined by the values of the resistors R1 and R2.

The video signal combined at the base of the transistor Q3 is output to the emitter and given to the base of the transistor Q4. Since the transistors Q3 and Q4 are composed of two stages of emitter followers, the emitter voltage of the transistor Q4 instantly responds to the blackest level voltage of the video signal applied to the base of the transistor Q3. The emitter of the transistor Q4 has a sufficiently large time constant of the resistor R6 and the capacitor C1 even if the voltage of the video signal becomes higher than the blackest level voltage. keeping. This voltage is input to the voltage supply unit for the black level setting terminal of the A / D converter composed of the transistors Q6 and Q7 through the transistor Q5.

Here, the base voltage of the transistor Q5 is the power supply voltage V _CC.
Is lower than the voltage divided by resistors R9 and R8, the base voltage of transistor Q6, that is, the black level setting voltage,
Therefore, the lower limit of the black level detected by the video signal can be set by the resistors R9 and R8.

Furthermore, the voltage to the black level setting terminal of A / D converter IC1 is
Since the voltage of the black level of the video signal applied to the base of the transistor Q1 (the black level of the video input to the A / D converter) is the same, the automatic black level setting circuit of this example operates correctly. This is a necessary matter. In Fig. 3, transistors Q1 and Q3, Q4 and Q5, Q6 and Q7 are paired, and the voltage between the emitter and the base is canceled.
The above requirements can be satisfied. This is easy because the blackest voltage detected by the transistor is not amplified.

Further, as shown in FIGS. 4 and 5, by placing a diode D or a resistor R between the emitter of the transistor Q5 and the base of the transistor Q6, the voltage of the black level setting terminal of the A / D converter IC1 is changed to a video signal. It is also possible to adjust the degree of black level setting by lowering the voltage of the highest black level by the amount of the diode D or the resistance R.

In FIG. 3, it is clear that the circuit including the black level voltage supply units Q6 and Q7 does not impair the configuration of the present invention even if the circuit is reduced according to the use of the A / D converter.

As described above, the same video signal that is input to the A / D converter is used as the video signal, and the voltage of the blackest level of the video part of the video signal and the black level setting terminal of the A / D converter are used. Since the voltage of the circuit is the same, it is not necessary to adjust the voltage of the black level setting terminal, and
It is possible to reduce variations in temperature drift and black level setting.

In addition, it is possible to set either the upper limit or the lower limit for detecting the blackest level of the video signal, or both, and the voltage applied to the black level voltage supply unit can be adjusted, so the video signal is A / D converted. Finally, it can be used as a flexible circuit.

Advantageous Effects of Invention According to the present invention as described above, the voltage of the black level setting terminal of the A / D converter automatically changes according to the voltage of the blackest level of the video signal input to the A / D converter. Therefore, the necessary portion of the analog input video signal can be efficiently digitally converted as compared with the case where the voltage of the black level setting terminal is fixed.

[Brief description of drawings]

FIG. 1 is a block diagram of an automatic black level setting circuit according to an embodiment of the present invention, FIG. 2 is a waveform diagram showing a signal state of a video signal in each part of FIG. 1, and FIG. 3 is an automatic black level of the present invention. FIG. 4 is a circuit diagram showing a concrete example of the level setting circuit, FIGS. 4 and 5 are circuit diagrams showing an essential part of an embodiment in which a part of FIG. 3 is modified, and FIG. 6 is a conventional level setting. FIG. 7 is a block diagram showing the function of the A / D converter, FIG. 7 is a diagram showing a waveform of a video signal in a horizontal period as a model, and FIG. 8 is a circuit diagram of a conventional level setting circuit. 1 ... video signal buffer section, 2 ... waveform shaping section, 3 ...
Image portion extracting unit, 4 ... Black level detecting unit, 5 ... Black level voltage stabilizing unit, 6 ... Black level voltage buffer unit, 7
...... White level voltage supply section, 8 ...... Black level voltage supply section,
9 ... A / D converter.

Claims (1)

[Claims] 1. A waveform shaping section for shaping the blanking pulse so as to output a waveform having a voltage higher than the pedestal level of the video signal to be A / D converted, and a section other than the video information section of the video signal. The output of the waveform shaping section that is output,
The video part extracting unit that combines and outputs the video signal, the black level detecting unit that detects the lowest voltage level from the output of the video part extracting unit, and the voltage detected by the black level detecting unit A black level voltage stabilizing unit that holds a predetermined period during which the video signal is A / D converted, and means for applying the voltage of this black level voltage stabilizing unit to the black level setting terminal of the A / D converter to which the video signal is input. An automatic black level setting circuit characterized by having and.