JP4386651B2 - Video signal processing circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a video signal processing circuit, and more particularly to a video signal processing circuit that amplifies a video signal.
[0002]
[Prior art]
As an amplification circuit of a video signal processing circuit that performs video signal processing, such as a DVD player and a camera-integrated VTR, there is, for example, one described in Patent Document 1. As shown in FIG. 8, a video signal input from an input terminal 81 is clamped by a clamp circuit 82 to a constant clamp voltage determined by a voltage from a DC voltage source 83 by a clamp circuit 82. 84, and output from its output terminal 85. In particular, the reference voltage from the DC voltage source 86 which is the reference of the differential amplifier circuit 84 is set to be the center of amplitude of the clamped video signal. It is to be fixed.
[0003]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 09-224173
[Problems to be solved by the invention]
[0005]
However, in the video signal processing circuit described in Patent Document 1, the differential amplifier circuit 84 amplifies the video signal gain-adjusted by the gain control circuit in the previous stage of the clamp circuit 82 although not shown. The gain of 84 is fixed, and the differential amplifier circuit 84 cannot adjust the gain by itself. In order to adjust the gain in such a differential amplifier circuit 84, for example, if the gain is switched from A times to B times as shown in FIG. 6B, output is performed to prevent distortion of the output waveform. It is necessary to increase the dynamic range (hereinafter simply referred to as the output range) more than necessary. For example, the bottom level V _out2 of the output waveform at the gain B times decreases from the bottom level V _out1 of the output waveform at the gain A times, and the bottom level of the output range needs to be lowered. The output range will be increased. In the figure, the input waveform is indicated by VIN, the output waveform is indicated by VOUT, the clamp voltage is V _clamp , and the reference voltage is V _ref .
[0006]
Further, in a conventional video signal processing circuit that requires gain adjustment, it is necessary to provide a gain control circuit separately from the amplifier circuit, and the configuration is complicated.
[0007]
An object of the present invention is to provide a video signal processing circuit that has a simple configuration that enables gain adjustment when a video signal is amplified, and that can obtain an output waveform without distortion.
[0008]
[Means for Solving the Problems]
The video signal processing circuit of the present invention includes a clamp circuit that fixes a bottom level of an input video signal to a predetermined clamp voltage, an amplifier circuit that amplifies the video signal clamped by the clamp circuit, and the amplifier circuit. A reference voltage circuit that selectively provides a plurality of reference voltages; and a control circuit that controls the gain of the amplifier circuit and the switching of the reference voltage applied from the reference voltage circuit to the amplifier circuit. The clamped video signal is input to the positive phase input terminal of the differential amplifier circuit, and a first variable resistor is connected between the negative phase input terminal of the differential amplifier circuit and the reference voltage circuit. A second variable resistor is connected between the negative phase input terminal of the differential amplifier circuit and the output terminal of the differential amplifier circuit, so that the first variable The gain is selected by switching the resistance value of the resistor and the second variable resistor, and the control circuit sets the gain and the reference voltage to the bottom level of the signal waveform of the video signal output from the amplifier circuit. It is characterized by switching so that the output range is kept constant.
[0010]
Further, the resistance values of the first and second variable resistors are R1 and R2, respectively, the voltage value of the clamp voltage is Vclamp, the voltage value of the reference voltage is Vref, and the output waveform of the differential amplifier circuit is When the voltage value of the bottom level is Vout , Vout is expressed by the relationship of Vout = Vclamp− (R2 / R1) × (Vref−Vclamp), and the control circuit is configured so that the above relationship is established . It is also preferable to switch the resistance value of the first and second variable resistors and the voltage value of the reference voltage.
[0011]
The video signal processing circuit of the present invention is preferably provided with a low-pass filter circuit that removes noise components outside the video signal band of the video signal input to the amplifier circuit.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
A configuration of a video signal processing circuit according to an embodiment of the present invention will be described with reference to FIG. As shown in FIG. 1, the video signal processing circuit 1 includes a clamp circuit 2 that clamps a video signal input from the input terminal IN to a predetermined clamp voltage, and a low-pass filter 3 that removes noise components outside the signal band of the video signal. An amplifier circuit 4 that amplifies the clamped video signal, a reference voltage circuit 5 that selectively supplies a plurality of reference voltages to the amplifier circuit 4, and a control that controls switching of the gain and reference voltage of the amplifier circuit 4 The circuit 6 is configured to output a video signal amplified with a gain selected by the control circuit 6 from an output terminal OUT.
[0013]
Next, details of the configurations of the amplifier circuit 4 and the reference voltage circuit 5 and their connection relation will be described with reference to FIG. As shown in FIG. 2, the amplifier circuit unit 4 inputs a video signal through the low-pass filter 3 using the positive phase input terminal + of the differential amplifier circuit 41 as an input terminal INPUT and the negative phase input terminal − of the differential amplifier circuit 41. The reference voltage circuit 5 is connected to the reference voltage circuit 5 via the variable resistor R1 to receive the reference voltage, and the variable resistance R2 is connected between the negative-phase input terminal − and the output terminal OUTPUT of the differential amplifier circuit 41 to provide a non-inverting amplifier circuit It consists of. The gain can be switched by switching the resistance values of the variable resistors R1 and R2. The variable resistor R1 is formed, for example, by connecting in parallel a series circuit including resistors R11, R12, and R13 and switching elements S11, S12, and S13 such as analog switches having a CMOS configuration that are connected in series with each other. Similarly, the variable resistor R2 is formed by connecting a series circuit including resistors R21, R22, and R23 and switching elements S21, S22, and S23 connected in series to each other in parallel. The reference voltage circuit 5 is a switching element S51 such as an analog switch of a CMOS configuration that selectively connects the negative phase input terminal − of the differential amplifier circuit 41 to the reference voltage sources V _ref1 , V _ref2 , and V _ref3 through the variable resistor R1. , S52, S53. The switching elements S11 to S53 are turned on and off in accordance with a control signal from the control circuit 6 to switch the gain and the reference voltage.
[0014]
Next, the operation of this example will be described. First, an outline of the operation will be described with reference to FIG. FIG. 3A shows a case where the reference voltage is switched from V _ref1 to V _ref2 , and FIG. 3B shows a case where the reference voltage is not switched. In the configuration of this example, as shown in the following equation (1), the bottom level (V _out ) of the output waveform is changed from the clamp voltage (V _clamp ) to the clamp voltage (V _clamp ) and the reference voltage (V _ref ). The level is lowered by a gain multiple of the difference ΔV.
[0015]
V _out = V _clamp − (R2 / R1) × (V _ref −V _clamp ) (1)
[0016]
For this reason, if the reference voltage is constant as shown in FIG. 3B, the bottom level of the output waveform changes when the gain is switched. Therefore, the output range of the output amplifier is higher than that in FIG. Must be taken large. However, by switching the reference voltage as well as the gain as shown in FIG. 3A, the output range can also be kept constant by keeping the bottom level of the output waveform constant.
[0017]
That is, the gain and the reference voltage are set in the amplifier circuit 4 and the reference voltage circuit 5, respectively, so as to satisfy the formula (1) while keeping the bottom level of the output waveform constant at a predetermined clamp voltage. These are appropriately switched by the control signal from 6. When the gain increases, the reference voltage is switched so that the difference between the clamp voltage and the reference voltage becomes small. When the gain decreases, the reference voltage is switched so that the difference between the clamp voltage and the reference voltage is increased.
[0018]
Next, an example of set values of the variable resistors R1 and R2 and the reference voltage sources V _{ref1 to} V _ref3 for realizing gain switching in this example will be shown. The clamp voltage (V _clamp ) of the input video signal is constant, for example, 2.0V, the bottom level of the output waveform is, for example, 1.0V, and each value is set as shown in the table of FIG. The resistors R11 to R13, R21 to R23, and the reference voltage sources V _{ref1 to} V _ref3 are used to switch the gain between the variable resistor R1 (= R11, R12, R13) and the variable resistor R2 (= R21, R22, R23). The reference voltage V _ref (= V _ref1 , V _ref2 , V _ref3 ) is also switched simultaneously with the gain switching. FIGS. 5A, 5B, and 5C show waveforms under the conditions of gains 1 to 3 in the table of FIG. That is, although not shown, the control circuit 6 includes a correspondence table of each gain, resistors R11 to R13, R21 to R23, and reference voltage sources V _{ref1 to} V _ref3 as shown in the table of FIG. When a signal for selecting a gain is received from the outside, the switching of the switching elements of the amplifier circuit 4 and the reference voltage circuit 5 is controlled according to the correspondence table to perform gain switching.
[0019]
FIG. 6 shows a comparison between the amplifier circuit 4 of the present example and the amplifier circuit of Patent Document 1 in which gain adjustment is attempted using the diagram corresponding to FIG. FIG. 6A shows a state of gain switching of the amplifier circuit 4 of this example, and FIG. 6B shows a state of gain switching in the amplifier circuit of Patent Document 1.
[0020]
As described above, in this example, since the output waveform can be stored in a predetermined output range and the gain can be switched, distortion of the output waveform can be prevented. In addition, since it is not necessary to secure a large output range and an amplifier circuit with a narrow output range can be used, it is possible to realize a video signal processing circuit with low power consumption while suppressing power consumption. Further, since it is possible to amplify the video signal while performing gain adjustment in the amplifier circuit 4, it is not necessary to separately provide a gain control circuit for gain adjustment, that is, an amplifier circuit. It contributes to that.
[0021]
In the above embodiment, for convenience, three types of gain switching have been described. However, the present invention is not limited to this. For example, as shown in FIG. 7, the variable resistor R1 has the same configuration as that of the above embodiment. The number of resistors R11 to R1n (n is an integer greater than or equal to 2) and n switching elements S11 to S1n are expanded, and similarly, the variable resistor R2 includes n resistors R21 to R2n, Similarly, the reference voltage circuit 5 is expanded to include n reference voltage sources V _{ref1 to} V _refn and switching elements S51 to S5n. Thus, it may be configured to perform n gain switching as required.
[0022]
In the above embodiment, the low pass filter 3 is provided. However, the low pass filter 3 may be omitted.
[0023]
【The invention's effect】
According to the present invention, the gain of the amplifier circuit that amplifies the video signal and the reference voltage of the amplifier circuit are switched so as to keep the output range of the video signal output from the amplifier circuit constant. In the video signal processing circuit to be used, the video signal can be amplified without distortion of the output waveform. It is only necessary to use one amplifier circuit, and it is possible to realize a video signal processing circuit that enables gain adjustment when a video signal is amplified with a simple configuration. Further, since an amplifier circuit with a narrow output range can be used, it is possible to realize a video signal processing circuit with low power consumption while suppressing power consumption.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a video signal processing circuit according to an embodiment of the present invention.
2 is a block diagram showing details of configurations of an amplifier circuit 4 and a reference voltage circuit 5 shown in FIG.
3 is an input waveform diagram and an output waveform diagram for explaining a gain switching operation of the video signal processing circuit shown in FIG. 1; FIG.
4 is a table for explaining set values of variable resistors R1, R2 and reference voltage sources V _{ref1 to} V _ref3 for gain switching of the amplifier circuit 4 and the reference voltage circuit 5 shown in FIG. 2;
5 is an input waveform and output waveform diagram by three types of gain switching based on the setting values of FIG. 4;
6 is an input waveform and output waveform diagram for comparing gain switching operations of the video signal processing circuit shown in FIG. 1 and the amplifier circuit described in Patent Document 1. FIG.
7 is a block diagram showing another configuration of the amplifier circuit 4 and the reference voltage circuit 5. FIG.
8 is a block diagram showing a configuration of an amplifier circuit described in Patent Document 1. FIG.
[Explanation of symbols]
2 Clamp Circuit 4 Amplifier Circuit 5 Reference Voltage Circuit 6 Control Circuit R1 First Variable Resistor R2 Second Variable Resistor V _{ref1 to} V _ref3 Reference Voltage Source

Claims (3)

A clamp circuit for fixing the bottom level of the input video signal to a predetermined clamp voltage;
An amplifier circuit for amplifying the video signal clamped by the clamp circuit ;
A reference voltage circuit that selectively provides a plurality of reference voltages to the amplifier circuit;
A control circuit for controlling the gain of the amplifier circuit and switching of the reference voltage applied from the reference voltage circuit to the amplifier circuit;
With
In the amplifier circuit, the clamped video signal is input to the positive phase input terminal of the differential amplifier circuit, and a first variable resistor is provided between the negative phase input terminal of the differential amplifier circuit and the reference voltage circuit. A second variable resistor is connected between the negative phase input terminal of the differential amplifier circuit and the output terminal of the differential amplifier circuit, and the resistors of the first variable resistor and the second variable resistor are connected. The gain is selected by switching the value,
The control circuit switches the gain and the reference voltage so that the bottom level of the signal waveform of the video signal output from the amplifier circuit is constant and the output range is kept constant. Processing circuit. The resistance values of the first and second variable resistors are R1 and R2, respectively, the voltage value of the clamp voltage is Vclamp, the voltage value of the reference voltage is Vref, and the bottom level of the output waveform of the differential amplifier circuit Vout is expressed by a relationship of Vout = Vclamp− (R2 / R1) × (Vref−Vclamp), and the control circuit is configured so that the above relationship is established. 2. The video signal processing circuit according to claim 1 , wherein a resistance value of a second variable resistor and a voltage value of the reference voltage are switched. 3. The video signal processing circuit according to claim 1, further comprising a low-pass filter circuit that removes noise components outside the video signal band of the video signal input to the amplifier circuit.

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