JP2548220B2 - Video signal processing device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clamp circuit required when A / D converting a video signal, such as a frame memory or a hard copy device for a television screen such as an image printer. It is widely available.

2. Description of the Related Art FIG. 3 is a block diagram showing a conventional example of a video signal processing device. FIG. 4 is a waveform diagram showing the signal waveform of each part. In FIG. 3, 301 is an input terminal to which a video signal k including a negative sync signal is input, 302 is a non-inverting amplifier that non-inverts and amplifies this video signal to an appropriate amplitude, and 303 is black from the input video signal k. A clamp pulse generation circuit for generating a pulse m at a level timing, 304 is a non-inverting amplifier 302
Clamping means for flanking the output 1 of the, 305 is a DC buffer of low output impedance without level shift, and 306 is D
Amplitude limiting circuit that limits the output amplitude of C buffer 305, 30
Reference numeral 7 is an A / D converter for A / D converting the output of the amplitude limiting circuit 306.

Further, 341 is a capacitor that constitutes the clamp means 304, and 34
2 is an analog switch for turning on / off the output terminal of the capacitor 341 and the ground voltage of the power supply by a pulse m, and 36
Reference numerals 1 and 362 are resistors that form the DC buffer 307, and reference numerals 363 and 364 are transistors that operate as emitter followers.

The video signal k applied to the input terminal 301 is non-inverted and amplified by the non-inverting amplifier 302 to a voltage suitable for the input voltage range of the A / D converter 307, and the amplified video signal 1 is DC-buffered through the capacitor 341. Added to 305. The clamp pulse generation circuit 303 generates a pulse m for turning on the analog switch 342 at the timing of the black level of the input video signal, the analog switch 342 charges or discharges the capacitor 341, and the potential n at the output end of the capacitor 341 becomes the ground potential. Will be equal. At times other than the black level timing, the analog switch 342 is turned off and the voltage charged in the capacitor 341 is kept constant. Therefore, the output 1 of the non-inverting amplifier 302 is input to the DC buffer 305 in the form of being DC-shifted by the voltage of the capacitor 341. To be done. The video signal converted to low impedance by the DC buffer 305 is limited by the amplitude limiting circuit 306 so that the white peak voltage is below the positive reference voltage of the A / D converter.
Input to 307. Since it is advantageous in terms of S / N to make the input voltage range of the A / D converter as large as possible, a positive reference voltage is usually used as the positive power supply voltage V _CC of the A / D converter (often 5 [ V]) and the negative reference voltage is grounded to 0 [V]. Therefore, the range from the black level of the video signal to the white peak is A / D converted.

Problems to be Solved by the Invention According to the method of the conventional example, the DC buffer 305 needs to pass a signal with a potential near the black level, that is, near 0 [V] with low impedance, and the emitter potential of the transistor 364 is Since it swings to a potential near 0 [V], a single power supply cannot be used to supply a sufficient current to the emitter resistor 362, and a power supply with positive and negative polarities is required. Therefore, there are disadvantages that the cost is high, the apparatus is large, and the weight is high.

Alternatively, if the clamp potential of the analog switch is set to 2 to 3 [V] by using a reference voltage source, it can be configured without a negative power source, but the number of bits of the A / D converter cannot be used effectively. There is a problem. The number of bits can be effectively used by setting the negative reference voltage of the A / D converter to the same potential as the clamp potential, but the analog input amplitude of the A / D converter becomes smaller, which results in poor S / N. Another problem arises. In addition, a new voltage source is needed to create the clamp potential.

In view of the above points, the present invention has an object to provide a video signal processing device that can be easily configured with a single power supply, has a good S / N, and can effectively use the number of bits of an A / D converter. To do.

Means for Solving the Problems An amplifier for inverting and amplifying an input video signal or for non-inverting amplification of an input inverting video signal, a first voltage source for applying a voltage to the amplifier, and a clamp from the first voltage source. A second voltage source for generating a reference voltage, a clamp means for clamping the black level of the inverted video signal output from the amplifier to the clamp reference voltage, a DC buffer, and an A / D converter for A / D converting the output of the DC buffer. Since the converter and the converter are provided, a negative power supply is unnecessary, and a single power supply constitutes an analog signal processing circuit in the preceding stage of the A / D converter.

The amplifier amplifies the input video signal to an amplitude suitable for the input range of the A / D converter to obtain an inverted video signal, and the first voltage source generates a voltage for operating this amplifier,
The second voltage source generates a clamp reference voltage, the clamp means clamps the black level of the inverted video signal to the clamp reference voltage, the DC buffer converts it into a low impedance signal, and the A / D converter converts the digital data. Convert to.

Embodiment An embodiment of the present invention is shown in FIG. The signal waveform of each part is shown in FIG. In FIG. 1, 101 is a terminal to which a positive video signal a is input, 102 is an inverting amplifier that amplifies this video signal to an appropriate amplitude b, and 103 is a pulse c from the input video signal a at a black level timing. , 104 is a clamp means for pedestal clamping the inverted video signal b, 141 is a capacitor which constitutes the clamp means 104, 142 is an analog switch which also constitutes the clamp means 104, and 105 is a DC level shift There is no DC buffer, 151 is a resistor that constitutes the DC buffer 105, 153 and 154 are transistors that operate as emitter followers, 152, 155, and 156 are resistors and transistors that configure a current mirror circuit, and 106 is an A / D converter. is there.

Further, 107 is a voltage source for generating a voltage V _CC for operating this video signal processing device, and 108 is a clamp reference voltage V _CR.
Is a second voltage source for generating.

The video signal a applied to the terminal 101 is A /
The inverted video signal b is inverted and amplified to have an amplitude suitable for the input voltage range of the D converter 106, and the DC signal is output through the capacitor 141.
It is added to the buffer 105. Clamp pulse generation circuit 103
Generates a pulse c for turning on the analog switch 142 at the timing of the black level of the input video signal, and the capacitor 141 is charged or discharged by this pulse, and the potential d of the output unit is
Is equal to V _CR . The output b of the inverting amplifier 102 is output because the analog switch 142 is turned off and the voltage charged in the capacitor 141 is kept constant except at the black level timing.
Is input to the DC buffer 105 in a DC shifted form. DC
The output current of the output transistor 154 of the buffer 105 is 0.3
A low impedance output can be obtained up to about [V]. This output is applied to the input terminal of A / D converter 106, and A / D converter 1
Since 06 performs A / D conversion from 0 [V] to V _CR , by inverting all bits of the output, digital image data whose black level of the video signal corresponds to digital data 0 can be obtained.

Further, when a peak voltage with an excessive amplitude is applied to the input terminal 101, the transistor 154 is cut off, so that it is possible to have the effect of a peak limiter. In the case of a normal video signal, such an excessive signal is not included, and even if the peak of the white part is cut, it has almost no effect on the entire image, which is rather convenient for a printer with a narrow dynamic range of the recording system. Is.

According to the configuration of this embodiment, the input amplitude of the A / D converter can be made large without using the negative power source, so that the S / N is good and the reference voltage range of the A / D converter is input. It is possible to configure a signal processing device having an advantage that A / D conversion can be performed by effectively using the number of bits because the maximum amplitude can be substantially matched.

EFFECTS OF THE INVENTION According to the configuration of the present invention, the S / N is good and the number of bits is effectively used with a simple configuration without using a negative power source.
It is possible to configure a video signal processing device that has the advantage that it can perform / D conversion.

[Brief description of drawings]

FIG. 1 is a block diagram of a video signal processing device according to an embodiment of the present invention, and FIG. 2 is a waveform diagram showing signal waveforms of respective parts thereof,
FIG. 3 is a block diagram of a conventional example of a video signal processing device, and FIG.
The figure is a waveform diagram showing the signal waveform of each part thereof. 101 ... video signal input terminal, 102 ... inverting amplifier, 103 ...
… Clamp pulse generation circuit, 104 …… Clamping means, 105
... DC buffer, 106 ... A / D converter, 107 ... first voltage source, 108 ... second voltage source, 141 ... capacitor, 142
…… Analog switches, 151 and 152 …… Resistance, 153,1
54,155,156 …… Transistor.

Claims (2)

(57) [Claims] 1. An amplifier for inverting and amplifying an input video signal or for non-inverting amplification of an input inverting video signal, a first voltage source for applying a voltage to the amplifier, and a clamp reference voltage from the first voltage source. A second voltage source, a clamp means for clamping the black level of the inverted video signal output from the amplifier to the clamp reference voltage, a DC buffer, and an A / D converter for A / D converting the output of the DC buffer. Video signal processing device equipped with. 2. The video signal processing device according to claim 1, wherein a positive reference voltage of the A / D converter is made to coincide with the clamp reference voltage.