JPS637081A - Video signal processor - Google Patents

Abstract

PURPOSE:To obtain a pedestal clamp device which is at high speed and accurate with a few pieces of parts in a low cost by combining a video signal amplification means constituted with a transistor, etc. and a detection means consisting of a low speed OP amplifier, etc. CONSTITUTION:A clamp pulse which fits the timing of the pedestal part of a video signal is generated synchronizing to a horizontal synchronizing signal by a clamp pulse generation means 2 and by using the clamp pulse the difference voltage between a voltage obtained by sample holding the pedestal part of the output of a video signal amplification means 3 in a sample holding means 4 and a pedestal reference voltage generated by a pedestal reference voltage generation means 1 is detected by a detection means 15. And the DC electric potential of the output of a video amplification means 3 is controlled to set the difference voltage to be '0', so that the pedestal level of the output of the video amplification means coincids with the pedestal reference voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a pedestal clamp device for clamping video signals, and is widely applicable to hard copy devices for television images such as image printers.

BACKGROUND OF THE INVENTION FIG. 3 shows a block diagram of a pedestal clamp device used in a conventional television receiver. A conventionally used pedestal clamp device connects a first amplifier IC) 1 with low output impedance and high speed and a second amplifier 102 with high input impedance and high speed and capable of DC amplification using a capacitor 1o3 to clamp the pedestal of the video signal. This capacitor 103 is charged by making the analog switch 104 conductive with a clamp voltage during the period, and during the period other than the video signal pedestal, the DC level is shifted by the potential difference charged in the capacitor 103, and the video signal is This is to make the pedestal potential of the signal match the pedestal reference voltage 105.

Problems to be Solved by the Invention As mentioned above, the conventional pedestal clamp device requires a first amplifier with low output impedance and high speed, and a second amplifier with high input impedance and high speed and capable of DC transmission. . However, it is difficult to construct a high-speed C amplifier with a small output offset voltage. For example, an OP amplifier has a very small output offset voltage, but one that operates at high speed is very expensive. Also, high-speed devices can be configured relatively easily if they are configured using discrete components, but
The offset voltage of the output must be reduced by adjusting a variable voltage filter, etc., and there are problems in that accuracy and temperature characteristics are poor, and changes over time are large.

Means for Solving the Problems A pedestal reference voltage generating means for generating a pedestal reference voltage, a clamp pulse generating means for generating a clamp pulse from a horizontal synchronizing signal, a video signal amplifying means for amplifying a video signal, and the clamp pulse. sample and hold means for sampling and holding the output signal of the video signal amplification means, and detection means for detecting the difference in voltage between the output voltage of the sample and hold means and the pedestal reference voltage. The output DC potential of the video signal amplifying means is controlled to match the pedestal of the video signal to the pedestal reference voltage.

Operation: The clamp pulse generating means generates a clamp pulse that matches the timing of the pedestal portion of the video signal in synchronization with the horizontal synchronizing signal, and using this clamp pulse, the pedestal portion of the output of the video amplifying means is converted to the sample. detecting a difference voltage between the voltage sampled and held by the holding means and the pedestal reference voltage generated by the pedestal reference voltage generating means, by the detecting means;
The pedestal level of the output of the video amplification means is made to match the pedestal reference voltage by controlling the DC potential of the output of the video amplification means so that this differential voltage becomes zero.

Embodiment 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 circuit diagram of the same device. In FIG. 1, 1 is a pedestal reference voltage generation means for generating a pedestal reference voltage, 2 is a clamp pulse generation means for generating a clamp pulse from a horizontal synchronizing signal, 3 is a video signal amplification means for amplifying a video signal, and 4 is a video signal amplification means for amplifying a video signal. A sample and hold means samples and holds the output signal of the signal amplification means, and a detection means 6 detects a voltage difference between the output voltage of the sample and hold means and the pedestal reference voltage.

In FIG. 2, 11 and 24 are coupling capacitors, 1
9, 20.21 and 22.23 are the resistors and capacitors that create the bias voltage 49 and the pedestal reference voltage 60, respectively; 12.13 and 6B and 14, 15 are the resistors and capacitors that constitute the low-pass filter; Bias resistor, 18 is feedback gain adjustment resistor, 33.34°35.36.3
7 and 41. 42 are resistors and transistors that make up the differential amplifier; 38 and 43 are resistors and transistors that make up the emitter follower for low output impedance; 40° and 39 are analog switches and capacitors that make up the sample-and-hold circuit; 25.26 and 27.
28° 29 and 30, 31.32 are horizontal synchronization signals 4
．． An inductor, a resistor, and a capacitor constitute a delay circuit for creating a clamp pulse 52 with a delay of about 6 μs. 46 is an OP amplifier 7" that compares the sampled voltage with the pedestal reference voltage and detects the difference. 46 is a video signal input terminal;
48 is a horizontal synchronizing signal input terminal, and 48 is a video signal output terminal.

The video signal applied to the 7 to 7 terminal 46 is applied to the transistor 41 forming a differential amplifier via the coupling capacitor 11 and resistors 12 and 13 forming a low-pass filter. −
The horizontal synchronization signals applied to the blade terminal 47 are 25, 26, 2
4 by a delay circuit composed of 7, 28, 29, and 30.
．． A clamp pulse 52 is obtained which is delayed by about 5 μs and matches the timing of the pedestal portion of the video signal. Then, this clamp pulse turns on and off the analog switch 40' (i-) via the switching transistor 44, samples the voltage at the pedestal portion of the output, charges the capacitor 39, and holds it. The constant is set so that it is sufficiently charged in the time of the clamp pulse width and the potential is held for the 1H period of the video signal.The sampled and held pedestal voltage 61 and the pedestal reference voltage 60 are the differential input voltage. It is compared by an oP amplifier 45, and its output is fed back via a feedback gain adjustment resistor 18 to the base of a transistor 42 constituting a differential amplifier.Pedestal voltage 61
When is higher than the pedestal reference voltage 5o, the base potential of the transistor 42 increases, the collector potential decreases, and the output DC potential decreases. Conversely, when the pedestal voltage 61 is lower than the pedestal reference voltage 5o, the base potential of the transistor 42 decreases, and therefore the collector potential decreases and the output DC potential decreases. By applying feedback in this manner, the pedestal potential of the output video signal and the pedestal reference voltage are kept equal in the steady state, and a pedestal-clamped video signal output is obtained.

As in the ox octopus embodiment, the signal input impedance can be increased by configuring the video signal amplification means with a differential amplifier, and this can be used to configure an active filter.
It has high utility value as it can simultaneously configure anti-aliasing filters etc. when converting video signals from A/D.

Effects of the Invention According to the present invention, a video signal amplification means constituted by transistors, etc., which are capable of high-speed DC transmission and have less than 9 pages;
By combining detection means such as an OP amplifier with extremely high accuracy but low speed, a high speed and extremely accurate pedestal clamp device can be constructed without using expensive parts and with a small number of parts.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a video signal processing device according to an embodiment of the present invention, FIG. 2 is a circuit diagram of the same device, and FIG. 3 is a block diagram showing the configuration of a conventional ρ11 of a pedestal clamp device. 1... Pedestal reference voltage generation means, 2...
... Clamp pulse generation means, 3 ... Video signal amplification means, 4 ... Sample hold means, 5
...Detection means.

Claims (3)

[Claims] (1) Pedestal reference voltage generation means for generating a pedestal reference voltage, clamp pulse generation means for generating a clamp pulse from a horizontal synchronizing signal, video signal amplification means for amplifying a video signal, and amplification of the video signal by the clamp pulse. sample and hold means for sampling and holding the output signal of the means; and detection means for detecting a voltage difference between the output voltage of the sample and hold means and the pedestal reference voltage; A video signal processing device, characterized in that the output DC potential of the amplifying means is controlled so that the pedestal of the video signal matches the pedestal reference voltage. (2) The video signal amplification means is constituted by a differential amplifier, and controls the output DC potential by applying the video signal to the first input and applying the output of the detection means to the second input. A video signal processing device according to claim 1. (3) The video signal processing device according to claim 2, wherein the video signal amplification means has filter characteristics.