JPH0628388B2 - Video signal white clip circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a video signal white clip circuit, and is particularly effective when used in an RF converter for a video signal that requires good linearity.

Background Art For example, in a VTR (video tape recorder),
In order to supply the reproduced video signal to an ordinary television receiver for reproduction, a VHF is used by using an RF converter.
Modulation to the radio signal of the band is performed. in this case,
In order to prevent the spread of sidebands and deterioration of image quality due to overmodulation, so-called white clip processing is performed to limit the white level of the video signal to the reference level (for example, "Hobby Electronics Home Video" published by the Japan Broadcasting Corporation). 51-53).

As means for performing the white clip processing of this video signal, the present inventors have examined the use of a differential switching circuit formed by commonly connecting a pair of transistors to a constant current circuit.

That is, the pair of transistors are commonly connected to a constant current circuit to be complementarily conductive, and the video signal is input to one of the pair of transistors and the reference level of the white clip level is input to the other of the pair of transistors, whereby It was studied to switch and output the signal and the reference level from a common connection point on the constant current circuit side according to the level of the video signal.

According to this, when the video signal is equal to or lower than the reference level, one of the transistors becomes conductive, and the video signal input to the one transistor is linearly transmitted to the common connection point and output as it is. However, when the video signal exceeds the reference level, the other transistor becomes conductive instead of one transistor,
The reference level input to the other transistor is output to the common connection point instead of the video signal, and white clipping processing of the video signal can be performed.

However, it has been clarified by the present inventors that the above-mentioned technique causes the following problems.

That is, the transfer characteristic of the switching circuit used in the above-mentioned video signal white clip circuit ideally exhibits complete linearity (linearity), but in reality, not only the linear portion but also the curved portion is small. Included.

In this regard, according to the investigations by the present inventors,
Due to the curved portion that appears due to the equivalent resistance of the transistor, the transfer characteristics of the transistor differ between when the input signal, that is, the video signal is at a low level and when it is at a high level, which causes the output signal, that is, the wide clip processing. It has been revealed that the video signal has a non-linear distortion that deteriorates the reproduction quality.

In particular, as shown by the broken line in FIG. 2, it is clear that the blunting of the waveform that appears when the level of the video signal approaches the white level deteriorates the differential gain and differential phase that play an important role in color reproduction. Became.

[Object of the Invention] It is an object of the present invention to provide a video signal white clip circuit which improves the transfer characteristics of a video signal, particularly the linearity in the vicinity of a white level, and enables good video reproduction.

The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

[Summary of the Invention] To briefly describe the summary of the invention disclosed in the present application,
It is as follows.

That is, when the conduction of the transistor pair of the differential switching circuit that performs white clipping of the video signal is switched from one to the other, direct current feedback is generated in the other transistor to output the video signal to the reference level. Improves transfer characteristics during switching.

Thus, the object is to improve the transfer characteristics of the video signal, especially the linearity in the vicinity of the white level, and enable good video reproduction.

[Embodiment] An embodiment of a video signal white clip circuit to which the technique of the present invention is applied will be described below with reference to FIGS. 1 and 2.

In FIG. 1, 1 is an RF modulator, 1'is a white clip circuit, Q1 to Q8, Q10 to Q16 and Q20.
~ Q23 is a bipolar transistor, CS1-CS
4, CS11, CS12, CS21 to CS23 are constant current circuits, R1 to R3, R11, RL1 and RL2 are resistors, V
REF1 to VREF3 are reference levels, and Vin is a video signal.

First, in the white clip circuit 1 ', the emitters (second controlled terminals) of the pair of transistors Q3 and Q4 are commonly connected to the constant current circuit CS2, and the selective output is made from the common connection point on the constant current circuit cs2 side. A differential type switching circuit which is designed to be taken out is used. This differential type switching circuit is a kind of analog switching circuit capable of switching selection of analog signals.

A pair of transistors Q forming this switching circuit
3, the video signal Vin is input to the base (control terminal) of one (Q3) of Q3, and the reference level VREF2 of the white clip is input to the base (control terminal) of the other (Q4). , Q4 are made conductive in a complementary manner according to the level of the video signal Vin,
As a result, the video signal Vin and the reference level VREF2
Are switched and output according to the level of the video signal Vin.

The transistors Q3 and Q4 are pnp bipolar transistors, and by limiting the current Io flowing from the power supply potential Vcc side to the common emitter to a constant value by the constant current circuit CS2, the transistor having the lower base input level with respect to the reference potential is selected. When the transistor is turned on, the turned-on transistor performs a linear transmission operation to output the signal or level input to its base to the common emitter.

Further, when the conduction is switched from the one transistor Q3 to the other transistor Q4, local DC positive feedback is generated in the other transistor Q4.

This DC positive feedback is coupled to the collector (first controlled terminal) side of the other transistor Q4, and a circuit (R1, R2) for detecting the current flowing through the collector by the current mirror and this detected current A circuit for biasing the input level of the other transistor Q4 in the direction of energizing the conduction of the other transistor Q4 (Q7,
Due to R3), only the other transistor Q4 is used.

Next, the operation will be described.

In FIG. 1, a composite video signal Vin input from a video device such as a VTR is a transistor Q2.
Is linearly transmitted by the emitter follower of and is input to the base of the transistor Q3. At this time, the transistor reference voltage VREF1 and the transistor Q1 supply a stable bias voltage to the base of the transistor Q2.

The reference level VREF2 that sets the white clip of the video signal Vin is linearly transmitted by the emitter follower of the transistor Q9 and input to the base of the transistor Q4 via the resistor R1 and the transistor Q7.

The transistor Q3 to which the video signal Vin is input and the transistor Q4 to which the reference level VREF2 are input are made conductive in a complementary manner depending on the magnitude relationship between the video signal Vin and the reference level VREF2. Then, the signal / level input to the transistor Q3 / Q4 that has been made conductive is linearly transmitted and output to the common emitter of the transistors Q3 and Q4.

That is, the level of the video signal Vin is the reference level VREF2.
If so, transistor Q3 is on and transistor Q4 is off. At this time, the transistor Q
When the video signal Vin input to 3 is selected, Q3
It is output to the common emitter of Q4.

On the other hand, when the level of the video signal Vin increases and exceeds the white clip level set by the reference level VREF2, contrary to the above, the transistor Q3 is off,
The transistor Q4 turns on. At this time, the reference level VREF2 input to the transistor Q4 is selected and output to the common emitter of Q3 and Q4.

In this way, the video signal Vin and the reference level VREF2 are switched and output according to the level of the video signal Vin, so that the maximum level of the video signal Vin is the reference level V.
Limited to the white clip level defined by REF2.

Here, the transistor Q3 is on and the transistor Q4 is
When the level of the video signal Vin approaches the reference level VREF2 with the transistor turned off, the transistors Q3 and Q4
The bare transfer characteristics of the differential switching circuit consisting of
As shown by the broken line in FIG. 2, it has a non-linear distortion, and this distortion is corrected as follows.

That is, the level of the video signal Vin is the reference level V
When the transistor Q4, which has been in the off state until now, is approaching the REF2 from below and is about to be turned on, a slight amount of current starts to flow through the transistor Q5 and the resistor R1 by the transistor Q4 which is about to be in the on state. The current that has started to flow is generated by the transistor Q6 and the resistor R2.
Is transferred by the current mirror to the current path of. This causes the transistor Q6 to draw the same current flowing through the transistor Q5 and the resistor R1 from between the transistor 7 and the resistor R3.

The resistor R3 connects the reference level VREF2 to the other transistor Q.
Transistors Q8 and Q7 transmitting emitter follower to 4
Are interposed in series.

Therefore, when the current is absorbed from the resistor R3, the base potential of the transistor Q7 drops, and the base potential of the transistor Q4 also drops accordingly.

Then, the decrease in the base potential of Q4 further accelerates the ON tendency of Q4. That is, the other transistor Q4
When a current flowing in the other transistor Q4 is detected, a direct current positive feedback for energizing the conduction of the other transistor Q4 is locally generated by the detected current.

As a result, as shown by the solid line in FIG. 2, the video signal Vi
Switching from n to the reference level VREF2 can be performed very quickly and smoothly without passing through the non-linear portion shown by the broken line in the figure.

As a result, the transfer characteristic at the time of switching the output from the video signal Vin to the reference level VREF2 is corrected. Therefore, the video signal Vin, which is the input signal, does not generate the nonlinear distortion that deteriorates the reproduced image, and the predetermined reference level VRE.
Linear transmission is performed with the white clip level set by F2. Since the DC positive feedback that brings about such a correction effect is locally limited only to a specific transistor Q4, the entire operation of the white clip circuit 1'is unstable due to the DC positive feedback. There is no such thing.

Further, the transistor Q4 to which the DC positive feedback as described above is applied exhibits an appropriate hysteresis characteristic when it is switched on / off. Due to this hysteresis, it is possible to obtain an effect of preventing the switching circuit using the transistor Q4 from performing an unnecessary switching operation due to ringing and noise components that are likely to be included in the video signal Vin.

The video signal Vin has a very wideband frequency component, and therefore the transmission path of the video signal Vin is also required to have a very wideband. However, in such a wideband transmission path, ringing and noise may occur. easily influenced.
On the other hand, the ringing and noise have a great influence on the quality of reproduced images, and avoiding these influences is also very important for obtaining good reproduced image quality.

As described above, the characteristic peculiar to the transistor Q4, that is, the transfer characteristic depending on the equivalent resistance, especially the linearity when the video signal Vin is in the vicinity of the white level, is improved, which enables good video reproduction. Become.

The video signal given a predetermined white level by the above-mentioned white clip circuit 1'is the transistor Q.
The signal is input to the RF modulator 1 as a modulation signal via the emitter follower 10.

The RF modulator 1 is configured using a balanced modulation circuit that is a composite differential circuit of transistors Q11 to Q16, and is RF amplitude-modulated by a video signal that is a modulation input signal.
The video signal Vout is output.

In this RF modulator 1, the transistor Q1
1, Q12 form a differential pair, one of which (Q11) is
The video signal from the white clip circuit 1'is input and the other (Q12) receives the reference level VREF3.
Based on this, a constant base voltage VB is applied.

Transistors Q13 and Q14, Q15 and Q16 form a differential pair, respectively, and Q13 and Q16, Q14 and Q15 are alternated by RF (radio frequency) carrier signals in the VHF band injected from terminals 2 and 3, respectively. On /
Can be turned off.

Load resistors RL1 and RL are provided to the common collectors of Q13 and Q15 and the common collectors of Q14 and Q16, respectively.
2 is connected, and the RF video signal Vout is taken out from one of the load resistors RL1 to the 6th terminal.

Here, the base voltage of one (Q11) of the transistors Q11 and Q12 forming the differential pair is set to be lower than the base voltage VB of the other (Q12). As a result, negative modulation is performed so that the amplitude of the RF carrier signal increases as the level of the video signal input to the base of Q11 decreases.

Since the RF video signal obtained as described above has a small amount of non-linear distortion when the white clip level is determined, it is possible to display a video of extremely good image quality on an ordinary television receiver.

[Effects] Of the effects of the invention disclosed in the present application, the outline of a typical one will be briefly described as follows.

When conduction is switched from one transistor of the differential switching circuit to the other transistor, positive DC feedback is applied to the other transistor to correct the transfer characteristic when the output is switched from the video signal to the reference level. As a result, it is possible to improve the transfer characteristics of the video signal, especially the linearity near the white level, and to achieve good video reproduction even when the video signal is at a level close to the white level.

Although the invention made by the present inventor has been specifically described based on the embodiments, the present invention is not limited to the embodiments and various modifications can be made without departing from the scope of the invention. There is no end.

For example, a field effect transistor may be used instead of the bipolar transistor.

[Field of Use] In the above description, the application of the invention made by the present inventor to the RF converter, which is the field of use of the invention, was mainly described, but the present invention is not limited thereto.

For example, it can be used for white level correction of a video signal demodulated from an RF video signal.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a video signal white clip circuit showing an embodiment of the present invention, and FIG. 2 is a characteristic diagram showing a transfer characteristic of the white clip circuit. 1 ... RF modulator, 1 '... video signal white clip circuit, Vin ... video signal, Q3, Q4 ... Transistors forming differential switching circuit, Q5
Q6, Q7, R1, R2, R3 ... Transistors and resistors that generate positive DC feedback, Vout ... RF modulation signal

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Mita 111, Nishiyokote-cho, Takasaki-shi, Gunma Hitachi Ltd. Takasaki Plant

Claims (1)

[Claims] 1. A first controlled terminal and a second controlled terminal having control terminals and first and second controlled terminals, respectively, according to an input signal applied between the control terminal and the second controlled terminal. The second controlled terminals of the pair of transistors whose currents are controlled between the terminals are commonly connected to the constant current circuit so as to be electrically conductive in a complementary manner, and a video signal is supplied to one control terminal of the pair of transistors. By inputting the reference level of the white clip level to the other control terminal, the video signal and the reference level are switched and output from the common connection point on the constant current circuit side according to the level of the video signal. Dynamic switching circuit, a circuit coupled to the first controlled terminal of the other transistor to detect a current flowing in the other transistor, and the detected current And a circuit for biasing the input level of one transistor in the direction of energizing the conduction of the other transistor, and when the conduction is switched from one transistor to the other transistor, positive DC feedback is provided in the other transistor. A video signal white clip circuit, characterized in that