JPH0865544A - Video signal reproducing device - Google Patents

Abstract

PURPOSE: To reduce the deterioration of picture quality due to repeated dubbing by adding a compensating synchronizing signal extracted from a reproducing video signal having a fixed average picture level to a synchronizing signal in a reproducing video signal having a high average picture level in an in-phase state. CONSTITUTION: When a reproducing luminance signal SYIN is inputted to a buffer 12, a coefficient multiplying circuit 14 multiplies the signal SYIN by a coefficient Kg and an integrating circuit 15 generates DC voltage VDC with a fixed level and supplies the voltage VDC to a differential amplifier circuit 17. When the S is higher than the VDC, the circuit 17 outputs power supply voltage, and when the SYIN is lower than the VDC, outputs a signal having the same phase as the SYIN. When an average picture level is low in the constitution, a synchronizing part of the SYIN is not detected by the circuit 17 and the SYIN inputted to a mixer circuit 13 is outputted to a buffer 19 as it is. When the average picture level is high, compensated synchronizing voltage proportional to the average picture level is generated and added to the synchronizing part through the circuit 13. Consequently the synchronizing signal can be processed in a state almost fixing its amplitude.

Description

Detailed Description of the Invention

[0001]

[Table of Contents] The present invention will be described in the following order. Fields of Industrial Application Conventional Technology (FIGS. 4 to 7) Problem to be Solved by the Invention (FIG. 7) Means for Solving the Problem Action Example (FIGS. 1 to 3)

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal reproducing apparatus, which is suitable for application to, for example, a video tape recorder.

[0003]

2. Description of the Related Art FIG. 4 shows a circuit configuration generally used as a reproduction luminance signal processing circuit of a video tape recorder 1. The reproduction signal reproduced by the reproduction head 2 is R
After being amplified by an F (radio frequency) amplifier 3, it is separated into a luminance signal component and a chrominance signal component, and processed by a luminance signal processing circuit 4 and a chrominance signal processing circuit (not shown). These signals are output to the Y / C mixer circuit 5
Are combined and output as a video signal.

Here, the luminance signal processing circuit 4 includes a high-pass filter 4A, a demodulation circuit 4B, a de-emphasis circuit 4C,
It is composed of a low-pass filter 4D and a plurality of amplification circuit stages 4E. Of these, the configuration of each amplifier circuit stage 4E is shown in FIG.
Shown in The amplifier circuit stage 4E is a cascade connection of the amplifier circuit 4E1 and the emitter follower output stage circuit 4E2,
Electrolytic capacitors C _IN and C provided on the input side and the output side
It is designed to be electrically connected to the front and rear circuits via _OUT .

Incidentally, the structure of the emitter follower output stage circuit 4E2 is shown in FIG. 6, and its operation state will be described with reference to FIG. In FIG. 7, the horizontal axis represents the voltage V _CE applied between the collector and emitter of the transistor Q1, and the vertical axis represents the collector current I _C. Also, the solid line in the figure represents the DC load line,
The alternate long and short dash line represents the AC load line. As shown in FIG. 6, the DC load resistance seen from the emitter side of the transistor Q1 is only the emitter resistance R _E , but the AC load resistance is the parallel impedance Z (= R _E of the emitter resistance R _E and the load resistance R _L). ‖R _L ). Therefore, the gradient of the AC load line becomes steeper than the gradient of the DC load resistance, and the load becomes heavy.

[0006]

However, in this circuit configuration, the average picture level (APL: Average Picture Lebe
When l) is high, there is a problem that a phenomenon in which part of the sync waveform is missing (sync clogging) is likely to occur. This is because when the average image level is high (for example, 100 [%]), the voltage of the sink portion becomes close to the power supply voltage V _C , so that the collector current stops flowing midway and a part of the sink waveform is lost ( This is because a so-called sync jam occurs). Incidentally, when the average image level is low (for example, 0 [%]), since the sync portion is near the operating point P, sync clogging is unlikely to occur.

Another cause of the sink clogging is that the gradient of the AC load line is steep (collector current I
_C and the load resistance R _L are small and the emitter resistance R _E is large), or the number of output stages of the emitter follower is large. Therefore, in order to prevent such sink clogging, the collector current I _C may be increased, the load resistance R _E may be decreased, or the number of stages may be reduced in order to eliminate the above cause. Also, NPN transistor Q1
The PNP transistor may be connected to Darlington to enhance the drive capability of the output stage of the emitter follower.

However, in this case, there is a problem that the current consumption increases and the circuit element (the number of parts) increases and the cost rises. Further, the feasibility is limited, and it is not ideal. Therefore, in the current circuit, it is inevitable that the sync waveform is gradually collapsed in the case of a signal having a high average image level. Therefore, when repeating dubbing with a video tape recorder, an automatic gain adjustment (AGC: Auto Gain Control) circuit in the luminance signal recording processing system adjusts the gain so that the level of the H sync becomes constant and records the video signal. Therefore, if the dubbing is repeated, the level of the video signal is unavoidably increased, and there is a problem that the image quality is deteriorated.

The present invention has been made in consideration of the above points, and it is an object of the present invention to propose a video signal reproducing apparatus capable of further reducing deterioration of image quality when a video signal is repeatedly dubbed.

[0010]

In order to solve such a problem, in the present invention, the reproduced video signal is level-shifted according to the average image level so that the average image level (V _bias ) after the level shift becomes a predetermined potential. The level shift means and the reproduced video signal (S _YIN ) output from the level shift means are compared with a predetermined DC voltage (V _DC ), and the reproduced video signal (S _YIN ) and the DC voltage (V _DC ) cross each other. In this case, a correction sync signal (V) having an amplitude proportional to the average image level (V _bias ) from the reproduced video signal (S _YIN )
Sync signal extraction means (17) for extracting and outputting _SYNC0 )
If, when the average picture level of the reproduced video signal (S _YIN) (V _bias) is low and outputs the reproduced video signal (S _YIN), average picture level of the reproduced video signal (S _YIN) (V _bias) is If it is higher, the synchronization signal extraction means (17)
And a sync signal expansion means (13) for adding the corrected sync signal (V _SYNC0 ) input from the in-phase to the sync signal of the reproduced video signal (S _YIN ) in phase to expand and output the amplitude of the sync signal. To

Further, according to the present invention, the clamp means for clamping the tip potential of the synchronizing signal included in the reproduced video signal to a constant potential and the reproduced video signal (S _YIN ) outputted from the clamp means are inputted to the reproduced video signal. comparing the threshold voltage generating means, and a reproduced video signal (S _YIN) and DC voltage (V _DC) for generating a signal DC voltage proportional to the average picture level (V _bias) in (S _YIN) (V _DC) Then, when the reproduced video signal (S _YIN ) and the DC voltage (V _DC ) cross each other, a correction synchronization signal (V _SYNC0 ) having an amplitude proportional to the average image level (V _bias ) of the reproduced video signal is extracted. When the average image level (V _bias ) of the reproduced video signal (S _YIN ) is low, the reproduced video signal is output as it is, and the average of the reproduced video signal (S _YIN ) is output. Image level (V _{When the bias} is high, the correction sync signal (V _SYNC0 ) input from the sync signal extraction means (17) is added in-phase to the sync signal of the reproduction video signal to expand the amplitude of the sync signal for output. A signal expansion means (13) is provided.

[0012]

[Action] By shifting a level of the reproduced video signal (S _YIN) so that the average picture level (V _bias) is constant potential, the reproduced video signal when the average picture level (V _bias) is high (S _YIN 2) is lower than the tip potential of the sync signal when the average image level (V _bias ) is low. Therefore, the reproduced video signal (S
When comparing _YIN ) with a predetermined DC voltage (V _DC ), the sync signal intersects the DC voltage (V _DC ) when the reproduced image signal (S _YIN ) has a high average image level (V _bias ) and the average A correction sync signal (V _SYNC0 ) having an amplitude proportional to the image level (V _bias ) is extracted. This correction synchronization signal (V
_{By adding SYNC0} ) to the sync signal of the reproduced video signal (S _YIN ) in phase, it is possible to effectively avoid the possibility that the sync signal waveform of the reproduced video signal is crushed even if the average image level (V _bias ) is high. As a result, good image quality can be maintained even if dubbing is repeated.

On the other hand, after the tip potential of the sync signal included in the reproduced video signal is clamped to a constant potential, the reproduced video signal is compared with the DC voltage (V _DC ) proportional to the average image level (V _bias ). In this case, when the average image level (V _bias ) of the reproduced video signal (S _YIN ) is high, the DC voltage (V _DC ) crosses the sync signal portion of the reproduced video signal,
A corrected sync signal (V _SYNC0 ) having an amplitude proportional to the average image level is extracted. After that, the amplitude of the synchronization signal can be expanded by the same processing as that described above. As a result, good image quality can be maintained even if dubbing is repeated.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings.

In this embodiment, a schematic structure of an amplifier circuit stage used in a luminance signal reproduction processing system of a video tape recorder is shown in FIG. 1, and its circuit example is shown in FIG. Amplifier circuit stage 11
First, the reproduction luminance signal S _YIN is first input to the buffer stage 12 of the first stage, and then the output thereof is divided into three signal paths and transmitted.

The first signal path is a main signal path for supplying the mixer circuit 13 with the emitter voltage of the transistor Q1 which constitutes the buffer stage 12 formed of the transistor Q1 and is level-shifted by a DC voltage proportional to the average image level. The reproduced luminance signal S _YIN is output. That is, as indicated by the thin line in FIG. 3A, the average image level of the reproduction luminance signal S _YIN output from the transistor Q1 is determined based on the DC bias voltage V _bias given to the base of the transistor Q1 and is always set to a constant potential. It is designed to be.

Next, the second signal path will be described.
This signal path outputs the output of the buffer stage 12 to the coefficient multiplication circuit 1
4, a path that is applied to the differential amplifier circuit 17 through the integrating circuit 15 and the voltage shift circuit 16 in order, and is configured to generate a DC voltage V _DC having a magnitude corresponding to the DC bias voltage V _bias . This DC voltage V _DC becomes a threshold voltage used for correcting the sync signal of the reproduction luminance signal S _YIN when the average image level is high, and in this example, has a constant potential.

Incidentally, each circuit existing in this signal path is constructed as follows. The coefficient multiplying circuit 14 is composed of resistors R1 and R2 connected in series, and outputs an output obtained by dividing the reproduction luminance signal S _YIN by a resistance ratio to the integrating circuit 15. By the way, in this processing, the coefficient K _{1 is set} to the reproduction luminance signal S
_This corresponds to the process of multiplying _YIN .

The integrating circuit 15 is composed of a resistor R3 and a capacitor C3, and its resistance value and capacitance are set so that the time constant τ becomes long. As a result, the DC voltage V of a constant level does not depend on the level of the average image level of the reproduction luminance signal S _YIN.
DC is generated. The voltage shift circuit 16 is a current mirror connection circuit of the transistors Q2 and Q3. By setting the value of the current flowing through the transistor Q2 and the resistors R3 and R2 in order, the resistor R3 and the capacitor C3 are set.
The potential appearing at the midpoint of connection is level-shifted. The output of the voltage shift circuit 16 is the DC voltage V _DC shown by the broken line in FIG.

Finally, the third signal path will be described. This signal path is a signal path for _{supplying the} reproduction luminance signal S _YIN to the differential amplifier circuit 17. The amplifier circuit stage 11 receives the DC voltage V _DC input via the second signal path and the third signal path by the differential amplifier circuit 17 formed of the differential pair of the transistors Q4 and Q5. It is determined whether or not the sync portion needs to be corrected by comparing it with the input reproduction luminance signal S _YIN .

Here, the differential amplifier circuit 17 uses the reproduction luminance signal S.
_When the potential of _YIN is higher than the DC voltage V _DC , the power supply voltage is output from the output end, whereas when the potential of the reproduction luminance signal S _YIN is lower than the DC voltage V _DC , the same phase as the reproduction luminance signal S _YIN is output from the output end. It is designed to output the signal of. Incidentally, in the case of this embodiment, the level of the DC voltage V _DC applied to the transistor Q5 is such that the average image level is approximately 100%.
_Is set to a potential such that the sync portion of the reproduced luminance signal S _YIN can be detected. Therefore, as shown in FIG. 3B, the differential amplifier circuit 17 generates a sync signal having an amplitude proportional to the average image level only when the average image level of the reproduced luminance signal S _YIN is approximately 100 [%]. It is designed to occur.

The amplifier circuit stage 11 is connected to the coefficient multiplication circuit 18
In, the sync signal is multiplied by K ₂ and compressed, and the compressed sync signal is given to the mixer circuit 13 as a correction sync V _sync0 to be added in phase to the sync signal of the reproduction luminance signal S _YIN . Incidentally, the coefficient K ₂ is a constant determined by the resistance ratio of the resistors R6 and R7. The mixer circuit 13 is composed of a capacitor C4 and is configured to add the output of the coefficient multiplication circuit 18 to the reproduction luminance signal S _YIN only when an AC component (that is, a correction sync signal) is generated.

Thus, the correction sync V _sync0 can be added to the reproduction luminance signal S _YIN only when the average image level of the reproduction luminance signal S _YIN is high. As a result, the reproduced luminance signal S is transferred from the mixer circuit 13 to the buffer stage 19 in the subsequent stage.
It is possible to output a reproduction luminance signal S _YOUT without a _clog in the sync waveform regardless of the average image level of _YIN .

The signal processing operation of the amplifier circuit stage 11 used in the luminance signal reproduction processing system in the above configuration will be described. First, when the average image level is low, in this case, the voltage of the sync portion of the reproduction luminance signal S _YIN has a higher potential than the DC voltage V _DC generated in the second signal path, so the differential amplifier circuit 17 Also in, the sync part is not detected.
Accordingly, the reproduction luminance signal S _YIN input to the mixer circuit 13
Is the buffer stage 1 of the subsequent stage as it is via the first signal path.
9 will be output.

On the other hand, the average image level is high and almost 100
When it is close to [%], in this case, the potential of the sync portion of the reproduction luminance signal S _YIN decreases, so the voltage of the sync portion input to the differential amplifier circuit 17 becomes lower than the DC voltage V _DC , and the average image level A correction sync V _sync0 is generated which is proportional to This correction sync V _sync0 is given to the mixer circuit 13 and added to the sync part where the amplitude becomes shorter as the average image level becomes higher. As a result, the amplitude of the sync waveform of the reproduced luminance signal S _YIN output to the subsequent stage via the buffer stage 19 can be stabilized substantially constant regardless of the average image level.

In addition, in the case of the amplifier circuit stage 11, since the time constant τ of the integrating circuit 15 used in the second signal path is long, the emitter potential of the transistor Q1 forming the buffer stage 12 is changed due to the variation of the temperature characteristic. Even if it fluctuates, this voltage fluctuation can be followed, and since the differential amplifier circuit 17 can also cancel the temperature characteristic, the amplitude value of the correction sink V _sync0 does not fluctuate depending on the operating temperature. It is possible to prevent the correction amount from having temperature characteristics.

According to the above configuration, it is possible to realize the amplifier circuit stage 11 which can transmit the H sink of the reproduction luminance signal S _{YIN to} the _subsequent stage without crushing it while keeping the power consumption low and the number of components low. As a result, even if the dubbing of the video signal having a high average image level is repeated, it is possible to eliminate the risk that the level of the video signal will increase, and it is possible to keep the quality of the recorded video signal high.

In the above embodiment, the reproduced luminance signal S _YIN level-shifted in proportion to the average image level.
However, the present invention is not limited to this, and the reproduction luminance signal S _YIN in which the sync tip potential is clamped is averaged. The sync signal may be extracted when the average image level is high by comparing with a direct current voltage that is a potential proportional to the image level. Even in this case, the corrected sync V _{sync0 in} which the amplitude of the sync waveform becomes larger as the average image level becomes higher can be generated, and the same effect as in the case of the above-described embodiment can be obtained.

Further, in the above-mentioned embodiment, the case where the amplitude variation of the H sync is reduced has been described, but the present invention is not limited to this and can be applied to the prevention of the amplitude variation of the V sync.

[0030]

As described above, according to the present invention, the reproduced video signal level-shifted so that the average image level becomes a constant potential irrespective of the reproduced video signal and the predetermined DC voltage are compared to perform correction synchronization. A video signal reproducing device that can perform signal processing while keeping the amplitude of the sync signal constant by extracting the signal and adding the corrected sync signal to the sync signal of the playback video signal with a high average image level in phase Obtainable.

Further, the reproduction video signal whose tip potential of the synchronization signal is clamped to a constant potential is compared with a DC voltage proportional to the average image level thereof to extract a correction synchronization signal, and the correction synchronization signal of the average image level is extracted. The same effect can be obtained also when adding in-phase to the sync signal of the high reproduced video signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit configuration of an amplifier circuit used in a video signal reproducing apparatus according to the present invention.

FIG. 2 is a connection diagram illustrating a circuit example of an amplifier circuit.

FIG. 3 is a signal waveform diagram for explaining a signal processing operation.

FIG. 4 is a block diagram showing a circuit configuration of a video signal reproducing device.

FIG. 5 is a block diagram showing a schematic configuration of each amplifier circuit.

FIG. 6 is a connection diagram showing a circuit configuration of a conventionally used amplifier circuit.

FIG. 7 is a signal waveform diagram showing operation characteristics of a conventionally used amplifier circuit.

[Explanation of symbols]

1 ... Video tape recorder, 2 ... Playback head, 3 ...
... RF amplifier, 4 ... Luminance signal processing circuit, 5 ... Y / C
Mixer circuit, 11 ... Amplifying circuit stage, 12, 19 ... Buffer stage, 13 ... Mixer, 14, 18 ... Coefficient multiplying circuit, 15 ... Integrating circuit, 16 ... Voltage shift circuit, 17
...... Differential amplifier circuit.

Claims (3)

[Claims] 1. A level shift means for level-shifting a reproduced video signal according to the average image level so that the average image level after the level shift becomes a predetermined potential, and a reproduced video signal output from the level shift means and a predetermined level. Sync signal extracting means for comparing the direct-current voltage of the above and the reproduced video signal with the direct-current voltage to extract and output a corrected synchronous signal having an amplitude proportional to the average image level from the reproduced video signal. When the average image level of the reproduced video signal is low, the reproduced video signal is output as it is, and when the average image level of the reproduced video signal is high, the correction synchronization signal input from the synchronization signal extraction means. Is added to the sync signal of the reproduced video signal in phase to expand the amplitude of the sync signal and output the sync signal. The video signal reproducing apparatus for. 2. A clamp means for clamping a tip potential of a sync signal included in a reproduced video signal to a constant potential, and a reproduced video signal output from the clamp means, which is input, and is proportional to an average image level in the reproduced video signal. Comparing the reproduced video signal and the DC voltage with the threshold voltage generating means for generating the DC voltage, and when the reproduced video signal and the DC voltage intersect, the average image level of the reproduced video signal. Sync signal extraction means for extracting and outputting a corrected sync signal having an amplitude proportional to, and when the average image level of the reproduced video signal is low, the reproduced video signal is output as it is, and the average image of the reproduced video signal is output. When the level is high, the corrected sync signal input from the sync signal extraction means is added in-phase to the sync signal of the reproduced video signal to obtain the sync signal of the sync signal. Video signal reproducing apparatus characterized by comprising a synchronization signal extension means for outputting the stretched width. 3. The video signal reproducing apparatus according to claim 1, wherein the synchronizing signal is a horizontal synchronizing signal.