JPH0415673B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal processing circuit suitable for a video tape recorder or the like.

In recent years, video tape recorders (hereinafter referred to as VTRs) have become widespread and are now being used in many ordinary households. in this way,
The widespread use of VTRs is due to various factors, one of which is thought to be the significant improvement in the quality of reproduced images. This improvement in image quality is due to the fact that VTRs can be constructed with high precision mechanically and electrically, and as a result, can achieve highly precise operation.

Such highly accurate operation is performed based on the synchronization signal of the video signal to be recorded and reproduced. In other words, when looking at home VTRs, in order to frequency modulate the video signal with a predetermined frequency shift, the gain is controlled so that the amplitude of the video signal is constant, and the leading edge of the synchronization signal of the video signal is set to a predetermined DC voltage. It is necessary to set the video signal to a specified amplitude level by fixing the level, and the automatic gain control circuit and clamp circuit for this purpose are operated based on the synchronization signal of the video signal. Further, the color signal is frequency-converted to a lower frequency band by a frequency conversion circuit and recorded, and a synchronization signal is used to generate a frequency conversion signal for this purpose. Furthermore, the servo system for controlling tape running, head rotation, etc. is also operated using the synchronization signal as a time reference.

As mentioned above, the synchronization signal of the video signal is used as a reference signal for the operation of the VTR, but the synchronization signal is separated from the video signal by a synchronization separation circuit provided in the video signal processing circuit. I'm trying to get the best out of it.

FIG. 1 is a block diagram showing an example of a conventional video signal processing circuit, in which 1 is an input terminal, 2 is a variable gain amplifier circuit, 3 is a low-pass filter, 4 is a buffer amplifier circuit, 5 is a clamp circuit, and 6 is a block diagram showing an example of a conventional video signal processing circuit. is a frequency modulation circuit, 7 is an output terminal, 8 is a low-pass filter, 9 is a synchronous separation circuit, 10 is a clamp circuit, 11 is a comparison circuit, 12 is a reference voltage source, 13 is an output terminal, 14 is a gain control signal detection circuit , 15 is an addition pulse generation circuit, 16 is a test wave signal generation circuit, 17 is a smoothing circuit, and 18 is an output terminal.

FIGS. 2A to 2F are waveform diagrams showing signals at various parts in FIG. 1, and signals corresponding to those in FIG. 1 are given the same reference numerals.

Next, the operation of this prior art will be explained.

In FIGS. 1 and 2, a color video signal from an input terminal 1 is supplied to a variable gain amplifier circuit 2,
The amplitude is controlled to be constant. The color video signal from the variable gain control circuit 2 is supplied from an output terminal 18 to a color signal processing circuit (not shown), and at the same time is supplied to a low-pass filter 3 to separate a luminance signal, which is then supplied to a buffer amplifier circuit 4. luminance signal a
is obtained.

The luminance signal a is supplied to the clamp circuit 5, and with the synchronization signal c obtained by the synchronization separation circuit 9 as a key pulse, the leading end of the synchronization signal is set to a predetermined DC level V _d
The luminance signal d is fixed at . The luminance signal d is supplied to a frequency modulation circuit 6, and a frequency modulated luminance signal with a predetermined frequency shift is obtained at an output terminal 7.

The luminance signal a from the buffer amplifier circuit 4 is further supplied to a low-pass filter 8 to obtain a luminance signal b from which undesired high-frequency components have been removed, and the luminance signal b is supplied to a sync separation circuit 9. In the synchronization separation circuit 9, the luminance signal b is clamped by a feedback clamp circuit 10 and supplied to the negative terminal of a comparison circuit 11. A reference voltage source 1 is connected to the positive terminal of the comparator circuit 11.
A reference voltage V _b is supplied from No. 2, and the synchronizing signal c is separated using this as a separation reference level. The synchronizing signal c is supplied from the output terminal 13 to a color signal processing circuit and various servo systems (not shown) together with the clamp circuit 5 and the gain control signal detection circuit 14.

From the synchronization signal c supplied to the gain control signal detection circuit 14, an addition pulse signal e delayed by a certain time τ ₂ from the trailing edge is formed in the addition pulse generation circuit 15 and is supplied to the test wave signal generation circuit 16. . The test wave signal generation circuit 16 is further supplied with a luminance signal d from the clamp circuit 5, and during the period of the addition pulse signal e, the luminance signal d is at a level between the level in this period and the level V _d of the leading edge of the synchronization signal. The difference is amplified by a predetermined times (140/40 times in the case of the NTSC system) the amplitude V _c of the synchronizing signal. The timing of the addition pulse signal e is the luminance signal d.
Therefore, a luminance signal f is obtained in which a pulse signal 19 having an amplitude corresponding to the amplitude V _c of the synchronizing signal is added to the back-portion period of the luminance signal d. Note that the amplitude of the pulse signal 19 becomes 100% white level when the amplitude V _c of the synchronizing signal is a normal amplitude. The luminance signal f is supplied to the smoothing circuit 17, smoothed, and supplied to the variable gain amplifier circuit 2 as a gain control signal.

When the amplitude of the color video signal from input terminal 1 is large and the amplitude of the synchronization signal is large, the luminance signal d
The amplitude of the pulse signal 19 added to the signal increases, and the gain control signal from the gain control signal detection circuit 14 acts to decrease the gain of the variable gain amplifier circuit 2, and conversely, the amplitude of the color video signal decreases. When the amplitude of the synchronization signal is small, the pulse signal 1
9 becomes smaller, and the gain control signal acts to increase the gain of the variable gain amplifier circuit 2.
Therefore, automatic gain control is performed so that the amplitude of the synchronizing signal is constant, a luminance signal with a constant amplitude is obtained, and a frequency modulated luminance signal with a specified frequency shift is obtained at the output terminal 7.

Here, the low-pass filter 8 is provided to remove high-frequency components of the luminance signal a that cause undesired pulses to be generated in the synchronization separation circuit 9. That is,
When receiving the input video signal in a weak electric field, the signal becomes a signal with S/N deterioration and waveform distortion, and as a result, depending on the luminance signal, the luminance signal a from the buffer amplifier circuit 4 may be As shown in Figure A, the waveform may be created. Therefore, if this luminance signal a is directly supplied to the synchronization separation circuit ₉ , as shown in FIG. . The low-pass filter 8 is provided to prevent the generation of such undesired pulses 20, 20', and is
It has the effect of relaxing the brissute (Fig. 3A).

Next, the separation reference level _b provided by the reference voltage source 12 in the comparator circuit 11 of FIG. 1 will be explained.

As a result of supplying the luminance signal a to the low-pass filter 8,
The leading edge of the synchronization signal of the luminance signal b becomes smooth, as shown in FIG. 2B. Therefore, as shown in FIG. 2c, the separated synchronization signal c is obtained with a delay of a time τ ₁ corresponding to the separation reference level V _b than the synchronization signal in the video signal a. This delay time τ ₁ varies depending on the amplitude V _a of the synchronization signal (FIG. 2A) and the separation reference level V _b , and the phase of the synchronization signal c separated by the synchronization separation circuit 9 fluctuates.

Incidentally, in home VTRs, in order to frequency-convert the color signal into a low frequency band signal in the color signal processing circuit, a signal having a frequency that is an integral multiple of the horizontal synchronization frequency, for example, 40 times, is used. In order to obtain a signal of such a frequency, a PLL (phase locked loop) circuit is provided which uses a horizontal synchronization signal as a reference signal. When a horizontal synchronization signal with phase fluctuation is supplied to such a PLL circuit, the frequency stability of the resulting signal deteriorates. In particular, the separation criteria level
When V _b is set close to the tip of the synchronizing signal, the phase fluctuation of the separated synchronizing signal c becomes large even with small fluctuations in the amplitude V _a of the synchronizing signal or the separation reference level V _b , which increases the PLL circuit. The frequency stability of the output signal will deteriorate. Furthermore, in the servo circuit system, if a synchronizing signal accompanied by phase fluctuation is supplied, this will appear as a fluctuation in the tape head running system, and the stability of the image will deteriorate. In order to prevent this, it is necessary to set the separation reference level V _b as high as possible so that the phase fluctuation of the separated synchronization signal can be suppressed to a sufficiently small level.

On the other hand, if the separation reference level V _b is increased, the separation reference level V _b may not touch the synchronous signal part due to fluctuations in the amplitude V _a of the synchronous signal, and therefore the synchronous signal cannot be accurately separated. Cases may arise where this is not possible. In order to eliminate this problem, it is necessary to set the separation reference level V _b to a level as close to the leading edge of the synchronization signal as possible, and to widen the allowable variation range.

In view of the above, the practically acceptable separation reference level _Vb is generally selected to be approximately 30% of the synchronizing signal amplitude from the leading edge of the synchronizing signal.

By the way, when the radio wave reception condition is poor, the video signal obtained at the input terminal 1 is considerably degraded, and as shown in FIG. 4A, the signal has a considerable amount of pre-shoots. At this time, in the above conventional technology, as shown in FIG. 4B, the low-pass filter 8
However, even if the _synchronous separation circuit 9 Unwanted pulses 21 will occur along with the signal.

Therefore, when the signal from the synchronization separation circuit 9 is supplied to the gain control signal detection circuit 14, a pulse signal 22 due to the undesired pulse 21 is added to the luminance signal d.
A luminance signal (FIG. 4D) is obtained, which is also added. Since the amplitude of the pulse signal 22 is significantly larger than the white level, the gain control signal obtained from the gain control signal detection circuit 14 acts to reduce the gain of the variable gain amplifier circuit 2. However, the amplitude of the luminance signal that has passed through the low-pass filter 8 becomes smaller, and the pre-cut portion becomes even more at the separation reference level.
If _Vb is exceeded, an undesired pulse other than the sync signal will be obtained from the sync separation circuit 9, and the variable gain amplifier circuit 2 will be erroneously locked in the minimum gain state. However, when recording and monitoring a color video signal supplied to a television receiver (not shown) through the variable gain amplifier circuit 2, or when recording and reproducing such a color video signal for monitoring, the image quality is There was a drawback that sufficient brightness could not be obtained, resulting in dark images.

The purpose of the present invention is to eliminate the drawbacks of the above-mentioned prior art,
An object of the present invention is to provide a video signal processing circuit that can significantly reduce malfunctions of an automatic gain control circuit.

To achieve this objective, the present invention provides that a synchronization separation circuit generates first and second synchronization signals at first and second separation reference levels, respectively, and A second separation reference level is set close to the leading edge level of the synchronization signal, and the second
The second synchronization signal separated at the separation reference level operates the automatic gain control circuit, and the first synchronization signal separated at the first separation reference level operates the color signal processing circuit, the servo circuit, etc. It is characterized by operating other circuits.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 is a block diagram showing an embodiment of the video signal processing circuit according to the present invention, in which 11' is a comparison circuit, 12' is a reference voltage source, and parts corresponding to those in FIG. 1 are given the same reference numerals. I will omit some of the explanation.

6A, B, and C are waveform diagrams showing signals of each part in FIG. 5, and signals corresponding to those in FIG. 5 are given the same reference numerals.

Next, the operation of this embodiment will be explained.

In FIGS. 5 and 6, similarly to the prior art shown in FIG. 1, the luminance signal a from the buffer amplifier circuit 4 has its pre-cut reduced by the low-pass filter 8, becomes the luminance signal b, and is sent to the sync separation circuit 9. Supplied. In the sync separation circuit 9, the luminance signal b is clamped by a clamp circuit 10 and supplied to comparison circuits 11 and 11'.

The comparison circuit 11 uses the reference voltage source 12 to
A separation reference level V _b that is almost the same as that of the comparison circuit 11 in the figure is set, and a synchronization signal with little phase fluctuation can be obtained by the separation reference level V _b . The comparison circuit 11' is set to a separation reference level V' _b which is closer to the tip of the synchronizing signal and lower than the separation reference level V _b of the comparison circuit 11 by the reference voltage source 12' _. The synchronizing signal g is obtained by this, and almost no undesired pulses are generated due to preshoots or the like.

The synchronizing signal from the comparison circuit 11 is supplied to the clamp circuit 5 as in the prior art shown in FIG. 1, and is also supplied from the output terminal 13 to a color signal processing circuit and various servo systems (not shown).

On the other hand, the synchronizing signal g from the comparison circuit 11' is supplied to the gain control signal detection circuit 14, which generates a gain control signal as described above. Comparison circuit 11'
does not generate undesired pulses, the gain of the variable gain amplifier circuit 2 operates accurately so that the amplitude of the synchronizing signal is constant according to the amplitude of the color video signal from the input terminal 1, and the gain of the variable gain amplifier circuit 2 operates accurately so that the amplitude of the synchronizing signal is constant according to the amplitude of the color video signal from the input terminal 1. As in, no malfunction will occur.

FIG. 7 is a circuit diagram showing a specific example of the synchronous separation circuit suitable for integration as shown in FIG. Transistors, 37 to 44 are resistors, 45 to 49 are constant current sources, 50 is a power supply voltage line, 51 is an external resistor, 52 and 53 are output terminals, and the parts corresponding to those in FIG. 5 are given the same symbols. ing.

In FIG. 7, the synchronous separation circuit 9 is integrated within an IC according to an integration range 23, and resistors 24, 51 and a capacitor 25 are externally attached to this.

The feedback type clamp circuit 10 includes an external resistor 24, an external capacitor 25, transistors 26 to 30, resistors 37 to 41, and a constant current source 45.
~47, the synchronization signal tip of the video signal which is the output of the low-pass filter 8, the separation reference level
V _b and V _b ' (Figure 6B) are fixed at specified levels. Separation level V _b is obtained at the connection point of resistors 39 and 40, and separation reference level V _b ' is obtained at the connection point of resistors 40 and 40.
41 connection points are obtained.

The comparator circuit 11 is composed of transistors 31 to 33, a resistor 42, and a constant current source 48, and the voltage at the connection point between the resistors 39 and 40 of the clamp circuit 10 is applied to the base of the transistor 31, so that the separation reference level V _b is Set. The synchronization signal separated by the comparison circuit 11 is obtained at the output terminal 52.

The comparison circuit 11' includes transistors 34 to 36,
Consisting of resistors 43, 44 and a constant current source 49,
The voltage at the connection point between the resistors 40 and 41 of the clamp circuit 10 is applied to the base of the transistor 34 to set the separation reference level V _b '. Comparison circuit 1
The synchronizing signal separated at 1' is obtained at output terminal 53.

As described above, the synchronization separation circuit can be integrated into an integrated circuit, and a small, lightweight, and low-cost video signal processing circuit can be realized. In addition, in Fig. 7,
Although the above effect is obtained by feeding back the separation reference level, it is clear that the same effect can be obtained by feeding back the video signal from the low-pass filter 8.

As explained above, according to the present invention, the separation reference level is set near the leading end of the synchronization signal to separate the synchronization signal, and the automatic gain control circuit is operated by the synchronization signal. Undesired pulses other than the synchronization signal are not supplied to the gain control circuit, and malfunctions of the automatic gain control circuit can be prevented, and at the same time, other predetermined separation reference levels can be set. This method separates a synchronization signal with little phase fluctuation, and uses this synchronization signal to operate other circuits that are subject to significant deterioration during phase fluctuation, such as color signal processing circuits and servo circuits. Even a circuit with a large value can be operated stably, and a video signal processing circuit with excellent functions can be provided without the drawbacks of the above-mentioned prior art.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an example of a conventional video signal processing circuit, Fig. 2 A to F are waveform diagrams showing signals of each part of Fig. 1, and Fig. 3 A and B are diagrams of one circuit of Fig. 1. 4A to 4D are waveform diagrams for explaining the malfunction of the automatic gain control circuit of FIG. 1, and FIG. 5 is a waveform diagram for explaining the operation of the automatic gain control circuit of FIG. FIGS. 6A, B, and C are block diagrams showing signals of each part of FIG. 5, and FIG. 7 is a circuit diagram showing a specific example of the synchronous separation circuit of FIG. 5. 1...Input terminal, 2...Variable gain amplifier circuit, 7
...Output terminal, 8...Low-pass filter, 9...Synchronization separation circuit, 11,11'...Comparison circuit, 12,1
2'... Reference voltage source, 14... Gain control signal detection circuit.

Claims (1)

[Claims] 1. An automatic gain control circuit to which an input video signal is supplied, and a synchronization separation circuit that separates a synchronization signal from the output video signal of the automatic gain control circuit, the synchronization signal being output from the synchronization separation circuit. In a video signal processing circuit that supplies the synchronization signal to the automatic gain control circuit and uses it as an operation reference signal for a desired external circuit, the synchronization separation circuit has a first separation signal set for the input video signal. A first synchronization signal is generated based on a reference level, and a second synchronization signal is generated based on a second separation reference level that is a level closer to the synchronization tip side of the input video signal than the first separation reference level. , wherein the first synchronization signal is the synchronization signal supplied to the desired external circuit, and the second synchronization signal is the synchronization signal supplied to the automatic gain control circuit. processing circuit.