JPS62115986A - Video signal processing circuit - Google Patents

Abstract

PURPOSE:To enable a non-adjustment of a recoding current by disposing an amplifier between a low pass filter and a detecting circuit and passing through the amplifier when using no burst signal amplifier circuit to control an automatic chroma control circuit by a control signal. CONSTITUTION:A burst signal level of a low frequency converted carrier chrominance signal outputted by an LPF is detected and according to this detecting result, a gain control of an ACC circuit 7 provided at a front stage side of a frequency converting circuit 9 is performed. After an amplitude is controlled in the ACC circuit 7, only the burst signal is usually 6dB amplified in a burst signal amplifier circuit 8. This signal is applied to a frequency converting circuit 9, the frequency is converted by a carrier signal fs to have a low frequency converting frequency of 629kHz and a frequency component of about 4.8MHz and only the low frequency converting frequency is separated in an LPF 20 including an IC. Since the ACC detection is carried out by the output of the LPF 20, a gain dispersion in the burst emphasis circuit 8, the frequency converting circuit 9 and the LPF 20 can be absorbed. Thereby, a recording signal level is not required to be adjusted.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a video signal processing circuit suitable for use in an apparatus for recording and reproducing color video signals such as a VTR.

It is about the road.

[Background of the invention]

FIG. 2 is a block diagram showing the recording signal processing path of a VH8 system VTR. The video signal sent from terminal 1 is AG
The amplitude is controlled by the C circuit 2. The luminance signal that has passed through I and PF3 is subjected to FM modulation by an FM modulation circuit 4, passes through an HPF5, is adjusted to an appropriate level by an attenuator 16, and is supplied to an adder circuit 11.

On the other hand, the carrier color signal is separated by BPFd, and the ACC circuit 7
After the amplitude is controlled by the burst signal amplification circuit 8, only the burst signal is amplified by +5 (IB).This signal is applied to the frequency conversion circuit 9.

The signal is then frequency-converted using the carrier signal f0. Further, when the signal is a black and white signal, the signal is mixed with the FM-modulated luminance signal by an adder circuit 11° after passing through a killer circuit 14 that cuts off the signal and LPFlo.

The data is then recorded on a magnetic tape (not shown) via the recording amplification circuit 12 and the magnetic head 13.

Since the detection of the ACC 7 is performed before the burst signal amplification circuit 8, the recording current of the magnetic head 13 varies in each block between the burst signal amplification circuit 8 and the recording amplification circuit 12.

Conventionally, the following method has been adopted as a means to deal with this variation.

(1) The recording current is adjusted before the addition circuit 11 to keep the value of the recording current flowing through the magnetic nodal 13 constant.

(2) Burst signal amplification circuit 8 and recording amplification circuit 12
The IC17 and external components are selected so that the variation of each block between them is below a certain value.

Separate.

Although method (1) has the disadvantage of requiring adjustment costs, it is superior in terms of performance because it allows the optimum recording current to be applied to the magnetic head 13.

Method (2) requires no adjustment cost, but ■C1
7 yield drop and definitely the best record.

There is an ode to the fact that current cannot flow through the magnetic head 13.

there were. Therefore, depending on the high-end and low-end models, (1) and (
2) was used differently.

By the way, reducing the cost of VTRs is an important issue.

As one means of achieving this, a method of integrating LpH'MO has been proposed. An example of this is shown in FIG. It is a secondary positive feedback type LPF, and terminal 18 is an input terminal, and terminal 19 is an output terminal. amplifier 2
0, a capacitance 21 with a capacitance value C1, a capacitance 23 with a capacitance value C2.
It is composed of a resistor 22 having a resistance value R1 and a resistor 24 having a resistance value R2. The cutoff frequency 10 is expressed by the following equation.

Therefore, fQ is affected by variations in the absolute values of resistance and capacitance. Since the resistance value varies within the IC by about +1.4% and the capacitance value varies by about ±30%, fo varies by about ±33%. The purpose of LPFlo is to generate FM modulated luminance signals 4±3MH2 and 1, and low-pass converted color signals 629±
This is separation from 500KH2.

Generally, it is set near fc-12MH2, but due to variations in the absolute values of the resistance value and capacitance value, the cutoff frequency f0 and gain also vary.

The variation in cutoff frequency does not pose a practical problem because there is a frequency difference between the cutoff frequency and the luminance signal.

The variation in gain is
For a PF gain variation of +1 dB, about ±3 d13 may occur. As a result, the total gain variation of each IC block between the burst signal amplification circuit 8 and the recording amplification circuit 12 reaches a level that cannot be ignored. In order to deal with this, we have no choice but to adopt the method (1) mentioned above, which requires an adjustment volume and increases the adjustment cost. When a filter is built into an IC in order to improve the recording performance, a recording current adjustment volume is required due to gain variations, which poses a problem in that overall cost cannot be reduced.

In addition, as a reference for the conventional system, "Home Video" (Masao Hara, Japan Broadcasting Publishing Association, P114-P115)
can be given.

[Purpose of the invention]

The object of the present invention is to eliminate the drawbacks of the above-mentioned prior art9,
It is an object of the present invention to provide a video signal processing circuit that absorbs variations in diblock gain and makes it possible to eliminate storage current adjustment.

[Summary of the invention]

In order to achieve the above object, the present invention provides L P F
#(To detect the burst signal level of the low-pass converted carrier color signal, and use this detection result.

Accordingly, the gain of the ACC circuit provided at the front stage of the frequency conversion circuit is controlled, thereby making it possible to eliminate the need to adjust the recording current.

[Embodiments of the invention]

FIG. 1 shows a specific embodiment of the present invention. The parts having the phosphizing function are numbered the same as in FIG. 2. The points that differ from Fig. 2 are described below.

BrF20 is built into IC23. The ACC1e L wave circuit 22 detects the output signal of the L P F 20.

The killer circuit 14 is located behind the LPF 20.

The operation will be explained below. The amplitude of the video signal input from the terminal 1 is controlled by the AGC circuit 2.

And LPF3. FM modulation circuit 4. The signal passes through the HPF 5, is made into a suitable legol by the attenuator 16, and is supplied to the adder circuit 11. On the other hand, the carrier color signal is separated by BrF3, and after the amplitude is controlled by the ACC circuit 7, the burst signal amplification circuit 8 normally converts only the burst signal to 5aB.
amplified. This signal is applied to the frequency conversion circuit 9,
Frequency by carrier signal f8 (approximately 4.2 MHz).

4. The low frequency conversion frequency of 629KHz and the ship 4.
It becomes a frequency component of 8MH2. Only the low conversion frequency of 629 KHz is separated using the ICC built-in BrF20. Further, after passing through the killer circuit 14, the signal is mixed with the FM-modulated luminance signal by the adder apposition 11. The data is then recorded on a magnetic tape (not shown) via the recording amplification circuit 12 and the magnetic head 13°.

Here, the LIPF 20 is constructed based on the second-order positive/feedback type LPF shown in Fig. 3.

The gain depends on the resistance value and capacitance value variation.

It's annoying. However, in order to perform ACC inspection using the output of BrF20, burst emphasis circuit 8 and frequency conversion circuit 9 are required.
, BrF20 gain variations can be absorbed. Therefore, the recording current of the magnetic head 13 is caused only by gain variations in each block of the killer circuit 21, the adder circuit 11, and the recording amplification circuit 12.

Therefore, there is no practical problem at all.

Further, the killer circuit 14 is located behind the BrF 20.

As will be explained in detail later, ACC detection is a method of detecting the amplitude of the burst signal portion, and is performed in the XX detection circuit 22.

As described above, the detection of ACC, which is the core of the present invention, is performed using L P F
Although we have described what is done with the output of 20, the means for realizing this will be explained in detail below.

First, the setting of the color signal recording level will be described. Next, emphasis recording of burst signals will be described.

The reproduction level of the reproduction carrier color signal is approximately proportional to the recording current level. Therefore, in order to improve the δ/N of the reproduced signal, the higher the recording current level is, the better. However, when the recording current level is increased, cross-modulation distortion occurs between the luminance signal recorded by FM modulation and the recorded luminance signal. Therefore, the recording current level of the carrier color signal is limited by the maximum amplitude of the carrier color signal. In other words, for any color signal,
The setting is made so that the cross-modulation distortion caused by the maximum amplitude can be kept to an allowable limit.

Now, as is well known, there is a burst signal part in the carrier color signal, and based on the amplitude of the burst signal part,
It determines the intensity of the color.

The maximum amplitude of the carrier color signal is for dark colors, which is approximately twice that of the burst signal.

By the way, in the VH8 VTR, there are three types of recording modes depending on the tape running speed. When the tape running speed is 53.55 mm/s, P mode, SP mode 1
The case of the speed is called EP mode.

In SP and EP modes, only the burst signal portion is amplified by 6 dB and recorded, and the original signal is restored upon playback. This control is performed using a terminal 28 and a switch 27. That is, Hz is applied to the control terminal 28 in the SP and EP modes. Then, the switch 27 is connected to 27α, and only the burst signal is amplified by 6aB by the burst signal amplification circuit 8.

In the LP mode, 1L' is applied to the control terminal on the 2nd day, and the switch 27 is connected to 27b.

The burst signal portion remains unchanged.

The present invention uses ACC in which the burst signal level is set to a constant value. Therefore, it should be noted that the relationship between the burst signal level and the signal level is different in the LP mode than in the SP and EP modes.

In other words, L without distinguishing between SP, LP, and EP modes.
When ACC detection is performed at the output part of P F 20,
The level of the signal portion is higher in LP mode than in SP and EP modes by the amount of burst signal emphasis (6d13).

Therefore, when the recording [m level is adjusted to the SP and EP modes, cross-modulation distortion becomes a problem due to the increase in the LP mode. Furthermore, when the Sm flow level is adjusted to the LP mode, S/N deterioration occurs in the SP and EP modes.

In the present invention, an amplifier 25 and a switch 26 are used to cope with this. The operation will be explained below. SP and E
In P mode, the switch 26 shown in Fig. 1 is used.
to frequency conversion circuit 9.

inputs the signal that has passed through the burst signal amplification circuit 8.

Further, since the switch 26 is in the state of 26α, the LPF
The output signal of 20 is input to the ACC detection circuit 22 as it is. These waveforms are shown in FIG. 4 (α). On the other hand,
In the LP mode, the switch 27 is in the state 27b and the burst portion is not emphasized. The signal that has passed through the L P F 2o passes through the amplifier 25 and then enters the ACC detection circuit 22 via the switch 26 (state of 26b). Here, the gain of the amplifier 25 is 64B. These waveforms are shown in FIG. 4(b).

As can be seen from the above, the output of LPF20 is SP,
The amplitude of the signal portion is the same in both LP and EP modes. The burst signal portion is emphasized in SP and EP modes.

FIG. 5 is a circuit diagram specifically showing the main parts of the IC block 23 in FIG. 1. L P F 20 .

Killer circuit 14. Amplifier 25. An actual IC circuit diagram of the switch 26 etc. is shown.

Transistors 51, 32, 33, 34, 35
, 36, etc. is an ACC detection switching circuit.

First, the configuration and operation of the ACC detection switching circuit will be explained. A voltage is applied to the terminal 25 only for the regeneration amount, and the transistor 51
, 53, and 34 are operated.

A voltage is applied to the terminal 26 only at the time of #'4, and the differential amplifier composed of transistors 32, 55, and 56 operates. To the terminals 29 and 30, carrier color signals of opposite phases are inputted from a reproduction Tagata terminal (not shown) so that the level of the 3,58 MH2 color signal returned to the carrier color signal during reproduction remains constant. LP at the base of transistor 35
A low frequency conversion color signal of 629KH2 that has passed through F20 is input. Therefore, a voltage is applied to the terminal 26 at the time of 7gp! '15, the output signal of the LPF 20 is input to the Sr nona 26 and the amplifier 25. During playback, a voltage is applied to the terminal 25, and the switch 26 and the amplifier 1ll are connected.
The reproduction output signal 5 is input to the liS device 25 .

Next, the killer circuit 14 will be explained. A voltage is applied to the terminal 27 only during recording. Also, the edge.

When the signal is black and white, a voltage higher than the base voltage of the transistors 5 and 32 is applied to the 1,02-day signal. Then, transistors 40 and 41 are turned on, so transistor 1 and transistor 3 are turned on.
8 and 39 are in the OFF state. Therefore, the signal that has passed through the LPF 20 is not output to the output terminal 24 of the IC. What should be noted here is transistors 40 and 41.
Since transistor 37 serves as a constant current source even when transistors 38 and 39 are ON, the DC voltage at output terminal 24 of the IC does not change. As a result, the killer circuit 21 is turned on.

It is possible to suppress transient potential changes when turning off.

The switch circuit 26 and the amplifier 25 are integrated. A voltage is applied to the terminal 50 only during recording and in the LP mode, and is not applied otherwise. No voltage is applied to the terminal 51 during reproduction and during recording in SP and EP modes.

When a voltage is applied to the terminal 50, the transistor 52,
A differential amplifier composed of 54 and 55 operates. By appropriately selecting the emitter and collector resistances of transistors 54 and 55, an amplifier with a gain of O dB is set. terminal 5
When voltage is applied to transistor 1, transistors 53 and 56
, 57 is operated. By appropriately selecting the emitter and collector resistances of transistors 56 and 57, the amplifier is set to have a gain of 6 dB.

Next, the ACC detector 22 will be explained. The signal output from the switch circuit 26 is clamped by a capacitor 63 and a transistor 62. Then, a comparator composed of transistors 59, 60, and 61 compares it with a reference voltage E4. Then, the high frequency component is removed by a capacitor 60 externally connected to the IC pin 59 via a transistor 62. Then, the ACC amplifier 7 is controlled.

Here, a voltage is applied to the terminal 58 only during the burst signal period. Therefore, ACC detection is performed only during the burst signal period.

〔Effect of the invention〕

According to the present invention described above, when a filter is built into an IC block, gain variations in the IC block can be absorbed, so there is no need to adjust the recording signal level. This is SP and EP mode that emphasizes the burst part and records it.
This is also possible in each of the LP modes in which emphasis recording is not performed.

Note that the above explanation has been about the NTSC mode, but in general, it is possible to use two modes with or without burst emphasis.
If there are two modes, the present invention is applicable.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of a video signal processing circuit according to the present invention, FIG. 2 is a block diagram showing an example of a conventional recording signal processing circuit, FIG. 3 is a circuit diagram showing an integrated LPF, and FIG. 4 is a waveform diagram for explaining FIG. 1, and FIG. 5 is a circuit diagram specifically showing the main part of the IC block in FIG. 1. 7...ACC amplifier 20...LPF 25...Amplifier 26...Switch 22...ACC detector 8...Burst amplification circuit 27...Switch attorney Masaru Ogawa Ojima 3 Figure 4

Claims (1)

[Claims] Automatically controlling the level of the carrier color signal such that the burst signal level is constant to record the carrier color signal of the color video signal separated from the color video signal and mixed with the frequency modulated luminance signal. a chroma control circuit; a burst signal amplification circuit that amplifies only the burst signal portion of the carrier color signal output from the automatic chroma control circuit;
The frequency-modulated luminance includes a frequency conversion circuit that converts the carrier color signal to a low frequency signal, and a low-pass filter that separates only the low-frequency converted carrier color signal from the output signal of the frequency conversion circuit. In the video signal processing circuit, the carrier color signal mixed with the signal and recorded is used as the low-pass converted carrier color signal output by the low-pass filter. A detection circuit is provided for detecting the level of the burst signal of the carrier chrominance signal subjected to range conversion and forming a gain control signal, and the gain control signal is configured to control the gain of the automatic chroma control circuit; An amplifier having a gain equal to the burst signal amplification amount of the burst signal amplification circuit is provided between the bandpass filter and the detection circuit, and when the burst signal amplification circuit is not used, the control signal passed through the amplifier is used to control the automatic operation. A video signal processing circuit characterized in that it is configured to control a chroma control circuit.