JPH0346881B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal recording circuit for a magnetic recording/reproducing device, and more particularly to a television adjustment circuit.

As a magnetic recording/reproducing device, the problems of the prior art will be explained below using a home VCR as an example. Figure 1 shows a block diagram of the video signal recording system of a home VTR. The operation of each part will be explained along the flow of signals. The video signal supplied to the video signal input terminal 1 has input level variations absorbed by the AGC circuit 2, color signals are removed by the next low-pass filter 3, and is input to the clamp circuit 4 via the clamp capacitor 18. The signal sync chip potential is fixed. This signal is transmitted to the next dynamic emphasis circuit 5 and main emphasis circuit 6.
, high-frequency components are emphasized. For this reason, the signal passing through the main emphasis circuit 6 has a large overshoot, so the next clip circuit 7 limits the white and black levels, and the signal is sent to the FM modulator via the current conversion transistor 8. 11, where the video signal is converted to an FM signal. After the low-frequency signal is removed from this signal by the next high-pass filter 12, it is mixed with the low-frequency conversion color signal supplied from the low-frequency conversion color signal input terminal 13, and the signal is amplified to a sufficiently high level by the recording amplifier 14. Thereafter, the data is recorded on the magnetic tape 16 via the magnetic head 15.

When converting a video signal into an FM signal in the FM modulator 11, the oscillation frequency _st corresponding to the sync chip S and the oscillation frequency _wp corresponding to the white peak P of the video signal are determined by the standard.
An example of this is shown in FIG. 2 together with a video signal waveform.
In the figure, the horizontal axis shows the voltage v at the emitter terminal 9.
The vertical axis shows the oscillation frequency of the FM modulator 11, and in this example, _st is 3.4MHz and _wp is 4.4MHz.
Therefore, the deviation is 1MHz.

Figure 3 shows an example of an FM modulator, which is an emitter-coupled astable multivibrator that is often used in integrated circuits. As is well known, the oscillation frequency of this circuit is proportional to the collector current IO of the current conversion transistor and inversely proportional to the capacitance C of the oscillation capacitor 19. Therefore, the deviation is proportional to the input video signal level of the current conversion transistor 8, inversely proportional to the resistance value of the emitter resistor 10, and is proportional to the capacitance C of the oscillation capacitor 19.
is inversely proportional to For this reason, the emitter resistor 10 and the oscillation capacitor 19 must have small variations and become peripheral components outside the integrated circuit.

However, even if it is designed with high precision, the gain from the clamp circuit 4 to the current conversion transistor 8 varies to some extent, and variations in peripheral components cannot be reduced to zero, so deviation adjustment and carrier set adjustment are required. There are three methods for the deviation adjustment as described above. The first is to change the input video level of the current conversion transistor 8.
The second method is to change the value of the emitter resistor 10, and the third method is to change the capacitance of the oscillation capacitor. The first method is relatively simple, and involves adjusting the value of the video level adjusting resistor 17 shown in FIG. 1 to achieve a specified deviation. The method shown in Figure 2 is
This can be done by using a variable resistor as the emitter resistor 10, but variation in temperature characteristics becomes a problem. Similarly, the third
This method has problems in terms of variations in temperature characteristics and component costs. Therefore, conventionally, the deviation adjustment has been performed using the first method. On the other hand, the carrier set can be adjusted externally using the oscillation frequency adjustment resistor 20.
The oscillation frequency was set to a specified value (in the example in Figure 2, _wp is 4.4MHz) by changing the current of the FM modulator. In this adjustment, the oscillation frequency is adjusted without changing the slope (deviation) of the straight line in FIG.

Conventionally, deviation and carrier set adjustments have been made in the manner described above, but when the deviation adjustment is performed, the input level of the dynamic enhancement circuit 5 changes, resulting in the following inconvenience. FIG. 4 shows the characteristics of the dynamic emphasis circuit 5. Curve I has an input level of O _dB
In the case of , the curves correspond to −10 _dB and −20 _dB , respectively. This circuit is a circuit that changes emphasis characteristics depending on the magnitude of the input. Therefore, when the input level changes due to deviation adjustment, the characteristics change, which causes a change in image quality.

The purpose of the present invention is to eliminate the above-mentioned conventional drawbacks,
An object of the present invention is to provide a new circuit capable of performing deviation adjustment without changing the input level of a dynamic enhancement circuit.

The feature of the present invention is that the emitter resistance of the current conversion transistor is composed of a thick film resistor with small temperature change.
Deviation adjustment is performed by trimming this resistance using a laser or the like.

An embodiment of a recording circuit for a home VTR according to the present invention is shown in FIG. Figure 5 shows the frump circuit.
A monolithic IC that includes an FM modulator is shown as an example. The feature of this circuit is that it is monolithic.
The peripheral parts of the IC are made up of chip capacitors and thick film resistors to create a hybrid IC, and by integrating a monolithic IC and a hybrid IC, the circuit is made smaller.

The monolithic IC 21 includes a clamp circuit 4, a dynamic emphasis circuit 5, a main emphasis circuit 6, a clip circuit 7, a current conversion transistor 8, and an FM modulator 11. In addition to the monolithic IC 21, the hybrid IC 22 is equipped with peripheral components C ₁ to C ₃ and R ₁ to R ₁₃ of the monolithic IC. Capacitor C ₁ ~
_C3 is a chip capacitor and resistors _R1 to _R13 are thick film resistors.

To briefly explain the signal flow, AGC circuit 2,
The video signal that has passed through the low-pass filter 3 (FIG. 1) is supplied to the B terminal of the hybrid IC 22 via a clamping capacitor 18. This signal enters the b terminal of the monolithic IC 21, passes through the clamp circuit 4, dynamic emphasis circuit 5, main emphasis circuit 6, clip circuit 7, and current conversion transistor 8, and is supplied to the FM modulator 11, where the FM signal is It is converted into and output from the k terminal of the monolithic IC. This FM signal is a hybrid
The signal is output from the E terminal through the IC and supplied to the high pass filter 12 (FIG. 1). capacitor
C ₁ and resistor R ₁ are elements that determine the characteristics of the dynamic emphasis circuit (time constant and amount of emphasis), and capacitor C ₂ and resistors R ₂ , R ₃ , and R ₄ are elements that determine the characteristics of the main emphasis circuit (time constant and amount of emphasis). This element determines the time constant and the amount of emphasis. Resistance R ₁ ~ _{R 4}
is an important resistance that determines the above-mentioned characteristics, and since it can be trimmed using a laser or the like, a very accurate resistance value can be obtained and characteristic variations can be suppressed to a small level. Resistors _R5 and _R8 are used to adjust the white clip level and dark clip level, and by changing the voltages at the f and g terminals, the white and black sides of the video signal shown in the waveforms in Figure 2 can be adjusted. This adjusts the height of the whiskers to a specified value.

The advantage of thick film resistors is that if the value of the sheet resistance is selected, a resistor with almost no temperature characteristics can be obtained, and furthermore, the variation in temperature characteristics is small. Furthermore, since trimming is possible as mentioned above, highly accurate resistance values can be obtained, and automatic adjustments can be made by changing the resistance value until the desired characteristics are achieved while measuring the characteristics of the circuit. be.
Therefore, the emitter resistance 9 of the current conversion transistor 8
By constructing the circuit with a thick film resistor and trimming this resistor, deviation adjustment can be performed without adjusting the level of the video signal, so that other circuits (such as the dynamic emphasis circuit) are not adversely affected.

An example of deviation adjustment will be explained. A video signal at a constant level is supplied to the input of the dynamic enhancement circuit 5, that is, the B terminal of the hybrid IC 22. (This can be done using the video level adjustment resistor 17 shown in Figure 1.)
While measuring the deviation value of the E terminal of the hybrid IC, which is the FM signal output terminal, the emitter is The resistance value of the resistor 10 is trimmed and the resistance value is changed until it reaches a specified deviation value. After that, resistor R ₁₂ or R ₁₃ is connected so that the frequency _wp corresponding to the white peak of the video signal becomes the specified value.
trim and adjust the carrier set.

When trimming a resistor, the change in resistance value is limited to from small to large. Therefore, the initial value of the emitter resistor 10 must be set to a slightly smaller value in advance, taking into account various variations. Furthermore, when the emitter resistance 10 is at its initial value, the deviation is large and the oscillation frequency is high, and with trimming the deviation is small and the oscillation frequency becomes low.

If f _st and f _wp are at the specified values at the end of the deviation adjustment, there is no need to adjust the carrier set, and if the amount of deviation is small, the time required for the carrier set adjustment will be short. From the above, it is desirable to select the initial values of the carrier set adjustment resistors R ₁₂ and R ₁₃ so that the oscillation frequency at the end of the deviation adjustment is a specified value, and the oscillation frequency at the end of the deviation adjustment is higher than the specified value. Trim R ₁₃ for low and R ₁₂ for low.

A second embodiment of the deviation adjustment method according to the present invention is shown in FIG. This diagram shows only the deviation adjustment part, and the rest is the same as Figure 5. This circuit differs from the deviation adjustment method shown in FIG. 5 in that deviation adjustment can be performed without changing the oscillation frequency (for example, by fixing the sync chip frequency). For example, if the voltage obtained by dividing the power supply voltage V _CC by resistors R ₁₄ and R ₁₅ is set to the same potential as the sync chip potential of the video signal at the emitter terminal 9 of the current conversion transistor 8, and the resistor R ₁₆ is trimmed, the sync chip will be supported. Deviation can be changed without changing the frequency _st . This is because at the sink chip level, even if the resistor _R16 is changed, the collector current of the current conversion transistor 8 does not change, but the gain (ie, deviation) of the current conversion transistor is affected by the resistance value of _R16 . FIG. 7 shows the relationship between the resistance value of the resistor _R16 and the Vf conversion of FM modulation. The smaller the value of resistance _R16 , the greater the slope of the straight line, and the larger the resistance value, the smaller the slope. In terms of the amount of deviation in the oscillation frequency at the end of the deviation adjustment, this method of deviation adjustment is advantageous when the deviation conversion due to the variation in the input video signal level of the conversion transistor 8 is larger than the variation in the capacitance of the oscillation capacitor 19, and vice versa. The method shown in FIG. 5 is advantageous.

In the circuit shown in FIG. 6, the deviation adjustment can also be made by trimming the resistor _R9 . In this case, by selecting the resistance ratio of resistors _R14 and _R15 , the oscillation frequency after the deviation adjustment is completed can be changed. Therefore, the resistance is adjusted so that the variable range of carrier set adjustment is balanced up and down.
Values of R ₁₂ and R ₁₅ can be set.

A third embodiment of the invention is shown in FIG. Deviation adjustment is done by trimming resistor _R9 . Emitter terminal 9 of current conversion transistor 8
and a coil is connected between resistors R ₉ and R ₁₆ ,
This circuit forms a low-pass filter that suppresses high-frequency components entering the FM modulator. This is to limit the band of the video signal, which is spread due to harmonic distortion caused by clipping, and to prevent the occurrence of moiré and the like. Even if the coil is connected, there is no effect on the direct current, and the deviation adjustment can be performed as described above.

Since the deviation adjustment circuit according to the present invention can be performed without changing the input level of the dynamic enhancement circuit, the characteristics of the dynamic enhancement circuit will not be changed by the deviation adjustment. Also, since deviation adjustment is performed at a fixed input level, adjustment can be made with a single hybrid IC. Additionally, since it is a variable resistor that trims a thick film resistor, it has the advantage of not having to worry about temperature characteristics unlike conventional resistors.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a recording circuit of a conventional magnetic recording/reproducing device, Fig. 2 is a voltage vs. frequency characteristic diagram for explaining deviation adjustment and carrier set adjustment, and Fig. 3 is a circuit diagram showing an example of an FM modulator. , FIG. 4 is a characteristic curve diagram showing an example of the characteristics of a dynamic enhancement circuit, FIG. 5 is a circuit diagram showing a first embodiment of a recording circuit according to the present invention, and FIG. 6 is a diagram showing a first embodiment of a recording circuit according to the present invention. A circuit diagram showing the main part of the second embodiment, FIG. 7 is a voltage vs. frequency characteristic diagram for explaining the relationship between the deviation adjustment and oscillation frequency shown in FIG. 6, and FIG. FIG. 3 is a circuit diagram showing the main parts of the third embodiment. 5: Dynamic emphasis circuit, 8: Current conversion transistor, 11: FM modulator, 19: Oscillation capacitor, 21: Monolithic IC, 2
2: Hybrid IC, _R1 to _R16 : Thick film resistor.

Claims (1)

[Claims] 1. An AGC circuit that absorbs variations in the level of an input video signal, a clamp circuit that fixes the sync chip of the video signal output from the AGC circuit to a predetermined potential, and an emphasis control circuit that absorbs variations in the level of an input video signal. An emphasis circuit including a dynamic emphasis circuit that emphasizes the video signal output from the clamp circuit with a predetermined dynamic emphasis characteristic that changes in amount, and a transistor that generates a collector current according to the output of the emphasis circuit. , a frequency modulator that generates an output signal with a frequency corresponding to the collector current of this transistor, a magnetic head that records the output of the frequency modulator on a magnetic tape, and a sync chip at the same potential as the video signal sync chip potential at the emitter of the transistor. A video signal recording circuit comprising: a bias source that generates a fixed voltage; and an adjustment resistor that is provided between the emitter of the transistor and the bias source and that adjusts the collector current of the transistor.