JPH0419627B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal recording circuit in a magnetic recording/reproducing device, and particularly to a video signal recording circuit having a circuit configuration suitable for use in an integrated circuit capable of low voltage operation.

Low-voltage operating circuits are necessary for lower power consumption and higher integration of ICs. However, since conventional video signal recording circuits are configured to generate large amplitude signals, they have the disadvantage that it is difficult to reduce the voltage.

First, the necessity of a low-voltage operating circuit is as follows. In other words, if the circuit can be configured with the same current consumption, the lower the power supply voltage, the better from the viewpoint of power consumption. Furthermore, the size of the elements within the IC is related to the withstand voltage; the higher the withstand voltage, in other words, the higher the operating voltage, the larger the element area becomes, resulting in a larger chip size and lower yield. Therefore, it is necessary to enable low-voltage operation and to miniaturize the elements within the IC.

Next, a conventional video signal recording circuit will be described. FIG. 1 is a block diagram showing a conventional video signal recording circuit. In the figure, 1 is a video signal input terminal, 2 is an amplifier, 3 is a buffer circuit, 4 is a low-pass filter, 8 is a clip circuit, 9 is an FM modulator, and 10 is an output terminal.

Here we will consider the video signal recording circuit of a home VTR. A video signal in which the peak of the synchronization signal is clamped to a constant DC potential (hereinafter, the synchronization signal will be of negative polarity) is supplied to the input terminal 1, and after being amplified by the amplifier 2, it is passed through the buffer circuit 3 to the low-pass filter. 4. The low-pass filter 4 is composed of resistors 5 and 6 and a capacitor 7. The signal passed through this low-pass filter 4 becomes the second input signal to the amplifier 2, and the amplifier 2, buffer circuit 3, and low-pass filter 4 constitute a feedback amplifier, and the frequency characteristics are This is the so-called pre-emphasis circuit that is emphasized. The frequency characteristic A _p (f) of the response in this pre-emphasis circuit is expressed by the following equation.

A _p (f)=A/1+A1+jωCR ₂ /1+jωC(R ₁ +R ₂ )…
...(1) where A is the gain of the amplifier 2, R ₁ and R ₂ are the resistance values of the resistors 5 and 6, respectively, and C is the capacitance of the capacitor 7.

Usually, the second term of the denominator on the right side of equation (1) is designed so that the condition of equation (2) is satisfied (A is sufficiently large).

1≪A1+jωCR ₂ /1+jωC (R ₁ + R ₂ )...(2) Therefore, equation (1) is expressed as equation (3), and the characteristics are expressed by the constants of the low-pass filter 3, that is, R ₁ , R ₂ , and C. It's decided.

A _p (f)=1+jωC(R ₁ +R ₂ )/1+jωCR ₂ ...(3) Expressing equation (3) in terms of time constant T and emphasis amount
Equations (4) and (5) are shown, and the frequency characteristics are shown in Figure 2 (the figure shows the VHS standard values for home VTRs).

T ₁ = C (R ₁ + R ₂ ), T ₂ = CR _{2 ...} ( ₄ ) After the amplitude of overshoot occurring on the white side and black side is limited by
The video signal is converted into an FM signal by the FM modulator 9 and output from the output terminal 10. This signal is recorded on a magnetic tape (not shown) via a magnetic head (not shown).

By the way, pre-emphasis the video signal.
As is well known, the purpose of FM modulation is to improve the signal-to-noise ratio (S/N), and the purpose of limiting the amplitude of the free-emphasized signal using a clip circuit is to prevent so-called inversion caused by overmodulation. It is.

Figure 3 shows an example of a circuit often used as an integrated FM modulator. The FM modulator is composed of a voltage-current conversion circuit 11 and an unstable multivibrator 12. The voltage-current conversion circuit 11 includes a voltage-current conversion transistor 13 and an emitter resistor 14 of the transistor. The unstable multivibrator 12 includes diodes 15 to 17, transistors 18 to 25, resistors 26 and 27, and a current source 2.
8, 29, voltage source 30 and oscillation capacitor 31
It consists of The video signal supplied to the base of the voltage-current conversion transistor 13 is converted into a change in collector current, and this current change controls the oscillation frequency of the unstable multivibrator 12.
The video signal is FM modulated. 12 is well known as an emitter-coupled non-stable multivibrator, so a description of its operation will be omitted.

FIG. 4 shows a conventional integrated video signal recording circuit. In the figure, 32 to 45 are transistors 46 to 54 are resistors, 55 is a current source, and 56, 5
7 is a voltage source, 58 and 59 are external variable resistors, and the same parts as those in FIGS. 1 and 3 are given the same symbols. Low-pass filter 4 and resistor 14
is attached externally.

Transistors 32 and 33 are differential amplifiers whose emitters are coupled together, and transistors 34,
The resistor 47, voltage source 56, and load resistor 46 constitute the amplifier 2 shown in FIG. transistor 35,
36, a resistor 48, and a voltage source 56 constitute the buffer circuit 3 of FIG. External resistor 5a, 5
b, 6 and a capacitor 7 constitute a low-pass filter 4. Transistors 37-41, resistors 50,5
1. Constant voltage source 56 and external variable resistor 58
transistors 43 to 45, resistor 54, voltage source 56, and external variable resistor 59 are clip circuits that limit the amplitude in the negative direction. The transistor 42, resistors 52 and 53, voltage source 57, and current source 55 are a level shift circuit. 13 is a voltage-current conversion transistor, 12 is an astable multivibrator, and 14 is an external emitter resistor of the voltage-current conversion transistor 13. Also, a, b, c, d, and e each indicate the terminal of the IC.

The circuit operation will be briefly explained below. The input terminal 1, that is, the base of the transistor 32, is supplied with a video signal whose synchronizing signal peak is fixed at a constant DC potential as shown in FIG. 5A. This signal is amplified by a differential amplifier of transistors 32 and 33, and is supplied from the collector of transistor 33 to resistor 49 via buffer transistor 35, and is output from terminal b. The signal taken out from terminal b passes through a low-pass filter 4 to a transistor 3.
The signal is supplied to a terminal a connected to the base of No. 3, and feedback is applied thereto, forming a pre-emphasis circuit. The frequency characteristics of this circuit are as follows:
The resistance values of 5b are R _1a and R _1b respectively, and R _1a and
If the parallel resistance of R _1b is set as R ₁ , the circuit is equivalent to the circuit shown in FIG. 1. For clarity R _1a =
If R _1b = 2R ₁ , the characteristic is given by equation (3).
However, the low frequency gain from input terminal 1 to terminal b is doubled. Therefore, the waveform of terminal b is
As shown in FIG. 5A, the high frequencies are emphasized and large overshoots occur on the white and black sides.

The signal emphasized in this way is connected to the resistor 49.
is supplied to the base of transistor 42 via. The anode (base, collector) of a diode-connected transistor 37 is connected to this base, and its cathode (emitter) is connected to the emitter of a transistor 38 serving as a voltage source. If the emitter voltage of transistor 38 is V ₁ , then the base voltage of transistor 42 is (V ₁
+V _BE ) (where V _BE is the voltage between the base and emitter of the transistor), the transistor 37 becomes conductive. Therefore, no matter how much the emitter voltage of transistor 35 rises, the base voltage of transistor 42 is fixed at the voltage (V ₁ +V _BE ), and the amplitude of the white side of the emphasized signal is clipped (white clip). call). To determine at what level the signal should be clipped, adjust the voltage at terminal d with an external variable resistor 58, and then use the transistor 38.
It can be changed by changing the emitter voltage _V1 .

The white-clipped signal is transmitted from the base of transistor 42 to the emitter. The emitter of the transistor 42 is connected to the emitter of a transistor 43, and the base of the transistor 43 is connected to the transistor 44 which serves as a voltage source.
Its emitter and collector are connected to the power supply V _CC . As before, if the emitter voltage of transistor 44 is _V2 , then the emitter of transistor 43 (i.e. the emitter of transistor 42)
When the voltage reaches (V ₂ −V _BE ), transistor 43 becomes conductive. Therefore, no matter how much the base voltage of transistor 42 drops, its emitter voltage is fixed at (V ₂ -V _BE ), and the amplitude of the black side of the signal is clipped (referred to as dark clip).
Similar to white clipping, the clipping level can be determined by adjusting the voltage at terminal e using an external variable resistor 5.
You can decide by changing it with 9.

The white clipped and dark clipped signals are connected to the resistors 52 and 53, the voltage source 57, and the current source 5.
5, the signal is appropriately divided and DC shifted, and then supplied to the base of the voltage-current conversion transistor 13. The voltage applied to the base is converted into a change in collector current, and by controlling the oscillation frequency of the astable multivibrator 12 with this current, the video signal is FM modulated.

The conventional circuit shown in FIG. 4 has two drawbacks. One of them is waveform distortion due to clipping. The waveform of the clip circuit output is shown in Figure 5c. VHS
Taking the standard as an example, if the video signal (from the peak of the sync signal to the 100% white signal, corresponding to A in Figure 5 C) is 100%, then the white clip level (corresponding to B in Figure 5 C) is 100%. ) is 160%, and the dark clip level (corresponding to C in Figure 5 C) is 160%.
It is specified as 40%. Emphasis standards are
Since T ₁ =1.3 μsec and X = 4, most of the signal is clipped at the rising edge which has a large amplitude and contains many high frequency components. Energy loss due to this clip appears as waveform distortion such as smear having a relatively long time constant in the reproduced signal, resulting in deterioration of image quality.

The second drawback is that it is difficult to reduce the voltage of the circuit because a large amplitude signal is generated at the emphasis output. Normally, clipping circuits utilize the nonlinearity of diodes, but in order to perform the clipping operation clearly, a large amplitude signal is required, so the video signal level is set to 1VP-P (the voltage from peak to peak is 1V) It is said that the amount of However, if the clip level is also considered, the amplitude of the clip circuit output reaches 2VP-P in the VHS standard. In order to amplify this signal without distortion, the transistor 3
The dynamic range of the differential amplifier consisting of 2 and 33 is required to be approximately 4VP-P or more. Since the output of the differential amplifier is a signal before clipping as shown in Figure 5A, the emphasis standard, T = 1.3μsec,
Considering that X=4, the amplitude becomes a large amplitude of 4VP-P or more. Therefore, even if an attempt was made to lower the voltage of the circuit, problems such as saturation of the transistors 32 and 33 occurred, making it difficult to realize.

In order to improve the first drawback mentioned above, a recording circuit having the configuration shown in FIG. 6 has been proposed. What is different from the configuration shown in FIG. 1 is that a clip circuit 8 is included in the loop of the feedback type emphasis, that is, the output of the clip circuit 8 is fed back to the amplifier 2 via the low-pass filter 4. It's about being there.

A recording circuit that realizes this configuration is shown in FIG.
From the emitter of the voltage-current transistor 13, that is, the terminal c, via the buffer transistor 60,
The configuration shown in FIG. 6 can be easily realized by simply returning the signal from the emitter to terminal a via the low-pass filter 4, and the IC shown in FIG. 4 can be used as is. Resistors 52 and 5
If the values of 3 are equal, the low frequency gain from input terminal 1 to terminal b will be the same as the circuit shown in Figure 4, and the frequency characteristics of the emphasis circuit will also be the constants of low-pass filter 4 shown in Figure 7. You can make it the same by doing this.

The reason why waveform distortion can be improved by inserting a clip circuit into the feedback loop of the emphasis circuit is that when the emphasized signal is clipped, the amount of feedback decreases, and as a result, more emphasis is applied. This is because the waveform spreads in the axial direction. This corresponds to less energy loss due to clipping, and although some signal is lost at the rising edge of the signal,
There is no smear with a large time constant, and waveform distortion is reduced.

As mentioned above, the first drawback of the conventional circuit can be improved with the circuit of FIG. However, the second drawback is even worse. As mentioned above, since the clipped signal is fed back through the low-pass filter, the gain of the differential amplifier is equivalently increased in this portion. As a result, a larger amplitude than that of the circuit shown in FIG. 4 is generated at the collector of the transistor 33. This is not a problem when the power supply voltage is high, but it becomes a major obstacle when lowering the voltage.

In FIG. 3, there is also a circuit that inserts a resistor between the collector of the voltage-current conversion transistor 13 and the emitters of the transistors 24 and 25 constituting the unstable multivibrator, and clips the signal at the collector. The voltage drop caused by the resistance hinders lowering the voltage. It is obvious that clipping in front of the voltage-current conversion transistor 13 is more advantageous for lowering the voltage.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art and to provide a circuit that can reduce the voltage of a video signal recording circuit in a magnetic recording and reproducing apparatus.

In the present invention, there is provided a first input terminal to which a video signal in which the leading end of the synchronization signal is clamped;
a differential amplifier having an input terminal; and an input terminal connected to the output terminal of the differential amplifier, the clip clipping the video signal amplified by the differential amplifier at predetermined upper and lower limit values. a buffer circuit for amplifying the output signal from the clip circuit; and a feedback pre-emphasis system for feeding back the output signal from the buffer circuit to the second input terminal of the differential amplifier via a low-pass filter. A feedback loop is formed by the circuit, an FM modulator that generates a frequency modulation signal according to the output signal from the buffer circuit, and a magnetic head to which the frequency modulation signal from the FM modulator is supplied. provided.

In the present invention, since a clip circuit is provided in this feedback loop, waveform distortion due to clipping is improved, and the clip circuit is directly connected to the output terminal of the differential amplifier to limit changes in the amplitude of the signal at this output terminal. Therefore, the dynamic range of the differential amplifier may be determined by the above-mentioned predetermined upper and lower limits.

An embodiment of the present invention will be described below with reference to FIGS. 8 and 9. FIG. 8 is a block diagram showing the configuration of one embodiment of the present invention, and FIG. 9 is a circuit diagram showing one embodiment of the present invention as an integrated recording circuit.

In FIG. 8, 1 is an input terminal, 2 is an amplifier,
8 is a clip circuit, 3 is a buffer circuit, 4 is a low-pass filter, 9 is an FM modulator, and 10 is an output terminal. The difference from the conventional one is that a clip circuit 8 is arranged so as to directly clip the output of the amplifier 2.Next, the operation will be explained using a specific circuit diagram shown in FIG.

In Figure 9, 32-34, 37-45, 6
3, 64 are transistors, 46, 47, 50-5
4, 62, 65 and 66 are resistors, 56 and 57 are voltage sources, 55 is a current source, 13 is a voltage-current conversion transistor, and 12 is an unstable multivibrator, all of which are integrated. 1 is an input terminal, 4 is an external low-pass filter, 14 is an external emitter resistor of the voltage-current conversion transistor 13, 58, 59
is an external variable resistor. a, b, c, d,
e and f indicate terminals of the IC.

Transistors 32 and 33 are differential amplifiers whose emitters are coupled together, and transistors 34,
The resistor 47, the voltage source 56, and the resistor 46 connected between the collector of the transistor 33 and the power supply V _CC
This constitutes the amplifier 2 shown in the figure. transistor 3
4, the resistor 47 and the voltage source 56 are the transistor 3
It operates as a constant current source connected between the emitters 2 and 33 and ground. The base of transistor 32, or input terminal 1, is configured to receive a first input signal, and the base of transistor 33, or terminal a, is configured to receive a second input signal.

Transistors 43 to 45, 63, resistors 54, 6
2 and an external variable resistor 59 are dark clip circuits that limit the amplitude in the negative direction. Power supply V _CC
an external variable resistor 59 connected between
A voltage is applied to terminal e from the dividing point. The base of the transistor 44 is connected to the terminal e, the collector is connected to the power supply V _CC , and the emitter is connected to the collector of the transistor 45 . The base of the transistor 45 is connected to a voltage source 56, and the emitter is connected to a resistor 54 between the ground, forming a current source. The cathode (emitter) of the diode-connected transistor 63 is connected to the emitter of the transistor 44, and the resistor 62 is connected to the anode (base collector) between the power supply V _CC and the base of the transistor 43. The anode serves as a voltage source. transistor 4
The collector of transistor 33 is directly connected to the power supply V _CC , and the emitter of transistor 33 is directly connected to the collector of transistor 33, that is, the output of the differential amplifier.

On the other hand, transistors 37 to 41, resistors 50,
51. An external variable resistor 58 is a white clip circuit that limits the amplitude in the positive direction. Power supply V _CC
an external variable resistor 58 connected between
A voltage is applied to terminal d from the dividing point. The base of the transistor 39 is connected to the terminal d, the collector is connected to the power supply V _CC , and the emitter is connected to the collector of the transistor 41 and the base of the transistor 38 . The base of the transistor 41 is connected to a constant voltage source 56, and the emitter is connected to a resistor 51 between the ground, forming a current source. The collector of the transistor 38 whose base is connected to the emitter of the transistor 39 is connected to the power supply V _CC
The emitter is connected to the collector of the transistor 40 and to the cathode (emitter) of the diode-connected transistor 37. The base of the transistor 40 is connected to a constant voltage source 56, and the emitter is connected to a resistor 50 between the ground and forms a current source. In contrast to the emitter of transistor 37, the emitter of transistor 38 serves as a voltage source. The anode (base-collector) of the diode-connected transistor 37 is directly connected to the collector of the transistor 33, ie, to the output of the differential amplifier.

The base of the transistor 42 is connected to the collector of the transistor 33 (that is, the emitter of the transistor 43 and the base-collector of the transistor 37). transistor 42 and resistor 52,
53, voltage source 57, and current source 55 serve as level division and DC shift. Furthermore, if the current source 55 and the voltage source 57 have temperature characteristics, temperature compensation can be performed. The collector of the transistor 42 is connected to the power supply V _CC , the emitter is connected to a resistor 52 , and the other end of the resistor 52 is connected to a current source 55 , one end of a resistor 53 in a series circuit consisting of a resistor 53 and a voltage source 57 , and a transistor 64 . Base is connected.

The collector of the transistor 64 is connected to the power supply V _CC , and the emitter is connected to a resistor 66 between the ground and the terminal f. The transistor 64 and the resistor 66 constitute the buffer circuit 3 shown in FIG. The terminal f is connected to the resistor 5 of the external low-pass filter 4, the other end of the resistor 5 is connected to the terminal a, that is, the base of the transistor 33, and the series circuit of the capacitor 7 and the resistor 6 is connected to the ground. It is connected to the. The low-pass filter 4 includes resistors 5, 6,
It consists of a capacitor 7.

The emitter of the transistor 64 is connected to the base of the voltage-current conversion transistor 13 via a resistor 65. The emitter of the transistor 13 is connected to a terminal c to which an external emitter resistor 14 is connected between ground, and the collector is connected to the unstable multivibrator 12.

An input terminal 1 is supplied with a video signal whose synchronizing signal peak is fixed at a constant DC voltage. This signal is amplified by a differential amplifier consisting of transistors 32 and 33 and taken out as a voltage from the collector side of transistor 33. This signal is transmitted to transistor 42
emitter follower, resistor 52, transistor 6
It receives appropriate DC level shift and signal division through the emitter follower of No. 4 and outputs it to the terminal f, and also receives the voltage of the FM modulator through the resistor 65.
A current is supplied to transistor 13.

The signal output to the terminal f is negatively fed back to the second input terminal of the differential amplifier, that is, the terminal a, via the low-pass filter 4, and the entire circuit is a feedback pre-emphasis circuit with a clip circuit in the loop. It consists of

At the collector of the transistor 33, a waveform signal is obtained in which the high frequency band is emphasized and overshoot is applied to the white side and the black side. Assuming that the base voltage of the clipping transistor 43 is V ₂ , when the voltage at the collector of the transistor 33, that is, the emitter of the transistor 43, drops to (V ₂ −V _BE ), the transistor 43 becomes conductive, so that the negative side of the signal The voltage is limited to (V ₂ −V _BE ) and a dark clip is applied. Also, the emitter voltage of the transistor 38 is V ₁
Then, when the collector voltage of the transistor 33 rises to (V ₁ +V _BE ), the diode-connected transistor 37 becomes conductive, so the positive side voltage of the signal is limited to (V ₁ +V _BE ) and becomes white. It gets clipped. The video signal level at the collector of transistor 33 is 1V _PP , the white clip level is 160%, and the dark clip level is 40%.
Then, the voltage generated at the collector will only be 2V _PP . In other words, only the necessary amplitude is created. Therefore, it is possible to lower the power supply voltage V _CC of the circuit than before.

Furthermore, since a clip circuit is placed in the loop and feedback is provided via a low-pass filter, the waveform distortion can be improved to the same extent as in the conventional example shown in FIG.

As described above, in this embodiment, not only waveform distortion can be improved, but also the signal amplitude can be reduced because the emphasis output is directly clipped, and the voltage can be lowered.

According to the present invention, it is possible to reduce the voltage of the recording circuit in a magnetic recording/reproducing device, which is effective in reducing power consumption. Furthermore, by lowering the voltage, the withstand voltage of the elements within the IC can be lowered, which is effective in reducing costs by reducing the chip size and improving yield.

[Brief explanation of drawings]

Figures 1 and 6 are block diagrams showing the configuration of a conventional video signal recording circuit, Figure 2 is a characteristic diagram showing an example of pre-emphasis characteristics, and Figure 3 is a circuit diagram of a typical integrated FM modulator. , FIG. 4 and FIG. 7 are specific circuit diagrams of a conventional integrated video signal recording circuit, FIG. 5 is a signal waveform diagram of each part necessary for explaining the circuit operation, and FIG. FIG. 9 is a block diagram showing one embodiment of the video signal recording circuit according to the invention, and FIG. 9 is a circuit diagram showing a specific circuit configuration of the embodiment shown in FIG. 8. Symbol explanation, 1...Input terminal, 2...Amplifier, 3
... Buffer, 4 ... Low-pass filter, 8 ... Clip circuit, 9 ... FM modulator.

Claims (1)

[Claims] 1. A differential amplifier having a first input terminal and a second input terminal to which a video signal to which a synchronizing signal has a clamped end is supplied; and a differential amplifier connected to the output terminal of the differential amplifier. a clipping circuit having a fixed input terminal and clipping the video signal amplified by the differential amplifier at predetermined upper and lower limit values; a buffer circuit that amplifies the output signal from the clipping circuit; a feedback pre-emphasis circuit that feeds back the output signal from the buffer circuit to the second input terminal of the differential amplifier via a low-pass filter; and an FM generator that generates a frequency modulation signal according to the output signal from the buffer circuit. A video signal recording circuit comprising: a modulator; and a magnetic head to which a frequency modulation signal from the FM modulator is supplied.