JPH0798934A - Magnetic recording and reproducing device - Google Patents

Abstract

PURPOSE:To reduce the increase of a noise due to the secular change of a video head by providing a variable low-pass filter whose frequency characteristic is controlled replying to a control signal in at least a noise canceller circuit or de-emphasis circuit. CONSTITUTION:In the noise canceller circuit 11B, the output video signal V from an LPF10 is supplied to an LPF21, and adder circuit 22, and a mixer circuit 24, respectively. The adder circuit 22 supplies a video signal DV including a noise component being a difference between video signals LV and V to a limiter circuit 23. The circuit 23 replies the supply of the signal DV, and supplies a video signal NV consisting of only the noise component to the circuit 24. The circuit 24 offsets the noise component by mixing the video signals V, NV with opposite phase, and supplies a video signal NCV with superior S/N to a picture control circuit 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording / reproducing apparatus, and more particularly to an analog type magnetic recording / reproducing apparatus such as a video tape recorder (VTR) for recording television video signals.

[0002]

2. Description of the Related Art An analog VTR, which is currently in the mainstream for business and home use, uses a low carrier FM modulation method for recording a video signal. This method uses a video signal to generate a carrier wave of 4 to 10 MHz by FM.
Modulation and recording on tape, which enables recording of wideband video signals, characteristics of level fluctuations that can be removed by a limiter, less deterioration of amplitude and phase characteristics, and difficulty of waveform distortion. Is used.

The frequency characteristic of the reproduced signal in this type of VTR depends on the characteristic of the video head, and when the recording / reproduction is repeated, the high frequency characteristic is deteriorated due to a change with time such as abrasion. The deterioration of the high frequency characteristic causes the deterioration of the resolution of the output image.

Conventionally, the above-mentioned deterioration of resolution is read out by the FM.
This is prevented by providing an AGC circuit that holds the signal level at a constant value.

Referring to FIG. 4 which is a block diagram showing a first example of a conventional magnetic recording / reproducing apparatus of this type, the first conventional example will be described.
In the magnetic recording / reproducing apparatus, a magnetic tape 1 which is a magnetic recording medium, a video head 2 which reads an FM signal recorded on the magnetic tape 1, a playback (PB) amplifier which amplifies the read FM signal, and this amplifier. , A peaking circuit 4 for correcting the frequency characteristic of the FM signal, and an AGC circuit 5 for holding the amplitude level of the frequency-corrected FM signal supplied from the peaking circuit 4 constant. An RF equalizer circuit 6 that equalizes the frequency characteristics of the output FM signal of the AGC circuit 5, a limiter circuit 7 that limits the amplitude of the equalized FM signal from the RF equalizer circuit 6, and a demodulated video signal that demodulates the equivalent FM signal. And a high-frequency emphasis (emphasis) characteristic for noise suppression of the demodulated video signal to restore the flat characteristic and reproduce the video signal. A de-emphasis circuit 9 for outputting a signal, a low-pass filter 10 for low-pass filtering a reproduced video signal, a noise canceller circuit 11 for removing noise in the reproduced video signal by canceling it, and a frequency of the reproduced video signal after noise removal And a picture control circuit 12 that corrects the characteristics and the phase characteristics.

To explain the operation, first, the FM signal recorded on the magnetic tape 1 is read by the video head 2.
The video amplifier 3 amplifies and switches the read reproduction FM signal and supplies it to the peaking circuit 4. Peaking circuit 4
Performs high frequency correction and supplies it to the AGC circuit 5. Next, the AGC circuit 5 receives the reproduction FM signal whose high frequency band has been corrected, controls the amplitude level to a constant value, and supplies it to the RF equalizer circuit 6. The RF equalizer circuit 6 performs a predetermined frequency equalization and supplies it to the limiter circuit 7. Limiter circuit 7
Limits the amplitude of the supplied FM signal after frequency equalization,
It is supplied to the FM demodulation circuit 8. The FM demodulation circuit 8 demodulates the supplied amplitude-limited FM signal, generates a demodulated video signal, and supplies the demodulated video signal to the de-emphasis circuit 9. The de-emphasis circuit 9 performs de-emphasis, which is an operation of correcting the frequency characteristic of the demodulated video signal, which has been emphasized in advance in a predetermined high range at the time of recording for noise suppression, with an inverse characteristic to restore the original flat characteristic. A noise canceller circuit 1 is passed through a low-pass filter 10 that removes residual FM components.
Supply to 1. The noise canceller circuit 11 further removes the high frequency noise component of the supplied video signal and supplies it to the picture control circuit 12. The picture control circuit 12 corrects the frequency characteristic and the phase characteristic of the reproduced video signal after noise removal in order to improve the image quality of the display image, and outputs the corrected video signal from the terminal TO.

The first conventional magnetic recording / reproducing apparatus is provided with the AGC circuit 5 for holding the amplitude level of the read FM signal at a constant value as described above, so that the high frequency characteristics due to the change with time of the video head 2 are increased. The decrease in the level of the FM signal due to the deterioration of is corrected. The AGC circuit 5 is the FM
The amplitude level is detected by the amplitude detection of the signal, and the amplification degree is changed so that the amplitude level becomes constant. However, the high-frequency noise component also increases at the same time due to the amplification degree that increases in accordance with the decrease in the level of the FM signal, and the noise component affects the details such as thin lines in the display image. Has some effect, but there is a limit to improvement in deterioration of image quality. Further, the relationship between the carrier level and sideband level of the reproduced FM signal and the envelope level are varied due to the recording state and the variation of the characteristics of the video head during reproduction, and the variation of the image quality due to the inversion phenomenon and the deterioration of the SN ratio is caused. Occurs.

JP-A-3-9716 for improving the above-mentioned drawbacks
The second conventional magnetic recording / reproducing apparatus disclosed in Japanese Patent No.
Equipped with an envelope detection circuit that detects the amplitude level of the signal and a noise detection circuit that detects the noise level, monitor the recording status or video head status, and optimize the equalizer characteristics, noise cancellation amount, and picture control amount according to these conditions. To maintain the best display quality.

Referring to FIG. 5, which shows a block diagram of a second conventional magnetic recording / reproducing apparatus, this second conventional magnetic recording / reproducing apparatus is different from the first conventional magnetic recording / reproducing apparatus shown in FIG. In addition to the components 1 to 10 and 12, an envelope detection circuit 13 that detects the envelope level of the output FM signal of the AGC circuit 5, a noise canceller circuit 11A instead of the noise canceller circuit 11, and a noise detection circuit 14
With.

The noise canceller circuit 11A has a well-known line correlation type structure and is an amplifier 1 for amplifying an input signal.
11, a delay line 112 that delays the video signal supplied from the amplifier 111 for one horizontal scanning period (1H) and outputs a delayed video signal, and a differential amplifier 113 that amplifies the difference between the video signal and the delayed video signal. And differential amplifier 1
A limiter circuit 114 for limiting the amplitude of the output signal of 13 to extract only noise components, and the above-mentioned video signal and limiter circuit 114
And a mixer circuit 115 that mixes the output signals of the above signals in the opposite phase.

To explain the operation, first, the basic operation of the circuit is the same as that of the first conventional magnetic recording / reproducing apparatus.
The noise canceller circuit 11A amplifies the output of the low-pass filter 10 by the amplifier 111 and outputs the video signal to the delay line 11
2, one input of the differential amplifier 113 and the mixer circuit 1
15 are supplied respectively. This video signal is delayed by the delay line 112 for 1H period, and the delayed video signal is delayed by the differential amplifier 1
13 to the other input. The differential amplifier 113 extracts the noise component by differentially amplifying the video signal and the delayed video signal, the limiter circuit 114 extracts only the noise component, and the mixer circuit 115 subtracts the noise component from the video signal to generate the noise component. To remove.

On the other hand, the envelope detection circuit 13 uses the AGC
The envelope level of the output FM signal of the circuit 5 is detected,
The RF equalizer circuit 6, the mixer circuit 115 of the noise canceller circuit 11A, and the picture control circuit 12 are controlled according to the output level. For example, during reproduction of a repeatedly used soft tape, the reproduction envelope level is lowered due to demagnetization, and the image quality is deteriorated due to the inversion phenomenon and the deterioration of the SN ratio. In this case, the envelope detection circuit 13 responds to the small envelope level by peaking near the carrier frequency to secure a margin for the inversion phenomenon of the equalization characteristic of the RF equalizer circuit 6 and the noise canceling amount of the noise canceller circuit 11A. Is increased and the correction amount of the picture control circuit 12 is decreased, the image quality is controlled with an emphasis on the SN ratio.

Further, when the SN ratio in the recording state is poor, such as the case of reception input recording of a broadcast channel having a weak electric field strength, the noise detection circuit 14 causes
A noise component of the baseband signal is detected from the output of the differential amplifier 113, and the equalization characteristic of the RF equalizer circuit 6 and the picture control circuit 1 are detected according to the noise level.
The correction amount of 2 is controlled to obtain the optimum image quality with a good SN ratio.

[0014]

In the first example, the conventional magnetic recording / reproducing apparatus described above increases in response to the decrease in the level of the FM signal accompanying the deterioration of the high frequency characteristics due to the change with time of the video head. However, there is a drawback in that it is not possible to prevent deterioration of image quality due to an increase in high frequency noise components due to the amplification degree.

Further, the relationship between the carrier level of the reproduced FM signal and the sideband level and the envelope level are varied due to the recording state and the variation of the characteristics of the video head at the time of reproduction, and the image quality is caused by the inversion phenomenon and the deterioration of the SN ratio. However, there is a drawback in that

In the second example for reducing the above-mentioned drawbacks, although there is a slight reduction effect for the increase of the noise level of the entire frequency band, the deterioration of the image quality due to the increase of the above-mentioned high frequency noise component is suppressed. It had the drawback of not being effective.

Further, there is a drawback that the circuit configuration of the noise canceller circuit is complicated.

[0018]

A magnetic recording / reproducing apparatus of the present invention comprises an FM signal reproducing means for extracting a reproduced FM signal from a magnetic recording medium, and an AGC for correcting the level of the reproduced FM signal.
An automatic gain adjustment circuit for outputting an FM signal, and the AGCF
A high frequency equalizer circuit that corrects the frequency characteristics of the M signal and outputs an equalized FM signal, an FM demodulation circuit that demodulates the equivalent FM signal and outputs a demodulated video signal, and a predetermined high-frequency emphasis characteristic of the demodulated video signal And a noise canceller circuit for canceling noise components of the reproduced video signal and outputting a noise canceling video signal, and a frequency characteristic and phase of the noise canceled video signal. In a magnetic recording / reproducing apparatus including a picture control circuit for correcting characteristics and an envelope detection circuit for detecting an envelope level of the AGCFM signal and outputting a control signal corresponding to the envelope level, the noise canceller circuit and the de-emphasis circuit are included. At least one of It is configured to include a variable low pass filter which is controlled the frequency characteristic in response to a serial control signal.

[0019]

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

A first embodiment of the magnetic recording / reproducing apparatus according to the present invention will be described with reference to FIG. 1, which is a block diagram in which components common to those in FIGS. 4 and 5 are designated by common reference characters / numerals. In the magnetic recording / reproducing apparatus of this embodiment shown in this figure, instead of the noise canceller circuits 11 and 11A, a cutoff circuit that outputs a low-pass filtered video signal LV in response to the supply of the output video signal V of the low-pass filter 10 is provided. A variable frequency low-pass filter 21, an adder circuit 22 that outputs a difference video signal DV that includes a noise component by taking the difference between the video signals V and LV, and a video signal NV that limits the amplitude of the video signal DV and that includes only the noise component. The noise canceller circuit 11B is provided with a limiter circuit 23 for outputting the signal and a mixer circuit 24 for mixing the video signals V and NV.

The low-pass filter 21 includes a variable capacitance diode 211 controlled by a control signal C from the envelope detection circuit 13 and a fixed capacitor 21 for blocking direct current.
It is a low-pass filter of variable cutoff frequency including 2.

The operation of this embodiment will be described. First, the basic operation of the circuit is the same as that of the first and second conventional magnetic recording / reproducing devices. Next, the noise canceller circuit 11B
Then, the output video signal V from the low-pass filter 10 is supplied to the low-pass filter 21 and one input of each of the adder circuit 22 and the mixer circuit 24. The adder circuit 22 uses the video signal L from the low-pass filter 21.
V and a video signal DV in response to the supply of the video signal V and including a noise component which is the difference between these video signals LV and V
Is supplied to the limiter circuit 23. In response to the supply of the video signal DV, the limiter circuit 23 supplies the video signal NV including only the noise component to the other input of the mixer circuit 24. The mixer circuit 24 cancels noise components by mixing the video signals V and NV in opposite phases, and supplies the video signal NCV having a good SN ratio to the picture control circuit 12 in the next stage.

As mentioned above, the low-pass filter 21 is
The cutoff frequency is varied by the variable capacitance diode 211 controlled by the control signal C from the envelope detection circuit 13. That is, the cutoff frequency decreases as the capacitance value of the variable capacitance diode 211 increases. As a result, the noise frequency component to be removed by the low pass filter 21 can be changed.

Referring also to FIG. 2 showing an example of the noise suppressing operation of the present embodiment, first, as an initialization, the AGC circuit 5 is configured so that the amplitude of the reproduced FM signal by the envelope detection circuit 13 as described in the prior art. The amplitude level is detected by the detection, and the amplification degree is changed so that the amplitude level becomes constant. That is, when the amplitude level decreases, a voltage signal corresponding to the decrease occurs in the output signal of the envelope detection circuit 13, and the voltage signal increases the gain of the amplifier of the AGC circuit 5 to increase the level of the output FM signal A. It is set to keep it constant.

When the envelope level of the reproduced FM signal read from the magnetic tape 1, that is, the output FM signal A of the AGC circuit 5 decreases due to the deterioration of the high frequency range due to the time-dependent change such as wear of the video head 2, the above voltage is generated. As the signal rises and the frequency component of the input signal becomes higher, as indicated by the broken line in FIG. 2A, the gain increases, and the envelope amplitude level, that is, the amplification degree of noise increases. on the other hand,
The voltage level of the control signal C output from the envelope detection circuit 13 is set to decrease in inverse proportion to the voltage signal. The voltage level of the control signal C output from the envelope detection circuit 13 decreases. As we all know,
Since the capacitance value of the variable capacitance diode 211 has a substantially inversely proportional relationship that it decreases as the applied voltage increases, the capacitance value increases in response to the decrease in the voltage level of the control signal C, and as a result, FIG. As shown in (B), the cutoff frequency of the low-pass filter 21 decreases. This allows
The pass frequency band component of the video signal LV decreases, and the high frequency noise component of the video signal DV, which is the difference from the video signal V, relatively increases as shown in FIG. Since the high frequency noise component of a certain video signal NV also increases, the high frequency noise component included in the video signal V can be effectively canceled as shown in FIG.

Referring to FIG. 3 which is also a block diagram of a second embodiment of the present invention, the difference of the present embodiment shown in this figure from the previously described first embodiment is that instead of the de-emphasis circuit 9 , A de-emphasis circuit 9A including a cut-off frequency variable low-pass filter 31 including a variable capacitance diode 311 similar to the low-pass filter 21.

The de-emphasis circuits 9 and 9A are FM
This circuit suppresses triangular noise caused by modulation. The high frequency range is emphasized in advance by the emphasis circuit before modulation during recording, and the high frequency range emphasized by the de-emphasis circuits 9 and 9A is attenuated after demodulation to restore the original flat characteristic. The low-pass filter 31 included in the de-emphasis circuit 9A varies the high-frequency attenuation characteristic according to the envelope level of the reproduced FM signal,
It is possible to suppress an increase in high frequency noise due to a change with time of the video head 2.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-mentioned embodiments and various modifications can be made. For example, similarly to the second conventional example, it is needless to say that the correction amount of the picture control circuit 12 can be further controlled by the control signal supplied from the envelope detection circuit without departing from the gist of the present invention.

[0029]

As described above, in the magnetic recording / reproducing apparatus of the present invention, at least one of the noise canceller circuit and the de-emphasis circuit has the variable low-pass filter whose frequency characteristic is controlled in response to the control signal. Therefore, there is an effect that it is possible to reduce the image quality deterioration due to the increase of the high frequency noise component due to the temporal change of the video head with a simple circuit configuration.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of a magnetic recording / reproducing apparatus of the present invention.

FIG. 2 is a characteristic diagram showing an example of a noise suppressing operation of the magnetic recording / reproducing apparatus of the present embodiment.

FIG. 3 is a block diagram showing a second embodiment of the magnetic recording / reproducing apparatus of the present invention.

FIG. 4 is a block diagram showing a first example of a conventional magnetic recording / reproducing apparatus.

FIG. 5 is a block diagram showing a second example of a conventional magnetic recording / reproducing apparatus.

[Explanation of symbols]

 1 Magnetic Tape 2 Video Head 3 Video Amplifier 4 Peaking Circuit 5 AGC Circuit 6 RF Equalizer Circuit 7, 23, 114 Limiter Circuit 8 FM Demodulation Circuit 9, 9A De-Emphasis Circuit 10, 21, 31 Low Pass Filter 11, 11A Noise Canceller Circuit 12 Pictures Control circuit 13 Envelope detection circuit 14 Noise detection circuit 22 Adder circuit 24,115 Mixer circuit 111 Amplifier 112 Delay line 113 Differential amplifier 211,311 Variable capacitance diode 212 Capacitor

Claims (2)

[Claims] 1. An FM signal reproducing means for extracting a reproduced FM signal from a magnetic recording medium, an automatic gain adjusting circuit for correcting the level of the reproduced FM signal and outputting an AGCFM signal, a frequency characteristic of the AGCFM signal, etc. High-frequency equalizer circuit for outputting a converted FM signal, an FM demodulation circuit for demodulating the equivalent FM signal and outputting a demodulated video signal, and a reproduction in which a predetermined high-frequency emphasis characteristic of the demodulated video signal is corrected and restored to a flat characteristic. A de-emphasis circuit that outputs a video signal, a noise canceller circuit that cancels a noise component of the reproduced video signal and outputs a noise canceling video signal, and a picture control circuit that corrects the frequency characteristic and the phase characteristic of the noise canceling video signal, The envelope level of the AGCFM signal is detected to detect the envelope level. In a magnetic recording / reproducing apparatus including an envelope detection circuit for outputting a control signal corresponding to a control signal, at least one of the noise canceller circuit and the de-emphasis circuit has a variable low frequency band whose frequency characteristic is controlled in response to the control signal. A magnetic recording / reproducing apparatus comprising a filter. 2. The magnetic recording / reproducing apparatus according to claim 1, wherein the variable low-pass filter includes a variable capacitance element that varies a capacitance value in response to a DC voltage of the control signal.