JPH0785284B2 - Magnetic reproducing device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a magnetic reproducing device, and more particularly to a reproduced FM.
The present invention relates to improvement of the S / N ratio of a video signal obtained by demodulating the signal.

[Prior Art] Fig. 6 shows a conventional video tape recorder (VTR).
3 is a block diagram showing a reproducing circuit of FIG. Referring to FIG. 6, this reproducing circuit includes a head 16 for reproducing a signal recorded on a video tape 15, a head amplifier 17, and an FM.
AGC circuit 13, FM equalizer 14, double limiter circuit 1
8, an FM demodulation circuit 19, a low pass filter 20, a de-emphasis circuit 21, a noise canceller circuit 22, a picture control circuit 23, and a chroma mixing circuit 24.

In operation, the signal reproduced from the video tape 15 by the head 16 is amplified by the head amplifier 17 and then FMAGC.
Given to circuit 13. The FMAGC circuit 13 controls the gain of each signal in order to adjust the imbalance of the signals generated between the reproduction signals obtained from the plurality of heads. FMAGC circuit 1
The output signal of 3 is given to the FM equalizer 14. The FM equalizer 14 corrects the distortion in the phase and frequency characteristics of the reproduced FM video signal, and then gives the corrected signal to the double limiter circuit 18. The output signal of the double limiter circuit 18 is given to the FM demodulation circuit 19, and the FM demodulated signal is given to the low-pass filter 20. The low-pass filter 20 removes the unnecessary FM carrier signal component and supplies it to the baseband processing circuit in the next stage. The baseband processing circuit includes a de-emphasis circuit 21, a noise canceller circuit 22, and a picture control circuit 23. The baseband-processed signal is supplied to the chroma mixing circuit 24 and mixed with the chroma signal to obtain a video signal.

[Problems to be Solved by the Invention] Generally, when a video tape is repeatedly reproduced, the level of a reproduction signal (particularly its upper sideband signal component) is decreased by a demagnetization effect, and thus the S / N ratio of the reproduction signal is decreased. . That is, the output signal level of the head amplifier 17 obtained from the demagnetized magnetic tape 15 decreases, and the carrier-to-noise (C / N) ratio also decreases, resulting in a decrease in the S / N ratio after FM demodulation. Was.

To deal with this, for example, if the gain of the FM equalizer 14 is set to a high value in advance, the S / N ratio of the demodulated signal can be improved when the level of the reproduced signal is low,
When the level of the reproduced signal is high, the upper sideband signal component is overemphasized and the S / N ratio of the demodulated signal is lowered.

The present invention has been made to solve the above problems, and as a result, in a magnetic reproducing apparatus, the level of a reproduced FM signal resulting from the demagnetized state of a magnetic medium is not simply affected. The purpose is to obtain a demodulated video signal with a good S / N ratio.

[Means for Solving the Problem] A magnetic reproducing apparatus according to the present invention is a reproducing means for reproducing a frequency-modulated FM signal recorded on a magnetic medium, and a minimum carrier frequency of the FM signal reproduced by the reproducing means. Detecting means for detecting the reproduction level of the component signal, comparing means for comparing the reproduction level detected by the detecting means with a predetermined reference level, FM arranged in parallel with the detecting means and reproduced by the reproducing means A gain controllable amplification means for emphasizing and amplifying the maximum carrier frequency component signal of the signal, a correction means for correcting the phase of a signal of a frequency near the maximum carrier frequency of the FM signal output from the amplification means, and the correction means Demodulation means for demodulating the output FM signal, and control means for controlling the gain of the amplification means based on the comparison result of the comparison means It is characterized by a door.

[Operation] In the magnetic reproducing apparatus according to the present invention, the reproduction level of the minimum carrier frequency component signal of the FM reproduction signal is compared with a predetermined reference level, and the demagnetization state of the magnetic medium is judged based on the comparison result, and the corresponding condition is determined. The maximum carrier frequency component signal of the FM reproduction signal is amplified by the gain. Moreover, since the phase of the frequency in the vicinity of the maximum carrier frequency of the FM reproduction signal thus obtained is correctly corrected, without being confused by the level of the reproduction FM signal due to the demagnetization state of the magnetic medium,
That is, regardless of this, a demodulated video signal having a good S / N ratio can be obtained without changing the hue of the demodulated video signal.

[Embodiment of the Invention] FIG. 1 is a circuit block diagram showing a reproducing circuit of a VTR showing an embodiment of the present invention. Referring to FIG. 1, the difference from the conventional circuit shown in FIG. 6 is that an amplifier circuit section 70 and a phase circuit section 80 are provided as an FM equalizer circuit, and in addition to this, a reproduction level is added. In response to the amplification circuit section 70
That is, a reproduction level detection circuit 90 for controlling the above is provided.

FIG. 2 is a circuit diagram showing an example of the reproduction level detection circuit 90 shown in FIG. Referring to FIG. 2, the reproduction level detecting circuit includes an amplifier circuit section 3 connected to receive the FM video signal reproduced from the head amplifier 17, and an output of the amplifier circuit section 3 via a coupling capacitor 47. And an comparing circuit section 5 connected to the output of the integrating circuit section 4.

In operation, the reproduced FM video signal is applied to the base of the npn transistor 31 forming the first stage amplifier via the resistor 30. The low-frequency chroma signal component of the signal amplified by the transistor 31 is dropped to the ground through the series connection of the coil 32 and the capacitor 33. The signal from which the chroma signal component has been removed is given to the base of the npn transistor 34 which constitutes the second stage amplifier. The signal amplified by the transistor 34 is given to the base of the npn transistor 39 which constitutes the amplifier of the third stage via the coupling capacitor 36. The resistors 37 and 38 form a bias circuit. A collector peaking circuit 40 composed of a resistor, a capacitor and a coil is connected to the collector of the transistor 39, and this circuit 40 allows a sync chip level frequency f _-1 (minimum carrier frequency of VTR, for example,
In the case of the VHS method, it is approximately 3.4 MHz), and only the signal component of this is further amplified. The output signal of the collector peaking circuit 40 is given to the integration circuit unit 4 via the capacitor 47.

The integrating circuit unit 4 includes an npn transistor 44 that constitutes an amplifier.
And its bias resistors 41 and 42. Resistors 41 and 42 bias the DC component of the base voltage applied to the base of transistor 44 near ground. As a result, the signal component of the sync chip level frequency f ₋₁ in the FM video signal reproduced by the integration circuit unit 4 is rectified and converted into a DC voltage signal indicating the level of the reproduced signal. This DC voltage signal is given to the comparison circuit section 5 at the next stage.

The comparison circuit unit 5 includes resistors 61 and 62 for generating a reference voltage, and a comparator 63. The comparator 63 is
Its negative input is connected to receive the aforementioned DC voltage signal and its positive input is connected to receive the reference voltage.
The comparator 63 compares the voltage levels of the two input signals, outputs a high level signal S8 when the voltage level of the DC voltage signal is lower than the reference voltage, and outputs a low level signal S8 when it is high. To do.

Next, with reference to FIG. 1 again, the operation of the FM equalizer to which the circuit units 70 and 80 are applied will be described.

Generally, a VTR for home use adopts a low carrier FM recording system (a FM system in which f = f _c ± Δf and Δf is smaller than f _c ) with a low frequency deviation of a carrier frequency. In this method, since the modulation index is smaller than 0.5 (Δf / f _c = 0.5), it is sufficient to consider only the carrier component and the upper and lower first sideband signal components as the signal components required for FM demodulation. Is generally known. However, the lower sideband of the low carrier FM signal is emphasized and the upper sideband is suppressed due to the recording frequency characteristic of the magnetic tape used and the reproducing frequency characteristic of the head. If the reproduced signal in such an unbalanced state is demodulated as it is, it does not pass through the zero-cross point or an inversion phenomenon occurs, which is not preferable. Therefore, an FM equalizer is required to correct such an imbalance of the reproduced signal.

FIG. 3 is a characteristic diagram showing the frequency characteristic of the reproduced FM video signal. In FIG. 3, the vertical axis represents the voltage of the reproduced signal, and the horizontal axis represents the frequency. As can be seen from this figure, the output signal of the FMAGC circuit 13 shown in FIG.
have. That is, compared with the reproduction level J ₀ of the carrier frequency f ₀ of the signal, the sync tip level playback level J _-1 (= lower sideband signal level) of the signal of the frequency f _-1 is emphasized, the maximum carrier frequency f ₁ Signal reproduction level J ₁ (=
The upper sideband signal level) is suppressed. As the demagnetizing action of the magnetic tape progresses, this state is further promoted, and as shown by the broken line in the figure, the reproduction level J ₁ of the signal of the frequency f ₁ drops, and at the same time, the frequency f _{-1 is} relatively increased. The reproduction level J _-1 of the signal is emphasized. That is, playback level J _-1
Level will rise. It should be noted that when the demagnetizing action is small, the relationship between the reproduction levels has the opposite tendency.

FIG. 4 is a characteristic diagram showing frequency characteristics of the equalizer circuit configured by the amplifier circuit section 70 and the phase circuit section 80 shown in FIG. In FIG. 4, the vertical axis represents gain and the horizontal axis represents frequency. The FM equalizer circuit shown in FIG. 1 has a low high-frequency reproduction capability when the reproduction level J _-1 of the sync-chip-level frequency f _-1 signal is above the reference level, that is, when the demagnetization effect is large. The characteristics are extended to the high range in response to the fact that the high range reproduction capability is large when the reproduction level J _-1 is below the reference level, that is, when the demagnetizing effect is small. Switch to the characteristic 41 that is set. This point will be described later in detail.

FIG. 5 is a characteristic diagram showing the frequency characteristic of the FM video signal improved by the equalizer circuit having the characteristic shown in FIG. In this way, the characteristic 43 having the lower sideband and the upper sideband that are symmetrical about the carrier frequency f ₀ is obtained.

In the amplifier circuit section 70 shown in FIG. 1, the npn transistor 71 constitutes an emitter peaking amplifier. The capacitor 72 (39 pF) and the coil 73 (33 μH) selectively amplify the carrier component near 4.43 MHz corresponding to 3.4 MHz + Δf. Playback level detection circuit 90 in FIG.
As described above, when the reproduction level J _-1 of the sync chip level frequency f _-1 of the reproduced FM video signal is higher than the reference level, the reproduction level detection circuit 90 outputs the low level signal S8. Output. Therefore, the transistor 78 turns off in response to the signal S8. In other words, the gain of the amplifier circuit 70 is lowered and becomes the characteristic of 40 in FIG. On the contrary, when the reproduction level J _-1 is lower than the reference level, the detection circuit 90
Outputs a high level signal S8. At this time, the transistor 78 is turned on, the resistance of the emitter peaking is reduced, the gain of the amplifier circuit 70 is increased, and the characteristic of 41 in FIG. 4 is obtained. Therefore, the 4.43 of the emitter peaking amplifier
The gain in MHz is raised from the characteristic 40 to the characteristic 41 as shown in FIG. Thus, the gain of the maximum carrier frequency f ₁ of the amplifier circuit 70 is increased when the reproduction level J _-1 of the signal of the sync tip level of the frequency f _-1 is lower than the reference level, the image signal obtained after the demodulation S / N
The ratio is improved. The capacitor 76 and the coil 77
Has a function of releasing low-frequency chroma signal components to the ground.

In the phase circuit unit 80, mutually inverted signals are output from the collector and emitter of the npn transistor 81. By passing through the series circuit of the coil 82 and the capacitor 83, the phase of the signal at the collector is changed by the phase determined by the impedance, and the signal is combined with the signal from the resistor 84 which is in the opposite phase to the signal. When viewed from the base of the transistor 81, the phase of the synthesized signal becomes nonlinear near the maximum carrier frequency f ₁ and deviates from the phase of the input signal. This phase shift reversely corrects the phase shift of the signal of the frequency near the maximum carrier frequency f ₁ due to the amplification and amplification of the signal of the maximum carrier frequency f _{1 by} the amplifier circuit 70, and as a result, the phase shift near the maximum carrier frequency f ₁ The phase is corrected.

By adjusting the phase in this way, it is possible to improve the frequency characteristics of the demodulation circuit connected later.

As described above, according to the present invention, the judgment of the demagnetization state of the magnetic medium is judged by the reproduction level of the maximum carrier frequency component of the FM reproduction signal, and the FM reproduction signal is gained with the gain according to the result. After the maximum carrier frequency component signal of is amplified by a predetermined amount, the phase of the maximum carrier frequency component signal of the FM reproduction signal is further corrected correctly, so the level of the reproduced FM signal caused by the demagnetization state of the magnetic medium can be simplified. As a result, a demodulated video signal with a good S / N ratio can be obtained without being affected by

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a VTR reproducing circuit showing an embodiment of the present invention. FIG. 2 is a circuit diagram showing an example of the reproduction level detecting circuit shown in FIG. Figure 3 shows
It is a characteristic view which shows the frequency characteristic of the reproduced FM video signal. FIG. 4 is a characteristic diagram showing frequency characteristics of the equalizer circuit configured by the circuit units 70 and 80 shown in FIG. FIG. 5 is a characteristic diagram showing frequency characteristics of an FM video signal with improved frequency characteristics. Fig. 6 shows the conventional VT
It is a block diagram which shows the reproduction circuit of R. In the figure, 3 is an amplification circuit section, 4 is an integration circuit section, 5 is a comparison circuit section, 70 is an amplification circuit section, 80 is a phase circuit section, and 90 is a reproduction level detection circuit.

Claims (1)

[Claims] 1. A frequency-modulated recording on a magnetic medium
Reproducing means for reproducing the FM signal, detecting means for detecting a reproducing level of the minimum carrier frequency component signal of the FM signal reproduced by the reproducing means, and a reproducing level detected by the detecting means and a predetermined reference level. Comparing means for comparing, amplifying means capable of controlling gain, which is installed in parallel with the detecting means, and which amplifies and amplifies the maximum carrier frequency component signal of the FM signal reproduced by the reproducing means, FM signal output from the amplifying means Correction means for correcting the phase of a signal having a frequency in the vicinity of the maximum carrier frequency, demodulation means for demodulating the FM signal output from the correction means, and control the gain of the amplification means based on the comparison result of the comparison means. A magnetic reproducing apparatus including control means for controlling.