JPH0391102A - Magnetic reproducing device - Google Patents

Abstract

PURPOSE:To improve the S/N of a demodulation video signal by controlling the gain of an amplifier means replying to a detecting means which detects the demagnetizing state of a magnetic tape. CONSTITUTION:An npn transistor 71 comprises an emitter peaking amplifier at a amplifier circuit part 70. A carrier component in the neighborhood of a frequency 4.4MHz is selectively amplified with a capacitor 72(39pF) and a coil 73(33muH), and a reproducing level detection circuit 90 outputs a signal S8 of high level when the level of an FM video signal is low. Therefore, a transistor 78 is turned on replying to the signal S8, and it follows that a resistor 75(680OMEGA) is connected in parallel with a resistor 74, and the gain of the emitter peaking amplifier at the frequency 4.4MHz is raised from characteristic 40 to characteristic 41. Thereby, the S/N of the video signal obtained after demodulation is improved.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a magnetic reproducing device, and in particular, to a magnetic reproducing device.
The present invention relates to improving the S/N ratio of a video signal obtained by demodulating an M video signal.

[Conventional technology] 6th F! lJ is a conventional video tape recorder (VTR).
FIG.

Referring to FIG. 6, this playback circuit includes a video tape 15
A head 16, a head amplifier 17, an FMAGC circuit 13, an FM equalizer 14, a double limiter circuit 18, and an FMu 131 for reproducing the signal recorded on the
1 circuit 19, a lowus 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 applied to the FMAGC circuit 13.

The FMAGC circuit 13 adjusts the signal imbalance that occurs between reproduction signals obtained from a plurality of heads.
Control the gain of each signal. The output signal of the FMAGC circuit 13 is given to an FM equalizer 14. The FM equalizer 14 corrects distortion in the phase and frequency characteristics of the reproduced FM video signal, and then supplies the corrected signal to the double limiter circuit 18. The output signal of the double limiter circuit 18 is applied to an FMS modulation circuit 19, and the FM demodulated signal is applied to a low-pass filter 20.

The low-pass filter 20 removes unnecessary FM carrier signal components and supplies them to the next-stage baseband processing circuit.

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 given to the chroma mixing circuit 24, and a video signal is obtained by mixing it with the chroma signal.

[Problems to be Solved by the Invention] Generally, when a videotape is repeatedly played back, the level of the playback signal decreases due to demagnetization, and therefore the S/N ratio of the playback signal decreases. 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 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 is high, the carrier component is emphasized, resulting in a decrease in the S/N ratio of the demodulated signal.

The present invention was made in order to solve the above-mentioned problems, and in a magnetic reproducing device, the S of the demodulated video signal is not affected by the level of the reproduced FM video signal.
The purpose is to improve the /N ratio.

[Means for Solving the Problems] A magnetic reproducing device according to the present invention includes a reproducing means for reproducing an FM video signal recorded on a magnetic tape, and an upper sideband signal included in the reproduced FM video signal. gain controllable amplification means for selectively emphasizing and amplifying components; and detecting the demagnetization state of the magnetic tape in response to a state signal indicating the demagnetization state of the magnetic tape included in the reproduced FM video signal. It includes detection means for detecting and control means for controlling the gain of the amplification means in response to the detection means.

[Operation] In the magnetic reproducing apparatus according to the present invention, the detection means detects the demagnetized state of the magnetic tape. The control means controls the gain of the amplification means in accordance with the demagnetization state of the magnetic tape. Therefore, since the FM video signal is amplified according to the level of the reproduced FM video signal, deterioration of the S/N ratio of the demodulated video signal can be prevented.

[Embodiment of the Invention] FIG. 1 is a circuit block diagram showing a reproduction circuit of a VTR showing an embodiment of the invention. Referring to FIG. 1, 86
The difference from the conventional circuit shown in the figure is an amplifier circuit section 70 and a phase circuit section 8 as an FM equalizer circuit.
0 is provided, and in addition to this, a reproduction level detection circuit 90 for controlling the amplifier circuit section 70 in response to the reproduction level is provided.

FIG. 2 is a circuit diagram showing an example of the reproduction level detection circuit 90 shown in FIG. 1. Referring to FIG.
An amplifier circuit section 3 connected to receive a video signal, an integration circuit section 4 connected to the output of the amplifier circuit section 3 via a coupling capacitor 47, and a comparison circuit section connected to the output of the integration circuit section 4. 5.

In operation, a reproduced FM video signal is applied via the resistor 30 to the base of the NPNH range meter 31 that constitutes the first stage amplifier. A low frequency chroma signal component of the signal amplified by transistor 31 is grounded via a series connection of coil 32 and capacitor 33. The signal from which the chroma signal component has been removed is applied to the base of the npn transistor 34 constituting the second stage amplifier. The signal amplified by the transistor 34 is applied via a coupling capacitor 36 to the base of an npn transistor 39 constituting the third stage amplifier. Resistors 37 and 38
constitutes a bias circuit.

The collector of the transistor 39 is connected to a collector peaking circuit 4 composed of a resistor, a capacitor, and a coil.
0 is connected, and this circuit 40 generates the sync chip level frequency (for example, in the case of VHS system, about 3.
4 MHz) is further amplified. The output signal of the collector peaking circuit 40 is applied to the integrating circuit section 4 via a capacitor 47.

Integrating circuit section 4 includes an npn transistor 44 constituting an amplifier and bias resistors 41 and 42 thereof.

The DC component of the base voltage applied to the base of transistor 44 by resistors 41 and 42 is biased near ground potential. As a result, the signal component of the frequency of the sync tip level of the FM video signal reproduced by the integrating circuit section 4 is rectified and converted into a DC voltage signal indicating the level of the reproduced signal. This DC voltage signal is applied to the comparison circuit section 5 at the next stage.

Comparison circuit section 5 includes resistors 61 and 62 and a comparator 63 for generating a reference voltage.

The comparator 63 is connected such that one side of the comparator 63 receives the above-mentioned DC voltage signal, and one side of the comparator 63 is connected so that it receives the reference voltage. The comparator 63 compares the voltage levels of the two manually input signals, and when the voltage level of the DC voltage signal is lower than the reference voltage, a high level signal S8 is generated.
When the signal is high, a low level signal S8 is output.

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

In general, home VTRs employ a low-carrier FM recording method with a low carrier frequency. In this method, since the modulation index is smaller than 0.5, it is generally known that the signal components necessary for F Mu13i only need to consider the carrier wave component and the upper and lower first sideband signal components. However, due to the recording frequency characteristics of the magnetic tape used, the reproduction frequency characteristics of the head, etc., the lower sideband of the low carrier FM signal is emphasized and the upper sideband is suppressed. If the reproduced signal in such an unbalanced state is demodulated as it is, it may not pass through the zero cross point or an inversion phenomenon may occur, which is undesirable. Therefore, an FM equalizer is required to correct such unbalance in the reproduced signal.

FIG. 3 is a characteristic diagram showing the frequency characteristics of the reproduced FM video signal. In FIG. 3, the vertical axis shows the voltage of the reproduced signal, and the horizontal axis shows the frequency. As can be seen from this figure, the output signal of the FMAGC circuit 13 shown in FIG. 1 has a characteristic 42. That is, the carrier signal component is set to 10, the lower sideband signal component J- is emphasized, and the upper sideband signal component J is suppressed.

FIG. 4 is a characteristic diagram showing the 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 bicollar 4 circuit shown in FIG. 1 has characteristic 40 when a reproduced signal of a predetermined level or higher is obtained, and switches to characteristic 41 when the level of the reproduced signal is lower than that. This point will be explained in detail later.

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

In the amplifier circuit section 70 shown in FIG. 1, the npn transistor 71 constitutes an emitter peaking amplifier. Capacitor 72 (39pF) and coil 73 (
33 μH), carrier components near 4.4 MH2 are selectively amplified. As mentioned above, when the level of the reproduced FM video signal is low, the reproduction level detection circuit 90
outputs a high level signal S8. Therefore, transistor 78 turns on in response to signal S8, and resistor 75 (6
80Ω) will be connected in parallel with the resistor 74. Therefore, the 4.43MHz of the emitter peaking amplifier
The gain at is raised from characteristic 40 to characteristic 41 as shown in FIG.

In this way, the carrier frequency gain increases when the playback level is low, so the S of the video signal obtained after demodulation increases.
/N ratio is improved. Note that the capacitor 76 and the coil 77 function to release the low frequency chroma signal component to the ground.

In the phase circuit section 80, mutually inverted signals are output from the collector and emitter of the npn transistor 81. The phase of the collector signal is controlled by the coil 82 and the capacitor 83, and the emitter signal is applied via the resistor 4. I can get it. The added signal is
When viewed from the base of the transistor 81, the phase is shifted, and only the signal component of a specific frequency can be shifted.

By adjusting the phase in this way, the frequency characteristics of the demodulation circuit connected later can be improved.

[Effects of the Invention] As described above, according to the present invention, the gain for amplifying the reproduced FM video signal is controlled according to the demagnetization state of the magnetic tape. A magnetic reproducing device capable of improving the S/N ratio of a demodulated video signal without being affected by the level has been obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a reproducing circuit of a VTR showing an embodiment of the present invention. FIG. 2 is a circuit diagram showing an example of the reproduction level detection circuit shown in FIG. 1. FIG. 3 is a characteristic diagram showing the frequency characteristics of the reproduced FM video signal. FIG. 4 shows circuit sections 70 and 80 shown in FIG.
FIG. 2 is a characteristic diagram showing the frequency characteristics of an equalizer circuit configured by the following. Figure 5 shows FM with improved frequency characteristics.
FIG. 3 is a characteristic diagram showing frequency characteristics of a video signal. Figure 6 shows
1 is a block diagram showing a conventional VTR playback circuit. FIG. In the figure, 3 is an amplifier circuit section, 4 is an integration circuit section, 5 is a comparison circuit section, 70 is an amplifier circuit section, 80 is a phase circuit section, and 90 is a comparison circuit section.
is a reproduction level detection circuit.

Claims (1)

[Scope of Claims] A reproduction means for reproducing an FM video signal recorded on a magnetic tape, wherein the FM video signal reproduced by the reproduction means includes a status signal indicating a demagnetization state of the magnetic tape. , gain controllable amplification means for selectively emphasizing and amplifying an upper sideband signal component included in a reproduced FM video signal; and detecting a demagnetized state of the magnetic tape in response to a reproduced state signal. A magnetic reproducing device comprising: a detection means for detecting the magnetic field; and a control means for controlling the gain of the amplification means in response to the detection means.