JPS61287387A - Preventing device for reversing phenomenon - Google Patents

Abstract

PURPOSE:To prevent a reversing phenomenon which keeping an S/N and a frequency characteristic optimum by preventing the reversing phenomenon by an FM equalizer and correcting the deteriorated S/N and the frequency characteristic by a deemphasis. CONSTITUTION:When a modulating characteristic becomes large, a sideband wave becomes large, operates to lower the modulating characteristic and a reversing phenomenon is scarcely generated. An FM signal changing in this manner passes through a limitter 6 and is modulated by an FM modulator 7 and inputted to a deemphasis 8. When the characteristic of an FM equalizer 5 is changed so as to limit the sideband wave of the FM signal having the large modulating characteristic and easily producing the reversing phenomenon, the S/N of the demodulated video signal becomes better and the frequency characteristic becomes worse, so that the characteristic of the deemphasis 8 is changed correspondingly to the change of the characteristic of the FM equalizer 5.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a device for preventing an inversion phenomenon in a video signal recording/reproducing device.

[Conventional technology]

In the spectrum of a frequency modulation (FM) signal, the upper and lower sidebands are distributed symmetrically around the carrier wave, but
When this FM signal is recorded and reproduced by an F'M signal magnetic recording and reproducing device such as a video tape recorder, the upper side band is expanded and the lower side band is suppressed.

When the imbalance between the upper and lower sidebands becomes extreme, an inversion phenomenon occurs. Furthermore, when the modulation index of FM modulation becomes high, an inversion phenomenon tends to occur.

Conventionally, as a measure to prevent the inversion phenomenon in video tape recorders, the balance between the upper and lower sidebands has been maintained by suppressing the lower sidebands with a 7M reproduction equalizer or by emphasizing the upper sidebands. However, the method of suppressing the lower sideband lowers the modulation index, resulting in deterioration of the frequency characteristics of the video signal after FM demodulation. This emphasizes a portion with a poor N ratio, resulting in a deterioration of the SA ratio of the video signal after FM demodulation.

[Problem that the invention seeks to solve]

As described above, the conventional 7M reproduction equalizer prevents the reversal phenomenon by adjusting the FM
There were problems such as deterioration of the frequency characteristics and SA ratio of the demodulated video signal.

This invention was made to solve the above-mentioned problems, and its purpose is to provide a device that can prevent the inversion phenomenon while also correcting the deterioration of the frequency characteristics and S/N ratio of the demodulated video signal. do.

[Means for solving problems]

The inversion phenomenon prevention device according to the present invention detects a case where the modulation index is high where an inversion phenomenon is likely to occur at the output level of the bandpass filter for extracting the lower sideband wave f, and suppresses the upper and lower sideband waves. The 7M equalizer is operated to correct the unbalance of the demodulated video signal, and the de-emphasis is thereby operated to restore the changed frequency characteristics and sA ratio of the demodulated video signal.

[For Kui]

The 7M reproduction equalizer and de-emphasis in the present invention take advantage of the fact that they can function almost equivalently in terms of frequency characteristics and S/N ratio in a reproduced video signal. In other words, the inversion phenomenon is prevented by changing the characteristics of the 7M reproduction equalizer and de-emphasis so that the frequency characteristics and S/N ratio do not change.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. 1st
The figure is a block diagram showing the configuration of this embodiment. In the figure, (1) is a magnetic tape, (2) is a magnetic tape fl'
(3) is a head amplifier (head amplifier) that amplifies the minute signal induced by the magnetic head (2), (41 ke? M signal 8 is delayed by a certain period of time) (5) is a 7M equalizer whose wave number characteristics change according to the output number of the detector (described later); (6) a limiter; (71fl FM demodulator); (8) is a detection unit (described later). 7M equalizer whose frequency characteristics change according to the output of the device, +91 #-j video output.

(10) is a bandpass filter that extracts the lower sideband of the IFM signal; (6) is a detector that detects the envelope, for example;
Second delay line that delays the detection output of the detector for a certain period of time, a3 side F'M equalizer input FM signal, α side FM
Equalizer control signal, aB ke F'M equalizer output yM
i issue, α! is the de-emphasis input video signal, αη is the de-emphasis control signal.

Next, the operation will be explained. In FIG. 1, an FM signal recorded on a magnetic tape (1) is reproduced by a magnetic head (2) and amplified by a head amplifier (3). The spectrum of this output is shown in FIG. 2 for the case of a single modulated FM signal. In the figure, a solid line indicates a case where the modulation index is small, and a broken line indicates a case where the modulation index is large. Jo
, J-z#-j\J1 lower sideband, J+l is the first
It is an upper sideband wave. As the modulation index increases as shown by the broken line, the first lower sideband wave exceeds the carrier wave due to minute fluctuations in the tape head system, resulting in an inversion phenomenon. Therefore, it can be said that the inversion phenomenon is more likely to occur in the case of a broken line with a high modulation index.

Therefore, the bandpass filter (10) in FIG.
By passing through υ, the magnitude of the lower sideband component is read and the control signal α is determined depending on the magnitude of the modulation index.

FIG. 3 shows an example of the characteristics of the bandpass filter 110+. In the figure, fc times the carrier frequency, for example, the center frequency is f
c-1.5MHz and the band is ±0.3M1 (z).On the other hand, the FM signal output from the head amplifier (2) is slowed down by the filter 001 and the envelope detector αυ, and then the control signal α and the 7M equalizer. (6) The first delay line (passes through 41 and the 7M equalizer (
5). A concrete example of the 7M equalizer (5) is shown in FIG. Figure 4A is a circuit diagram in which the FM signal (to) is input to the base of transistor Qx through resistor R1 and output from its collector, while the control signal α is input to the base of transistor Q2 through resistor R4. As a result, the frequency characteristics change as shown in FIG. 4B. When the modulation index becomes larger, the sideband becomes larger, and the output of the envelope detector αD becomes a kennel. This causes the potential of the transistor Q2 to rise, causing resonance between the inductance Ll, capacitance C1, and transistors Q and Q. As the effect of the system increases, the characteristics change as follows: A → A → For example, if A't is -0 dB at the peak, it becomes +3 dB.) The resonant frequency is the carrier frequency fc.
If it is in the vicinity, the modulation index will be lowered and the reversal phenomenon will be less likely to occur. Then it changes like this 7':
The F'M signal is a limiter (6) and a t-way FM demodulator (7).
The demodulated signal is then input to the de-emphasis (8).

On the other hand, the second delay line Q2t-path de-emphasis (8) is controlled in order to synchronize the time with the control signal α41 and the demodulated video signal αO output from the envelope detector (6). A specific example of de-emphasis (8) is shown in FIG. Figure 5 is a circuit diagram showing demodulated video signal αQ and resistance R.
The control signal αη7> is inputted to the base of the transistor Q3 through the resistor Rs, and is outputted from its collector. changes.

This is because the S/N of the demodulated video signal changes when the characteristics of the 7M equalizer are changed to suppress the sidebands of the 7'1m FM signal, which increases the modulation index and prevents inversion from occurring.
The ratio is better and the frequency characteristics are worse, so the original 8/N
The constants are determined to maintain the ratio and frequency characteristics, and Fig. 4B
Corresponding to the characteristic of the 7M equalizer (6) changing from A to A to one as shown in FIG. If A is OdB and Tsu is +3 dB at the highest frequency.) In other words, when the output of envelope detector 0 increases, the base potential of transistor Q4 increases, and resistor R? , capacitance c11. This reduces the de-emphasis effect of transistor Q4.

In the above embodiment, transistor Q2. Although Q and 4 have the effect of a variable resistor, they can also be configured with FIT. The characteristics in this case are shown in Figure 6. In the figure, (a) is the characteristic of the 7M equalizer (5), where force → force → ri (
For example, if the force is OdB, the modulation index is lowered by changing it to -3aB), and the characteristic of de-emphasis (8) in (b) is changed from force to key (for example, if the force is 0 (IB), +x, 5dB), the same effect as in the above embodiment can be achieved.Furthermore, the 7M equalizer (6)
The characteristic .degree. may be made to emphasize the upper sideband, and the characteristic in this case is shown in FIG. In the figure, (a) is the characteristic of the 7M equalizer (5). By changing the characteristics as →S →S (for example, if S is OdB, then S is +5aB), the imbalance between the upper and lower sidebands is corrected, and the imbalance in (b) is Characteristics of emphasis (8) wosa→shi→su (r.

For example, if S'i'OdB is changed to SH-1,5dB), the 7M equalizer (6) will degrade SA.
ratio can be improved.

Furthermore, although the envelope detector I is used in the above embodiment, a peak detector may also be used.

Furthermore, although the above embodiment describes a VTR, video signals of other video discs, still video cameras, etc. can be transmitted to FM.
A device that modulates recording and reproduction may also be used, and the same effects as in the above embodiment can be achieved.

〔Effect of the invention〕

As described above, according to the present invention, by utilizing the fact that the 7M equalizer and de-emphasis have the same effect on the 8/N ratio and frequency characteristics, the 7M equalizer can prevent the inversion phenomenon between degraded ElNlN. The wave number characteristics can be corrected by de-emphasis, so if you use a 7M equalizer and de-emphasis t-R meter to optimize the S/N ratio and frequency characteristics, you can correct the S/N.
It is possible to prevent the inversion phenomenon while optimizing the ratio and frequency characteristics.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.
Figure 2 is a frequency spectrum diagram of 7M waves, Figure 3 is a characteristic diagram of the bandpass filter in the above embodiment, and Figure 4 is AI-j.
A circuit diagram showing a specific example of the 7M equalizer used in this embodiment, FIG. 4B is its characteristic diagram, and FIG. 5 is a circuit diagram showing a specific example of de-emphasis used in this embodiment.
Characteristic diagrams of the F'M equalizer and de-emphasis in other embodiments of the present invention, Figures 6 (a) and (b) respectively FIG. 6 is a characteristic diagram of a 7M equalizer and de-emphasis in still another embodiment of the present invention. In the figure, (11 is a recording medium (magnetic tape), (2)
(6) is a reproduction FM equalizer, (7) is an F'M demodulator, (8) is a hard emphasis, +to+
tri-band H filter, (9) is a detector (envelope detector). In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] (1) A bandpass filter that extracts the lower sideband of a reproduced FM signal obtained by reproducing a video signal recorded by frequency modulation (FM), a detector that detects the output level of this bandpass filter, and the output of this detector a reproduction FM equalizer whose characteristics are changed according to the characteristics of the reproduction FM equalizer, and a reproduction video signal obtained by further FM demodulation of the reproduction FM signal through the reproduction FM equalizer, and a reproduction FM equalizer whose characteristics change according to changes in the characteristics of the reproduction FM equalizer; An inversion phenomenon prevention device comprising a de-emphasis whose characteristics are changed by the output of the detector so as to correct the frequency characteristics and signal/noise ratio of the reproduced video signal.