JPH0335472A - Inverted phenomenon preventing device - Google Patents

Abstract

PURPOSE:To prevent an inverted phenomenon without degrading an S/N by extracting only the part, where a zero cross point is missed, of an FM modulation wave and reviving the zero cross point. CONSTITUTION:A signal component to suppress the lower side wave component of the FM modulation wave by a band pass filter 2 and the signal component of an input FM modulation wave 1, for which a phase is matched with that of the above mentioned signal component, are subtracted by a subtracter 4 and the signal of the suppressed lower side wave component is outputted. An extracting part 6 extracts only the signal in the part, where the zero cross point is missed, from this output signal and amplification is executed corresponding to the degree of a signal level. Then, the missing signal of the zero cross point is revived. The input FM modulation wave 1 of the matched phase is synthesized again with the missing signal, which is restored by the extracting part 6, by an adder 9 and accordingly, the FM modulation wave, for which there is no missing part of the zero cross point or deviation, can be acquired. Thus, the inverted phenomenon can be suppressed without degrading the S/N.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an inversion phenomenon prevention device, and particularly to an FM demodulation means of a video recording and reproducing device that records and reproduces a video signal by FM modulating it. The present invention relates to an inversion phenomenon prevention device that prevents omission of zero crossings in reproduced FM modulated waves when recorded.

[Conventional technology]

Conventionally, in a video signal recording and reproducing apparatus that records an FM modulated wave obtained by FM modulating a video signal on a recording medium such as a magnetic tape and reproduces the recorded FM modulated wave, an FM modulated wave with a high degree of FM modulation is recorded. It is known that when this happens, a problem called an inversion phenomenon occurs in the video signal waveform that is reproduced.

For example, in the playback system of a video signal recording and playback device as shown in FIG.
The M modulated wave is reproduced by a recording/reproducing head (20). This reproduced weak FM signal is amplified by 1 in an amplifier (21), its frequency characteristics are corrected in a correction section (22), the amplitude is limited by a limiter circuit (23), and the FM demodulation section (24)
), and the video signal is reproduced by a reproduction section (25).

In this manner, in the video signal recording and reproducing apparatus shown in FIG. 9, when the video signal changes from the black level to the white level as shown in FIG.
This results in a waveform showing an inversion phenomenon as shown in Figure (a). Looking at this reversal phenomenon visually, the image that changes from the original black level to its own level is shown in Figure 8 (d), but the demodulated image is shown in Figure 8 (c). The black level and white level video signals are reversed at the boundary so that f
You can see that the elephant is disturbed.

The cause of the stiffness reversal phenomenon is thought to be as follows.

That is, in the video signal recording and reproducing apparatus, the lower wave of the FM modulated wave is emphasized due to the electromagnetic conversion characteristics.

In this way, in the FM modulated wave, when the lower side wave is emphasized compared to the upper side wave, a zero cross point is lost. However, since the limiter circuit (23) shown in FIG. 9 detects this zero-crossing point, if the zero-crossing point is missing, a normal FM modulated wave cannot be output to the output side of the limiter circuit (23). . Even if such a limiter circuit output is demodulated by the FM demodulator (24), a normal video signal cannot be obtained, and a sharp drop in level occurs, resulting in a waveform as shown in FIG. 8(a). It becomes. In other words, it is thought that the lack of a zero crossing point causes an inversion phenomenon.

Conventionally, a limiter method was used to prevent this reversal phenomenon, as described in "NHK Home Video Technology" (by Katsuya Yokoyama).
There is a reversal prevention circuit called the double limiter (DL-FM) system, which is described in pages 98 to 100 of Japan Broadcasting Publishing Association (ed.). FIG. 10 shows a block diagram of this double limiter system, and FIG. 11 shows waveform diagrams for each part of the block diagram of FIG.

In FIG. 10, (24) is a high-pass filter, (2
5) is a limiter circuit as an amplitude limiting means, (27) is a low-pass filter, (28) is an adder, (26) is a phase shifter for adjusting the phase of the signal to be added, and (29) is for amplitude limiting. It is a limiter circuit.

Next, its operation will be explained. FIG. 11(a) shows the FM modulated wave to be recorded. Figure 11(b) shows that the lower side wave is emphasized and reproduced relative to the upper side wave due to the lower side wave emphasizing effect caused by the modulated wave in Figure 11(a) passing through the electromagnetic conversion system. This is a reproduced FM modulated wave. In addition, in FIG. 11, the zero-crossing points in FIG. 11(a) are indicated by symbols A-J and indicated by broken lines. In Figure 11(b),
Zero cross points E and F are missing, and if demodulated as is, an inversion phenomenon will occur.

FIG. 1O shows a circuit that can restore the missing zero-crossing points E and F by inputting FIG. 11(b). In this circuit, the upper side wave is emphasized by inserting the high-pass filter (26), so the waveform shown in FIG. 11(C) without missing zero-crossing points is obtained.
The amplitude of this is limited by the limiter circuit (27), resulting in the waveform shown in FIG. 11(d).

However, in video signal recording and reproducing devices, the upper side wave generally has a higher C/N ratio (carrier power/n1se).
11(d) obtained by emphasizing the upper side wave becomes a signal slightly deviated from the original zero-crossing point due to noise. The shift of the zero-crossing point in this reproduced FM modulated wave is determined by the S/N ratio (so
This causes deterioration of the power ratio (under power/n1se power ratio). For this reason, Fig. 11(d) can be changed to F
When MI tone is applied, the reproduced image signal has a poor S/N ratio. Therefore, by using the low-pass filter (29) that emphasizes the lower side wave with a good C/N ratio, the signal shown in FIG. 11(e) in which the deviation of the zero-crossing point due to noise is extremely small can be obtained.

The signals in FIG. 11(d) and FIG. 11(e) are transmitted through the phase shifter (28).
Phase matching is performed by the adder (30), and the waveform shown in FIG. 3(f) is obtained. The waveform in FIG. 11(f) has a smaller deviation of the zero cross point due to noise than the waveform in FIG. 11(d), so compared to demodulating the waveform in FIG. If the circuit (31) demodulates the signal by limiting the amplitude as shown in FIG. 2(g), reproduction with a better S/N ratio can be achieved.

In this way, the conventional double limiter method for preventing the reversal phenomenon can restore the missing zero-crossing points E and F, and can keep the deterioration of the S/N ratio relatively small.

[Problem to be solved by the invention]

Since the conventional reversal phenomenon prevention device is configured as described above, a low-frequency suppressed signal with a poor C/N ratio is added by the high-pass filter to locations where no zero-crossing points are missing. . This poses a problem in that the zero-crossing point shifts due to noise, resulting in a deterioration of the S/N ratio.

Also, the shift of this zero cross point is due to the total noise <4t-
This occurs even when the frequency characteristics of the FM transmission system are not flat. In particular, in the double limiter method, the FM is
Since the frequency characteristics of the transmission system change, the zero-crossing point shifts.

Such a shift in the zero-crossing point causes a change in the frequency characteristics of the demodulated video signal or causes distortion. Further, a change in the addition ratio in the double limiter method changes the degree of the effect of suppressing the inversion phenomenon. Therefore, there is a problem in that the frequency characteristics of the demodulated video signal change or distort depending on the degree of the effect of suppressing the inversion phenomenon.

This invention was made with the aim of solving the above-mentioned problems, and it is possible to prevent the reversal phenomenon by reliably restoring the missing zero-crossing point, and to minimize the deviation of the zero-crossing point. The purpose of the present invention is to provide an inversion phenomenon prevention device that can reduce the deterioration of the S/N ratio.

[Means to solve the problem]

The reversal phenomenon prevention device according to the present invention provides a regenerated input F
a bandpass filter that suppresses the lower side wave component of the M modulated wave;
a first delay circuit that matches the phase of the input FM modulated wave with the input FM modulated wave suppressed by the band-pass filter; and a subtracter that subtracts the output signal of the band-pass filter from the output signal of the first delay circuit. Then, noise is removed from the output signal of the subtracter to extract only the signal of the missing part of the zero crossing point, and when the signal level of the missing part is small, the gain is set to high, and when the signal level is large, the gain is set to an extraction section that outputs a signal amplified by lowering the frequency, a second delay circuit that matches the phase of the input FM modulated wave with the output signal extracted by the extraction section, and an output signal of the second delay circuit. The present invention is characterized by comprising an adder that adds the output signal of the extraction section to restore the zero-crossing point, and a limiter circuit that limits the amplitude of the output signal from the adder to suppress level fluctuations.

[Effect]

In the inversion phenomenon prevention device according to the present invention, a subtracter subtracts a signal component whose lower side wave component of an FM modulated wave is suppressed by a bandpass filter and a signal component of an input FM modulated wave that is in phase with this signal component. outputs the suppressed lower side wave component signal.

The extractor extracts only the signal of the missing portion of the zero-crossing point from this output signal, performs amplification according to the degree of the signal level, and restores the missing signal of the zero-crossing point.

By combining the phase-aligned input FM modulated wave with the missing signal restored by the extractor using an adder, an FM modulated wave without any missing parts or deviations at the zero crossing point can be obtained.
There is no deterioration of the S/N ratio, and the inversion phenomenon can now be suppressed.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a reversal phenomenon prevention device according to the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a reversal phenomenon prevention device according to an embodiment of the present invention. In the figure, (1) is a 33-wave input FM wave, and (2) is a band pass filter that suppresses low frequencies. This band pass filter (2) may be a high pass filter.

(3) is a first delay circuit having the same delay time as the bandpass filter (2), (4) is a subtracter, and (6) is a zero crossing point from the output signal (5) of the subtracter (4). The extraction unit (7) that extracts the missing portion is the output signal of the extraction unit (6). (8) is a second delay circuit having the same delay amount as the delay time from (1) to (7), and (9) is a second delay circuit.
An adder that adds the output of the delay circuit (8) and the output signal (7), (10) is an amplitude limiter, and (11) is the output signal of the amplitude limiter (10) that is input to the FM demodulator. This is the signal.

The operation of the reversal phenomenon prevention device of this embodiment consisting of the above (14 components) will be explained.

Figure 2 shows the signal waveforms of each part in Figure 1, and (a) in Figure 2 shows the FM modulation wave to be recorded.
b) shows the reproduced FM modulation in which the lower side wave is emphasized relative to the upper side wave due to the lower side wave emphasizing effect caused by the FM modulated wave shown in (a) passing through the electromagnetic conversion system. It's a wave. The waveform lacks the zero cross points E and F in the part indicated by the arrow, and if demodulated as is, an inversion phenomenon will occur. In FIG. 2, the zero-crossing points in FIG. 2(a) are designated by symbols A-J and indicated by broken lines.

The input FM modulated wave (1) in FIG. 1 has a waveform shown in FIG. 2(b). The input FM modulated wave (1) is input to each of a bandpass filter (2), a first delay circuit (3), and a second delay circuit (8).

The output signal of the band-pass filter (2) has the waveform shown in FIG. 2(C), and the zero-crossing point in the area indicated by the arrow has been restored. Moreover, the output signal of the first delay circuit (3) is .:1
Since the delay amount is set to be the same as that of the bandpass filter (2), the phases of both input signals to the subtracter (4) match.The output signal (5) of this subtractor (4) is Figure 2 (
The waveform is shown in d). The part of this waveform indicated by the arrow M corresponds to the arrow L in FIGS. 2(b) and 2(c), and is the result of subtracting K and L. Therefore, the larger the level difference between the K and L parts, the larger the pulse level of the M part, and the smaller the difference, the lower the level of the output pulse.

The parts other than M in Fig. 2(d) are random noise included in the waveforms in Fig. 2(b) and (C) and the band-pass filter (2).
) and the first delay circuit (3) due to the difference in gain frequency characteristics between the signals in FIG. The gain frequency characteristic of the first delay circuit (3) is flat, and the difference with the bandpass filter (2) becomes low-frequency and high-frequency components, which are output to the output signal (5) of the subtracter (4). appear. Since the subtracter (4) performs alternating current subtraction, the low frequency components are originally reduced, and only the high frequency components remain. Therefore, in the waveform of FIG. 2(d), only the zero-cross missing detection portion indicated by the arrow M and the high-frequency noise component appear.

Next, the output signal (5) from the subtracter (4) is input to an extraction unit (6) that extracts the missing portion of the zero crossing point, which is a characteristic means of the present invention. This extractor (6) may have several configurations. Its specific configuration is shown in Figure 3 (a
) and shown in FIGS. 4(a) and 4(b).

First, in m3 diagram (a), (13) is the input signal (
5) A low-pass filter that removes high-frequency noise components, (
14) is a slice circuit that removes signals below a certain level, and (16) is a limiter amplifier having gain characteristics as shown in FIG. 3(b). As can be seen from the characteristic diagram, this limiter amplifier performs amplification with a high gain when the signal level of the missing portion of the extracted zero-crossing point is small, and with a low gain when the signal level is large.

This makes it possible to generate a signal at an appropriate level to restore the missing zero-crossing point.

In addition, in FIGS. 4(a) and 4(b), (12) is a differential amplifier, and (13) and (14) are the same low-pass filter and slice circuit as in FIG. 3(a). (15)
is a buffer amplifier for amplifying the output signal (7) of the slice circuit and inputting a signal at an appropriate level to the inverting input side of the differential amplifier (12).

The operation will be explained below. In FIG. 3, the input signal to the low-pass filter (13) is input to the subtracter (4) in FIG.
), which contained the high-frequency noise shown in FIG. 2(d). Therefore, the high-frequency noise is attenuated by a low-pass filter (13), and the second
As shown in the waveform shown in Figure (e), the differential amplifier between the missing detection portion M at the zero crossing point and the noise level is increased. Thereafter, a slice circuit (14) removes low-level signals (noise components) and outputs only the M portion of pulses.

Here, the pulse level of the M portion is the second
It appears in response to the level difference between K and L in Figures (b) and (C), so when the level difference between K and L is small, the level of M becomes small.

After that, when the output signal (7) of the extractor (6) is added to the original FM modulated wave by the adder (9) in FIG. When the level of M is small, the gain is increased to ensure that the zero cross point is restored.

Furthermore, the concept is the same for the configurations shown in FIGS. 4(a) and 4(b), and when the output level of the slice circuit (14) is small, the amount fed back via the buffer amplifier (15) is This is the principle that the level of the output signal (7) of the extractor (6) becomes higher. Figure 4 (
The difference between a) and Fig. 4(b) is that in Fig. 4(a), a low-pass filter (13) is involved, so the differential amplifier (1
2), a phase difference occurs between the non-inverting input and the inverting input, but in FIG. 2(b), the configuration is basically such that no phase difference occurs.

As described above, since the output signal (7) of the extractor (6) is controlled by the level of the missing detection pulse at the zero-crossing point, the output signal (7) of the extractor (6) is sent to the second delay circuit in the adder (9) in FIG. (8)
The above pulse is added to the FM modulated wave whose phase has been matched through to suppress level fluctuations and improve the FM demodulator input signal (1
Output as 1).

As described above, according to the embodiment of the Ts1 diagram, by eliminating the shift of the zero cross point, which was insufficient in the conventional double limiter method, it is possible to reduce the deterioration of the S/N ratio, the change in the frequency characteristics of the video signal after demodulation, or This makes it possible to suppress the occurrence of distortion.

By the way, in the embodiment shown in FIG. 1, since the output of the subtracter (4) depends on the level difference between K and L shown in FIGS. 2(b) and (c), the characteristics of the bandpass filter may not necessarily be optimal. If the L at the zero cross part does not recover properly, or if the FM
When noise in the same band as the carrier is mixed, as mentioned above, the level difference in the L portion becomes small and the missing portion of the zero crossing point may not be detected sufficiently.

Therefore, another embodiment is shown in FIG.

Components (1) to (10>) of the embodiment shown in FIG.
Since this embodiment is exactly the same as the embodiment shown in FIG. 1, the explanation will be omitted.

Characteristic parts of this embodiment include a first limiter circuit (17) and a second limiter circuit (17) that respectively perform amplitude limitation after the first delay circuit (3) and bandpass filter (2).
18). FIG. 6 shows waveforms of various parts in this case. In addition, Fig. 6 (a
) and (c) are signal waveforms opposite to those in FIG. 2(b) and (C).

Next, the operation will be explained. First, the signal in Figure 6(a) is input to the band pass filter (2), and the fluctuations in the a(range) are removed, resulting in a waveform in which the missing part of the cello cross point is restored as shown in Figure 6(C). is obtained, and the following second limiter circuit (
18), the transfer is restricted and the result is as shown in Figure 6(d). In addition, the signal as shown in FIG. 6(a) is transmitted to the first delay circuit (3).
The signal output from the first limiter circuit (17) becomes as shown in FIG. 6(b). Here, Fig. 6 (a
) is the waveform in (b) of the same figure corresponding to K, and the waveform of the same figure (b) is
The waveform in FIG. 3(d) corresponding to L in c) is indicated by L. As can be seen from this correspondence, by passing through the limiter circuit, the K part becomes a high potential signal like ゛, and the L part becomes a low potential signal like L゛, so K and L
The level of the output from the subtracter (4) in the part shown in FIG. 1 is certainly higher than in the case of the embodiment shown in FIG. Therefore, the level difference between the noise component at the input of the extraction unit (6) and the missing part M of the zero crossing point becomes large, and malfunctions are reduced.
It becomes possible to restore the zero-crossing point more accurately.

However, in this embodiment, an adverse effect may occur due to the gain of the limiter circuit (in other words, the limiter level). This will be explained using FIG. 7. Figure 7 (
The waveform a) is an input FM modulated wave lacking a zero-crossing point, and shows a state in which a high-frequency nose f is present. Here, in the first limiter circuit (17) of FIG. 5, the case of FIG. 7(b) where the limiter gain is high and the case of FIG. 7(C) where the limiter gain is low will be considered separately.

In Figure 7(a), the limiter gain is expressed as a limiter level, but when the limiter gain is high, the limiter level is like X, which is quite close to the AC ground (ACO
Amplitude limitations apply even to signals close to V), that is, zero-cross missing portions. As a result, an output as shown in FIG. 7(b) is obtained. Further, when the limiter gain is low, the limiter level is like Y, and the zero-cross missing portion in the figure remains as it is and is output (FIG. 7(C)).

Therefore, the higher the limiter gain is, the more advantageous it is to eject zero-cross missing parts, but as shown in Figure 7(b), when the limiter gain is high, the noise component will also be amplified, and the subtracter This noise appears in the output of (4) in the form of being superimposed on the zero-cross missing part, and this noise also remains in the FMM adjustment input, making it impossible to extract the zero-cross missing part accurately and completely suppressing the inversion phenomenon. It becomes difficult to remove.

On the other hand, in the embodiment shown in FIG. 5, by suppressing the limiter gains of the first limiter circuit (17) and the second limiter circuit (18) to a low level, the adverse effects caused by noise amplification as described above are eliminated. At the same time, by appropriately using the limiter circuit and feedback circuit described above in the extraction section that extracts the zero-cross missing portion, it is possible to create and output a sufficiently high-level zero-cross missing correction pulse (11). .

〔Effect of the invention〕

As explained above, the reversal phenomenon prevention device according to the present invention extracts only the missing zero-crossing point of the FM modulated wave and restores the zero-crossing point.
It is now possible to prevent the inversion phenomenon without deteriorating the N ratio.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a reversal phenomenon prevention device according to an embodiment of the present invention, FIGS. 2(a) to (f) are waveform diagrams of each part in FIG. 1, and FIG. 3 is an embodiment of an extraction section. 4, (a) is a block diagram of the extracting section, (b) is a characteristic diagram of the limiter amplifier, FIGS. 4(a) and (b) are block diagrams showing other embodiments of the extracting section, and FIG. The figure is a block diagram of another embodiment of the present invention, Figures 6 (a) to (g) are waveform diagrams of various parts in Figure 5, and Figures 7 (a) to (c) are limiter gains in Figure 5. 8(a) to 8(d) are diagrams explaining the reversal phenomenon. FIG. 9 is a block diagram of a conventional reproduction system. FIG. 10 is a block diagram of a conventional double limiter system. 11(a) to (g) are waveform diagrams of each part in FIG. 10. In the figure, (1) is the input FM modulated wave, (2) is the bandpass filter, (3) is the first delay circuit, (4) is the subtracter, (6) is the extractor, and (8) is the second Delay circuit (9) is an adder, and (10) is a limiter circuit. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] In a playback system of a video signal recording/playback device that records a video signal by FM modulating the video signal, there is provided a bandpass filter that suppresses a lower side wave component of a reproduced input FM modulation wave; a first delay circuit that matches the phase of the input FM modulated wave suppressed by the bandpass filter; a subtracter that subtracts the output signal of the bandpass filter from the output signal of the first delay circuit; and the subtracter. Removes noise from the output signal and extracts only the signal of the missing part of the zero crossing point, and when the signal level of the missing part is low, the gain is increased, and when the signal level is large, the gain is lowered and amplified. a second delay circuit that adjusts the phase of the input FM modulated wave with the output signal extracted by the extraction section; and an output signal of the second delay circuit and an output signal of the extraction section. What is claimed is: 1. An inversion phenomenon prevention device comprising: an adder for restoring a zero-crossing point by adding the above, and a limiter circuit for suppressing level fluctuations by limiting the amplitude of an output signal from the adder.