JPS62146468A - Dropout detecting circuit - Google Patents

Abstract

PURPOSE:To detect an optimum dropout corresponding to the kind of a generated noise by obtaining a dropout detecting signal when the frequency of a frequency converting signal is changed because of the increase of disturbance of a reproduced RF signal waveform. CONSTITUTION:A boosting circuit 15 in a dropout detecting circuit 1 has amplifying characteristics increasing output levels on both sides larger than a carrier signal level in the frequency range of upper and lower side band signal components of a chroma signal around a carrier signal component and outputs a boosted output signal having such a characteristic to an output terminal. The boosted output signal is applied to a comparator 16 as a comparing signal SEO, and when the instantaneous value of the signal SEO is increased larger than a dropout detecting reference signal VDC obtained from a reference power supply 17, a compared output SCOM having logic '1' level is sent to a frequency detecting circuit 18 as a frequency converting signal. When the frequency of the output SCOM becomes lower than the prescribed frequency, the circuit 18 generates a detecting output SDET with logic '0' level and sends the output as a dropout detecting output DOOUT through a dropout detecting width expanding circuit 19.

Description

[Detailed description of the invention] The present invention will be explained in the following order.

A: Industrial field of application B: Overview of the invention C: Conventional technology Problems to be solved by the invention ErtiIB Means for solving the points (Figs. 13 and 21) F Effects (Figs. 13 and 21) ) G Example (Figures 1 to 27) (G1) Relationship between cause of dropout and spectral signal component of reproduced RF signal (G2) Effect of dropout on reproduced RF signal (1)
Changes in frequency characteristics of VTR (Figures 1 to 3) (2) When the chroma signal is large (Figures 4 to 6) (3) When the chroma signal is small (Figures 7 to 9) ) (4) When the chroma signal is very small and the luminance signal is large (Figures 10 to 12) (G3) Example of dropout detection circuit (1) Dropout detection principle (Figures 7 to 10) ) (2) Configuration of the embodiment (Figs. 13 and 14) (3) Operation of the embodiment (1st
5 to 27) (4) Effects of the Embodiment (G4) Other Embodiments H Effects of the Invention A Industrial Application Field The present invention relates to a dropout detection circuit, and is particularly applicable to a direct FM recording type video tape recorder. It is suitable for application.

B. Summary of the Invention The present invention provides a video signal reproducing device that demodulates a video signal from a reproduced RF signal obtained by FM modulating a video signal, recording and reproducing the same, and when a dropout occurs, the reproduced RF By detecting the occurrence of a dropout based on an increase in the disturbance in the signal waveform, it is possible to detect a dropout under optimal conditions for the type of dropout.

C. Conventional technology In video tape recorders (VTRs) using the FM (frequency modulation) recording method, video signals are directly recorded as they are by using an 8 (MHz) carrier, for example.
M modulated and this FM modulated signal (this is called the RF multiplied signal)
A VTR that records video diagonally on a magnetic tape using a rotating video head is used in high-end machines such as VTRs for broadcasting equipment.

In this type of VTR, the R
When playing with an F-fold signal video head, there are cases where the magnetic tape and the video head do not come into proper sliding contact. For example, if dust gets between the magnetic tape and the video head, or if the surface of the magnetic material coated on the magnetic tape is scraped off, the friction between the video head and the magnetic tape may The contact is not sufficient (and as a result, the signal level of the reproduced RF signal decreases).

This phenomenon is generally called dropout, and when a weak dropout occurs, it only degrades the S/N of the demodulated playback screen, but when a strong dropout occurs, the playback screen An unsightly noise bar appears on top.

Conventionally, in order to prevent particularly unsightly noise bars from appearing on the playback screen as much as possible, the part of the noise bar where dropout has occurred is compensated for by reusing the playback signal before IH. . For example, Japanese Patent Publication No. 54-27216 discloses a configuration in which dropout compensation is directly performed on an RF multiplied signal, and Japanese Patent Publication No. 55-19478 discloses a configuration in which dropout compensation is directly performed on an RF multiplied signal, and Japanese Patent Publication No. 55-19478 discloses a video signal obtained by demodulating a reproduced RF signal. An arrangement for compensating for dropouts in a signal is disclosed. However, in any case, the conventional configuration detects whether or not the signal level of the reproduced RF signal has decreased by a predetermined amount, resulting in the following problem.

D Problems to be Solved by the Invention In general, when a dropout occurs in a reproduced RF signal,
Among the spectral signal components, the amount of decrease in high frequency signal components tends to be greater than the amount of decrease in low frequency signal components, and depending on the degree of dropout defects, drops due to shallow defects on the surface of the magnetic tape. out (hereinafter referred to as shallow dropout; at this time, the reproduced RF signal drops by about -8 (dB)),
Dropouts due to deep defects (hereinafter referred to as deep dropouts; at this time, the reproduced RF signal is, for example, -16
(dB)), the amount of decrease tends to be larger.

Therefore, for example, when a dropout occurs in a reproduced RF signal that has a chroma signal (its amplitude is large) representing a highly saturated color such as a color par, the amount of decrease in the carrier signal component is (signal), the modulation index appears to be larger, and as a result, "whiteout" noise corresponding to the defect occurs on the playback screen. The brightness of this "whitening" noise is brighter in a deep dropout than in a shallow dropout because the amount of decrease in the carrier signal component is greater, and therefore the "whitening" noise tends to be more noticeable.

In addition, deep dropouts with a large dropout occur, especially for colors with extremely high saturation (for example, cyan), which causes the carrier signal component to drop too much and the signal level of the carrier signal component to lower the lower band. When the signal level becomes lower than the signal level of the wave signal component, the FM demodulation circuit assumes that the carrier has switched to the lower sideband frequency and inverts the signal level of the demodulated signal from the white level to the black level. At this time, a "black and white inversion" phenomenon appears on the playback screen, resulting in a significant deterioration of image quality.

In order to detect dropouts that occur in various ways as described above, the dropout detection level can be adjusted to -8 (
dB), it is possible to detect not only the occurrence of a shallow dropout but also the occurrence of a deep dropout and a drop in the reproduced RF signal by, for example, -16 (dB).

However, if the dropout detection level is set low in this way, the following problem will occur when a reproduced RF signal representing a general image with a sufficiently small chroma signal is reproduced.

In other words, most ordinary images (referred to as normal images) have low chroma as a whole, and therefore the amplitude of the chroma signal is very small. Further, a normal picture video signal often has a high luminance signal level and a weak H correlation. Considering that the H phase is weak, it is desirable to avoid dropout compensation operations such as reusing the pre-IH video signal as much as possible in order to prevent deterioration in image quality. Even if "whitening" noise occurs on the playback screen, it is not very noticeable, so it will not be a big problem if you leave it alone.

The present invention has been made in consideration of the above points, and utilizes the fact that the effects on the spectrum signal components included in the reproduced RF signal vary depending on the dropout mode that occurs in the magnetic tape. This paper attempts to propose a dropout detection circuit that can reliably detect only dropouts that need to be compensated for.

Means for Solving Problem E In a video signal-reproducing apparatus that demodulates a video signal from a reproduced RF signal S0 obtained as a result of FM modulating a video signal and recording and reproducing it, a reproduced RF signal SIF is used.
A comparison signal S, having a waveform corresponding to the waveform of . ■ Dropout detection reference signal at a predetermined level. . By comparing the comparison signal S,. A frequency-converted signal S, whose frequency is determined based on the point in time when the waveform of S crosses the dropout detection reference signal VDO. and frequency detection means for sending out a dropout detection signal S□7 based on a change in the frequency of the frequency-converted signal SCON.
When the frequency of the frequency conversion signal 5cOH changes due to an increase in the disturbance of the waveform of □, the dropout detection signal SDEa is obtained.

When an F-effect dropout occurs, the disturbance in the waveform of the reproduced RF signal changes depending on the depth of the dropout by exerting different effects on the spectrum signal components depending on the depth of the dropout.

Therefore, the point at which the waveform of the comparison signal Sso crosses the dropout output reference signal VDO varies depending on the type of dropout that has occurred. Therefore, when the frequency of the frequency conversion signal S eON changes, the dropout detection reference signal ■. . It can be determined that a type of dropout corresponding to the signal level has occurred, and thus dropout detection can be performed under the optimal conditions for the type of dropout.

G example An embodiment of the present invention will be described in detail below with reference to the drawings.

(G1) Relationship between the cause of dropout and the spectrum signal component of the reproduced RF signal Generally, the FM signal wave eFM is expressed by the following formula (%), and the reproduced RF signal reproduced from the magnetic tape is (
1) It consists of the signal of formula. Here ω. is the carrier angular frequency,
ω is the modulation angular frequency and Δω is the angular frequency deviation. Also(
In equation 1), Δω/ω is expressed by the modulation index β from the relationship. Here, f is the modulation angle/wave number, and ΔF is the frequency deviation.

When formula (1) is expanded using the Bessel function of the first kind, 6 F n =J o (β) cosωc1+Jl(
β) cos(ω0+ω,) is -J, (β) cos(ω.-ωp) t...
(3) can be rewritten as However, in practice, the RF signal modulation index β, which is the FM modulated signal recorded on the magnetic tape in a VTR, is selected to be a relatively small value, so the carrier signal Component Jo(β)cos
ωc 1 and the upper and lower sideband signal components Jl(β)cos
(ωゎ+ωp) t− and −Jl(β)cos(ω.

-ω, )t may be considered to be superimposed.

The modulation index β is expressed by k □ β−□ (5). Here, a is the amplitude, k is the modulation sensitivity, and the modulation sensitivity is 100%.The frequency displacement ΔF during modulation is 100C.
%].

The reproduced RF signal obtained by recording and reproducing the RF multiplied signal made of such an FM modulated signal and the transmission system made of a VTR
The signal has an amplitude characteristic A expressed by the following formula.
It can be evaluated by Here ρ. represents the amplitude of the carrier frequency component J0, and ρ and ρ represent the amplitudes of the upper and lower sideband signal components, respectively.

In this way, the amplitude characteristics of the transmission system regarding the RF multiplied signal are
The average value of the upper and lower sideband signal components J, and J-1 (ρ
0. ρ−I)/2 and the amplitude ρ of the carrier signal component Jo
. It can be expressed by the ratio of

(G2) Effect of dropout on reproduced RF signal (1)
Changes in Frequency Characteristics of VTR The frequency characteristics of the reproduced RF signal obtained as a result of recording and reproducing the RF multiplied signal expressed by equation (4) for a transmission system consisting of a VTR are as shown in Figure 1. It has a drooping characteristic as shown by the downward-sloping curve KO with respect to the signal component J0, and the upper and lower sideband signal components J+1 and l with respect to the carrier signal component J0.
The difference in the output level of , is U, (dB) and , (dB), respectively. In addition, J, , J, iJ
−+ is the Bessel function term J6(β) in equation (4), Jl
This corresponds to the term (β), −J, and (β), respectively.

In this example, the frequency of carrier signal component J0 is 8
(MHz), upper and lower sideband signal components J, l and J
-1 is the chroma signal, the frequencies are 4 (MHz) and 12
(Suppose Ml(z).

When a shallow dropout occurs in a transmission system with such frequency characteristics, as shown in Figure 2, a curve with a larger downward drop to the right than the normal case in Figure 1 will appear. It exhibits the frequency characteristics shown below. Therefore, in this case, the difference Dt (dB) between the signal level of the carrier signal component J0 and the signal level of the lower sideband signal component J-1, and the signal level of the carrier signal component J0 and the signal level of the upper sideband signal component 11 are determined. The difference Uz (dB) with U, (an) is much larger than the differences U, (an) and U, (an) in the case of FIG.

If deeper dropouts occur, the regenerative RF
As shown in FIG. 3, the frequency characteristic of the signal exhibits a frequency characteristic as shown by 2 in a curve in which the amount of drooping downward to the right is larger than that in the case of shallow dropout in FIG. 2. Therefore, the difference D3 (dB) between the signal level of the carrier signal component J0 and the signal level of the lower sideband signal component 1°,
The difference U3 (dB) between the signal level of the carrier signal component J0 and the signal level of the upper sideband signal components J, l is the second
The difference in the case shown in the figure is much larger than (dB) and Ut (dB).

From Figures 1 to 3, the signal levels of the carrier signal component J and the upper sideband signal component J decrease as the depth at which dropout occurs increases. On the other hand, it can be seen that the signal level of the lower sideband signal component J- does not decrease significantly even if the depth at which dropout occurs increases.

From this change in characteristics, as the depth of dropout occurrence increases, the signal level of the carrier signal component J0 gradually approaches the signal level of the lower sideband signal component J-1, and as a result, the lower sideband signal component As the signal level of carrier signal component J0 increases, the signal level of carrier signal component J0 increases to lower sideband signal component 1. It can be seen that there are cases where the signal level becomes lower than the signal level of

The degree of influence on the reproduced RF signal when dropout occurs depends on the magnitude of the amplitude of the upper and lower sideband signal components J+1 and J-1 (i.e. chroma signal) included in the reproduced RF signal. occurs.

(2) When the chroma signal is large If the reproduced RF signal (Fig. 4 (B)) contains a chroma signal with a large amplitude Bl+ such as a cyan color par with high saturation (Fig. 4 (C) )) is the 4th one under normal conditions.
As shown in Figure (A), under the influence of the frequency characteristics shown in Figure 1, the signal level of the lower sideband signal component J-+ changes to a signal level R+ (dB) corresponding to the first-order Bessel function value.
(e.g. 13.4 (dB)) to DI (aB)
(for example, 3.6 (dB)") and approaches the carrier signal component J0. Here, the upper and lower sideband signal components J+1 and J-1 are at signal levels corresponding to the first-order Bessel function values. This means that the reproduced RF signal is an ideal FM
This means that the modulation signal has a waveform (of a constant amplitude that the carrier signal component has).

Under these conditions, as shown in Fig. 5, if a shallow dropout occurs, the reproduced RF signal S□ (Fig. 5 (B))
is a large decrease amount DR+ (aB) (for example, 8 Cd
B)), and the lower sideband signal component J-1
(Fig. 5 (A)) is affected by the frequency characteristics shown in Fig. 2, and the signal level R, (d
There is a significantly large increase in iD2 (dB) (e.g. 8.6 (dB)) from B) to 'J-T' on the bamboo.

As a result, the signal level of the lower sideband signal J-5 approaches the signal level of the carrier signal component J by Dz (dB) compared to the case of an ideal FM modulated signal. As is clear from Table 1, the modulation index β
This means that the amplitude B of the video signal VD becomes larger (Fig. 5 (C)), which ultimately results in the occurrence of "whitening" noise. ing.

β J, J, 1, J-.

0 1.0000 00, 1
0.9975 0.04990.2
0.99 0.09950.3
0.9776 0.14830.
38 0.9638 0.186
50.4 0.9604 0.
19600.5 0.9385
0.24230.6 0.912
0.28670.7 0.881
2 0.3290.8 0.8
463 0.36880.9
0.8075 0.40591.0
0.7652 0.4401 As shown in Table 1 and FIG.
)) and the lower sideband signal J-.

(Fig. 6 (A)) is affected by the frequency characteristics shown in Fig. 3, resulting in a much larger increase Ds (dB) from the signal level R+ (dB) (for example, 13.9 (dB)).
) rises by ).

As a result, the signal level of the lower sideband signal J,,1 increases by D3 (dB) compared to the case of an ideal FM modulated signal, thereby increasing the carrier signal component J.

exceeds the signal level of At this time, the demodulated video signal VD (FIG. 6(C)) drops to the black level LIILK, thus causing a "black and white inversion" noise on the playback screen.

(3) When the chroma signal is small, the reproduced RF signal (Figure 7 (B)) has lower saturation than in Figure 4, but is much higher than the normal image), such as a blue color par. If a small chroma signal is included (Fig. 7 (C)), it will be shown in Fig. 7 (A) under normal conditions.
As shown in FIG. 1, the signal level of the lower sideband signal component 1 is influenced by the frequency characteristics shown in FIG.
(dll) (e.g. 20 (dB)) to 01 (
aB) and approaches the carrier signal component.

When a shallow dropout and a deep dropout occur from this state, the signal level of the lower sideband signal component J-1 increases as shown in FIGS. 8 and 9 corresponding to FIGS. 5 and 6. Under the influence of the frequency characteristics shown in FIGS. 2 and 3, the amount of increase Dt and D!, respectively, is increased. (dB) and approaches the carrier signal component J0, resulting in "whiteout" noise in the demodulated video signal VD (FIGS. 8(C) and 9(C)).

(4) When the chroma signal is very small and the luminance signal is large. Unlike the case of color par described above, the chroma signal is very small in the normal image video signal, as shown in FIG. 10 (D). Moreover, it has a signal structure in which the luminance signal is large.

When dropout does not occur, the reproduced RF signal has the frequency characteristics described above with respect to FIG. 1, as shown in FIG. 10(A).

By the way, when recording and reproducing a normal picture video signal, the recorded video signal VIIEc has an amplitude of 100% between 7 (MHz) and 9 (Mllz), as shown in FIG. 10(B), for example. Since pre-emphasis is applied to the assigned video signal, an emphasized waveform that protrudes at frequencies above 9 MHz is formed at the rising edge of the video signal, and at the same time at the falling edge of the video signal. For the part, an emphasized waveform is formed that protrudes at frequencies below 7 [MHz], and thus the S/
It is designed to improve N.

However, since a VTR has a downward-sloping frequency characteristic as described above with reference to Figure 1, the recorded video signal
. If the frequency of increases to the frequency near the upper sideband signal component J+1 of the chroma signal, the decrease due to the frequency characteristics Ul
The signal level of the reproduced RF signal 5IIF (FIG. 10(C)) decreases to a value COM close to (for example, 6 (dB)).

Conversely, if the recording video signal V REC drops to a frequency close to the lower sideband signal component J-1 of the chroma signal, the signal of the reproduced RF signal SRF will increase by the increase in frequency characteristics (for example, 3.6 (dB)). level increases.

However, this decrease amount U, (aB) and increase amount DI (
The influence of amplitude fluctuations on the order of dB) can be removed using a limiter in the FM reproducing circuit;
), the reproduced video signal VD can be prevented from having that effect, and thus a very small chroma signal 5C appears on the luminance signal 5Rtl as shown in FIG. 10(D).
11 can be obtained by superimposing the reproduced video signal VD.

However, as mentioned above with respect to Fig. 2, R-8 (dB
) Shallow dropout occurs and the same recording video signal ■□. Even if recorded, there will be a decrease of 1 kUt to 12 due to the frequency characteristics in the peak waveform part where the frequency increases
(dB), the signal level of the reproduced RF signal 5IIF (FIG. 11(C)) is COM.

-20[dB].

However, even in this case, since the drop in the carrier signal component J0 is still small, the drop amount COM is reduced through the limiter circuit.
By removing the influence of t, a reproduced video signal VD can be obtained.

However, as shown in FIG. 12, when a deep dropout occurs, the carrier signal component J0 obtained from the VTR drops significantly, so the reproduced RF signal 5IIF (first
In FIG. 2(C)), the amplitude of the signal SDO in the portion where dropout occurs becomes so small that it cannot be reproduced, and as a result, noise NS occurs in the reproduced video signal VD.

(G3) Embodiment of Dropout Detection Circuit (1) Dropout Detection Principle Based on the above considerations, the present invention does not detect a decrease in the amplitude of the reproduced RF signal 5IIF, but detects when a dropout occurs in the reproduced RF signal SIF. By focusing on the fact that waveform disturbances that change depending on the depth of the dropout occur in the detected signal portion, we developed a dropout detection signal whose frequency changes based on the waveform disturbance that occurs when a dropout occurs. By forming such a structure, it is possible to perform appropriate dropout compensation depending on the depth of dropout occurrence.

Incidentally, from the conventional thinking, for a reproduced RF signal with a large chroma signal, in order to detect both a shallow dropout and a deep dropout when they occur, it is necessary to increase the amplitude of the reproduced RF signal 5IIF by a decrease amount DRI. (−
8 (dB)), it would be possible to determine this as a dropout (see Figure 5).
6, 8, 9).

However, in this case, as described above with reference to FIG. Of 0,
Even if the peak-shaped pre-emphasized waveform part shifts slightly to a higher frequency, this may be judged as a dropout, and in such cases, compensation using the IH video signal is frequently repeated. This is undesirable because it may cause resolution deterioration.

However, this does not mean that the amount of decrease in the reproduced RF signal is DRz
(-16(dB)) (Figure 6(B),
If this is determined as a dropout (FIG. 9(B)), it is impossible to prevent "whitening" noise from occurring when a shallow dropout occurs (FIGS. 5 and 8).

(2) Configuration of the Embodiment In FIG. 13, ``■'' is a dropout detection circuit which receives a reproduced RF signal SRF reproduced by a head 4 from a tape 3 of a VTR 2 as a transmission system through a reproduction amplifier circuit 5 and a reproduction equalizer 6.

This reproduced RF signal SIF is amplitude-limited by a limiter circuit 7, converted into a frequency signal sro determined based on the zero cross point of the reproduced RF signal 5IIF, and then supplied to an FM demodulation circuit 8, which generates a demodulated video signal. After the VD is de-emphasized in the de-emphasis circuit 9, it is sent out as a reproduction output signal VDout.

The dropout detection circuit 1 includes a boost circuit 15 having boost characteristics as shown in FIG. The boost circuit 15 has output levels on both sides of the carrier signal component J0 at the carrier signal component J0 within the frequency range of the upper sideband signal component J++ and the lower sideband signal component J-I of the Habochroma signal. It is an amplifier circuit that has higher frequency characteristics.

(Then, the reproduced RF signal 5 is output to the output terminal of the boost circuit 15.
A boost output signal is obtained by increasing the signal levels of the upper and lower sideband signal components J and J-I included in IIF relative to the signal level of the carrier signal component J0, and this It is applied to the comparison circuit 16 as a comparison signal 5l11. At the reference input terminal of the comparator circuit 16, a dropout detection reference signal ■ consisting of a DC voltage is supplied from a reference power supply 17. . is given and a comparison signal S,. The instantaneous value of is the dropout detection reference signal V,. When it becomes higher, the logic ``
1'' level comparison output SCON is sent to the frequency detection circuit 18 as a frequency conversion signal. This frequency detection circuit 18 receives a comparison output S, which is a frequency-converted signal sent from the comparison circuit 16. , when the frequency of
Send as.

Here, the boost circuit 15 includes a sensitivity control means 31 as a control means for changing and controlling the boost ratio of the lower sideband signal component to the carrier signal component.

This sensitivity control means 31 operates with a switching input terminal PKI which inputs a switching signal for operating the boost circuit 15 with the characteristic shown as 1 in the boost characteristic curve described above with reference to FIG. It is a change-over switch having a change-over input terminal PKt into which a change-over signal for switching is inputted.

In the boost characteristic curve 2, for example, 0 (dB
) and the boost ff of the carrier signal component J0
i B S z shall be equal to or greater than O (dB). By doing this, the sensitivity control means 31 changes from the switching input terminal PKI to PK.
When switched to Z, the comparison human power S, by changing the ratio of the signal levels of the carrier signal component J0 and the lower sideband component J-+. Waveform disturbance! (and therefore dropout detection sensitivity).

(3) Operation of the embodiment (3-1) When shallow dropout occurs (a)
In the case where the chroma signal is large In the above configuration, as shown in FIG. 15, for example, the reproduced RF signal SI of color par, where the upper and lower sideband signal components J, I and J-r are large with respect to the carrier signal component J.
Suppose that F is obtained at the output end of the reproduction equalizer 6. Note that the sensitivity control means 31 of the boost circuit 15 has a characteristic curve K.
It is assumed that the high-sensitivity detection side contact PKI corresponding to 1 (FIG. 14) has been switched.

The signal level of the reproduced RF signal 5IIF obtained at this time is the carrier signal component J11 (-0 (as)), the signal level R1 (=12 (
dB) ), the rise in the lower sideband signal component J-.

(=3.5 (dB)) (Therefore, the signal level of the lower sideband signal component J-, is 8.5 (dB)), the decrease in the upper sideband signal component J, l = (6 (dB))
(Therefore, the signal level of the upper sideband signal components J and I is 18
(dB) ”), amplitude characteristic A (=0.25), modulation index β (=O,,19), amplitude B of video signal VD (=
78.9%).

The boost circuit 15 performs boost processing on this reproduced RF signal, thereby producing a comparison signal S shown in FIG. 16 at its output terminal.
,. Output. Here, the signal level of the lower sideband signal component J-r included in the comparison signal SIO is DI, (= 4
．． 5 (dB)) to 4 [dB], and the signal level of the upper sideband signal component J41 is
(-9 (dB)) boosted by 9 (dB)
become. In this state, if dropout does not occur in the reproduced RF signal 5IIF as shown in FIG. 17(A), the reproduced output signal VDoua is shown in FIG. 17(B).
A video signal having a chroma signal with an amplitude of B (=78.9%) is transmitted as shown in FIG.

At this time, the comparison signal SI whose amplitude has been boosted by the boost circuit 15 is supplied to the output terminal of the boost circuit 15.
O is sent. What should be noted here is the comparison signal S.

The peak value of carrier signal The frequency of component J0 (
The peak value level of the carrier signal, which fluctuates at 8 [MHz]), fluctuates over time (this is called waveform disturbance).

However, the width of this waveform disturbance is sufficiently small when dropout does not occur in the boost circuit 1.
Since the boost characteristic of 5 is selected, this comparison signal S,. In the comparison circuit 16, the dropout detection reference signal ■. . There is no effect on the comparative output SCO when comparing with.

While the reproduced RF signal S□ is being reproduced in this manner, a dropout occurs in the shallow part of the magnetic tape as shown in FIG. 20 (A), and the reproduced RF signal S□ becomes D
Assuming that the signal level decreases by R, -8 (dB),
In the same manner as described above with reference to FIG. 5, the signal level of the spectrum of the reproduced RF signal is determined as shown in FIG. In this example, if a shallow dropout causes a drop of D R+ = 8 (dB) in the reproduced RF signal SIF, then the lower sideband signal component 1. The signal level of R,=12 (aB), DR,-8(an),
Since Dz = 0.5 (dB), it becomes -11.5 (dB).

Also, the upper sideband signal component J. The signal level of 1 is R1=
12 (dB), DRI =8 (dB), UZ
Since it is -22 (dB), it becomes -34 (dB).

This reproduced RF signal S0 is sent to the boost circuit 15,
As shown in FIG. 19 for the lower sideband signal component J-+ and the upper sideband signal J41, the lower sideband signal component J-+ is boosted by D1+=4.5 (dB), and the upper sideband signal component About J+H U, , egg9
Boost by (dB). As a result, the lower and upper sideband signal components 1. The signal levels of J□ and J□ are −7(
dB) and -25 (dB).

It should be noted here that in Fig. 19, the signal level of the lower sideband signal components J, , l (=-7 (dB)) is higher than the signal level of the carrier signal component J0 (=-8 (dB)). This state means that the carrier signal component J0 has been inverted to the frequency (=4 [MHz)] of the lower sideband signal component 1., as described above with reference to FIG.

By the way, when such carrier inversion occurs, the waveform of the boost output signal SIO is greatly disturbed, resulting in a result that differs greatly from the ideal waveform of the FM modulation signal.

In other words, in general, an ideal frequency modulation signal is one in which a carrier signal waveform having a constant amplitude changes its frequency moment by moment according to the amplitude of the modulation signal, and at this time, the upper and lower sideband signals for the carrier signal component J0 Ingredient J (
11 and J-, the Bessel function values (and therefore the signal levels)
are equal signal levels (R,=1 in Fig. 19)
2 (dB) to a signal level reduced by DRl = 8 (dB)).

However, the reproduced RF signal SR in which dropout occurred
The spectrum of F is affected by the downward-sloping frequency characteristics of VTR2 (Fig. 1) and DR, = 8 due to dropout.
(dB), the upper and lower sideband signal components J, 1 and l, relative to the carrier signal component J0,
However, since the ratio is not an ideal ratio that can be expressed by a Bessel function value, disturbances occur in the waveform of the reproduced RF signal SRF.

In addition to this, a boost circuit 15 generates upper and lower sideband signal components J. , and the upper and lower sideband signal components J to the carrier signal component J0 to partially boost J-t.
. , and J-0 will be subjected to boost processing that drastically changes compared to the ideal FM modulation signal, further emphasizing the waveform disturbance that has occurred in the reproduced RF signal S+++-. The result will be something like this.

Especially the lower sideband signal component 1. Since the signal level of the carrier signal component J0 is higher than that of the carrier signal component J0, the comparison signal S, as shown in FIG. Dropout occurrence period T,. The waveform W at . will be extremely disordered.

In practice, the way the waveform is disturbed is that the waveform of the carrier signal component J0 (3 (MHz) in this example) is lowered by the drop DRI caused by dropout, and then the lower sideband signal component J-9 is lowered by the dropout DRI. (4 (MHz) in this embodiment) produces a waveform that is deformed in the same manner as when the level is shifted up and down over time.

As a result, during the dropout occurrence period Too, every other waveform portion WD0. of the waveform based on the carrier signal component J0. is level-shifted in the direction of becoming higher than the O level, while the waveform portion Wl, between. 8 is level-shifted in the direction of decreasing the signal level with respect to the O level.

As a result, when looking at the dropout waveform W8° as a whole, every other waveform portion WI of each waveform portion of the carrier signal component. A waveform in which 1 stands out at a signal level higher than the O level is obtained.

The comparison signal 5ilo having such a dropout occurrence period T0° is the dropout detection reference signal VI set to a level near the 0 cross signal level of the limiter circuit 7.
IO and generates a comparison output sco14 which rises to a logic "1" level when the comparison signal SIO crosses the dropout detection reference signal VDO to a high instantaneous value.

As a result, the dropout occurrence period T1. . In,
Waveform part WDO1 is dropout detection reference signal VDO
Having a higher instantaneous value produces a comparison output signal s coM with a frequency approximately half the frequency of the carrier signal component J0. On the other hand, the dropout occurrence period T. . In other periods, the instantaneous value of the waveform portion having the frequency of the carrier signal component J0 is the dropout detection reference signal ■. . By crossing the comparison output S COM with the frequency of the carrier signal component J0
is obtained from the comparison circuit 16.

The frequency detection circuit 1 detects the change in the frequency of this comparison output COM.
7 is detected, and after the detection output SDEa is subjected to predetermined processing in the 1' dropout detection width expansion circuit 19, it is sent out as the dropout detection output D00t+T.

In this way, if a shallow dropout occurs and the reproduced RF signal drops by about 8 (dB) when the chroma signal is large, such as a color par signal, the comparator circuit will respond accordingly. This can be reliably detected by lowering the frequency of the comparison output S eOM of 16.

('b) When the chroma signal is very small, on the other hand, when the chroma signal is small and the luminance signal is large, as in a normal picture, the Bessel functions of the upper and lower sideband signal components J+1 and J- The signal level R2 corresponding to the value is
As low as Rz = -28 (dB) (Figure 22), therefore amplitude coefficient A = 0.0397, modulation index β - 0,078
Therefore, the amplitude B of the chroma signal is as shown in Fig. 24 (A
), B is reduced to approximately 12.5%.

Therefore, during normal operation, the comparison signal S of the dropout detection circuit 1. As for the spectrum signal components of Wave signal components J and l are Ull = 9 (dB)
will be boosted only. As a result, the signal level becomes higher than the signal level R2=-28 (dB) of the upper and lower sideband signal components J and J-1 in the case of an ideal FM modulated wave.

In this state, a shallow dropout occurs and the reproduced RF signal SRF becomes DRl =
8 [d8], based on the frequency characteristics shown in Fig. 5, the lower sideband signal component J, , ;6<8
2 = 0.4 CdB), resulting in a signal level of 27.4 (dB), and the upper sideband signal component J
, 1 decreases by Uz = 12 (aB) and the signal level becomes -48 (dB).

Therefore, as the apparent modulation index B increases, the amplitude of the reproduced video signal VD increases as shown in FIG. 24(B).
B=18.4C%], “whitening”
Noise occurs. This reproduced RF signal is transmitted to the boost circuit 15.
As a result, as shown in FIG. 26, the lower sideband signal component J-, is boosted at D + + = 4.5
(dB) and the signal level is -22.9CdB.
)become. Moreover, the upper sideband signal component J, l is U + 1
= 9 (dB) and the signal level is -39 (
dB). At this time, the signal level of the sideband signal component in an ideal FM modulation signal is -(RZ +DR+ ”
) = -36 (dB), so the comparison signal Sl'. As shown in FIG. 27, the degree of waveform disturbance in every other waveform W, o, and W. . 2 are both dropout detection reference signals■. It has a waveform that crosses 0.

Therefore, the frequency of the comparison output SCOi of the comparison circuit 16 becomes the frequency (8 (Mllz)) of the carrier signal component J0, and as a result, the frequency detection circuit 18 does not perform a detection operation, so that the dropout detection output DOOLIT is not sent out.

In this way, even if a shallow dropout occurs, as in the case of FIGS. 22 to 27, if the chroma signal is small, the dropout detection circuit 1 will not judge it as a dropout. .

(3-2) When a deep dropout occurs When a deep dropout occurs, as described above with respect to FIGS. 6 and 9, the decrease DR with respect to the reproduced RF signal
, is large, reaching, for example, DR, = 16.5 (dB), and as described above with reference to FIG. The signal level reaches such a level that the FM demodulation circuit 8 cannot demodulate the video signal.

In such a state, the boost circuit 15 does not perform a boost operation on the carrier signal component J0 and outputs the comparison signal S at the same signal level as described above with reference to FIG. The signal is then sent to the comparator circuit 16.

However, since the comparator circuit 16 is given a signal level of -16 (dB) as the dropout detection reference signal V0°, the output terminal of the comparator circuit 16 always has a constant level (therefore, the frequency is 0) for comparison. Output S. This results in the following: Therefore, the frequency detection circuit 18 outputs the comparison output S
,. Since A is below a predetermined frequency (ie, 4 (MHz)), a detection operation is performed, and thus the dropout detection circuit 1 sends out a dropout detection output DOout.

(3-3) The operation beyond the sensitivity adjustment operation of the boost circuit 15 is to adjust the sensitivity switching means 31 provided in the boost circuit 15 of the dropout detection circuit 1 to the upper sideband signal component J.
, I and the lower sideband signal component J-1 are set to the high-sensitivity detection side contact Kl that boosts both, but if the sensitivity switching means 31 is switched to the boost characteristic 2 side, dropout occurs. The dropout detection sensitivity of the detection circuit 1 can be reduced.

In other words, 2 in the boost curve is the lower sideband signal component J-1
Instead, with little boost to the signal level of
It has a characteristic of boosting the signal level of carrier signal component J0. Therefore, when the sensitivity switching means 31 is switched to the 2nd side of the curve, the boost circuit 15 selects the carrier signal component J0 and the lower sideband signal component 1. of the spectrum signal components included in the comparison signal 5Ilo. By reversing the boost ratio of the comparison signal SS, it is possible to weaken the increase ratio of the lower sideband signal component J-1 to the carrier signal component when dropout occurs, and thereby
Control can be performed to weaken the degree of disturbance in the waveform of O.

Thus, the comparison output Sco in the comparison circuit 16.

State, carrier signal component J. The switching from the frequency of the lower sideband signal component J-1 to the frequency of the lower sideband signal component J-1 is limited when a deeper dropout occurs, and thus the detection sensitivity of the dropout detection circuit l can be controlled.

(4) Effects of the Embodiment If configured as in the embodiment described above, the disturbance in the RF multiplied signal waveform is used to convert this into a frequency signal, and then the change in frequency is detected, thereby making it possible to accurately detect the change in frequency. It is possible to detect the occurrence of a drop error.

In this way, when the chroma signal is small, by taking advantage of the fact that the waveform disturbance is weak, it is possible to avoid unnecessary dropout detection for the video signal where the chroma signal is small and the luminance signal is large.

Furthermore, by boosting the sideband signal component of the chroma signal included in the reproduced RF signal as necessary, the sensitivity of the dropout detection operation of the dropout detection circuit 1 can be increased as necessary. Therefore, by configuring the boost circuit 15 of the dropout detection circuit 1 to be able to change and control the relative boost rates for the carrier signal component J0 and the sideband signal components J, 1, and J-1, the chroma signal By specifying the combination conditions of the size of the dropout and the depth of the dropout, it is possible to easily realize a dropout detection circuit that can detect various dropouts under optimal conditions.

(G4) Other Embodiments In the above-mentioned embodiments, a case has been described in which a changeover switch for switching between two sensitivities is used as the sensitivity control means 31, but the number of sensitivity switching steps may be two or more. good. Furthermore, instead of switching the sensitivity stepwise, the sensitivity may be switched continuously by continuously changing the boost characteristic curve of the boost circuit 15.

In addition, in the above, the case where the sideband signal component is boosted in order to control the sensitivity has been described, but in other words, the ratio of the sideband signal component to the carrier signal component is reduced. Even if change control is performed, the same effect as in the above case can be obtained.

H Effects of the Invention As described above, according to the present invention, a frequency-converted signal having a corresponding frequency is formed based on the waveform disturbance caused by the RF multiplied signal due to dropout, and a change in the frequency is detected, thereby eliminating dropout. By detecting the occurrence of the dropout, it is possible to detect the optimal dropout according to the type of noise that occurs in the playback video signal, depending on the depth of the dropout that occurs and the size of the chroma signal. A dropout detection circuit can be easily realized.

[Brief explanation of drawings]

Figure 1 is a curve diagram showing the normal frequency characteristics of the VTR, which is the transmission system, and Figure 2 is a curve diagram showing the frequency characteristics of the VTR when a shallow dropout occurs.
Figure 3 is a curve diagram showing the frequency characteristics of the VTR when a deep dropout occurs. Figure 4 is a spectrum characteristic diagram and signal waveform showing the reproduced signal under normal conditions when the chroma signal is large. Figure 5 is a spectrum characteristic diagram and signal waveform diagram showing the reproduced signal when a shallow dropout occurs when the chroma signal is large, and Figure 6 shows the reproduced signal when a deep dropout occurs when the chroma signal is large. Spectrum characteristic diagram and signal waveform diagram. Figure 7 shows the spectrum characteristic diagram and signal waveform diagram showing the reproduced signal under normal conditions when the chroma signal is small. Figure 8 shows the spectrum characteristic diagram and signal waveform diagram when a shallow dropout occurs when the chroma signal is small. Figure 9 is a spectrum characteristic diagram and signal waveform diagram showing the reproduced signal when a deep dropout occurs when the chroma signal is small. Figure 10 is a spectrum characteristic diagram and signal waveform diagram showing the reproduced signal when a deep dropout occurs when the chroma signal is small. Figure 10 is the chroma signal is very small. FIG. 11 is a spectrum characteristic diagram and a signal waveform diagram showing the reproduced signal when a shallow dropout occurs when the chroma signal is very small.
FIG. 12 is a spectrum characteristic diagram and signal waveform diagram showing a reproduced signal when a shallow dropout occurs when the chroma signal is very small, and FIG. 13 is a block diagram showing an embodiment of the dropout detection circuit 1 according to the present invention. , 14th
The figure is a characteristic curve diagram showing the boost characteristics of the boost circuit 15 in Figure 13, Figure 15 is a spectrum characteristic diagram showing the reproduced RF signal under normal conditions when the chroma signal is large in the first operation example, and Figure 16 is a characteristic curve diagram showing the boost characteristics of the boost circuit 15 in Figure 13. A spectrum characteristic diagram showing the output signal of the boost circuit 15 during normal operation in the first operation example, FIG. 17 is a signal waveform diagram showing the signals of each part during normal operation in the first operation example, and FIG. FIG. 19 is a spectrum characteristic diagram showing the reproduced RF signal when a shallow dropout occurs in the first operation example. FIG. 20 is a spectrum characteristic diagram showing the output signal of the boost circuit 15 when a shallow dropout occurs in the first operation example. FIG. 21 is a signal waveform diagram showing the signals of each part when a shallow dropout occurs in the first operation example. FIG. 21 is a signal waveform diagram showing the waveform of the output signal of the boost circuit 15 in the first operation example. FIG. 23 is a spectrum characteristic diagram showing the reproduced RF signal under normal conditions when the chroma signal is very small in the second operation example; FIG. 23 is a spectrum characteristic diagram showing the output signal of the boost circuit 15 under normal conditions in the second operation example; FIG. 24 is a signal waveform diagram showing the signals of each part in the second operation example, FIG. 25 is a spectrum characteristic diagram showing the reproduced RF signal when a shallow dropout occurs in the second operation example, and FIG. FIG. 27 is a spectrum characteristic diagram showing the boost output signal when a shallow dropout occurs in the second operation example. FIG. 27 is a signal waveform diagram showing the waveform of the output signal of the boost circuit 15 in the second operation example. 1... Dropout detection circuit, 2...
・VTR15...Reproduction amplification circuit, 6...
・Reproduction equalizer, 7...Limiter circuit, 8.
...FM demodulation circuit, 9...De-emphasis circuit, 15...Boost circuit, 16...
...Comparison circuit, 17...Reference power supply, 18...
. . . Frequency detection circuit, 19 . . . Dropout detection output width expansion circuit, 31 . . . Sensitivity control means.

Claims (2)

[Claims] (1) In a video signal reproducing device configured to demodulate a video signal from a reproduced RF signal obtained as a result of recording and reproducing FM-modulated video signals, a comparison signal having a waveform corresponding to the waveform of the reproduced RF signal is used. and a dropout detection reference signal at a predetermined level, thereby forming a frequency-converted signal whose frequency is determined based on the point at which the waveform of the comparison signal crosses the dropout detection reference signal. and a frequency detection means for transmitting a dropout detection signal based on a change in the frequency of the frequency-converted signal, and the frequency-converted signal is detected when dropout occurs and the waveform disturbance of the reproduced RF signal increases. A dropout detection circuit that obtains the dropout detection signal when the frequency of the dropout detection circuit changes. (2) The level of the dropout detection reference signal is set to the dropout detection level for the luminance signal of the reproduced RF signal when the chroma signal of the video signal is very small and the luminance signal is large. A dropout detection circuit according to claim 1.