JPH0728431B2 - Video signal playback device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described in the following order.

A Industrial Field B Outline of the Invention C Prior Art D Problems to be Solved by the Invention E Means for Solving Problems (Figs. 13 and 21) F Action (Figs. 13 and 21) Fig. G Example (Figs. 1 to 27) (G1) Relationship between dropout cause and reproduced RF signal spectrum signal component (G2) Effect of dropout on reproduced RF signal (1) VTR frequency characteristic Changes (Figs. 1 to 3) (2) When chroma signal is large (Figs. 4 to 6) (3) When chroma signal is small (Figs. 7 to 9) (4) Chroma signal is Very small and large luminance signal (Figs. 10 to 12) (G3) Example of dropout detection circuit (1) Dropout detection principle (Figs. 7 to 10) (2) Configuration of example ( (Figs. 13 and 14) (3) Operation of the embodiment (Figs. 15 to 27) (4) Effect of the embodiment (G4) Other implementation FIELD The present invention on the effect A industry H invention relates to video signal reproducing apparatus, and is suitably applied particularly to a video tape recorder of direct FM recording system.

B. Summary of the Invention The present invention is a video signal reproducing apparatus adapted to demodulate a video signal from a reproduced RF signal obtained as a result of FM-modulating and recording and reproducing a video signal. , By controlling the ratio of the sideband signal component to the carrier signal component to emphasize the waveform disturbance, when dropout occurs and the waveform disturbance of the reproduced RF signal becomes large, Disturbance-based dropout detection can be further facilitated.

C Prior Art In a video tape recorder (VTR) of this type of direct FM (frequency modulation) recording system, the video signal is directly FM-modulated using, for example, a carrier of 8 [MHz],
A VTR that records this FM modulated signal (called RF signal) diagonally on a magnetic tape using a rotating video head.
However, it is used in high-end machines such as VTRs for broadcasting equipment.

In this type of VTR, when the RF signal recorded on the magnetic tape is reproduced on the video head, the magnetic tape and the video head may not slide properly. For example, when dust enters between the magnetic tape and the video head, or when the surface portion of the magnetic material applied to the magnetic tape is scraped off, the video head and the magnetic tape are slid. The contact is not sufficient, and as a result, the signal level of the reproduced RF signal is lowered.

Such a phenomenon is generally called dropout.When a weak dropout occurs, the S / N of the demodulated playback screen is only degraded, but when a strong dropout occurs, an unsightly noise bar is displayed on the playback screen. Appears.

Therefore, conventionally, in order to prevent an unsightly noise bar from being generated on the playback screen as much as possible, the noise bar portion in which dropout has occurred is compensated by reusing the playback signal of 1H before. For example, Japanese Patent Publication No. 54-27216 discloses a configuration in which dropout compensation is directly performed on an RF signal, and Japanese Patent Publication No. 55-19478 discloses a video signal obtained by demodulating a reproduced RF signal. A configuration for dropout compensation is disclosed. However, in any of the conventional configurations, it is arranged to detect whether or not the signal level of the reproduction RF signal has decreased by a predetermined amount, and therefore the following problems occur.

D. Problems to be Solved by the Invention In general, when a dropout occurs in a reproduction RF signal, the amount of decrease in the high frequency signal component of the spectrum signal component tends to be larger than the amount of decrease in the low frequency signal component. Depending on the degree of the defect of the dropout, the defect due to the shallow defect of the surface portion of the magnetic tape (hereinafter referred to as the shallow dropout, at which time the reproduction RF signal is lowered by about −8 [dB]) is a deeper defect. Dropout (hereinafter referred to as deep dropout, at which time the reproduction RF signal is, for example, -16 [dB]
The degree of decrease) tends to be larger.

Therefore, when dropout occurs with respect to a reproduction RF signal having a chroma signal (amplitude thereof is large) representing a color with high saturation such as a color bar, the lowering amount of the carrier signal component (that is, the chroma signal) occurs when dropout occurs. signal)
, The apparent modulation index is increased, resulting in "whitening" noise corresponding to the defect on the playback screen. The brightness of the "whitening" noise becomes brighter in the deeper dropout than in the shallower dropout because the amount of reduction of the carrier signal component is larger, so that the "whitening" noise tends to be conspicuous.

In addition, especially for color bars with extremely high saturation (for example, cyan), a deep dropout with a large decrease occurs, which causes an excessive decrease in the carrier signal component, causing the signal level of the carrier signal component to fall into the lower sideband. When the signal level becomes lower than the signal level of the signal component, the FM demodulation circuit inverts the signal level of the demodulation signal from the white level to the black level, assuming that the carrier has switched to the frequency of the lower sideband. At this time, a "black and white reversal" phenomenon appears on the reproduction screen, which causes a remarkable deterioration in image quality.

In detecting dropouts that occur in various manners as described above, the dropout detection level is adjusted to, for example, -8 [d] in accordance with the amount of decrease in the reproduction RF signal during shallow dropout.
If set to [B], of course, shallow dropouts will occur,
A deep dropout occurs and the playback RF signal is, for example, −16.
When [dB] drops, this can be detected.

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

That is, a general image (which is referred to as a normal image) often has low saturation as a whole screen, and thus the amplitude of the chroma signal is very small. In addition, a video signal of a normal image often has a high luminance signal level and weak H correlation. Therefore, from the viewpoint that the H correlation is weak, it is desirable from the viewpoint of preventing the deterioration of the image quality that the dropout compensation operation for reusing the video signal of 1H before is not performed as much as possible. Because of its low saturation, even if "white-out" noise occurs on the playback screen, it is not very noticeable, and it is not a problem even if it is left alone.

The present invention has been made in consideration of the above points, and the reproduction is performed by the difference in the form of dropout occurring in the magnetic tape.
An attempt is made to propose a dropout detection circuit that can reliably detect only dropouts that need to be compensated practically by utilizing the different influences of spectrum signal components included in RF signals. Is.

E Means for Solving the Problems In order to solve the problems, according to the present invention, an FM modulated signal obtained by FM-modulating a video signal is recorded and reproduced, and a reproduced RF signal S _RF obtained as a result is reproduced. In a video signal reproducing device adapted to demodulate a video signal, in order to send out a comparison signal S _BO having a large waveform disturbance, of the spectrum signal component of the reproduction RF signal S _RF , the carrier signal component J ₀ and the side band Waveform disturbance enhancing means 15 for changing the relative signal levels of the wave signal components J ₊₁ and J _-1 , and a frequency conversion signal that changes based on the time when the waveform of the comparison signal S _BO crosses the dropout detection reference signal V _DO. to deliver S _COM, comparison signal and the comparison means 16 for comparing the S _BO waveform a predetermined level of dropout detection reference signal V _DO of, based on the change in frequency of the frequency converted signal S _COM dropout detection No. comprising a dropout detection means 1 and a frequency detection means 18 for sending a DO _OUT, dropout detecting means 1, a frequency conversion signal S _COM by disturbance of dropout occurs reproduction RF signal S _RF waveform is greater Dropout detection signal DO when the frequency of
_{Send OUT} .

F-action When dropout occurs, the disturbance of the waveform of the reproduction RF signal S _RF becomes large. This waveform disturbance is caused by the waveform disturbance emphasizing means 15 by the carrier signal component and the sideband signal component.
It is emphasized by changing the ratio of J ₋₁ or J ₊₁ .

As a result, depending on the type of dropout, when the dropout you want to detect occurs (for example, when the chroma signal is large, when the shallow dropout occurs), play
RF signal S When the disturbance of the waveform of _RF is emphasized so that this can be easily detected, and when you do not want to detect dropout (for example, the chroma signal is very small and the luminance signal is large like in normal images) When a shallow dropout occurs, the detection sensitivity can be set and adjusted so that the dropout detection operation is not performed.

G Embodiment One embodiment according to the present invention will be described below in detail with reference to the drawings.

(G1) Relationship between dropout cause and spectrum signal component of reproduced RF signal Generally, FM signal wave e _FM is The reproduced RF signal reproduced from the magnetic tape is represented by the equation (1). Here, ωc is the carrier angular frequency, ωp is the modulation angular frequency, and Δω is the angular frequency deviation. In equation (1), Δω / ωp is It is represented by the modulation index β from the relationship of Where f is the modulation frequency and ΔF is the frequency excursion.

When Eq. (1) is expanded using the Bessel function of the 1st kind, e _FM = J ₀ (β) cosωct + J ₁ (β) cos (ωc + ωp) t −J ₁ (β) cos (ωc−ωp) t. ………………… (3) can be rewritten. However, since the modulation index β of the RF signal, which is the FM modulation signal recorded on the magnetic tape in the VTR, is actually selected to be a relatively small value, the following equation e _FM = J ₀ (β) cosωct + J ₁ ( β) cos (ωc + ωp) as represented by _{t -J 1 (β) cos (} ωc-ωp) t ...... (4), carrier signal component J ₀ (β) cosωct
And the upper and lower sideband signal components J ₁ (β) cos (ωc + ωp)
It can be considered that t and −J ₁ (β) cos (ωc−ωp) t are superimposed.

Modulation index β is Is represented by Where a is the amplitude, k is the modulation sensitivity,
Modulation sensitivity is 100%, frequency deviation ΔF at the time of modulation is 100
It is a value that is multiplied by [%].

The reproduced RF signal obtained by recording and reproducing the RF signal composed of such an FM modulated signal in the transmission system composed of the VTR is
The VTR amplitude characteristic is It can be evaluated by the amplitude characteristic A represented by
Where ρ ₀ is the amplitude of the carrier frequency component J ₀ , ρ ₊₁ and ρ
_-1 represents the amplitude of the upper and lower band signal components, respectively.

In this way, the amplitude characteristic of the transmission system for the RF signal is the average value (ρ ₊₁ +) of the upper and lower sideband signal components J ₊₁ and J _−1.
It can be expressed by the ratio of ρ ₋₁ ) / 2 to the amplitude ρ ₀ of the carrier signal component J ₀ .

(G2) Effect of dropout on reproduced RF signal (1) Change in VTR frequency characteristic Frequency of reproduced RF signal obtained as a result of recording and reproducing the RF signal represented by equation (4) for the transmission system consisting of VTR The characteristics are
As shown in FIG. 1, the carrier signal component J ₀ has a drooping characteristic as shown by a downward-sloping curve K ₀ , and upper and lower sideband signal components J ₊₁ and J with respect to the carrier signal component J ₀ . The output level difference of _-1 is U ₁ [dB] and D ₁ [d
B]. Note that J ₀ , J ₊₁ and J ₋₁ are the terms of the Bessel cell term J ₀ (β), the term of J ₁ (β), and −J ₁ (β) of the equation (4).
Correspond to the respective terms.

In the case of this embodiment, the frequency of the carrier signal component J ₀ is 8
[MHz], upper and lower sideband signal components J ₊₁ and J ₋₁ are chroma signals, and frequencies are 4 [MHz] and 12 [MHz].

When a shallow dropout occurs in the transmission system having such a frequency characteristic, as shown in FIG. 2, a curve K1 having a larger downward sloping amount than in the normal case of FIG. 1 is shown. It exhibits frequency characteristics. Therefore, in this case, the difference D ₂ [dB] between the signal level of the carrier signal component J _{0 and} the signal level of the lower sideband signal component J _{−1 and} the signal level of the carrier signal component J ₀ and the upper sideband signal component The difference U ₂ [dB] from the signal level of J ₊₁ is the difference D ₁ [dB] and U _{1 in} the case of FIG.
It is significantly larger than [dB].

When a deeper dropout occurs, the frequency characteristic of the reproduction RF signal is, as shown in FIG. 3, a curve K2 in which the droop amount to the right is larger than that in the shallow dropout of FIG. It exhibits the frequency characteristics as shown. Therefore, the difference D ₃ (dB) between the signal level of the carrier signal component J _{0 and} the signal level of the lower sideband signal component J _{-1 and} the signal level of the carrier signal component J _{0 and} the signal of the upper sideband signal component J ₊₁ The difference from the level U ₃ [dB] is the difference D ₂ [dB] and U ₂ [d] in the case of FIG.
B] is much larger.

From FIGS. 1 to 3, the signal levels of the carrier signal component J ₀ and the upper sideband signal component J ₊₁ decrease more as the depth at which the dropout occurs becomes deeper. It can be seen that the signal level of the lower sideband signal component J ₋₁ does not decrease so much even if the dropout occurs deeply.

From this change in characteristics, as the dropout occurrence depth increases, the signal level of the carrier signal component J ₀ becomes
When the signal level of the lower sideband signal component J _-1 gradually approaches the signal level of the lower sideband signal component J _-1 (and thus the chroma signal), the carrier signal component J ₀
It can be seen that there is a possibility that the signal level of the signal will be lower than the signal level of the lower sideband signal component J ₋₁ .

The extent of the dropout occurrence on the playback RF signal depends on the magnitude of the amplitude of the upper and lower sideband signal components J ₊₁ and J _-1 (that is, the chroma signal) included in the playback RF signal. Generate different noise.

(2) When the chroma signal is large When the reproduced RF signal (FIG. 4 (B)) includes a chroma signal having a high saturation such as a cyan color bar having a large amplitude B ₁₁ (see FIG. 4 ( C)) is, under normal conditions, as shown in FIG. 4A, the signal level of the lower sideband signal component J ₋₁ is affected by the frequency characteristics of FIG. Corresponding signal level R ₁ [dB] (eg 13.4
It rises from [dB]) by D ₁ [dB] (for example, 3.6 [dB]) and approaches the carrier signal component J ₀ . Here, the fact that the upper and lower sideband signal components J ₊₁ and J _-1 are at signal levels corresponding to the first-order Bessel function value means that the reproduced RF signal has an ideal FM modulation signal waveform (carrier signal component). Has a constant amplitude) that has.

Under this condition, when a shallow dropout occurs as shown in FIG. 5, the reproduction RF signal S _RF (FIG. 5 (B)) has a large decrease amount DR ₁ [dB] (for example, 8 [dB]). As the lower sideband signal component J _-1 (Fig. 5 (A)) decreases,
Due to the influence of the frequency characteristic of FIG. 2, the signal level R ₁ [dB] corresponding to the first-order Bessel function value rises by a remarkably large increase amount D ₂ [dB] (for example, 8.6 [dB]).

As a result, the signal level of the lower sideband signal J _-1 is ideal F
This means that the signal level of the carrier signal component J ₀ was approached by D ₂ [dB] as compared with the case of the M modulation signal. This means that, as is clear from Table 1, the modulation index β is apparently increased, which causes the amplitude B of the video signal VD to rise.
Is large (FIG. 5 (C)), which means that "whitening" noise is eventually generated.

β J ₀ J ₊₁ , J _-1 0 1.0000 0 0.1 0.9975 0.0499 0.2 0.99 0.0995 0.3 0.9776 0.1483 0.38 0.9638 0.1865 0.4 0.9604 0.1660 0.5 0.9385 0.2423 0.6 0.912 0.2867 0.7 0.8812 0.329 0.8 0.8463 0.3688 0.9 0.8075 0.4059 1.0 0.7652 0.4401 Table 1 As shown in FIG. 6, when a deep dropout occurs, the reproduction RF signal S _RF (FIG. 6 (B)) decreases by a further large decrease amount DR ₂ [dB] (for example, 16 [dB]), and sideband signal J _-1 (FIG. 6 (a)) is, the signal level R ₁ [dB] even more remarkably large increase amount D ₃ under the influence of the frequency characteristic of FIG. 3 [dB] (for example 13.9 [DB]) only.

As a result, the FM level of the lower sideband signal J _-1 is ideal.
The signal level of the carrier signal component J ₀ is exceeded by increasing by D ₃ [dB] as compared with the case of the modulation signal. At this time, the demodulated video signal VD (Fig. 6 (C)) falls to the black level L _BLK , thus causing "black and white inversion" noise on the reproduced screen.

(3) When the chroma signal is small In the reproduced RF signal (FIG. 7 (B)), the saturation is lower than in the case of FIG. 4 (but much higher than the normal image), such as a blue color bar, If there are small chrominance signal (Figure 7 (C)) is at the time of normal as shown in Figure 7 (a), the lower sideband signal component under the influence of the frequency characteristic of FIG. 1 J _{- first} signal level is closer to the carrier signal component from the signal level R ₂ corresponds to the primary Betsuseru function values of an ideal FM signal [dB] (for example, 20 [dB]) rises only D ₁ [dB] .

When a shallow dropout and a deep dropout occur from this state, the signal level of the lower sideband signal component J _-1 becomes the second level as shown in FIGS. 8 and 9 corresponding to FIGS. 5 and 6. Under the influence of the frequency characteristics shown in FIGS. 3 and 4, the amount of increase increases by D ₂ and D ₃ [dB], respectively, and approaches the carrier signal component J ₀ . The resulting demodulated video signal VD
In FIG. 8 (C) and FIG. 9 (C), “whitening” noise occurs.

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

When dropout does not occur, the reproduced RF signal has the frequency characteristic described above with reference to FIG. 1 as shown in FIG.

By the way, when recording and reproducing a video signal of a normal image, the recorded video signal V _REC is, for example, as shown in FIG.
Since the pre-emphasis is applied to the video signal to which the amplitude of 100 [%] is allocated between 7 [MHz] and 9 [MHz], the rising portion of the video signal is 9 [MH].
z] [7] [MH] is formed at the falling portion of the video signal, with an emphasized waveform protruding above the frequency.
The emphasized waveform is formed so as to project to a frequency below z], thus improving the S / N.

However, since the VTR has the frequency characteristic of falling to the right as described above with reference to FIG. 1, if the frequency of the recorded video signal V _REC rises to a frequency near the upper sideband signal component J _{+1 of the} chroma signal, The signal level of the reproduction RF signal S _RF (FIG. 10 (C)) decreases to a value COM ₁ close to the decrease U ₁ (for example, 6 [dB]) due to the frequency characteristic.

Conversely, if the recorded video signal V _REC drops to a frequency near the lower sideband signal component J _{-1 of the} chroma signal, the reproduction RF signal S _RF signal is increased by the amount D ₁ of the frequency characteristic (for example, 3.6 [dB]). Level increases.

However, since the fluctuation of the amplitude of the decrease amount U ₁ [dB] and the increase amount D ₁ [dB] can be removed by using the limiter in the FM reproducing circuit, as shown in FIG. 10 (D). The playback video signal VD may not have that effect,
Thus, as shown in FIG. 10 (D), it is possible to obtain the reproduced video signal VD in which the very small chroma signal S _CR is superimposed on the luminance signal S _RU .

However, when a dropout of about DR = 8 [dB] as described above with reference to FIG. 2 occurs, even if the same recording video signal V _REC is recorded, the decrease amount U ₂ due to the frequency characteristic in the peak waveform portion where the frequency becomes high is generated. = 12 [dB], the playback RF signal S _RF (Fig. 11 (C)) has a signal level of COM.
_{2 It} decreases to about 20 [dB].

However, even in this case, since the reduction of the carrier signal component J ₀ is still small, the reproduction video signal VD can be obtained by removing the influence of the reduction amount COM ₂ via the limiter circuit.

However, as shown in FIG. 12, when a deep dropout occurs, the carrier signal component J ₀ obtained from the VTR is remarkably reduced, and therefore, of the reproduced RF signal S _RF (FIG. 12 (C)), The amplitude of the signal S _DO at the portion where dropout has occurred becomes so small that it cannot be reproduced, and as a result, noise NS occurs in the reproduced video signal VD.

(G3) Example of dropout detection circuit (1) dropout present invention from the above study detection principle does not detect a decrease in the amplitude of the reproduction RF signal S _RF, dropout occurs among the reproduction RF signal S _RF Paying attention to the point that the disturbance of the waveform that changes according to the dropout occurrence depth occurs in the signal part,
By forming a dropout detection signal in which the frequency changes based on the disturbance of the waveform generated when the dropout occurs, the dropout compensation can be appropriately performed according to the occurrence depth of the dropout.

However, according to the conventional way of thinking, reproduction with a large chroma signal
Regarding the RF signal, in order to detect both the shallow dropout and the deep dropout, in order to detect both, when the decrease amount DR ₁ (= 8 [dB]) of decrease in the amplitude of the reproduction RF signal S _RF occurs, Should be determined to be a dropout (see FIGS. 5, 6, and 8).
(Fig. 9).

However, in this case, as described above with reference to FIG. 10, when the chroma signal is very small and the luminance signal is large (the video signal of the normal image belongs to this case), the pre-emphasis recorded video signal V _REC Of these, the peak pre-emphasis waveform part may be judged to be a dropout even if it slightly shifts to a high frequency.
This is not desirable because it may cause deterioration of resolution due to repetition of compensation by the video signal.

However, this means that the amount of reduction in the reproduced RF signal is DR ₂ (= 1
6 [dB]) (Fig. 6 (B), Fig. 9 (B)), if this is determined as a dropout, "whitening" noise occurs when a shallow dropout occurs. Cannot be prevented (FIGS. 5 and 8).

(2) Configuration of the embodiment In FIG. 13, reference numeral 1 is a dropout detection circuit, which reproduces a reproduction RF signal S _RF reproduced from a tape 3 of a VTR 2 as a transmission system by a head 4 and a reproduction amplifier circuit 5 and a reproduction equalizer 6
Receive through. The reproduced RF signal S _RF is limited in amplitude by the limiter circuit 7, converted into a frequency signal S _FQ determined based on the 0 cross point of the reproduced RF signal S _RF , and then FM.
The demodulated video signal VD given to the modulation circuit 8 is de-emphasized and then sent out as a reproduction output signal VD _OUT .

The dropout detection circuit 1 has a boost circuit 15 having a boost characteristic as shown in FIG. Boost circuit 15, around the carrier signal component J _0, the sideband signal component J ₊₁ and the lower sideband signal component J both sides of the output level of carrier signal component J ₀ in a range of frequencies of _-1 on the substantially chroma signal The amplifier circuit has a frequency characteristic that becomes higher than the signal level of.

Thus, at the output terminal of the boost circuit 15, the signal levels of the upper and lower sideband signal components J ₊₁ and J _-1 included in the reproduction RF signal S _RF are relative to the signal level of the carrier signal component J _0. A boosted output signal is obtained, which is provided to the comparison circuit 16 as the comparison signal S _BO . Comparison circuit 1
The dropout detection reference signal V _{DO, which} is a DC voltage, is applied from the reference power supply 17 to the reference input terminal of 6, and the comparison signal S _BO
When the instantaneous value of is higher than the dropout detection reference signal V _DO , the comparison output S _COM of logic “1” level is sent to the frequency detection circuit 18 as a frequency conversion signal. This frequency detection circuit 18 is a detection output which becomes, for example, a logic "0" level when the frequency of the comparison output S _COM as the frequency conversion signal sent from the comparison circuit 16 becomes lower than a predetermined frequency.
S _DET is generated and sent as a dropout detection output DO _OUT through the dropout detection output width expansion circuit 19.

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

The sensitivity control means 31 operates with a switching input terminal P _K1 for inputting a switching signal for operating the boost circuit 15 with the characteristic shown in the boost characteristic curve K1 described above with reference to FIG. made by switching switch having a switching input P _K2 for inputting a switching signal for.

In the boost characteristic curve K2, the boost amount of the lower sideband signal component J _-1 of the reproduction RF signal S _RF is set to 0 [dB], and the boost amount BS ₂ of the carrier signal component J ₀ is set to 0 [dB].
That is all. By doing this, when the sensitivity control means 31 is switched from the switching input terminal P _K1 to P _K2 , the ratio of the signal levels of the carrier signal component J ₀ and the lower sideband component J _-1 is changed to change the comparison input S _BO. The amount of turbulence of the waveform (and hence the dropout detection sensitivity) is controlled.

(3) Operation (3-1) shallow if dropout occurs (a) construction of more than when the chroma signal is greater embodiment, as shown in FIG. 15, the upper and lower with respect to carrier signal component J ₀ It is assumed that a reproduction RF signal S _RF of, for example, a color bar having large sideband signal components J ₊₁ and J ₋₁ is obtained at the output end of the reproduction equalizer 6. The boost circuit
It is assumed that the sensitivity control means 31 of 15 is switched to the high sensitivity detection side contact P _K1 for the characteristic curve K1 (FIG. 14).

The signal level of the reproduced RF signal S _RF obtained at this time is the carrier signal component J ₀ (= 0 [dB]), the signal level R ₁ (= 12 [dB]) corresponding to the first-order Bessel function value, and the lower sideband wave. Increase in signal component J _-1 D ₁ (= 3.5 [dB]) (hence the signal level of lower sideband signal component J _-1 is 8.5 [dB]), upper sideband signal component
The decrease amount of J ₊₁ U ₁ = (6 [dB]) (thus, the signal level of the upper sideband signal component J ₊₁ is 18 [dB]), and the amplitude characteristic A (= 0.2
5), modulation index β (= 0.49), amplitude B of video signal VD
(= 78.9%).

The boost circuit 15 boosts the reproduced RF signal to output the comparison signal S _BO shown in FIG. 16 to the output terminal thereof. Here, the signal level of the lower sideband signal component J ₋₁ included in the comparison signal S _BO is amplified by D ₁₁ (= 4.5 [dB]) to 4 [dB], and the upper sideband signal component J _−1. The signal level of ₊₁ is boosted by U ₁₁ (= 9 [dB]) to 9
It becomes [dB]. In this state, if the reproduction RF signal S _RF has no dropout as shown in FIG. 17 (A), the reproduction output signal VD _OUT has the amplitude (B) (= 78.9) as shown in FIG. 17 (B). %) Chroma signal is transmitted.

Then, at this time, the comparison signal S _BO whose amplitude is boosted by the boost circuit 15 is sent to the output terminal of the boost circuit 15. It should be noted here that the peak value of the comparison signal S _BO is a signal in which the ratio of the upper and lower sideband signal components to the carrier signal component J ₀ is different from the ideal FM modulation signal represented by the Betzell function value. Since the level is controlled to change, the level of the peak value of the carrier signal that fluctuates at the frequency (8 [MHz]) of the carrier signal component J ₀ fluctuates with the passage of time (this is referred to as waveform disturbance). Call) is occurring.

However, the turbulence width of this waveform is set to be sufficiently small when dropout does not occur.
Since the boost characteristic of is selected, this comparison signal S
_BO is compared in the comparison circuit 16 with the dropout detection reference signal V
_The comparison output S _COM is not affected during _DO comparison.

In the state where the reproduction RF signal S _RF is reproduced in this way, as shown in FIG. 20 (A), a dropout occurs in the shallow portion of the magnetic tape, and the reproduction RF signal S _RF becomes DR ₁ = 8.
If the signal level is lowered by [dB], the signal level of the spectrum of the reproduction RF signal is determined as shown in FIG. 18 in the same manner as described above with reference to FIG. In the case of this embodiment, the reproduction RF signal S _{RF is changed} to D by the shallow dropout.
If a drop of R ₁ = 8 [dB] occurs, the signal level of the lower sideband signal component J ₋₁ is R ₁ = 12 [dB], DR ₁ = 8 [dB],
Since D ₂ = 0.5 [dB], it becomes −11.5 [dB].

The signal level of the upper sideband signal component J ₊₁ is R ₁ = 12 [d
B], DR ₁ = 8 [dB], U ₂ = 22 [dB], so -34 [dB]
become.

The reproduction RF signal S _RF, in the boost circuit 15, as shown in FIG. 19 for the lower sideband signal component J _-1 and the upper sideband signal J _+1, lower sideband signal component J _-1 for D ₁₁ = 4.5 〔d
B] only, and for the upper sideband signal component J ₊₁
U ₁₁ = Boost by 9 [dB]. As a result, the signal levels of the lower and upper sideband signal components J ₋₁ and J ₊₁ are −
It becomes 7 [dB] and -25 [dB].

It should be noted here that the lower sideband signal component in FIG. 19 is
The signal level of J _-1 (= -7 [dB]) is the carrier signal component J
_Since the signal level is higher than ₀ (= −8 [dB]), as described above with reference to FIG.
This means that the carrier signal component J ₀ has been inverted to the frequency (= 4 [MHz]) of the lower sideband signal component J _-1 .

By the way, when such carrier inversion occurs, the waveform of the boost output signal S _BO is greatly disturbed, and the ideal F
The result is that it is far from the waveform of the M-modulated signal.

That is, in general, an ideal frequency modulation signal is one in which a carrier signal waveform having a constant amplitude changes its frequency from moment to moment in accordance with the amplitude of the modulation signal, and at this time, upper and lower sidebands with respect to the carrier signal component J _0. The signal levels (therefore, signal levels) of the signal components J ₊₁ and J _-1 that are equal to each other (from R ₁ = 12 [dB] to DR in FIG. 19)
₁ = 8 [dB] reduced signal level).

However, the spectrum of the reproduction RF signal S _RF in which the dropout has occurred has the frequency characteristic of the VTR2 falling to the right (first
(Fig.) And the dropout of DR ₁ = 8 [dB] due to dropout, the upper and lower sideband signal components J ₊₁ and J _-1 with respect to the carrier signal component J ₀ are Betzell function values. Although it does not reach the ideal ratio that can be expressed by, the waveform of the reproduced RF signal S _RF is disturbed.

In addition to this, the boost circuit 15 has upper and lower sideband signal components.
The J ₊₁ and J _-1 to partially boost, compared the ratio of the upper and lower sideband signal components J ₊₁ and J _-1 is an ideal FM signal extremely relative carrier signal component J ₀ Since the boosting process that causes a change is performed, the result is that the waveform disturbance that has occurred in the reproduction RF signal S _RF is further emphasized.

In particular, since the signal level of the lower sideband signal component J ₋₁ is higher than the signal level of the carrier signal component J ₀ , the waveform of the comparison signal S _{BO in} the dropout occurrence period T _DO is shown in FIG. W _DO will be extremely disturbed.

Actually, the disturbance of the waveform is that the waveform of the carrier signal component J ₀ (8 [MHz] in this embodiment) is reduced by the drop amount DR ₁ caused by the dropout, and the lower sideband signal component With the frequency of J _-1 (4 [MHz] in this embodiment), a waveform is generated which is similar to the one obtained when the level is vertically shifted with the passage of time.

As a result, during the dropout occurrence period T _DO , every other waveform portion of the waveform based on the carrier signal component J ₀ W _DO1
Is shifted in the direction of becoming higher than 0 level, while the waveform portion W _DO2 between them is level shifted in the direction of decreasing the signal level with reference to 0 level.

As a result, when the dropout waveform W _DO is viewed as a whole, a waveform is obtained in which every other waveform portion W _DO1 of each waveform portion of the carrier signal component projects to a signal level higher than 0 level.

The comparison signal S _BO having such a dropout occurrence period T _DO
Is compared with the dropout detection reference signal V _DO set at a level near the 0 cross signal level of the limiter circuit 7, and when the comparison signal S _BO crosses the dropout detection reference signal V _DO and reaches a high instantaneous value, Generates comparison output S _COM that rises to "1" level.

As a result, during the dropout occurrence period T _DO , the waveform portion W _DO1 has an instantaneous value higher than that of the dropout detection reference signal V _DO , so that the comparison output having a frequency of approximately half the frequency of the carrier signal component J ₀ is generated. Generate signal S _COM . On the other hand, in the period other than the dropout occurrence period T _DO , the instantaneous value of the waveform portion having the frequency of the carrier signal component J ₀ crosses the dropout detection reference signal V _DO , so that the carrier signal component J _O A comparison output S _COM having a frequency of will be obtained from the comparison circuit 16.

The frequency detection circuit 17 detects the change in the frequency of the comparison output COM, and the detection output S _DET is subjected to predetermined processing in the dropout detection width expansion circuit 19 and then sent out as a dropout detection output DO _OUT .

In this way, when the chroma signal is large like the color bar signal, for example, when the shallow dropout occurs and the reproduction RF signal is lowered by about 8 [dB], the comparison circuit 16 is accordingly operated. Since the frequency of the comparison output S _COM of is decreased, this can be detected with certainty.

(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 the normal image, the upper and lower sideband signal components
Signal level R corresponding to the Bessel function values of J ₊₁ and J _-1
2 is as low as R ₂ = -28 [dB] (Fig. 22). Therefore, the amplitude coefficient A = 0.0397 and the modulation index β = 0.078, so that the amplitude B of the chroma signal is reduced to B = 12.5% as shown in FIG. 24 (A).

Therefore the normal, spectrum signal component of the comparison signal S _BO of dropout detection circuit 1, as shown in FIG. 23, D ₁₁ is the lower sideband signal component J _-1 as compared with the case of Figure 22
= 4.5 [dB] boosted and upper sideband signal component J
₊₁ is boosted by U ₁₁ = 9 [dB]. As a result, the upper and lower side wave signal components J ₊₁ and J _-1 for an ideal FM modulated wave
The signal level becomes higher than the signal level R _{2 of} −28 [dB].

In this state, a shallow dropout will occur and play
Assuming that a drop of DR ₁ = 8 [dB] occurs in the RF signal S _RF as shown in FIG. 25, based on the frequency characteristic of FIG.
The lower sideband signal component J _-1 is increased by B ₂ = 0.4 [dB] to raise the signal level to 27.4 [dB], and the upper sideband signal component J
₊₁ decreases by U ₂ = 12 [dB] and the signal level is -48 [dB]
become.

As a result, the apparent modulation index B rises, so that the amplitude of the reproduced video signal VD is B = B as shown in FIG. 24 (B).
By rising to 18.4%, "whitening" noise occurs. This reproduced RF signal is boosted in the boost circuit 15, and as a result, as shown in FIG. 26, the lower sideband signal component J ₋₁ is increased by D ₁₁ = 4.5 [dB] and the signal level is −2.
It becomes 2.9 [dB]. The upper sideband signal component J ₊₁ is U ₁₁ = 9
It is increased by [dB] and the signal level becomes -39 [dB].
At this time, the signal level of the sideband signal component in the ideal FM modulation signal is − (R ₂ + DR ₁ ) = − 36 [dB],
The degree of disturbance of the waveform of the comparison signal S _BO is such that every other waveform W _DO1 and W _DO2 crosses the dropout detection reference signal V _DO, as shown in FIG.

Therefore, the frequency of the comparison output S _COM of the comparison circuit 16 becomes the frequency (8 [MHz]) of the carrier signal component J ₀ , and the frequency detection circuit 18 does not perform the detection operation, so that the dropout detection output DO _OUT is transmitted. Not done.

Thus, 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 does not determine it as a dropout.

(3-2) When a deep dropout occurs When a deep dropout occurs, as described above with reference to FIGS. 6 and 9, the drop amount DR ₂ with respect to the reproduction RF signal is large, for example DR ₂ = 16.5 [ dB],
As described above with reference to FIG. 12, the signal level of the reproduced RF signal is so weak that the limiter circuit 7 cannot detect it, and the FM demodulator circuit 8 cannot demodulate the video signal. .

Boost circuit when this state 15, as described above for Figure 14, compared with intact signal level without boost operation for carrier signal component J ₀ signal S _BO
Is sent to the comparison circuit 16.

However, the drop-out detection reference signal V
_Since a signal level of, for example, -16 [dB] is given as _DO , the comparison output S _{COM of a} constant level (thus, the frequency is 0) is always obtained at the output end of the comparison circuit 16.
Accordingly, the frequency detection circuit 18 performs the detection operation because the comparison output S _COM is below the predetermined frequency (that is, 4 [MHz]),
Thus, the dropout detection circuit 1 outputs the dropout detection output DO _OUT .

(3-3) Sensitivity Adjustment Operation of Boost Circuit 15 In the above operation, the sensitivity switching means 31 provided in the boost circuit 15 of the dropout detection circuit 1 is operated by the upper sideband signal component J ₊₁ and the lower sideband signal component. The operation when the high-sensitivity detection side contact K1 that boosts J _-1 together is described, but if the sensitivity switching means 31 is switched to the boost characteristic K2 side, the dropout detection sensitivity of the dropout detection circuit 1 is reduced. be able to.

That is, the boost curve K2 has a characteristic that the signal level of the lower sideband signal component J ₋₁ is hardly boosted, and instead, the signal level of the carrier signal component J ₀ is boosted. Therefore, when the sensitivity switching 31 is switched to the curve K2 side, the boost circuit 15 boosts the carrier signal component J ₀ and the lower sideband signal component J _-1 among the spectrum signal components included in the comparison signal S _BO. By reversing the ratio, it is possible to weaken the rising ratio of the lower sideband signal component J _-1 to the carrier signal component at the time of dropout occurrence, thereby controlling so as to weaken the degree of disturbance of the waveform of the comparison signal S _BO. be able to.

Thus, the comparison output S _COM in the comparison circuit 16 is switched from the frequency of the carrier signal component J _{0 to} the frequency of the lower sideband signal component J _-1 only when a deeper dropout occurs, and thus, the dropout detection circuit. The detection sensitivity of 1 can be controlled.

(4) Effects of Embodiments With the above-described embodiments, it is possible to accurately measure the change in the frequency after converting the waveform of the RF signal into a frequency signal by utilizing the disturbance. It is possible to detect the occurrence of dropout.

Thus, when the chroma signal is small, the weak waveform disturbance can be used to prevent unnecessary dropout detection for a video signal having a small chroma signal and a large luminance signal.

Further, by boosting the sideband signal component of the chroma signal included in the reproduction RF signal as necessary, the sensitivity of the dropout detection operation of the dropout detection circuit 1 can be increased as necessary. For this reason, the boost circuit 15 of the dropout detection circuit 1 is configured to change and control the relative boost rate with respect to the carrier signal component J ₀ and the sideband signal components J ₊₁ and J _-1 . It is possible to easily realize a dropout detection circuit capable of detecting various dropouts under the optimum conditions by designating a combination condition of the size of the dropout and the depth of the dropout.

(G4) Other Embodiments In the above embodiment, the sensitivity control means 31 is
Although the case where the switching switch for switching between two sensitivities is used has been described, the number of sensitivity switching stages may be two or more. Further, instead of switching the sensitivity stepwise, the sensitivity may be continuously switched by continuously changing the boost characteristic curve of the boost circuit 15.

Further, in the above description, the case where the sideband signal component is boosted for controlling the sensitivity has been described, but the point is to lower the carrier signal component, in other words, the ratio of the sideband signal component to the carrier signal component. Even if the change is controlled, the same effect as the above case can be obtained.

H Effect of the Invention As described above, according to the present invention, the dropout
The frequency of the generated dropout is detected by forming a frequency-converted signal having a corresponding frequency based on the disturbance of the waveform generated in the RF signal, and detecting the change in the frequency to detect the dropout. It is possible to easily realize a dropout detection circuit capable of optimally detecting a dropout in accordance with the type of noise generated in the reproduced video signal in accordance with the conditions of the above, and the magnitude of the chroma signal.

According to the present invention, in particular, according to the present invention, after the disturbance of the waveform of the reproduction RF signal is emphasized by the waveform disturbance emphasizing means, the frequency conversion signal is obtained, so that the detection sensitivity of the dropout is adjusted as necessary. Can be set to a higher value.

[Brief description of drawings]

Fig. 1 is a curve diagram showing the frequency characteristics of a VTR that is the transmission system when normal, Fig. 2 is a curve diagram showing the frequency characteristics of VTR when a shallow dropout occurs, and Fig. 3 is a VTR frequency when a deep dropout occurs. FIG. 4 is a curve diagram showing the characteristics, FIG. 4 is a spectrum characteristic diagram and a signal waveform diagram showing a reproduced signal in a normal state when the chroma signal is large, and FIG. 5 is a reproduced signal when a shallow dropout occurs when the chroma signal is large. Spectrum characteristic diagram and signal waveform diagram, FIG. 6 is a spectrum characteristic diagram and signal waveform diagram showing a reproduced signal when a deep dropout occurs when the chroma signal is large, and FIG. 7 is a reproduced signal when the chroma signal is small and normal Fig. 8 shows the spectrum characteristic diagram and signal waveform diagram, and Fig. 8 shows the reproduced signal when a shallow dropout occurs when the chroma signal is small. Fig. 9 is a spectrum characteristic diagram and signal waveform diagram, Fig. 9 is a spectrum characteristic diagram and signal waveform diagram showing a reproduced signal when a deep dropout occurs when the chroma signal is small, and Fig. 10 is a normal state when the chroma signal is very small. Fig. 11 is a spectrum characteristic diagram and signal waveform diagram showing the reproduced signal in Fig. 11. Fig. 11 is a spectrum characteristic diagram and signal waveform diagram showing the reproduced signal when a shallow dropout occurs when the chroma signal is very small. FIG. 13 is a block diagram showing an embodiment of the dropout detection circuit 1 according to the present invention, FIG. 14 is a block diagram showing the reproduction signal when a shallow dropout occurs when the voltage is very small, and FIG. Fig. 15 is a characteristic curve diagram showing the boost characteristic of the circuit 15. Fig. 15 shows the positive characteristic when the chroma signal is large in the first operation example. 16 is a spectrum characteristic diagram showing the reproduced RF signal at the time of operation, FIG. 16 is a spectrum characteristic diagram showing the output signal of the boost circuit 15 at the time of normal operation in the first operation example, and FIG. 17 is each part at the time of normal operation in the first operation example. Signal waveform diagram showing the signal of
FIG. 18 is a spectrum characteristic diagram showing a reproduced RF signal when a shallow dropout occurs in the first operation example, and FIG. 19 is a spectrum characteristic diagram showing an output signal of the boost circuit 15 when a shallow dropout occurs in the first operation example, FIG. 20 is a signal waveform diagram showing signals of respective parts when a shallow dropout occurs in the first operation example, and FIG. 21 is a signal waveform diagram showing output signal waveforms of the boost circuit 15 in the first operation example. FIG. 23 is a spectrum characteristic diagram showing a reproduction RF signal in a normal state when the chroma signal is very small in the second operation example, and FIG. 23 is a spectrum showing an output signal of the boost circuit 15 in a normal state in the second operation example. FIG. 24 is a signal waveform diagram showing the signal of each part in the second operation example, and FIG. 25 is a shallow dropout occurrence in the second operation example. FIG. 26 is a spectrum characteristic diagram showing a reproduced RF signal in the second operation example, FIG. 26 is a spectrum characteristic diagram showing a boost output signal when a shallow dropout occurs in the second operation example, and FIG. 27 is an output signal of the boost circuit 15 in the second operation example. It is a signal waveform diagram showing the waveform of. 1 ... Dropout detection circuit, 2 ... VTR, 5 ... Reproduction amplifier circuit, 6 ... Reproduction equalizer, 7 ... Limiter circuit, 8 ... FM demodulation circuit, 9 ... Deenmphasis 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. A video signal reproducing apparatus adapted to record an FM-modulated signal obtained by FM-modulating a video signal and demodulate the video signal from a reproduced RF signal obtained as a result of the reproduction. In order to send the increased comparison signal, the waveform disturbance enhancing means for changing the relative signal levels of the carrier signal component and the sideband signal component of the spectrum signal component of the reproduction RF signal, and the waveform of the comparison signal are Comparing means for comparing the waveform of the comparison signal with a dropout detection reference signal of a predetermined level, in order to output a frequency conversion signal that changes based on a point of time when the dropout detection reference signal is crossed; and a frequency of the frequency conversion signal. And a frequency detecting means for transmitting a dropout detecting signal based on the change of the dropout detecting signal. The pullout detection means is configured to send out the dropout detection signal when the frequency of the frequency conversion signal changes due to the occurrence of dropout and the disturbance of the waveform of the reproduction RF signal increases. Video signal playback device. 2. The waveform disturbance emphasizing means comprises sensitivity control means for changing and controlling the ratio of the lower sideband signal component to the carrier signal component in the spectrum signal component of the reproduction RF signal in order to control the detection sensitivity. The video signal reproducing device according to claim 1, characterized in that it has.