JPS6357873B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a dropout detection circuit for a recorded information reproducing apparatus.

Some recorded information reproducing apparatuses reproduce a recorded signal recorded on an information recording medium such as a video tape or a video disc using an FM modulation method as an FMRF signal. This FMRF signal includes
Dropouts, or signal loss, occur due to various causes such as scratches on the surface of the information recording medium and dust adhering to the information recording medium. This dropout is unavoidable, and on this premise, the recorded information reproducing apparatus is equipped with a dropout detection circuit for detecting this dropout.

An example of a conventional dropout detection circuit is shown in the first example.
2, 1 is a comparator, 2 is a retriggerable mono multivibrator, and the FMRF signal has its DC component removed via a coupling capacitor 3, and the positive input of comparator 1 It is configured to be input to the terminal. A threshold voltage E is applied to the positive input terminal of comparator 1, and its negative input terminal is grounded.
The output level of the FMRF signal and the reference level E' of the threshold voltage E are compared as shown in a of FIG. 2, and the rectangular wave shown in b of FIG. This is what is output to the destination. Here, a in FIG. 2 shows the waveform of the FMRF signal input to the comparator 1,
2b shows a rectangular wave as a comparison signal output from the comparator 1, and c in FIG. 2 shows the output waveform of the dropout detection signal output from the retriggerable mono multivibrator 2. be.

If no dropout occurs, the FMRF signal is input to the comparator 1 regularly with an average period, and correspondingly, the comparator 1 outputs a square wave regularly with an average period. The retriggerable mono multivibrator 2 is triggered by the fall of the comparison signal from high level to low level, and its output pulse width is approximately 1.5 times the average period of the FMRF signal. is set to . Therefore, if a dropout as shown by the symbol A in Figure 2a occurs in the FMRF signal, the rectangular wave as a comparison signal will be output from the retriggerable mono multivibrator 2 as shown in Figure 2B. In order to maintain a high level for a period longer than the pulse width, it is triggered by the fall of the comparison signal from high level to low level just before dropout A occurs, and maintains a high level for the set output pulse width. After that, it becomes low level. Here, the symbol B in c in FIG. 3 indicates the delay time from the start point of dropout A. After that, when dropout A disappears, the retriggerable mono multivibrator 2 is triggered by the fall of the comparison signal from high level to low level immediately after its disappearance, and goes from low level to high level, and maintains this level. I will do it. Corresponding to dropout A, the retriggerable mono multivibrator 2 changes from high level to low level, so this can be used as a dropout detection signal.

However, this conventional dropout detection circuit has the following disadvantages.

The conventional dropout detection circuit is a
In order to avoid a problem where, when a dropout as shown by the symbol C occurs, it is determined that there is no dropout even though dropout C has actually occurred due to noise generated near the zero point level Z. Although the reference level is set higher than the zero point level Z by E', the dropout D near the reference level E' as shown in Fig. 3b and the dropout F as shown in Fig. 3d are It is not detected accurately, and only the dropout G as shown in c in Fig. 3 is accurately detected.
Depending on the form of the dropout, it may be difficult to accurately detect the dropout. In particular, as shown by d in Figure 3, when the dropout section straddles the zero point level Z, part F' of the dropout F can be detected, but the remaining part cannot. be.

The present invention has been made in view of the drawbacks of the prior art described above, and an object of the present invention is to provide a dropout detection circuit that can improve dropout detection accuracy.

The configuration of the present invention will be explained below with reference to the drawings.

FIG. 4 shows a circuit diagram of a dropout detection circuit according to the present invention, and FIG. 5 shows a timing chart thereof. In this figure, 4 is a Windke comparator. This window comparator 4 has a comparator 5 and a comparator 6. The positive input terminal of comparator 5 and the negative input terminal of comparator 6 are connected via a coupling capacitor _C1 for DC component removal.
FMRF signal is now input. The center bias of this FMRF signal is set to the zero point level Z by the resistor _R1 , and the negative input terminal of the comparator 5 and the positive input terminal of the comparator 6 have a threshold voltage at the zero point level Z as a border. E and a threshold voltage -E are applied. Resistor R ₂ , resistor R ₃ , and variable resistor VR ₁ constitute a voltage divider circuit for setting this threshold voltage, and its reference level ±E′ is
By adjusting VR ₁ , it can be changed within a predetermined range. Here, the output level of the FMRF signal is kept at a constant peak-to-peak value by an auto gain control circuit (not shown) provided before the window comparator 4. . The comparator 5 outputs an H signal when the output level of the FMRF signal is higher than the reference level E' as shown in a in FIG. 5, and outputs an L signal when it is lower than the reference level E'. The comparator 6 determines that the output level of the FMRF signal is
As shown in FIG. 5a, an H signal is output when the level is lower than the reference level -E', and an L signal is output when the level is higher than the reference level -E'.

The output signals of the comparators 5 and 6 are input to inverter elements 7 and 8, where the output signals of the comparators 5 and 6 are inverted. In FIG. 5, b indicates the inverted output signal of the comparator 5, and c indicates the inverted output signal of the comparator 6. This inverted output signal is input to the R-S flip-flop circuit 9. This R-S flip-flop circuit 9 is
It has a NAND circuit 10 and a NAND circuit 11, and is set when the inverted output signal b falls from a high level to a low level, and is reset when the inverted output signal c falls from a high level to a low level. A rectangular signal shown in d in FIG. 5 is output. This rectangular signal shown in d of FIG.
It is designed to be input to the A terminal of. This retriggerable mono multivibrator 12 has a fifth
It is triggered by the falling of the rectangular signal from high level to low level as shown in d in the figure.Resistor _R4 and capacitor _C2 have the function of setting the output pulse width. ,here,
The output pulse width is set to approximately 1.3 times the average period of the FMRF signal, and when the retriggerable mono multivibrator 12 is not triggered within the time of this output pulse width after being triggered and reaching a high level, the retriggerable mono multivibrator 12 This is a low level. This output pulse width is equal to the input FMRF
Set to less than twice the average period of the signal.

5e shows the waveform of the output signal as a dropout detection signal output from the Q terminal of this retriggerable mono multivibrator 12, and the output signal from this Q terminal is connected to the resistor _R6.
The signal is input to one input terminal of a NAND gate 14 constituting a logic circuit via a delay circuit 13 composed of a diode D ₁ and a capacitor C ₄ .
This delay circuit 13 connects an output signal from a mono multivibrator (described later) and an output signal from a retriggerable mono multivibrator 12 to a NAND gate 14.
This is to prevent whisker pulses from occurring when input to The output signal of the terminal of the retriggerable mono multivibrator 12 is the fifth
The output signal shown in e of the figure is inverted,
The inverted signal of the signal shown at f in FIG. 5 is input to the A terminal of the mono multivibrator 15, and is applied to the rise of the output signal shown at e in FIG. 5 from the low level to the high level. The output pulse width is set by the capacitor _C3 and the resistor _R5 , during which the monomultivibrator 15
is at H level. f in Figure 5 is
This shows the waveform of the output signal output from the terminal of this mono multivibrator 15, and this fifth
The output signal shown at f in the figure is input to another input terminal of the NAND gate 14. The NAND gate 14 becomes high level when the output signal shown in e of FIG. 5 is at high level and the output signal shown in f of FIG. 5 is at low level. The output signal shown in is output as a complete dropout detection signal. Note that in FIG. 4, Vcc indicates the power supply voltage.

The operation of this dropout detection circuit will be explained next.

(b) When a dropout occurs near the reference level E', as shown by the symbol H in the FMRF signal in Figure 5a.

Since the inverted output signal of the comparator 6 remains at a high level for a predetermined period of time as shown in FIG. 5c, the output signal of the R-S flip-flop circuit 9 is as shown in FIG. As a result, the output signal of the NAND gate 14 remains at a high level as shown in g in FIG. 5, and dropout is detected.

(b) When a dropout occurs near the reference level -E', as shown by symbol I in the FMRF signal of a in FIG. 5.

In this case, the inverted output signal of the comparator 5 will maintain a high level for a predetermined period of time as shown in b in FIG. As shown in FIG. 5, the output signal of the NAND gate 14 remains low level only when the output signal shown in e of FIG. 5 is at a high level and the output signal shown in f of FIG. 5 is at a high level. level, and then becomes a high level, and a dropout as shown by reference numeral I in a of FIG. 5 is detected.

(C) A case where dropout occurs in the vicinity of the zero point level Z, as shown by the symbol J in the FMRF signal in FIG. 5a.

In this case, the inverted output signal of the comparator 5 remains at a high level for a predetermined period of time as shown in b in FIG. Although the high level is maintained, the R-S flip-flop circuit 9 is set when the inverted output signal b falls from high level to low level, and when the inverted output signal c falls from high level to low level. Since it is reset at
The output signal No. 4 obtains a logic state in which it becomes a low level only when the output signal shown in e of FIG. 5 is at a high level and the output signal shown in f of FIG. 5 is at a high level. The level becomes high, and a dropout indicated by the symbol J in a of FIG. 5 is detected.

(D) As shown by the symbol K in the FMRF signal of a in FIG. 5, a dropout occurs from near the reference level E', across the zero point level Z, to near the reference level -E'.

In this case, the inverted output signal of the comparator 6 maintains a high level during the interval K', and the inverted output signal of the comparator 5 maintains a high level during the interval K'', as shown in FIG. d, the R-S flip-flop circuit 9 changes from a high level to a low level during this dropout period, and the retriggerable mono multivibrator 12 changes from a high level of the R-S flip-flop circuit 9 to a low level. During this dropout K period, the NAND gate 14 goes from a low level to a high level as shown in FIG. 5e. Since the output signal is at a high level and becomes a low level only when the output signal shown at f in FIG. 5 is at a high level, the NAND gate 14 remains at a high level during the dropout period K. As a result, dropout K can be detected accurately.

Here, this dropout detection signal is
After the FMRF signal dropout begins,
This FMRF signal is output with a delay of about one cycle,
It is configured to end after a predetermined amount of time has elapsed.
It is said that the dropout detection signal is delayed with respect to the point at which a dropout occurs in the FMRF signal, but since a delay occurs in the process of demodulating this FMRF signal as a video signal,
It is possible to make the dropout occurrence point of this FMRF signal coincide with the start point of the dropout detection signal.

As explained above, the present invention provides a window comparator, sets threshold levels on both sides of the zero point level, and detects whether or not dropout has occurred on either side. As a result, the dropout detection accuracy can be improved compared to the conventional method.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a conventional dropout detection circuit, FIG. 2 is a timing chart of the conventional dropout detection circuit, and FIG. 3 is a diagram showing various aspects of dropout to explain the disadvantages of the conventional dropout detection circuit. 4 is a block diagram of a dropout detection circuit according to the present invention, and FIG. 5 is a timing chart thereof. 4...Window comparator, 5, 6...Comparator, 7, 8...Inverter element, 9...R
-S flip-flop circuit, 10, 10... NAND circuit, 12... Retriggerable mono multivibrator, 14... NAND gate, 15... Retriggerable mono multivibrator, C ₁ ... Coupling capacitor, E, -E ...Threshold voltage, Z...Zero point level, E', -E'...Reference level, H, I, J, K...Dropout.

Claims (1)

[Claims] 1 Threshold voltages are set on both sides of the zero point level, and video tape,
played from an information recording medium such as a video disc.
The FMRF signal is input to the window comparator, which compares the signal level of the FMRF signal with the reference level of the threshold voltage and outputs a set signal and a reset signal, respectively. An R-S flip-flop circuit that is turned off when a signal is input, and an R-S flip-flop circuit that is triggered by the output signal of this R-S flip-flop circuit and that is input
a retriggerable mono multivibrator whose output pulse width is set within a range greater than the average cycle of an FMRF signal and smaller than twice the cycle;
A mono multivibrator that is set to a larger output pulse width than the set output pulse width of this retriggerable mono multivibrator and is triggered by the output pulse of the retriggerable mono multivibrator, and a retriggerable mono multivibrator and and a logic circuit for inputting each output pulse of the mono multivibrator to obtain a logic state in which the retriggerable mono multivibrator is triggered and a dropout detection period except for a period in which the mono multivibrator is not triggered. A dropout detection circuit for a recorded information reproducing device, characterized in that: