JPS6339195B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video tape recorder (hereinafter abbreviated as VTR) in which a playback video head is mounted on a head drive element such as a piezoelectric element, and the playback video head can be driven in the direction of the rotational axis of a cylinder. ) relates to an automatic tracking device that enables safe automatic tracking operation by performing automatic tracking operation only when the voltage applied to the head drive element during variable speed reproduction is within a predetermined range.

In the case of so-called variable speed playback in which a magnetic tape is played back at a speed different from the recording speed, the scanning locus of the playback video head (hereinafter referred to as playback head) has a slope different from that of the recording track. This situation is shown in FIG. In Figure 1, G ₁ is the scanning trajectory in still mode, G ₂ is the scanning trajectory in double speed mode, and A
is the recording track. The following method has been reported as a method for correcting these different inclinations.

That is, this method detects the difference in inclination between the recording track and the scanning locus of the reproducing head based on the tape running direction and running speed, generates a triangular wave according to the difference in inclination, and applies the triangular wave to the head drive element. This makes it possible to make the inclination of the recording track and the inclination of the scanning locus of the reproducing head the same even in variable speed reproduction. This is schematically shown in FIG. In Fig. 2, a is a head switch signal synchronized with the rotation of the cylinder, b is a still mode,
c shows the waveform at 1x speed, d shows the waveform at 2x speed, and e shows the waveform at 3x speed. Here, the dotted line portion indicates the portion where the reproducing head is not in contact with the tape, and may have any waveform.

A circuit that generates such a waveform for variable speed playback has already been reported, but this waveform is used to correct the deviation in the slope of the recording track and the reproducing head scanning locus, and is designed to ensure that the reproducing head and recording track are in good condition. It doesn't necessarily mean tracking. In other words, the deviation of the X value is not guaranteed at all. Therefore, in order to achieve good tracking, the above-mentioned waveform generation circuit for variable speed reproduction and an automatic tracking circuit for X value correction are required. That is, in variable speed reproduction, good tracking can be achieved by combining the signal from the variable speed reproduction waveform generation circuit and the signal from the automatic tracking circuit and applying this combined signal to the head drive element.

However, the following problem occurs here. Piezoelectric elements such as piezo elements used in head drive elements generally change non-linearly with applied voltage, and this tendency becomes particularly pronounced when a high voltage is applied. This means that the higher the voltage applied to the head drive element, the greater the amount of deviation between the reproducing head and the recording track. Further, if a change in sensitivity occurs due to aging of the piezoelectric element, this tendency becomes even more pronounced, causing the automatic tracking operation to malfunction.

Next, the operation of the automatic tracking circuit and the reason for its malfunction will be specifically described. Assume now that the VTR is playing back at the same tape speed as when recording. When an applied voltage is applied to the head drive element such that the voltage increases stepwise in each frame, as shown in FIG. It is easy to understand that the displacement with respect to the recording track causes the reproduced signal output to change as shown in FIG. 3d. Here, the head switch signal (hereinafter referred to as HSW) shown in Figure 3a is a 30Hz square wave synchronized with the rotation of the cylinder, and when HSW is at a high level, the playback head comes into contact with the tape and plays the tape. period. In addition, in FIG. 3c, the shaded area indicates the area where the reproducing head 1 scans and reproduces the recording track of the same azimuth, and F ₁ ,
F ₂ ,...F ₈ are the first frame, second frame...
It represents the 8th frame. Therefore, it is necessary to constantly control the applied voltage so that the reproduced high frequency signal (hereinafter referred to as RF signal) output from the VTR is maximized, and this is achieved as follows.

First, after sampling and holding the detected output of the RF signal (hereinafter referred to as envelope), the voltage applied to the head drive element is changed, for example, increased.
Compare with the envelope of the next frame.

If the envelope of the next frame is large, the applied voltage is further increased and compared with the next envelope. That is, the applied voltage is changed in the same direction until the envelope decreases compared to the previous frame. If the envelope of the rear frame is smaller than the envelope of the front frame, the direction of displacement of the voltage applied to the head drive element may be reversed.

Figure 4a schematically shows the relationship between the applied voltage applied to the head drive element and the envelope level, and Figure 4b shows the change in applied voltage for each frame during operation of this device. So, Figure 4c
is a schematic representation of the change in the envelope level at that time. The envelope increases from the 1st frame to the 5th frame, and the applied voltage also increases monotonically, but the envelope decreases in the 6th frame, and accordingly, the direction of change in the applied voltage in the 7th frame is reversed and decreases. . As a result, the envelope of the seventh frame increases again. 8th
When the envelope decreases again in the frame, the direction of change in the applied voltage also reverses accordingly. As can be seen from the fifth frame, the applied voltage reaches a stable point while changing slightly around the voltage at which the envelope is maximum. This means that the tracking condition is the best.

In such automatic tracking operation,
The amount of variation in the envelope due to a one-step change in the applied voltage needs to be detected in units of one frame. However, a shift occurs between each frame due to a change in the sensitivity of the applied element, and this shift appears as a variation in the envelope. If this variation exceeds the variation in the envelope due to a one-step change in the applied voltage, the automatic tracking operation is disabled. This will cause a lot of malfunction. In other words, in such an automatic tracking circuit, the amount of change in one step of the applied voltage is
The amount of deviation needs to be larger than the amount of deviation due to blackening of the sensitivity of the piezoelectric element.

FIG. 5 shows the applied voltage during slow motion reproduction. Frame f ₁ scanning the same track
~ f ₃ , the applied voltage also shifts as the tape moves,
At the time of track update (f ₄ ), the applied voltage changes significantly (point P) and the track moves to the next track. Here, the waveform indicated by a dotted line indicates a portion where the head is not in contact with the tape. In this way, when the applied voltage changes, the amount of sensitivity deviation due to the difference in applied voltage between adjacent frames is reduced to 1 in the automatic tracking circuit.
This means that it needs to be smaller than the amount of change in steps. This point will be explained more specifically.

Let us now assume that the recording track interval recorded by the same head is lμ, the amount of one-step change by the automatic tracking circuit is Sμ, and the amount of sensitivity deviation of the piezoelectric element is k%. At this time, in order to prevent the automatic tracking circuit from malfunctioning when updating the track, the following equation must hold.

S>1/100kl Here, for example, if l=120 and S=3, then k<
2.5, and a sensitivity deviation of 2.5% or more will cause a malfunction. This poses a big problem when performing automatic tracking operations.

The present invention solves the above problem by operating the tracking circuit when the voltage applied to the head drive element is within a predetermined range, thereby achieving stable tracking without being affected by the sensitivity deviation of the element. The main point is to realize the action.

That is, since the amount of sensitivity deviation between frames depends on the difference in applied voltage between frames, by operating the automatic tracking circuit only when a constant applied voltage is applied, the sensitivity deviation is not affected. Therefore, by operating the automatic tracking circuit when the applied voltage enters a certain minute range (assumed to be α%), the influence of the sensitivity shift can be extremely reduced. In other words, if the tracking operation is to be performed when the applied voltage falls within the range of α% of the total applied voltage, the following equation should hold.

S>1/100・1/100k・l・α Here, if l=120, s=3, and k=10, then α<25. That is, even if a sensitivity change occurs by 10%, malfunctions can be prevented by performing the tracking operation within a range of 25% of the applied voltage. This is shown in FIG.

FIGS. 6a and 6b show the applied voltage, the applied voltage range B to C in which the tracking operation is performed, and the output from the automatic tracking circuit. However, it is assumed that the automatic tracking circuit operates when approximately the center point of one field (indicated by a dot in FIG. 6a) falls within the applied voltage range α%. Looking at this, it can be seen that the automatic tracking circuit operates only when almost the same applied voltage is applied.

Next, a block diagram of essential parts of an embodiment of the present invention is shown in FIG. The outputs from the variable speed playback waveform generation circuit 2 and the automatic tracking circuit 3 are combined and output to the VTR.
1 to the head driving element 5. The applied voltage detection circuit 4, which receives the applied voltage as input, operates the automatic tracking circuit 3 when the applied voltage is within a predetermined range, and does not operate the automatic tracking circuit 3 when the applied voltage is outside the predetermined range. Control as follows.

An example of the circuit configuration of the applied voltage detection circuit 4 is shown in FIGS. 8A and 8B. In FIG. 8A, an applied voltage is inputted from the input terminal 6, and an output that is window-compared with the reference voltage signals s ₁ and s ₂ is outputted from the output terminal 7. Here, the operational amplifiers 8 and 9 are open collectors. It is desirable for the applied voltage detection circuit to output a signal when no high voltage is applied to the piezoelectric element due to the characteristics of the element, and the reference potentials s ₁ and s ₂ are set near OV. The reference potential difference e ₁ determines the applied voltage width.

FIG. 8B shows another example of the applied voltage detection circuit.
The applied voltage is input through the input terminal 10, and the resistor 11,
After passing through a low-pass filter consisting of a capacitor 12 and receiving it at a voltage follower consisting of an operational amplifier 15, it is connected to two comparators 1.
A window comparator composed of 6 and 17 compares the signal with the input signal and outputs it from the output terminal 18. Here, the comparators 16 and 17 are open collectors, and _e2 is a reference potential difference. That is, when an input signal enters between the approximate average value _s3 of the input signal and _s4 having a predetermined potential difference _e2 from _s3 , the output terminal 18 outputs "H level". An example of this is shown in FIGS. 9a and 9b. Note that the resistors 13 and 14 in FIG. 8 are merely used to divide the input voltage in order to allow the window comparator to operate within an optimal range, and may be omitted.

Even when a circuit other than the above-mentioned circuit is used as an automatic tracking circuit, more stable automatic tracking operation can be performed by adopting the present invention. For example, by forcing the reproducing head to vibrate at a certain reference frequency in a direction perpendicular to the rotational direction of the cylinder,
A method has been reported in which the tracking state of the playback head and recording track is detected by comparing the phase of the envelope of the RF signal being played at that time with the above reference frequency, and the playback head is displaced in a direction for better tracking. has been done. Even in such automatic tracking circuits, it is inevitable that malfunctions occur due to sensitivity shifts due to changes in applied voltage during track updating, and as shown in the present invention, tracking is performed only when the applied voltage is within a predetermined range. By performing an automatic tracking operation at the same time, a more stable tracking operation can be performed.

As described above, according to the present invention, there is provided a waveform generation circuit for variable speed playback which inputs a playback signal from a magnetic tape and generates a waveform signal corresponding to the scanning direction and scanning speed of the magnetic tape based on the playback signal; an automatic tracking circuit that inputs the reproduction signal and generates a predetermined automatic tracking voltage signal so that the reproduction head scans the center line of the magnetic tape based on the reproduction signal; and a waveform of the variable speed reproduction waveform generation circuit. A head driving element to which an applied voltage for head movement created from a composite signal of a signal and a voltage signal of the automatic tracking circuit is applied, and an applied voltage detection circuit to detect the applied voltage, the applied voltage being When within the specified range,
By providing a control means for operating the automatic tracking circuit based on a signal from the applied voltage detection circuit, it is possible to prevent the head driving element from operating in a range where the displacement becomes non-linear with respect to the applied voltage. Therefore, the automatic tracking circuit does not malfunction, and the tracking state of the playback head can be significantly stabilized, making it possible to realize a video tape recorder that can obtain stable images during variable speed playback. .

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the recording track and head scanning locus during variable speed playback, Fig. 2 a, b, c, d, and e are waveform diagrams showing examples of output from the waveform generation circuit for variable speed playback, and Fig. 3 Figures a, b, c, and d are diagrams showing the relationship between the displacement of the reproducing head due to the applied voltage and the RF signal at that time. Figures 5a, b, and c are waveform diagrams showing examples of output from the variable speed reproduction waveform generation circuit during slow motion, Figures 6a and b are diagrams showing tracking operation in the applied voltage range, and Figure 7 is A main part block diagram of an embodiment of the present invention, FIGS. 8A and 8B are circuit diagrams showing an example of the configuration of an applied voltage detection circuit, and FIGS. 9A and 9B are waveforms showing an example of the output of the applied voltage detection circuit. It is a diagram. 1... VTR, 2... Waveform generation circuit for variable speed playback,
3... Automatic tracking circuit, 4... Applied voltage detection circuit, 5... Head drive element.

Claims (1)

[Scope of Claims] 1. A playback head for scanning a magnetic tape to obtain a playback signal from information on the magnetic tape, and inputting the playback signal and controlling the scanning direction and scanning of the magnetic tape based on the playback signal. a waveform generation circuit for variable speed reproduction that generates a waveform signal corresponding to the speed; and a predetermined automatic tracking voltage that inputs the reproduction signal and causes the reproduction head to scan the center line of the magnetic tape based on the reproduction signal. an automatic tracking circuit that generates a signal; a head drive element to which an applied voltage for head movement created from a composite signal of the waveform signal of the variable speed reproduction waveform generation circuit and the voltage signal of the automatic tracking circuit is applied;
an applied voltage detection circuit that detects the applied voltage, and when the applied voltage is within a predetermined range,
An automatic tracking device comprising: a control means for operating the automatic tracking circuit based on a signal from the applied voltage detection circuit. 2. The automatic tracking device according to claim 1, wherein the applied voltage detection circuit includes two reference voltage sources and a comparator. 3. The automatic tracking device as set forth in claim 1, wherein the applied voltage detection circuit is configured to include a low-pass filter and a comparator, and outputs a signal whose information is the average value of the applied voltage.