JPS6215931B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a rotating head type magnetic recording/reproducing apparatus (VTR) that uses a rotating magnetic head to record and reproduce signals sequentially as a recording locus inclined with respect to the longitudinal direction of a magnetic tape. The present invention relates to a drive circuit system for the electromechanical transducer of a VTR that performs tracking control during playback by providing the rotating magnetic head in the electromechanical transducer and forming the electromechanical transducer. helical scan
When playing back a VTR at a tape speed different from the tape speed at which it was recorded, such as double speed, slow motion, still playback, etc., the trajectory of the playback head does not match the recording track, so noise may appear in parts of the image. or the image may have a poor signal-to-noise ratio. In order to eliminate this drawback, a tracking method has been adopted so that the relative positions of the magnetic head and the recording track do not deviate.

As one method, for example, an electro-mechanical transducer is used as a support arm for a magnetic head, and the position of the magnetic head is displaced to track a recording track.

As an electro-mechanical transducer, we use a material that has extremely large piezoelectric and electrostrictive effects, such as barium titanate porcelain, and when an electric effect is applied in a certain direction, which is a characteristic of the material, its shape expands in the direction of the electric field. ,
Taking advantage of the property of shrinking in the direction perpendicular to this, the piezoelectric element plate is polarized, and the oppositely polarized surfaces are bonded together via electrodes to form a so-called bimorph structure.

That is, in FIG. 1A, two piezoelectric element plates 1 are polarized using electrodes on both sides, and a bimorph structure is created in which the two opposite polarity surfaces are glued together, and one end of the piezoelectric element plates 1 is fixed to a rotating body with a fixture 3. At the same time, the other end is equipped with a magnetic head 4, which is placed in contact with the magnetic tape 5, and with the terminal 6 connected to the electrode at the joint part as a reference, the terminal 7 connected to the front and back electrodes is provided with a magnetic head 4. When a potential is applied, the piezoelectric element plate deforms as shown in FIG. 1B, and the magnetic head 4 moves in a direction perpendicular to the direction of rotation of the magnetic head.

FIG. 1C is a schematic diagram of a piezoelectric element having a bimorph structure and a magnetic head attached thereto.

Next, Figure 2 shows the relationship between the playback head trajectories during playback when the tape speed is zero, that is, in a still state, for a track recorded at standard tape speed by a VTR using a two-head helical scan high-density azimuth recording system. This will be explained in .

In FIG. 2, when recording tracks 9A and 9B recorded on the magnetic tape 8 by heads having different azimuth angles are reproduced at the standard recording tape speed, the tracks 10A and 9B are recorded on the magnetic tape 8 as shown by dotted lines.
Since the reproducing head scans each recording track as a scanning locus of 10B, a good reproduction signal can be obtained, but when the tape speed is zero, that is, when still reproduction is performed, the reproducing head locus becomes a solid line 11. In order to reproduce this so that a good reproduction signal can be obtained, for example, A
The head can be left at the starting point, and as it scans, the playback head can be pulled back in the direction of arrow 12;
It is necessary to shift it in the direction 3 and then gradually return it to its original position. The loci of movement of each head A and B at this time are as shown in FIG. In FIG. 3, the vertical axis represents the amount of movement, and the numerical value represents the track pinch. Further, the horizontal axis represents time, and its numerical value represents the number of fields. The solid line is the trajectory, and the dotted line is the return trajectory. Therefore, by applying a potential that causes this locus to be drawn to the piezoelectric bimorph shown in FIG. 1, the reproduced signal can be made perfect. Although the applied potential vs. deviation characteristic has a slight hysteresis characteristic, if the deviation is not that large, it can be considered that there is an almost proportional relationship, so if the relative position of the tape and head is completely constant, then This purpose can be achieved by applying a triangular waveform voltage as shown in FIG. However, when still playback is performed in normal operation, the positional relationship between the tape and the head is not necessarily maintained constant, and therefore it is necessary to automatically track the recording track. A block diagram of a conventional automatic search device for this purpose is shown in FIG. In this figure, an adder 16 uses the output signal of an oscillator 15 that oscillates at a specific frequency (generally 100 to 500 Hz) as a search signal.
The voltage is applied to the piezoelectric element drive circuit 17 through the piezoelectric element 18, vibrates the piezoelectric element 18, and mechanically vibrates the magnetic head 19 attached thereto. If the magnetic head is misaligned from the recording track on the magnetic tape, amplitude fluctuations at the same frequency as the vibration frequency will occur in the magnetic head output signal envelope. Therefore, the magnetic head output signal is amplified by an amplifier 20, detected by a detector 21, and then only this frequency component is separated by a bandpass filter 22, etc., and a synchronous detector 23 performs both detection and polarity determination to smooth the signal. The smoothed error signal is applied to the adder 16 via the circuit 24 and the piezoelectric element 1
8 to shift the position of the magnetic head 19 to track the recording track. Note that 25 is a phase shifter for matching the phase of the reference signal supplied to the synchronous detector with the phase of the envelope fluctuation signal.
On the other hand, a waveform generator 26 generates a synchronous pulse signal separated from the reproduced video signal, a control pulse signal, or a pulse signal in a synchronous relationship with these, for example, in a waveform as shown in FIG.
In addition to this, a waveform synchronized therewith is added to the adder 16 and applied to the piezoelectric element 18 along with the search signal and the detection error signal via the piezoelectric element drive circuit 17.

Note that A in FIG. 3 is a waveform corresponding to the A head, and B is a waveform corresponding to the B head.

In this way, a simple method is common in which the playback head is scanned almost parallel to the recording track using a waveform created in advance, and the remaining track deviation is corrected using a search signal. . The waveform at this time is shown in FIG.

In FIG. 5, A is the output signal waveform of the waveform generator 26, and B is the search signal waveform output from the oscillator 15. When these two signals are added to drive the piezoelectric element, tracking deviation occurs. For example, the reproduced output signal causes fluctuations in the envelope as shown in C. By separating only this fluctuation component, D
Then, perform synchronous detection to obtain E, and smooth it to F.
Signal G is obtained by adding the A and B signals together with this error signal F. This waveform is the voltage applied to the piezoelectric element 18 when tracking is finally corrected.

The above explanation was given for 1 head, but 2
The same is true for the head. Furthermore, the concept is exactly the same for slow motion playback and double speed playback, except that the waveform of the output from the waveform generator 26 changes depending on the tape speed.

However, commonly used electromechanical transducers using piezoelectric elements with a bimorph structure usually have a high dielectric constant, and therefore the capacitance between the electrodes is quite large, reaching tens of thousands of PF depending on the shape. For this reason, with a high impedance drive circuit, the fixed waveform becomes dull and the required amount of movement cannot be obtained, resulting in an increase in the number of circuit elements and power consumption, but it is unavoidable to drive with a low impedance drive circuit. An example of the specific circuit is shown in FIG.

In FIG. 6, a DC-coupled drive waveform (see FIG. 3) is added to the terminal 30, and the transistor 31
The potential across the load resistor 32 is converted into a low impedance by the PNP transistor 33 and the NPN transistor 34 to drive the electromechanical transducer 18 and move the position of the reproducing head 19. If it is driven with high output impedance, the waveform 35 shown by the dotted line in Figure 7 will be obtained, and the necessary waveform 3 will be obtained.
6 required a low output impedance circuit.

If this electromechanical transducer is bent to move the playback head by about ±100 μm, the effective length of the electromechanical transducer is 200 mm and the thickness is 0.5 mm.
A voltage of about 400V is required, and the withstand voltage of ordinary small-signal transistors is about 250V at most, so it is necessary to use high-power transistors or use cascade connections, which has the disadvantage of increasing the size of the circuit elements and increasing the number of elements. .

The present invention eliminates these drawbacks and enables driving even with a high output impedance circuit.

A waveform diagram and a block diagram for explaining the operating principle of the present invention are shown in FIG. 8 and FIG. 9, respectively. A in Figure 8
B is a signal such as a control pulse or a pulse for a reproduction head switching signal which is in a synchronous relationship with the vertical signal of the reproduced video signal, and B is an output signal obtained by applying this signal as a trigger pulse to a flip-flop.
C is a sawtooth waveform (output waveform of the waveform generator 26 in FIG. 4) created using waveform B, and is used as a fixed waveform in still reproduction. D is the polarity inversion of waveform B. When the solid line waveform E, which is the sum of C and D, is supplied to the electromechanical transducer through a high impedance drive circuit, due to the capacitance existing in the conversion element, the waveform at both ends becomes the dotted line waveform E. This becomes the drive waveform of the electromechanical transducer during still playback.

In FIG. 9, a trigger pulse that is in synchronization with the vertical signal is applied from an input terminal 37, and a flip-flop 38 generates a rectangular waveform, which is then converted into a sawtooth waveform by a charging/discharging circuit consisting of a resistor 39 and a capacitor 40, which is necessary for still reproduction. A transistor which supplies a rectangular wave to the base of a transistor 41 constituting a mixing amplifier, and also supplies a rectangular wave to the emitter via a resistor 42, and mixes the two to constitute a high impedance electromechanical transducer drive circuit. 4
Add to the base of step 3. On the other hand, a search signal and an error signal obtained therefrom are supplied from a terminal 44 to cause the reproducing head to on-track.

As described above, according to the present invention, the necessary drive stage for the high-voltage transistor is a simple amplifier, so the circuit becomes simple and the cost becomes low.

Furthermore, since the rectangular waves used to create the sawtooth waveform need only be added by inverting their polarities, the compensation circuit becomes simple.

[Brief explanation of the drawing]

Fig. 1A is a side view showing an example of an electro-mechanical conversion element, B is a side view during the same operation, C is a perspective view of the same, and Fig. 2 shows the locus of the reproducing head to explain auto-tracking. Figures 3A and 3B are voltage waveform diagrams applied to the electro-mechanical conversion element for auto-tracking, Figure 4 is a block diagram showing an example of a conventional tracking device, and Figure 5 is an explanation of the same operation. A waveform diagram, FIG. 6 is an electrical connection diagram showing an example of a conventional electro-mechanical conversion element drive circuit, FIG. 7 is a waveform diagram explaining the same operation, and FIG. 8 is a waveform diagram explaining the operation of the present invention.
FIG. 9 is an electrical wiring diagram showing one embodiment of the present invention. 15... Oscillator, 16... Adder, 17... Piezoelectric element drive circuit, 18... Piezoelectric element, 19... Magnetic head, 20... Amplifier, 21... Detector, 2
2...Band pass filter, 23...Synchronized detector, 24...Smoothing circuit, 25...Transfer device, 26...
...Waveform generator, 38...Flip-flop.

Claims (1)

[Claims] 1 It has an electromechanical transducer with a large capacitance between electrodes, a high impedance drive circuit that applies a drive voltage between the magnetic head and the electrodes provided at the end of the electromechanical transducer, and is synchronized with a vertical synchronization signal. A sawtooth wave is created from a first rectangular wave created from related pulses, and this sawtooth wave and a second rectangular wave obtained by inverting the first rectangular wave are combined and input to the high impedance drive circuit. A magnetic recording/playback device characterized by: