JPH07111767B2 - Magnetic head tracking device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic head tracking device for reproducing a signal recorded in a track on a magnetic tape.

[Outline of Invention]

Using a head position (height) control means such as a bi-moiph plate, one track scanning is performed for correction of +1 bit or -1 bit with respect to tracking correction data in response to an increase or decrease of a tracking error signal obtained from a rotary magnetic head. The head height is corrected in 1-bit steps, and when the tape running speed is less than a predetermined value, correction is performed once for several track scans, so that the average correction amount is 1 step. It is a tracking device in which finer adjustment is performed and the tracking accuracy is improved without increasing the number of quantization steps.

[Conventional technology]

To accurately scan diagonal tracks on magnetic tape,
A VTR that uses a servo pilot signal recorded on each track is known. Servo pilot signal f ₁ ~f ₄
It consists of 4 frequencies and is recorded cyclically in track order. During playback, the crosstalk components (f ₁ and f ₃ or f ₂
The head height is controlled so that the levels of f ₄ ) become equal to each other. An electrostrictive element such as a bimorph plate is used for controlling the head height, and the head height is adjusted by a tracking error signal in the track width direction (direction perpendicular to the scanning direction). When such a tracking servo device is used, the angle difference between the head scanning locus and the track can be corrected during slow or fast reproduction, and a reproduction screen without noise bars can be obtained.

[Problems to be solved by the invention]

Since the tracking error signal is obtained as the difference between the crosstalk levels of the servo pilot signals from both adjacent tracks, the S / N is not good. Therefore, controlling the bimorph plate with such a tracking error signal results in an increase in the number of trace errors. It is possible to use a filter to attenuate noise as a conventional method, but there is a problem that the servo response characteristic deteriorates (frequency characteristic up to about 100Hz is required).

Therefore, a differential quantization (delta modulation) method is used in which the digital control data is changed by ± bit only by looking at the polarity of the tracking error with respect to the track center (the direction in which the error increases or decreases). This method is a kind of non-linear digital filter processing, and the track arrangement direction (longitudinal direction of the tape) is set for each of a plurality of control points on the track.
The spatial filtering process is performed to reduce the effect of noise.

However, in this method, a quantization error occurs at ± 1 bit or less, and therefore, in order to perform highly accurate tracking, the control value of 1
The amplitude of the bimorph plate corresponding to the bit needs to be very small. On the contrary, for fast response,
It is necessary to increase the amplitude (gain) corresponding to 1 bit. For example, the maximum amplitude of the bimorph plate is 120 μm (p-
p), using 8 bits (256 steps) of control data, one step is about 0.5 μm. In this case, at the time of high-speed noiseless reproduction, if the tracking correction amount for one time is set to ± 2 bits, ± 3 bits ...
A relatively large gain (amplitude / 1 step) of m, 1.5 μm, etc. can be easily obtained.

However, it is difficult to obtain a gain of 0.5 μm or less corresponding to 1 bit. Therefore, for example, in slow reproduction, the same track is scanned many times, but with a gain of 0.5 μm, the adjustment amount for one time is too coarse, and stable tracking cannot be performed.

In order to secure the maximum amplitude of 120 μm and to reduce the gain of one step to 0.5 μm or less, the number of bits of control data must be increased to 12 bits, for example. However, this is not a good idea because the cost of the processing system (CPU, D / A, RAM, etc.) becomes high.

In view of this problem, the present invention obtains a sufficiently small gain without increasing the number of bits, thereby expanding the dynamic range (gain variable range) of the servo system,
The purpose is to enable head tracking with high accuracy.

[Means for solving problems]

The magnetic head tracking device of the present invention comprises a control means (bimorph plate 14) for controlling the position of the rotary magnetic head 19 in the width direction of the track 9 and digital control values A _{1 to} A ₈ for each control point in the track longitudinal direction. Memory means to hold
15, D / A conversion means 6 for converting the output of the memory means 15 into an analog value and giving it to the control as a tracking correction signal, and the memory means according to the polarity of the tracking error signal obtained from the magnetic head 10. Correction means (CPU2
0) and above the predetermined tape running speed, the contents of the memory means are corrected by +1 bit or -1 bit for each track scanning, and below the predetermined tape running speed, once per a plurality of track scanning. And a correction interval varying means (CPU 20) for performing the above correction at a ratio of.

[Action]

When the digital control value of the head height is set to, for example, 8 bits, the response amplitude of the control means (bimorph plate) is allocated to 8 bits (256 steps), and the amplitude of one step (minimum gain) is determined. Normally 1 step (± 1 bit)
Is performed for each track scanning, and when the tape speed is less than a certain value, the correction is performed once for several tracks. This results in an average finer height adjustment than the minimum gain.

〔Example〕

FIG. 1 shows a block diagram of essential parts of the tracking device of the embodiment. As shown in the track pattern diagram of FIG. 2, servo pilot signals of four frequencies f _{1 to} f ₄ are cyclically recorded on tracks 9 of the magnetic tape 8. The position of the head is controlled so that the levels of the crosstalk components (f ₂ and f _{4 in} the example of FIG. ₂ ) from the adjacent tracks of the servo pilot signal included in the reproduction signal obtained by the rotary head 10 become equal. Is the principle of tracking servo.

A tracking error called an ATF error is obtained by the detection system shown in FIG. That is, the reproduced pilot signal and the reference pilot signal are multiplied by the multiplier 11 to obtain the beat between the crosstalk component and the reference pilot. In the example of FIG. 2, the crosstalk pilots are f ₂ and f ₄ , the reference pilot is f ₁ , and the beats are f ₁ −f ₂ = f _H and f ₁ −f ₄ = 3f.
It becomes _H. When this crosstalk beat is envelope-detected by the detectors 12a and 12b and both level components are subtracted by the subtractor 13, an ATF error is obtained.

In principle, the ATF error is given to the bimorph plate 14 that cantilevers the head 10 as shown in FIG. 4, and the height of the head 10 is in the track width direction T _W perpendicular to the track longitudinal direction T _L.
Controlled by.

In the dynamic tracking system shown in Fig. 1, first AT
The F error is A / D converted by the A / D converter 1, and the reference value corresponding to the track center is subtracted by the subtracter 2. The ATF error is then subjected to spatial filtering in filter 2 in the tape longitudinal direction and the track longitudinal direction. This filter 3 is a digital filter composed of a CPU and has a non-linear characteristic with variable gain.

The sampling frequency of the A / D converter 1 is, for example, 480 Hz, and eight control points are provided corresponding to the sampling points in the track longitudinal direction as shown in FIG. The A / D converted data is taken into the memory 15 in the CPU, and the tracking correction data is calculated at track intervals based on the data A _{1 to} A ₈ in the memory. That is, assuming that the A / D converted data is AD _i and the reference value of the track center is REF, processing (1) AD _i −REF> 0 ... A _i + 1 → A = 0 ... A _i → A _i <0 …… Performs A _i -1 → A _i . That is, when the error data fetched by the CPU is positive, 1 bit is added to the contents A _i (A _{1 to} A ₈ ) of the memory 15, and when the error data is negative, 1 bit is subtracted.
If there is no tracking error, the correction for A _i is not performed.

The tracking correction data is obtained as AD _i −REF. Further, the correction data of the output of the filter 3 is led to the adder 4 and added to the ramp waveform RAMP created by the ramp generator 5. The ramp waveform has a time width corresponding to the section of the head switching pulse H-SW representing the section in which the head 10 is hitting the tape, and its amplitude is changed according to the reproduction speed information. For example, the amplitude of the ramp waveform is +1 track pitch for double speed reproduction, and -1 track pitch for still reproduction. The amplitude of the ramp waveform is zero during normal reproduction.

The output of the adder 4 is RAMP + A _i −REF (i = 1 to 8) as described above (Process 2), and this output is the D / A converter 6 as the bimorph drive voltage as shown in FIG. , Is given to the bimorph board 14 through the drive circuit 7. By applying the correction signal to the bimorph plate 14, the tracking servo system in FIG. 1 becomes a closed loop, and the servo system is stabilized in the state where the data correction of ± 1 bit for each control point of the track does not occur.

In a two-head machine, the above-described processing is actually performed alternately on channels CH1 and CH2 as shown in FIG. A, c, e, g for control points 0 to 7 of channel CH1
Channel CH for each track scan (field) of
The second control points 8 to 15 are processed in the fields b, d, f ....

At the time of high speed search such as cue / review, the sixth
As shown in the figure, the correction step for each frame in the processing (1) is performed from ± 1 bit to ± 2 bits (3 × speed),
± 3 bits (5x speed) ...

Further, in the case of slow reproduction such as 1/2 or 1/4, the correction system of ± 1 bit may cause one servo correction step to be too coarse and the servo system may not be stable. Therefore, the processing (1) for each frame is thinned out and corrected in accordance with the slow speed. For example, in the case of 1/2 throw, with respect to the channel CH1 of FIG. 7, the correction processing of ± 1 bit is performed only in the fields of a, e ... Every other frame. Channel CH2 as well
With respect to, the correction processing is performed only in the fields of b, f .... As a result, the response gain of the bimorph plate 14 corresponding to the correction of ± 1 bit is reduced to 1/2 on average. To further reduce the gain, the thinning rate may be increased.

It is very easy to perform thinning correction by data processing in the CPU, and no hardware change is required. In addition, as described above, when expressing the maximum amplitude of 120 μm of the bimorph plate 14 with 8 bits (256 steps), the gain is 0.5 μm / step, but without increasing the number of bits by thinning correction.
A fine control gain of 0.5 μm or less can be easily obtained.

In the example shown in FIG. 6, when the tape speed is 1 (standard), the correction step is ± 1 bit / frame, that is, 1
It is ± 1 bit per track scan, but this correspondence may be set optimally according to the system. For example, correction of ± 1 bit / frame may be performed at the double speed, and thinning correction may be performed at a tape speed lower than that.

8 and 9 show a specific tracking circuit,
In FIG. 8, the A / D converted data sampled by the A / D converter 1 is taken into the CPU 20, and the above-mentioned nonlinear filtering processes (1) and (2) are performed, and the processed output (correction signal) is obtained.
After analog conversion by the D / A converter 6, it is given to the bimorph plates 14a and 14b through the sample and hold circuits 6a and 6b of each channel and the drive circuits 7a and 7b.

In FIG. 9, the comparator 19, the CPU 20 and the D / A converter 6
An A / D converter is configured, and the D / A converter 6 also serves as a converter for deriving the corrected data after processing. Other configurations are the same as those in FIG. The CPU 20 continues to send the digital value to the comparator 19 through the D / A converter 6 until it coincides with the ATF error, and captures the digital value when the coincidence is detected by the comparator 19 as A / D conversion data. CPU this data
It is processed in 20 and is derived to each channel as tracking correction data through the D / A converter 6.

〔The invention's effect〕

As described above, the present invention corrects the digital correction data by ± 1 bit for each track scanning according to the polarity of the tracking error, and when the tape speed is less than a certain value, the correction intervals are thinned to be equivalently 1 on average. Since a fine correction amount such as / 2 bit, 1/4 bit, etc. is obtained, the dynamic range of head height control (minimum correction amount without increasing the number of quantization steps (number of bits)) And the maximum correction amount) can be increased.

Further, since the correction interval can be simply changed by data processing, it is possible to configure a head tracking system with low cost and high accuracy without increasing the burden on hardware.

[Brief description of drawings]

FIG. 1 is a block circuit diagram of a servo system showing an embodiment of a magnetic head tracking device of the present invention, FIG. 2 is a track pattern diagram on a magnetic tape, and FIG. 3 is a block circuit diagram of a tracking error detection system. FIG. 4 is a schematic diagram of a bimorph head, FIG. 5 is a waveform diagram showing an operation of tracking correction, FIG. 6 is a graph of error correction, FIG. 7 is a waveform diagram showing driving of a two-channel bimorph head, and FIG. FIG. 9 and FIG. 9 are block circuit diagrams showing specific examples of the tracking circuit. In the symbols used in the drawings, 1 ... A / D converter 2 ... Subtractor 3 ... Filter 4 ... Adder 5 ... Ramp generator 6 ... D / A converter 7 ... Drive circuit 8 ...... Magnetic tape 9 ...... Track 10 ...... Rotating head 12a, 12b ...... Detector 13 ...... Subtractor 14 ...... Bimorph board 15 ...... Memory 19 ...... Comparator 20 ...... CPU.

Claims (1)

[Claims] 1. A control means for controlling the position of a rotary magnetic head in the track width direction, a memory means for holding a digital control value for each control point in the track longitudinal direction, and an output of this memory means converted into an analog value. D / A conversion means to be given to the control means as a tracking correction signal, and correction means for correcting the contents of the memory means by +1 bit or -1 bit in accordance with the polarity of the tracking error signal obtained from the magnetic head. Above the predetermined tape running speed, the contents of the memory means are corrected by +1 bit or -1 bit for each track scanning, and below the predetermined tape running speed, once every plural track scanning. A tracking device for a magnetic head, comprising: a correction interval varying means for performing the above correction.