GB1580008A

GB1580008A - Automatic head tracking system

Info

Publication number: GB1580008A
Application number: GB23554/78A
Authority: GB
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1977-06-04
Filing date: 1978-05-26
Publication date: 1980-11-26
Also published as: SE7806528L; FR2393391B1; DK147867B; DE2824277C2; AU3670078A; AU515299B2; ES470451A1; NL193155C; FR2393391A1; NL7806112A; AT376535B; DE2824277A1; NL193155B; SE438216B; ATA404178A; IT7824177A0; BR7803529A; DK247678A; DK147867C; CH634433A5

Abstract

During the reproduction of signals, a magnetic head (1) is moved along the recording track. An electrically deflectable securing means (2) for the magnetic head is fed an electric signal for deflecting the magnetic head. The deflecting signal is generated in a deflecting-signal generator (11, 12) according to the deflection of the magnetic head with respect to a position of rest. An envelope detector (5) serves for detecting the envelope of the signal which is emitted by the magnetic head. With a multiplying circuit (22), the detected envelope is demodulated synchronously with the deflecting signal, in order to obtain from it the tracking-error signal, which represents the deflection from the position of rest. In the adding circuit (8), the tracking-error signal and the deflecting signal are added. <IMAGE>

Description

(54) AUTOMATIC HEAD TRACKING SYSTEM (71) We, SONY CORPORATION, a Corporation organised under the laws of Japan of 7-35 Kitashinagawa, 6-chome, Shinagawa-ku, Tokyo, Japan do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates generally to apparatus for reproducing video or other information signals recorded in a track on a record medium, for example, as in a video tape recorder in which the video signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape. More particularly, the invention is directed to an improved system or arrangement by which a magnetic head or other transducer is made to accurately track or scan the track or tracks in which the video or other information signals are recorded.

It is conventional to record video or other information signals along successive parallel tracks which are skewed or extend obliquely on a magnetic tape, for example, as in a helical scant VTR. It has been proposed, for example, as disclosed in (U.S. No. 4151570) to provide a system by which an information or data signal transducer, for example, in the form of a rotary magnetic head, is continuously maintained in a desired position in respect to the recorded information signal track or tracks on a magnetic tape in a helical scan VTR. In such previously proposed system, the position of the information signal transducer or head relative to the track is monitored during the scanning of the track through the reproduction of the recorded information signals while a small oscillatory motion or dither is imparted to the transducer or head via its supporting element or arm which is, for example, in the form of a piezo-electric bender element or bi-morph leaf. The oscillatory motion or dither is induced in the supporting element or arm by applying to the latter a suitable drive signal which causes the transducer to fluctuate or oscillate transversely about its normal scanning path. The oscillation of the transducer introduces deviations in the envelope of the reproduced information signals resulting from the scanning of the record track. Such deviations take the form of an amplitude modulation of the envelope of the reproduced signals, with the change in magnitude of the envelope being representative of the amount of transverse displacement of the transducer from the optimum transducing or centered position with respect to the track, and with the direction of transverse displacement of the transducer from the optimum transducing position being represented by the phase of the envelope amplitude modulation at the fundamental frequency of the oscillatory motion or dither.

To obtain such transducer or head position information, the modulated RF envelope signal reproduced by the transducer is applied to an amplitude modulation envelope detector which recovers the dither signal fundamental and its sidebands, whereupon the output of the envelope detector is applied to a synchronous amplitude modulation detector which detects the amplitude and polarity of the output of the envelope detector with reference to the original or constant dither or oscillation signal by which the head is simultaneously made to oscillate transversely. The synchronous amplitude modulation detector provides a tracking error signal which is added to the dither or oscillation signal to provide the drive signal for effecting oscillation of the head or transducer. Generally, the amplitude of the tracking error signal is proportional to the transverse distance from the null position of the oscillated head to the track centre while the polarity of the tracking error signal is indicative of the direction of such displace ment of the null position from the track centre. Therefore, the tracking error signal, when added to the dither or oscillation signal, tends to align the null position of the transducer with the centre of the track.

It will be apparent that, in the aboveproposed automatic head tracking system, the output of the envelope detector contains various unwanted frequency components due to mechanical vibration of the bi-morph leaf supporting the head and such unwanted frequency components adversely affect the accuracy of the tracking error signal obtained when the output of the envelope detector is compared with the constant dither or oscillation signal in the synchronous amplitude modulation detector.

The existence of problems due to mechan ical vibration of the bi-morph leaf supporting the reproducing head or transducer has been recognized, for example, in U.S. Patent No.

4,080,636. In the system disclosed in such patent, the output of the reproducing head or transducer is processed in the manner described above, that is, such output is envelope-detected and then compared with the constant dither or oscillation signal which is applied to the bi-morph leaf so as to obtain the tracking error signal which is ultimately added to the dither or oscillation signal for providing the drive signal of the bl-morph leaf. In addition to the foregoing, the aboveidentified patent discloses a negative feedback loop for developing an electrical damping signal which is also applied to the bimorph leaf so as to dampen its vibrations or oscillations, particularly at the resonance frequency thereof. In the system being described, the electrical damping signal is derived from a signal generator or sensor which is integral with the bi-morph leaf for generating a signal which is representative of the instantaneous deflected position of the transducer or head, and which is converted to a transducer velocity signal by means of a differentiator. The transducer velocity signal is then passed through a low pass filter which attenuates the signals attributable to second and higher order resonance characteristics of the bi-morph leaf, and which is followed by a phase lead network operative to shift the phase of signals received from the filter so that those signals having a frequency near the resonance frequency of the bi-morph leaf will have a net phase shift of 00. Finally, the output of the phase lead network is applied to an inverting or negative feedback amplifier so as to obtain the damping signal which is added to the previously-described drive signal. Thus, the signal representative of the instantaneous deflected position of the head or transducer as obtained from the signal generator or sensor integral with the bimorph leaf is only employed for producing the feedback or electrical damping signal by which mechanical vibration of the bi-morph leaf at its resonance frequency is damped.

However, such damping action does not elimate or correct inaccuracies appearing in the tracking error signal by reason of the fact that the latter is still derived from a comparison of the fixed dither oscillation signal with the envelope detected output of the reproducing head or transducer which contains mechani cally induced vibrations or other undesired frequency components.

According to the present invention, there is provided an apparatus for reproducing information signals recorded in a track on a record medium comprising a transducer movable along said track for reproducing the information signal recorded therein, a deflectable mounting for said transducer which is operative in response to the reception of an electrical drive signal for deflecting said transducer in a direction which is transverse in respect to the direction along said track, and a control circuit including an oscillator for providing a dither oscillation signal which, on application to said deflectable mounting for the transducer, causes said transducer to oscillate in said transverse direction about a null position, an envelope detector for detecting the envelope of the output of said transducer as the latter moves along the track and oscillates in said transverse direction, a device for synchronously demodulating the detected envelope from said envelope detector by means of a signal having a frequency component of the dither oscillation signal frequency so as to obtain a tracking error signal representative of the deviation of said null position of the transducer from the centre of the track considered in said transverse direction, and an adding circuit adding said tracking error signal to said dither oscillation signal so as to provide therefrom said electrical drive signal; characterized in that a deflection signal generator is provided for generating a deflection signal in correspondence to the deflection of said transducer from a rest position, and said device synchronously demodulates the detected envelope from the envelope detector with reference to said deflection signal so that said drive signal from the adding circuit can accurately maintain the null position of the transducer in alignment with the centre of the track.

It will be noted that the tracking error signal is derived by a comparison of the envelope detected output of the reproducing transducer which includes frequency components resulting from mechanical vibration of the bi-morph leaf or other support for the transducer. with the deflection signal which similarly includes such frequency components due to mechanical vibration of the bi-morph leaf, so that the unwanted frequency components due to mechanical vibra tion or the like are automatically eliminated from the tracking error signal by cancelling each other.

In one embodiment of the invention, the means by which the deflection signal is generated in correspondence to the deflection of the transducer includes a capacitor defined by a fixed electrode and by a movable electrode on the bi-morph leaf or other support of the transducer so that the capacitance of such capacitor is varied with displacement of the transducer in the transverse direction, and an amplifier circuit having a high input impedance and being connected with the variable capacitor to provide the desired deflection signal in correspondence to changes in the capacitance resulting from displacement of the movable electrode in respect to the fixed electrode.

In another embodiment of the invention, for generation of the deflection signal there is provided a strain gauge secured on the bimorph leaf to be stressed in response to deflection of the transducer, and a circuit by which the resulting change in resistance of the strain gauge is converted to a corresponding voltage change of the output or deflection signal.

The invention will be further described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic block diagram illustrating an automatic head tracking system according to the prior art; Fig. 2 is a schematic view illustrating a section of a magnetic tape with a record track extending obliquely thereon, and with the path of a reproducing head or transducer being represented in broken lines; Fig. 3 is a graphic representation of the output signal from a reproducing head or transducer when scanning the record track in the manner shown on Fig. 2; Fig. 4 is a diagrammatic view illustrating an automatic head tracking system according to an embodiment of this invention; Figs. 5A to SE are waveforms to which reference will be made in explaining the operation of the system on Fig. 4; Fig. 6 is a diagrammatic view illustrating an automatic head tracking system according to another embodiment of the invention; and Fig. 7 is a circuit diagram showing details of a portion of the system illustrated on Fig.

6.

Referring to the drawings in detail, and initially to Fig. 1 thereof, it will be seen that, in an existing automatic head tracking system of the type disclosed in the previously mentioned U.S. Patent No. 4151570 each rotary magnetic head 1 of a helical scan VTR is mounted on a rotary portion of the usual guide drum (not shown) by way of a bimorph leaf 2 which can flex in the axial direction of the guide drum. As is conventional, the video or other information signals are recorded in successive parallel tracks which are skewed or extend obliquely on a magnetic tape T, for example, as is indicated in respect to the single record track t on Fig. 2.

Therefore, when the recorded magnetic tape T is guided in a helical path about a substantial portion of the periphery of the guide drum, the rotary magnetic head 1 may scan more or less along the record track t.

In the existing head tracking system of Fig.

1 for continuously maintaining the rotary head 1 in a desired position in respect to the record track t, the position of head 1 is monitored during the scanning of the track through evaluation of the reproduced signals Sa from head 1 while a small oscillatory motion or dither is imparted to head 1 by applying a suitable drive signal Sf to the bi-morph leaf 2. The resulting oscillation of the head 1 in the direction transverse to the direction along the track t, as indicated in broken lines at 1a on Fig. 2, introduces deviations in the envelope of the reproduced video or other information signal Sa (Fig. 3) resulting from the scanning of record track by head 1. Such deviations take the form of amplitude modulation of the envelope of the reproduced signals Sa, with the change in magnitude of the envelope being representative of the amount of transverse displacement of the head 1 from the optimum transducing or centered position with respect to the track t, and with the direction of the transverse displacement of the head from the optimum transducing position being represented by the phase of the envelope amplitude modulation at the fundamental frequency of the oscillatory motion or dither.

In order to obtain the above-described head position information, the existing head tracking system shown on Fig. 1 applies the reproduced signals Sa from head 1 through a reproducing amplifier 3 to an output circuit 4 which processes the reproduced video signal and to an amplitude modulation envelope detector 5 which recovers the dither signal fundamental and its sidebands. Thereupon, the output Sb of envelope detector 5 is applied to a synchronous amplitude modulation detector 7 which detects the amplitude and polarity of the output Sb of the envelope detector with reference to the original or constant dither or oscillation signal Sc by which the head 1 is made to oscillate trans- versely. The synchronous amplitude modulation detector 7 provides a tracking error signal Sd which is applied to an adding circuit 8 for addition therein to the dither or oscillation signal Sc having a frequency fc of about 450 Hz. The resulting added signal Se is acted upon by a driving amplifier 9 to provide the driving signal Sf for the bi-morph leaf 2.

Generally, the amplitude of the tracking error signal Sd is proportional to the trans verse distance from the null position of the oscillated head 1 to the centre of the scanned track t, while the polarity of tracking error signal Sd iS indicative of the direction of such displacement of the null position from the track centre. Therefore, the tracking error signal Sd, when added to the dither or oscillation signal Sc, tends to align the null position of the oscillated head 1 with the centre of the track t. However, it will be apparent that, in the system illustrated on Fig. 1, any mechanical vibration of the bi-morph leaf 2 supporting head 1 will cause the output of the envelope detector 5 to contain various unwanted frequency components. When such output Sb of the envelope detector 5 is compared, in synchronous amplitude modulation detector 7, with the constant dither or oscillation signal Sc from fixed oscillator 6 which, of course, does not include the unwanted frequency components due to mechanical vibration, the tracking error signal Sd obtained from the modulation detector 7 is influenced by the unwanted frequency components and the accuracy of tracking is adversely affected.

Referring now to Fig. 4, in which parts corresponding to those described above with reference to Fig. 1 are identified by the same reference numerals it will be seen that, in an automatic head tracking system 10 according to the present invention, the deflection of head 1 from its rest position as determined by the bi-morph leaf 2 supporting the head is instantaneously and accurately detected by a deflection signal generating circuit 11. In the illustrated embodiment, the circuit 11 is shown to include a deflection detecting transducer in the form of a capacitor 12. The capacitive transducer or capacitor 12 is constituted by a fixed electrode 12a suitably mounted adjacent the free end portion of bi-morph leaf 2 and a movable electrode 12b mounted on leaf 2 so as to confront fixed electrode 12a and thereby vary the capacitance of transducer 12 with displacement of head 1 in the direction transverse to the length of the record track being scanned by the head. More particularly, the capacitance of transducer 12 varies inversely with the distance between fixed electrode 1 2a and movable electrode 1 2b which is connected to ground. In a practical example of this invention, the electrode 12a is a square with a dimension of 3 to 4 mm. along each side. and the spacing between electrodes 12a and 12b ranges between 0.2 and 0.5 mm.

The circuit 11 is shown to further comprise an amplifier with a high input impedance for converting changes in the capacitance of transducer 12 into a corresponding voltage signal which varies suitably to directly and instantaneously represent the deflection of head 1 from its rest position. More particularly, in the illustrated embodiment of the invention, circuit 11 is shown to include resistors 13 and 14 connected in series between fixed electrode 12a and the gate of a junction FET 15 acting as an amplifier and having its source-drain circuit connected to power supply terminals Vi and V2 through an output resistor 16. A second junction FET 17 intended to serve as a high resistance, for example, of more than 10. MII, has one of its source and drain electrodes connected to a junction between resistors 13 and 14, while the other of its drain and source electrodes is an open circuit and the gate of FET 17 is connected to the power supply terminal V2.

The output signal across resistor 16 is applied through a capacitor 18 to a band pass filter 19. An electrical shield 20 envelopes amplifier 11 for electrical shielding the latter from the electric field resulting from the application of the driving signal to the piezo-ceramic support arm or bi-morph leaf 2 on which head 1 is mounted.

As in the known head tracking system described above with reference to Fig. 1, in the system 10 according to the present invention, the reproduced frequency modulated signal or output Sa of head 1 is applied through a reproducing amplifier 3 to an output circuit 4 and to an envelope detecting circuit 5. Once again, the signal Sa is amplitude-modulated with a dither or oscillation signal Sc of the fixed frequency fc produced by an oscillator 6 and which may be about 450 Hz. The amplitude variations of the output or envelope signal Sb (Fig. 5A) represent, among other things, tracking errors which exist between the path of movement of head 1 and the record tracks t being scanned by the head. However, the envelope signal Sb also includes unwanted frequency components besides the tracking error information at the dither or oscillation signal frequencies Fc, such as, for example, frequency components at the primary and secondary resonant frequencies and at the anti-resonant frequency of bi-morph leaf 2 and various other frequency components due to transient responses. Such unwanted frequency components adversely affect detection of, and correction for the tracking error between the scanning path of head 1 and the record track t on the magnetic tape.

The envelope signal Sb from envelope detector 5 is applied to a band pass filter 21 which is adapted to pass a frequency band centered at the dither or oscillation frequency fc and which contains the tracking error information. More particularly, the upper cut-off frequency of band pass filter 21 is selected to be somewhat lower than the frequency 2fc, while the frequency fc is several times greater than the selected lower cut-off frequency of filter 21. The resulting output signal Sb' from band pass filter 21 is applied to one input of a multiplier 22 which may be constituted by a balanced modulator and which has another input receiving the output Sg' of band pass filter 19.

Band pass filter 19 has substantially the same characteristic as is described above for band pass filter 21, that is, filter 19 is adapted to pass a band of frequencies centered at the dither or oscillating signal frequency fc. The deflection signal Sg (Fig. 5B) from amplifier 11 which corresponds to the instantaneous deflection of head 1 from a rest position may have the frequency component fe corresponding to the oscillation of the bi-morph leaf 2 superimposed on a signal Si of triangular waveform which, as hereinafter described, may be additionally applied to the bi-morph leaf 2 during slow or still motion reproduction of the recorded video signals.

In the absence of such signal Si, the scanning path of head 1 would be at an angle in respect to the direction of each record track t being scanned by the head.

In any event, filter 19 is effective to remove from the deflection indicating signal Sg' (Fig. 5C) obtained at its output the lowfrequency component of any such triangular waveform Si. However, signal Sg' still contains the unwanted frequency components, such as components at the first and second order resonant and anti-resonant frequencies of bi-morph leaf 2 and various other frequency components due to the transient response in addition to the component at the dither or oscillation frequency Fc. Since the deflection signal Sg or Sg' represents the deflection of head 1 from its rest position, rather than the position of the head 1 relative to the centre of the track t being scanned, it will be apparent that the deflection signal Sg' applied to multiplier or balanced modulator 22 does not include any information in regard to the tracking error. It will be appreciated that the frequency, phase and amplitude of the unwanted frequency components contained in deflection signal Sg' substantially correspond to the frequency, phase and amplitude, respectively, of the corresponding unwanted frequency components contained in the envelope signal Sb' obtained from filter 21.

By reason of the above, the multiplier 22 which, as previously mentioned, may be a balanced modulator, provides an output signal Sj (Fig. SD) which represents the difference, or sum of the frequencies of the signals applied to the two inputs of multiplier 22. Thus, multiplier 22 is effective to eliminate from its output Sj the components with the dither or oscillation frequency Fe and with the unwanted frequencies, such as the first and second order resonant and antiresonant frequencies of the bi-morph leaf 2.

The resulting output signal Sj contains the information in respect to the tracking errors and also a frequency component having the frequency 2fc generated by the multiplier 22 and which may be eliminated in a band eliminating filter 23. The filter 23 may be effective to block the passage therethrough of frequency components in a band centered at the frequency 2fc and extending above and below the latter frequency by only a fraction of the frequencies fc. As a result of the foregoing, a tracking error signal Sk (Fig. 5E) representing the deviation of head 1 from the record track t being scanned thereby is obtained from band eliminating filter 23.

Alternatively, the band eliminating filter 23 may be replaced by a low pass filter having a suitable characteristic to block the 2fc frequency component.

The tracking error signal Sk is applied through an amplifier 24 to an adding circuit 8 in which it is added to the dither or oscillation signal Sc from oscillator 6. Further, the adding circuit may receive the signal Si of triangular waveform which is applied in the slow or still motion reproducing mode of the VTR to compensate for the angular deviation of the path of rotary head 1 relative to the longitudinal direction of each record track t resulting from the fact that the speed of longitudinal advancement of the tape T in the slow or still motion reproducing mode is different from the normal speed of advancement of the tape during recording of each record track t. Finally, as in the previouslydescribed existing system, the output of adding circuit 8 is applied to a drive amplifier 9 to provide the drive signal Sf by which bi-morph leaf 2 is driven so that the null position of head 1, when oscillated in the direction transverse to the direction along a record track, will correspond to the centre of the track considered in such transverse direction.

Referring now to Fig. 6, in which parts corresponding to those previously described with reference to Fig. 4 are identified by the same reference numerals, it will be seen that, in an automatic head tracking system 10' according to another embodiment of the invention, the guide drum associated with the rotary head or transducer 1 is shown in broken lines and generally identified by the reference numeral 25. Such guide drum assembly 25 includes a rotatable upper drum portion 26 and a stationary lower drum portion 27 with a radially opening circumferential slit 28 being defined between drum portions 26 and 27. The bi-morph leaf 2 of piezo-ceramic material is fixed at its base or inner end to the bottom surface of the rotatable upper drum portion 26, so that the magnetic head 1 in the form of a chip mounted at the free or outer end of the bimorph leaf 2 extends through slit 28 and is movable vertically or in the direction parallel to the axis of rotation of upper drum portion 26 in response to the deflection of bi-morph leaf 2 by the application of the drive signal Sf thcreto. Once again, a magnetic tape (not shown) is wrapped helically, that is, at an angle to the plane of slit 28, about a substantial portion, for example, about one half, of the periphery of drum 25 so that, upon rotation of head 1 with upper drum portion 26, head 1 scans obliquely across the magnetic tape and is simultaneously moved or oscillated in the direction normal to the scanning direction in response to the deflection of bi-morph leaf 2.

In the automatic head tracking system 10' according to this invention, a strain gauge 29 is secured to the surface of bi-morph leaf 2 to be stressed variably in response to deflection of the latter and to cooperate with an associated circuit 11' for generating the deflection signal Sg which indicates the extent and direction of deflection of head 1 from its rest position. In the system 10' as shown on Fig.

6, the deflection signal generating circuit 11' and a reproducing amplifier 3 for amplifying the output of head 1 are included in a circuit assembly 30 which is suitably arranged within the rotary upper drum portion 26 of the guide drum.

Referring now to Fig. 7, it will be seen that the reproduced signal from magnetic head 1 is amplified by reproducing amplifier 3 and is then transmitted from circuit assembly 30 in the upper drum portion 26 by way of a rotary transformer 31. Further, circuit assembly 30 is shown on Fig. 7 to be provided with slip rings indicated at 32 and 33 and respectively adapted to be engaged by brushes or the like connected to a source of operating voltage and to ground, respectively. Additional slip rings indicated at 34 and 35 are provided for engagement by brushes which respectively deliver and receive the deflection signal Sg and the drive signal Sf.

The strain gauge, which may be of the type employing a resistance wire, is illustrated at 29 on Fig. 7 in the position of its connection to circuit 11' and is also illustrated at 29' on Fig. 7 in the position of its physical attachment to bi-morph leaf 2.

The strain gauge 29 employing a resistance wire exhibits a change in its resistance value in response to deflection of flexing of the bi-morph leaf 2 on which the strain gauge is secured. The circuit 11' associated with strain gauge 29 is shown to include a field effect transistor 36 connected between slip ring 32 and strain gauge 29 so as to act as a constant current source for the latter. The change of resistance of the strain gauge 29 which occurs in response to deflection of bi-morph leaf 2 is converted to a corresponding change in the voltage which constitutes the deflection signal Sg after being suitably amplified by an amplifier 37. The amplifier 37 is shown to include an operational amplifier 38, resistors 39 and 40 and a capacitor 41, and has its output connected to slip ring 34.

Returning now to Fig. 6, it will be seen that the system 10' is there shown to be similar to the previously described system 10 apart from the means by which the deflection signal Sg is generated. Thus, in the system 10', the reproduced frequency modulated signal or output Sa of head 1, after passage through reproducing amplifier 3, is applied by way of rotary transformer 31 to envelope detecting circuit 5, and the resulting output or envelope signal Sb' from envelope detector 5 by means of the detection signal Sg' which represents the instantaneous position of the head 1 and thus includes all the unwanted frequency components also contained in the envelope signal. The foregoing is to be distinguished from the known system of Fig. 1, in which the multiplier or balanced modulator 7 compares the evelope signal Sb with the fixed dither or oscillating signal Sc which does not include the unwanted frequency components arising from mechanical vibration or the like of the bi-morph leaf 2. By reason of such distinction, the multiplier or balanced modulator 22 of the system according to this invention automatically eliminates the unwanted frequency components from the output signal Sj, with the result that the signal Sk accurately represents the tracking error so as to permit the head 1 to be automatically maintained precisely at the optimum tracking position by means of a relatively simple system.

Although the movable support 2 for the head 1 has been specifically described as being constituted by a bi-morph leaf, it will be understood that the support 2 may be constituted by any other type of piezoelectric element, or by a magneto-strictive or moving coil element.

Having described specific embodiments of the invention with reference to the accompanying drawings, it is to be appreciated that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined in the appended claims.

WHAT WE CLAIM IS: 1. An apparatus for reproducing information signals recorded in a track on a record medium comprising a transducer movable along said track for reproducing the information signal recorded therein, a deflectable mounting for said transducer which is operative in response to the reception of an electrical drive signal for deflecting said transducer in a direction which is transverse in respect to the direction along said track, and a control circuit including an oscillator for providing a dither oscillation signal which, on application to said deflectable mounting for the transducer, causes said transducer to oscillate in said transverse direction about a null position, an envelope detector for detecting the envelope of the output of said transducer as the latter moves along the track and oscillates in said transverse direction, a device for synchronously demodulating the detected envelope from said envelope detector by means of a signal having a frequency component of the dither oscillation signal frequency so as to obtain a tracking error signal representative of the deviation of said null position of the transducer from the centre of the track considered in said transverse direction, and an adding circuit adding said tracking error signal to said dither oscillation signal so as to provide therefrom said electrical drive signal; characterized in that a deflection signal generator is provided for generating a deflection signal in correspondence to the deflection of said transducer from a rest position, and said device synchronously demodulates the detected envelope from the envelope detector with reference to said deflection signal so that said drive signal from the adding circuit can accurately maintain the null position of the transducer in alignment with the centre of the track.

2. The apparatus according to claim 1, in which said device for synchronously demodulating said detected envelope from said envelope detector includes a multiplier having inputs receiving said detected envelope from the envelope detector and said deflection signal, respectively.

3. The apparatus according to claim 2, in which said multiplier is constituted by a balanced modulator.

4. The apparatus according to any one of the preceding claims, in which said control circuit further includes a first band pass filter through which said detected envelope from the envelope detector is applied to said device for synchronously demodulating the same, and a second band pass filter through which said deflection signal is applied to said device for synchronously demodulating said detected envelope; and each of said first and second band pass filters has a pass band with a lower cut-off frequency which is a fraction of the frequency of said dither oscillation signal and an upper cut-off frequency which is less than two times said frequency of the dither oscillation signal.

5. The apparatus according to any one of the preceding claims, in which said control circuit further includes a band elimination filter through which said tracking error signal is applied to said adding circuit the same to said dither oscillation signal, and said band elimination filter is operative to eliminate a predetermined band of frequencies centered about two times the frequency of said dither oscillation signal.

6. The apparatus according to claim 1, in which said deflectable mounting for the transducer includes a support arm carrying said transducer and being deflectable in said transverse direction, and said deflection signal generator is connected with said support arm to generate said deflection signal in response to deflection of said support arm.

7. The apparatus according to claim 6, in which said deflection signal generator includes a capacitive-type sensor having a fixed electrode and a movable electrode on said arm, and an amplifier circuit having a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. signal Sb' from envelope detector 5 by means of the detection signal Sg' which represents the instantaneous position of the head 1 and thus includes all the unwanted frequency components also contained in the envelope signal. The foregoing is to be distinguished from the known system of Fig. 1, in which the multiplier or balanced modulator 7 compares the evelope signal Sb with the fixed dither or oscillating signal Sc which does not include the unwanted frequency components arising from mechanical vibration or the like of the bi-morph leaf 2. By reason of such distinction, the multiplier or balanced modulator 22 of the system according to this invention automatically eliminates the unwanted frequency components from the output signal Sj, with the result that the signal Sk accurately represents the tracking error so as to permit the head 1 to be automatically maintained precisely at the optimum tracking position by means of a relatively simple system. Although the movable support 2 for the head 1 has been specifically described as being constituted by a bi-morph leaf, it will be understood that the support 2 may be constituted by any other type of piezoelectric element, or by a magneto-strictive or moving coil element. Having described specific embodiments of the invention with reference to the accompanying drawings, it is to be appreciated that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined in the appended claims. WHAT WE CLAIM IS:

1. An apparatus for reproducing information signals recorded in a track on a record medium comprising a transducer movable along said track for reproducing the information signal recorded therein, a deflectable mounting for said transducer which is operative in response to the reception of an electrical drive signal for deflecting said transducer in a direction which is transverse in respect to the direction along said track, and a control circuit including an oscillator for providing a dither oscillation signal which, on application to said deflectable mounting for the transducer, causes said transducer to oscillate in said transverse direction about a null position, an envelope detector for detecting the envelope of the output of said transducer as the latter moves along the track and oscillates in said transverse direction, a device for synchronously demodulating the detected envelope from said envelope detector by means of a signal having a frequency component of the dither oscillation signal frequency so as to obtain a tracking error signal representative of the deviation of said null position of the transducer from the centre of the track considered in said transverse direction, and an adding circuit adding said tracking error signal to said dither oscillation signal so as to provide therefrom said electrical drive signal; characterized in that a deflection signal generator is provided for generating a deflection signal in correspondence to the deflection of said transducer from a rest position, and said device synchronously demodulates the detected envelope from the envelope detector with reference to said deflection signal so that said drive signal from the adding circuit can accurately maintain the null position of the transducer in alignment with the centre of the track.

high input impedance and being connected with said capacitive-type sensor to provide said deflection signal in correspondence to changes in the capacity resulting from displacement of said movable electrode in respect to said fixed electrode.

8. The apparatus according to claim 8, in which said deflection signal generator further includes electrical shielding extending around said amplifier circuit for shielding the latter from the electric field resulting from the application of said drive signal to said deflectable mounting for the transducer.

9. The apparatus according to claim 6, in which said support arm is constituted by a bi-morph leaf which is cantilevered at one end and has said transducer secured to the other end of said leaf, and said electrical drive signal is applied to said bi-morph leaf for flexing the same and thereby causing the deflection in said transverse direction.

10. The apparatus according to any one of claims 6 to 9, in which said deflection signal generator includes a strain gauge secured on said support arm so as to be stressed in accordance with the deflection of said arm, and a circuit for providing said deflection signal in accordance with said stressing of the strain gauge.

11. The apparatus according to claim 10, in which said strain gauge is of the resistance wire type to provide a resistance varying with said stressing of the strain gauge, and said circuit for providing the deflection signal includes a constant current source connected with said strain gauge to provide a voltage signal varying with said resistance and an amplifier for amplifying said voltage signal.

12. The apparatus according to any one of claims 10 and 11 taken together with claim 9, in which said bi-morph leaf includes first and second piezo-ceramic elements and a central electrode therebetween, said transducer and strain gauge are mounted on the outer surface of the second piezo-ceramic element which is connected to ground. and a circuit applies the drive signal to said first piezo-ceramic element and to said central electrode.

13. The apparatus according to any one of the preceding claims, in which said record medium is a magnetic tape having said track extending obliquely thereon and being preceded and followed by similar tracks which are parallel thereto and also have information signals recorded therein, said tape extends helically about at least a portion of the periphery of a guide drum and is adapted to be longitudinally advanced, at least a portion of said guide drum is rotatable, and said transducer is a magnetic head mounted by means of said deflectable mounting on said rotatable portion of the guide drum so as to rotate with the latter and thereby scan along the one of the tracks positioned in proximity thereto by advancement of the tape.

14. Apparatus for reproducing information signals recorded in a track on a record medium, substantially as hereinbefore described with reference to and as shown by Figures 4 to 7 of the accompanying drawings.