GB2209238A - Rotary head deflection apparatus - Google Patents

Abstract

The moving coils 42a, 42b of rotary head deflection units are connected over slip-rings and driven from waveform generators 61a, 61b, the supply acting as current sources for frequencies about the fundamental of the generated waveforms but acting as voltage sources about the higher resonance frequency of the deflection units. The current feed- back paths L1a, L1b, round amplifiers 64a, 64b, are effective at low frequencies, feed-back paths L2a, L2b, turning the amplifiers into voltage sources as the impedance of capacitors 71a, 71b diminishes. <IMAGE>

Description

2209230 HEAD-CARRIER DRUM CONTROL APPARATUS The present invention

generally relates to a magnetic tape recording and/or reproducing apparatus generally referred to as a video tape recorder and, more particularly, tc a hcad-carrier drum control apparatus, used in the video tape recorder, of a type comprising at least one rotary magnetic recording and/or reproducing head supported for displacement in a direction generally widthwise of a length of magnetic recording medium to accomplish a proper tracking for the purpose of accomplishing a high quality video reproduction.

It is well known that most video tape recorders of helical scan type now available in the market employs a generally cylindrical head-carrier drum assembly comprising rotary and stationary drum halves, the rotary drum half carrying a magnetic recording and/or reproducing head assembly for rotation together therewith An example of the prior art head-carrier drum assembly is reproduced in Fig 7 of the accompanying drawings in a schematic longitudinal sectional representation and will now be discussed with reference thereto.

Referring now to Fig 7, the head-carrier drum assembly shown therein comprises a motor-coupled drive shaft 1, a stationary or lower drum half 2 fixedly mounted on a support framework (not shown) and carrying a pair of spaced apart bearings 6 through which the drive shaft 1 extends coaxially for rotation independently of the lower drum half 2, and a rotary or upper drum half 3 mounted on the drive shaft I through a support stock 9, rigid with the drive shaft 1, for rotation together therewith and in coaxial relation with the lower drum half 2, the lower drum half 2 having an outer diameter exactly equal to the outer diameter of the upper drum half 3 The upper drum half 3 carries a plurality of, for example, two, magnetic recording and/or reproducing heads 5 a and 5 b for rotation together therewith spaced 180 from each other about the axis of rotation of the upper drum half 3 that is defined by the drive shaft 1 These magnetic transducing heads 5 a and 5 b are secured from below to the upper drum half 3 by means of respective fixture plates 4 with tips of said transducing heads 5 a and 5 b confronting a circumferen- tially extending clearance between the lower and upper drum halves 2 and 3 Within the head-carrier drum assembly, sector-shaped transformers 7 are supported by the support stock 9 for rotation together with the drive shaft 1 or the upper drum half 3, and correspondingly sector-shaped transformers 8 cooperable with the upper transformers 7 are rigidly mounted on the lower drum half 2 A length of magnetic tape generally identified by 13 and reeled at opposite ends to supply and take-up reels (now shown) is, during use, wrapped helically around the head-carrier drum assembly while extending at a predetermined angle relative to the longitudinal axis of the head-carrier drum assembly.

In this illustrated prior art structure, the magnetic transducing heads 5 a and 5 b are positioned at respective predetermined locations spaced 180 from each other about the drive shaft 1 and retained by the upper drum half 3 with their tips protruding a slight distance outwards from the outer peripheral surface of the upper drum half 3 The magnetic transducing heads 5 a and 5 b are electrically connected with the upper transformers 7 through connectors b 1 a and l Ob, then through printed wiring boards Ila and llb and finally through connectors 12 a and 12 b, respectively During a fixed speed rotation j 4 of the upper drum half 3 driven by the drive shaft 1 while the length of magnetic tape 13 is transported at a predetermined speed in one direction from the supply reel towards the take-up reel, the magnetic transducing heads a and 5 b while alternately switched into operation scan that portion of the length of magnetic tape 13 which is then wrapped around the head-carrier drum assembly, thereby to either record or reproduce one or both of video information and audio information.

As described above, each upper transformer 7 is fixed to the support stock 9 rotatable together with the drive shaft 1 and is spaced a slight distance from and confronts the associated lower transformer 8 which is electrically connected with a signal processor (not shown in Fig 7) During operation, the sector-shaped upper and lower transformers 7 and 8 are magnetically coupled to transmit a signal therebetween.

Referring still to Fig 7, the upper drum half 3 has a magnet 75 mounted atop the upper drum half 3 adjacent an outer peripheral portion thereof, which magnet is cooperable with a magnetic position sensor 76 supported immediately above the path of movement of the magnet 75 for detecting magnetic lines of force emanating from the magnet 75 thereby to provide a signal indicative of the position of the upper drum half 3 relative to the lower drum half 2.

The prior head-carrier drum assembly of the construction described above operates in the following manner.

During information recording or reproduction, -the magnetic transducing heads Sa and 5 b successively scans the length of magnetic tape 13 in a direction slantwise with respect to the longitudinal axis of the length of magnetic tape 13 while depicting successive traces which are parallel to each other as shown in Fig.

8 It is to be noted that the traces of movement of the magnetic transducing heads relative to the movement of the length of magnetic tape 13 are generally referred to as the recorded tracks on the length of magnetic tape 13 when the latter is used for reproduction of previously recorded information and to as the recording tracks on the length of magnetic tape 13 when the latter is used for recording information thereon.

In Fig 8, reference numeral 13 a represents a trace of movement of the length of magnetic tape 13; reference character VI represents a the speed of movement of the length of magnetic tape 13 during information recording or reproduction; reference numeral SA represents a trace of movement of any one of the magnetic transducing heads 5 a and 5 b relative to the length of magnetic tape 13; and reference character VO represents the speed of movement of any one of the magnetic transducing heads 5 a and 5 b As shown, the trace 13 a of movement of the length of magnetic tape 13 and the trace SA of movement of any one of the magnetic transducing heads 5 a and 5 b generally intersect with each other at a predetermined angle.

Accordingly, tracks depicted by any one of the magnetic transducing heads 5 a and 5 b on the length of magnetic tape 13 during the information recording or normal information reproduction are such as generally identified by A in Fig 8 (a) and the space P between the neighboring traces A represents a track pitch However, where the speed of movement of the length of magnetic tape 13 is during the information reproduction increased from the value V 1 to a higher value V 2 such as occurring when an operator selects a high speed search mode to skip a video program being then reproduced and to locate the succeeding video program, or during a high speed picture reproduction or quick-motion picture reproduction, the traces of movement of any one of the magnetic transducing heads 5 a and 5 b depicted on the length of magnetic tape 13 will be such as shown by B in Fig 8 (b).

Thus, it is well known that, where while the peripheral velocity of any one of the transducing heads 5 a and 5 b is fixed at a predetermined value the length of -7.

magnetic tape 13 is transported at a speed different from that during the normal information reproduction such as occurring during any one of the search mode, a quick- motion picture reproduction, a slow-motion picture reproduction and a still or frozen picture reproduction, any one of the magnetic transducing heads 5 a and 5 b fails to properly follow the recorded tracks on the length of magnetic tape 13 to such an extent that signals picked up by the magnetic transducing heads 5 a and 5 b are lowered in level The consequence is the reproduction of video and audio information of reduced quality and also the appearance of noise bars in the video display.

In order to substantially eliminate the above discussed problems, attempts have hitherto been made to permit the magnetic transducing heads to be displaceable according to the change of the speed of movement of the length of magnetic tape, or tape speed for short, relative to that during the normal picture reproduction or, more precisely, relative to the tape speed at which the length of magnetic tape had been transported during the information recording For example, the Japanese Patent Publication examined No 56-50329, published November 28, 1981, discloses the use of two head position control elements each capable of displacing in dependence on the magnitude of an electric signal applied thereto and having a respective movable member The magnetic transducing heads are secured to the rotary drum half through the respective movable members of the head position control elements so that, when an electric signal appropriate to a particular operating mode such as a high speed search mode or a slow-motion picture reproduction is applied to the head position control elements, the magnetic transducing heads Sa and 5 b can be displaced in a direction parallel to the axis of rotation of the rotary drum half to allow the magnetic transducing heads to properly follow the discrete parallel tracks on the length of magnetic tape.

United States Patent No 4,318,142, patented March 2, 1982, to Raymond F Ravizza, discloses the automatically compensated movable head servo circuit wherein, in order to cause the magnetic transducing head to accurately follow the tracks during the picture reproduction and, at the completion of the picture reproduction from the tracks, to properly position the magnetic transducing head in position to either reproduce the next adjacent successive track, reproduce the same track or reproduce another track so that the appropriate special motion effect can be achieved, a technique is employed to apply a small oscillatory motion to the magnetic transducing head to cause it to vibrate laterally of the track, to examine the resulting modulation of the c\.

reproduced signal's envelope to generate a tracking error correction signal and to apply the error correction signal to the magnetic transducing head.

United States Patent No 4,451,859, patented May 29, 1984, to Stan L Noel discloses the magnetic head position control system wherein a constant control potential is applied to the positionable magnetic trans- ducing head during the information recording to hold the magnetic transducing head fixed and the position of the magnetic transducing head relative to the tracks on the length of magnetic tape is controlled during the picture reproduction in dependence on signals reproduced from the tracks on the length of magnetic tape.

Any one of the above discussed prior art systems employs a drive unit including the head position control elements necessitated either to displace the magnetic transducing heads such as in the first mentioned publica- tion or to cause the magnetic transducing head to finely oscillate such as in the second and last mentioned publications, which drive unit referred to above has a mechanical resonant frequency peculiar to such drive unit.

Since in any one of the above discussed prior art system the drive unit is driven at a frequency associated with the cycle of rotation of the head-carrier drum, that is, the rotary drum half, with no particular care paid to the mechanical resonant frequency peculiar to the drive unit, the drive unit tends to undergo mechanical resonance at a high frequency region and, therefore, the trace of move- ment of the magnetic transducing heads during the picture reproduction fluctuates, as shown by the broken line in Fig 9 (b), relative to the trace of movement of the mag- netic transducing heads depicted during the information recording as shown by the solid line in Fig 9 (b) The consequence is that no proper tracking can be maintained and, therefore, the envelope of the reproduced signal oscillates, as shown in Fig 9 (c), accompanied by the generation of noises at each neck region (region of small amplitude) of the envelope, thereby to constitute a cause of the reduced signal-to-noise ratio.

Also, when the drive unit is driven by a voltage of a frequency including a fundamental drive frequency thereof, a loss of the resistance of the head position control elements multiplied by the second power of the control current applied to the head position control elements of the drive unit, accompanied by the self- heating of the head position control elements Once this self-heating occurs, the ambient temperature changes to such an extent as to result in change in resistance of the head position control elements and, also, change in contact resistance between contact elements and associated electrodes, both used to supply the control current there through to the drive unit Once the resistance of the head position control elements and/or the contact resistance between the contact elements and the associated electrodes vary as discussed above, the supply of the control current to the drive unit is no longer stabilized, resulting in improper head-to-tape tracking accompanied by the generation of noise bars in the video display.

Therefore, the present invention has been devised with a view to substantially eliminating the above described problems and disadvantages inherent in the prior art head-carrier drum control systems and is aimed at providing an improved head-carrier drum control system wherein the occurrence of an improper tracking during the picture reproduction which would result from the change in resistance of the drive unit and the mechanical resonance of the drive unit is effectively minimized irrespective of change in tape speed, thereby to provide a substantially noiseless picture reproduction in the video display at high signal-to-noise ratio.

A second important object of the present invention is to provide an improved head-carrier drum control system of the type referred to above, wherein the control signal can be rectilinearly supplied to the drive unit irrespective of the status of the drive unit, thereby to ensure the noiseless picture reproduction in a stabilized manner.

In order to accomplish the above described objects of the present invention, the improved head- carrier drum control system is herein disclosed, which comprises a rotatably supported, generally cylindrical head-carrier drum assembly including rotary and stationary drum portions; a drive unit accommodated in the head- carrier drum assembly and including a coil, disposed in a magnetic circuit, and a positionable magnetic transducing head cooperable with the coil to displace in a direction generally widthwise of a length of magnetic tape; a contact feeler through which a control signal is supplied to the drive unit; and an electrode means held in contact with the contact feeler and connected electrically with the coil.

In accordance with the present invention, the head-carrier druin control system is provided with a drive circuit for supplying an electric current (control signal) required to drive the drive unit at a frequency associated with the cycle of rotation of the head-carrier drum assembly and at a frequency at least about a fundamental drive frequency through a current-based drive operation to cause a trace of movement of the transducing head depicted during information reproduction to properly follow a trace of movement of the transducing head depicted during information recording.

According to the present invention, regardless of the self-heating of the coil, which would result from when the coil is driven by an electric current of relatively high output impedance and, also, regardless of the change of both of the resistance of the coil and the contact resistance between the contact feeler and the electrode means resulting from the change in ambient temperature, the supply of the control current of a predetermined value can be supplied in a stabilized manner to the drive unit and, as a result thereof, the trans- ducing head can be repositioned to cause the transducing head to properly follow any one of tracks recorded on the length of magnetic tape, that is, to accomplish a proper tracking Because of this, picture can be reproduced in the video display with no substantial noise bars appearing in the video display.

Also, the provision of the drive circuit for supplying an electric voltage (control signal) required to drive the drive unit at a frequency associated with the cycle of rotation of the head-carrier drum assembly and at a frequency at least about the frequency of mechanical resonance of the drive unit through a voltage-based drive operation is effective in that, when the drive unit is to be driven by the control signal having higher harmonic components (or noises) of the high frequency about the mechanical resonant frequency through a voltage-based drive operation, a counter electromotive force having a frequency component equal to the mechanical resonant frequency can be generated across the coil incident to the mechanical resonance of the coil occurring in the magnetic circuit This counter electromotive force so generated counteracts the drive voltage of a relatively low output impedance to exhibit a short-circuited braking effect by which the mechanical resonance of the drive unit including the transducing head can be effectively controlled.

Therefore, the tracking control during the picture reproduction can be stabilized to ensure the noiseless picture reproduction in the video display.

Moreover, according to another preferred embodiment of the present invention, the head-carrier drum control system is provided with a first drive circuit for supplying an electric current required to drive the drive unit at a fundamental drive frequency associated with the cycle of rotation of the head-carrier drum assembly through a current-based drive operation and a second drive circuit for supplying an electric voltage required to drive the drive unit at a frequency at least about a /Smechanical resonant frequency through a voltage-based drive operation, so that the gain relative to a load resistance during the drive with the current -can be made equal to the gain relative to a load resistance during the drive with the voltage According to this second embodiment, the linearity of the control signal to be applied to the electrode means can be maintained during the drive with the current and also during the drive with the voltage, thereby to permit the control signal to be linearly supplied to the drive unit to eventually accomplish the proper tracking regardless of the status of drive, i e, the drive with the current or the drive with the voltage.

In any event, the present invention will become more clearly understood from the following description of a preferred embodiment thereof, when taken in conjunction with the accompanying drawings However, the embodiment and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined solely by the appended claims In the drawings, like reference numerals denote like parts in the several views, and:

Fig I is a schematic longitudinal sectional view of a portion of a head-carrier drum assembly embodying the present invention; Fig 2 is a top plan view of that portion of the head-carrier drum assembly shown in Fig 1; Fig 3 is a schematic longitudinal sectional view showing a portion of the head-carrier drum assembly in detail; Fig 4 is a bottom plan view of that portion of the head-carrier drum assembly shown in Fig 3; Fig 5 is a block circuit diagram showing a control circuit; Fig 6 is a graph illustrating a signal characteristic exhibited when the drive is effected with a current and when the drive is effected with a voltage; Fig 7 is a longitudinal sectional view of the prior art head-carrier drum assembly;

Fig 8 is a diagram showing different arrangements of traces of movement of the magnetic transducing heads depicted on the length of magnetic tape; and Fig 9 is a graph illustrating a waveform of a cyclic signal and the relationship between the trace of movement of the transducing heads and a signal envelope when the transducing heads undergo mechanical resonance.

It is to be noted that the component parts identified by I to 3 and 6 to 13 in Figs 1 to 4 are identical with the component parts identified by like reference numerals in and described with reference to Fig.

7 and, therefore, will not be reiterated for the sake of brevity.

Referring first to Figs 1 and 2, and in the embodiment shown therein according to the present invention, each of the magnetic transducing heads 5 shown therein is supported by a drive unit 4 a which is in turn connected to the rotary or upper drum half 3 for movement in a direction substantially parallel to the drive shaft I so that the respective magnetic transducing head 5 can be permitted to displace together with the drive unit 4 a in a direction generally widthwise of the length of magnetic tape 13.

A contact feeler 14 having a plurality of contact brushes is fixedly supported by the stable member (not shown) above the rotary or upper drum half 3 with contact brushes slidingly engaged to corresponding electrodes forming an electrode means 15 The electrode means 15 including the electrodes slidingly engaged with the contact brushes of the contact feeler 14 is mounted, or otherwise formed, on the support stock 9 for rotation together therewith and, therefore, a control current to be supplied to the drive unit 4 a can be supplied through the brushes of the contact feeler 14 and then through the associated electrodes of the electrode means 15 The electrode means 15 is in turn connected electrically with the drive unit 4 a through the printed wiring board 11 and then through a connector 16.

With particular reference to Figs 3 and 4, the upper drum half 3 has a cross-sectional shape generally similar to the inverted shape of a figure "U" having an annular cavity 50 defined therein so as to open downwards towards the stationary or lower drum half 2 The drive unit 4 a for each transducing head 5 is accommodated within the cavity 50 in the upper drum half 3 and comprises a generally elongated leaf spring 41 made of nonmagnetizeable material carrying the respective transducing head 5 at a free end thereof, the opposite end of said leaf spring 41 being secured to the upper drum half 3 in a manner as will be described later by means of a set bolt 46 The leaf spring 41 has a generally cylindrical coil assembly 42 mounted thereon and having a centerline parallel to the drive shaft 1 so as to protrude towards the cavity 50 The drive unit 4 a is comprised of the transducing head 5, the leaf spring 41 and the coil assembly 42.

Secured to the upper drum half 3 by means of a screw member 49 and positioned inside the cavity 50 immediately above the cylindrical coil assembly 42 is a generally cap-shaped yoke member 44 made of ferromagnetic material This yoke member 44 is of a generally U-shaped cross-sectional configuration opening downwards towards the leaf spring 41 and has a cylindrical magnet 43 secured thereto in coaxial relationship so as to protrude loosely into the hollow of the cylindrical coil assembly 42 as best shown in Fig 3 while an annular gap is formed between the permanent magnet 43 and the coil assembly 42.

As will become clear from the subsequent description, a magnetic force emanating from the permanent magnet 43 and that emanating from the coil assembly 42 cooperate with each other to drive the leaf spring and, hence, the transducing head 5 in a direction close towards and away from the permanent magnet 43 thereby to displace reposition the transducing head 5.

The yoke member 44 has a ferromagnetic plate 45 having an opening defined therein and secured from below to the yoke member 44 with the opening aligned coaxially with the permanent magnet 43, which ferromagnetic plate 45 cooperates with the permanent magnet 43 to form a magnetic circuit this ferromagnetic plate 45 has an extension arm a extending in a direction away from the transducing head 5 The opposite end of the leaf spring 41 remote from the transducing head 5 is secured to the extension arm 45 a by means of the set bolt 46 threaded thereto with an annular spacer 47 positioned between the extension arm a and the leaf spring 41 and also with an annular washer 48 positioned between the head of the set bolt 46 and the leaf spring 41.

It will readily be understood that the free end of the leaf spring 41 is displaceable in a direction generally parallel to the drive shaft I relative to the opposite end thereof that is secured to the upper drum half 3 through the extension arm 45 a and the yoke member 44 The annular gap defined between the coil assembly 42 and the permanent magnet 43 has a width sufficient to avoid any possible interference therebetween during the displacement of the coil assembly 42 relative to the permanent magnet 43, which displacement takes place when a control current is supplied to the coil assembly 42 to develop a magnetic flux interacting with that produced by the permanent magnet 43 Incident to the displacement of the coil assembly 42, the leaf spring 41 is correspondingly displaced with the consequent displacement of the transducing head 5 in a direction generally widthwise of the length of magnetic tape 13 - The drive unit 4 a according to the present invention can be assembled in the following manner The leaf spring 41 has a centering perforation 52 of a generally circular shape defined at a generally central portion thereof for the passage of a cylindrical jig shown by the phantom line 53 in Fig 3 When and while the cylindrical jig 53 is passed through the centering perforation 52 until it is brought into contact with the cylindrical permanent magnet 43, the cylindrical coil assembly 42 can be accurately centered relative to the permanent magnet 43 within an annular gap g defined between the permanent magnet 43 and the peripheral lip region of the opening defined in the ferromagnetic plate The assembly including the yoke member 44 carrying the permanent magnet 43 and the leaf spring 41 carrying the coil assembly 42 is then fitted to the extension arm a by fastening the set bolt 46 to secure the opposite end of the leaf spring 41 to the extension arm 45 a, thereby completing the drive unit 4 a in which the cylindrical coil assembly 41 is exactly centered with the permanent magnet 43.

The drive unit 4 a so assembled with the yoke member 44 and the ferromagnetic plate 45 as hereinabove described is then inserted into the cavity 50 in the upper drum half 3 with the yoke member 44 oriented upwards, and the screw member 49 is then fastened to secure the yoke member 44 firmly against an annular top wall of the upper drum half 3, thereby completing the fitting of the drive unit 4 a to the upper drum half 3.

After the complete fitting of the drive unit 4 a, the necessity may arise to finely adjust the position of the transducing head 5 relative to the head-carrier drum assembly and also the amount of protrusion of the tip of the transducing head 5 radially outwardly from the head- carrier drum assembly For this purpose, the annular top wall of the upper drum half 3 has three adjustment holes 51 a, 51 b and Sic defined therein These adjustment holes 51 a to 51 c and the transducing head 5 are so positioned, when viewed from top as shown in Fig 4, that the imaginary line drawn so as to connect between the adjustment holes 51 a and 51 b and the imaginary line drawn so as to connect between the transducing head 5 and the adjustment hole 51 c can intersect orthogonally with each other at a point which should align with the longitudinal axis of the permanent magnet 43 A bearing hole 49 a through which the screw member 49 extends to secure the drive unit 4 a against the annular top wall of the upper drum half 3 as hereinbefore described is defined at a position spaced apredetermined distance D radially inwardly from the point of intersection of the imaginary lines referred to above.

After the drive unit 4 a employed in association with each transducing head 5 has been temporarily secured to the upper drum half 3 within the cavity 50 by passing the screw member 49 through the bearing hole 49 a, positioning wedge members (not shown) are inserted into the respective adjustment holes 51 a to 51 c so that rotational forces can be produced from the adjustment holes 51 a and 51 b to adjust the posture of the transducing head 5 in a direction circumferentially of the head- carrier drum assembly and to cause the adjustment hole 51 c to adjust the amount of protrusion of the tip of the transducing head 5 radially outwardly from the outer peripheral surface of the upper drum half 3 By so doing, the position of the drive unit 4 a relative to the upper drum half 3 can be accurately adjusted, this positional relationship being maintained when the screw member 49 is subsequently firmly fastened to draw the drive unit 4 a rigidly against the annular top wall of the upper drum half 3.

Referring to the magnetic circuit of high magnetic flux comprised of the permanent magnet 43, the yoke member 44 and the ferromagnetic plate 45, the cylindrical coil assembly 42 positioned inside the annular gap g between the permanent magnet 43 and the peripheral lip region of the ferromagnetic plate 45 around the V - opening thereof, when supplied with the control signal, develops a magnetic force which cooperates with the permanent magnet 43 to displace the leaf spring 41 and, hence, the magnetic transducing head S In order to avoid the magnetic transducing head 5 from being adversely affected by portion of the magnetic flux leaking from the coil assembly 42 towards the transducing head 5, the position of the extension arm 45 a of the ferromagnetic plate 45 is so selected as to be distant from the magnetic transducing head 5 as hereinbefore described.

Hereinafter, the operation will be described.

Assuming that the speed of movement of the length of magnetic tape 13 is increased from the tape speed VI to the tape speed V 2 for the purpose of effecting, for example, the high speed search, and when the magnetic transducing head 5 is displaced from a fixed position in a direction shown by the arrow T in Fig 8 (c), that is, in a direction generally widthwise of the length of magnetic tape 13, to change the trace of movement of the transducing head 5 from the positi 5 A to the position B, the trace of relative movemen itween the length of magnetic tape 13 and the transducing head 5 will be such as shown by C in Fig 8 (c) and coincides with the track A recorded on the length of magnetic tape 13-during the picture recording, thereby accomplishing a proper tracking.

A similar description applies even where the tape speed is changed to a value other than the tape speed mentioned above Thus, when during the reproduction mode the tape speed is selected to a value other than the tape speed at which the discrete parallel recorded tracks are formed on the length of magnetic tape 13, the proper tracking can be achieved by displacing the transducing heads 5 in the same direction generally widthwise of the length of magnetic tape 13 to reposition the transducing heads 5 so as to cause the latter to properly follow the recorded tracks.

The control circuitry of the head-carrier drum control system is illustrated in Fig 5, reference to which will now be made It is however to be noted that, while in the foregoing description made in connection with the drive unit 4 a reference is made only to one of the transducing heads 5, the control circuitry shown in Fig 5 is shown to have two circuits of substantially identical construction employed one for each of the magnetic transducing heads 5 In view of this, in Fig 5, the two transducing heads spaced 180 from each other about the axis of rotation of the upper drum half 3 are identified by 5 a and 5 b, respectively, and component parts of one of the circuits associated with the magnetic transducing head a are identified by respective reference numerals having a suffix "a" whereas those of the other of the circuits associated with the magnetic transducing head 5 b are identified by respective like reference numerals having a suffix of "b".

Referring now to Fig 5, each of the circuits associated respectively with the magnetic transducing heads 5 a and 5 b includes a cyclic signal generator 61 a or 61 b adapted to be controlled by a switching signal The switching signal referred to above is generated from a switching signal generator 77 including the magnetic position sensor 76 As hereinbefore described, the magnetic position sensor 76 is used for detecting the passage of the magnet 75 rotatable together with the upper drum half 3 thereby to provide a signal indicative of the position of the upper drum half 3.

An output from the cyclic signal generator 61 a or 61 b is supplied to a respective drive circuit including an operational amplifier 64 a or 64 b, a differential amplifier 69 a or 69 b, a current limiter 65 a or 65 b, resistors 62 a, 63 a, 68 a, 70 a or 62 b, 63 b, 68 b, 70 b, and a capacitor 71 a or 71 b The drive circuit provides as its output signal the control current to the associated contact brush 14 a or 14 b which is in turn slidingly engaged with the associated electrode 15 a or 15 b connected electrically with the respective cylindrical coil assembly 42 a or 42 b.

As shown, the coil assemblies 42 a and 42 b associated with the respective magnetic transducing heads Sa and 5 b are connected at one end posi tioning on a control side with the respective electrodes 15 a and 15 b while the coil assemblies 42 a and 42 b are connected at the other end positioning on a non-control sid-e with each other and, also, with the reference electrode 15 c to which a reference potential is applied through the grounded contact brush 14 c The control side referred to above means the side towards the driving circuit from the coil assembly 42 a or 42 b, and the non-control side means the side towards opposite of the driving circuit from the coil assembly 42 a or 42 b and being fixed at the reference potential.

The magnetic field developed by each of the coil assemblies 42 a and 42 b when the control current is supplied thereto reacts with the magnetic force developed by the associated permanent magnet 43 to displace the associated transducing head 5 a or 5 b in a direction generally widthwise of the length of magnetic tape 13.

The amount of displacement of each transducing head Sa or b is so chosen as to accomplish the proper tracking, that is, to permit the respective transducing head during the picture reproduction to properly follow the recorded track on the length of magnetic tape 13 The control current referred to above is supplied to the associated coil assembly 42 a or 42 b even when the transducing head 5 a or b is not held in sliding contact with the length of magnetic tape 13 The video and/or audio signal picked up by each transducing head 5 a or 5 b is supplied by the effect of an electromagnetic conversion taking place between the associated rotary and stationary transformers 7 a, 7 b and 8 a, 8 b to an associated head amplifier 72 a or 72 b Respective amplified signals emerging from the head amplifiers 72 a and 72 b are in turn supplied to a common signal processor 78 through a switch 73 controlled by the switching signal to pass the amplified signals alternately through such switch 73 and are eventually reproduced in the video display in a manner known to those skilled in the art.

Each of the current limiters 65 a and 65 b is comprised of a respective window comparator 66 a or 66 b and a current limiting means 67 a or 67 b Each current limiter a or 65 b is so constructed as to detect the current flowing across the associated resistor 68 a or 68 b employed to adjust the current through the coil assembly, that is, the current expected to flow across the associated coil assembly 42 a or 42 b, and then to regulate the current of the control signal to a current value required for the displacement of the associated transducing head 5 a or 5 b to establish the following relationship.

I (Desired Speed Ratio) 1 I x (Track Pitch P)< (Limit of Displacement of Head)< (Structural Limit of Displacement of Head) For this purpose, a voltage across the respective resistor 68 a or 68 b is fed-back to the associated current limiter a or 65 b through the associated differential amplifier 69 a or 69 b by way of the operational amplifier 64 a or 64 b.

It is to be noted that the Desired Speed Ratio referred to above means a speed ratio relative to the normal tape speed during the reproduction and may take a negative value when the length of magnetic tape is transported in a direction reverse to the direction thereof from the supply reel onto the take-up reel; the Limit of Displacement of Head referred to above means the limit by which, consequent upon the result that the relationship between the value of the current flowing across each of the coil assemblies 42 a and 42 b and the amount of displacement of any one of the transducing heads a and 5 b exhibits a linearity, an accurate operation of the drive unit 4 a can be guaranteed; and the Structural Limit of Displacement of Head referred to above means the limit at which any one of the transducing heads Sa and 5 b 30.

will not interfere with those component parts that surround such one of the transducing heads 5 a and 5 b.

It is also to be noted that each of the resistors 63 a or 63 b is selected to have a resistance higher than that of the associated resistor 70 a or 70 b, and each of the capacitors 71 a or 71 b is selected to have an Lmpedance higher than the resistance of the associated coil assembly 42 a or 42 b.

Hereinafter, the operation of the control circuitry will be described.

Each of the synchronizing signal generators 61 a and 61 b generates a signal, shown in Fig 6 (a), of a waveform coinciding with the cycle of rotation of the magnetic transducing heads 5 a and 5 b, which signal is in turn applied to the corresponding operational amplifier 64 a or 64 b shown in Fig 5 The respective operational amplifier 64 a or 64 b outputs an inverted version of the signal applied thereto from the associated synchronizing signal generator 61 a or 61 b and applies it to the associated coil assembly 42 a or 42 b Accordingly, referring to Fig 8 (c), the displacement of each of the transducing heads Sa and 5 b in the direction shown by the arrow T will be zero at the starting point (left-hand lower point) of the relative trace of movement; increasing progressively as the relative trace of movement extends towards the terminating point (right-hand upper point) while depicting the desired trace of movement as shown by the solid line C.

Each of the drive circuits forming the control circuitry includes a current feedback loop LI and a voltage feedback loop L 2 connected parallel to each other; the current feedback loop Li including the associated differential amplifier 69 a or 69 b for detecting the voltage across the associated resistor 68 a or 68 b, that is, the current expected to flow across the associated coil assembly 42 a or 42 b, and the voltage feedback loop L 2 including the associated capacitor 71 a or 71 b for the feedback of a voltage across the associated resistor 68 a or 68 b.

Where the frequency is high, the impedance of each capacitor 71 a or 71 b approaches a zero value and, accordingly, the voltage feedback dominates over the current feedback and the drive operation based on the voltage takes place On the other hand, where the frequency is low, the impedance of each capacitor 71 a or 71 b approaches an infinite value and, accordingly, the current feedback dominates over the voltage feedback and the drive operation based on the current takes place The relationship between the fundamental drive frequency of each of the transducing heads 5 a and 5 b operable respectively with the coil assemblies 42 a and 42 b and the mechanical resonant frequency of the drive unit 4 a is such that the fundamental drive frequency is lower than the mechanical resonant frequency Therefore, the current- based drive operation is carried out at a frequency about the fundamental drive frequency, but the voltage-based drive operation is effected at a frequency about the mechanical resonant frequency, so that the counter electromotive force generated across the associated coil assembly 42 a or 42 b during the voltage-based drive at which the output impedance is low can be absorbed to effect the short-circuited braking.

The control signal shown in Fig 6 (a) to drive the transducing heads 5 a and 5 b comprises higher harmonic components or noises superimposed to the fundamental drive frequency component Therefore, when the higher harmonic components have a frequency component equal to the mechanical resonant frequency of the drive unit 4 a, the mechanical resonance of the drive unit 4 a may occur with the consequence of improper head-to-tape tracking.

However, the resonance of the drive unit 4 a due to the supply of the higher harmonic components having high frequency is absorbed by the short-circuited braking effect as described above, with the consequence of proper head-to-tape tracking.

On the other hand, the drive circuit supplies the fundamental drive frequency component of the drive current through the current-based drive operation.

Therefore, regardless of the self-heating of the coil assemblies 42 a and 42 b due to high output impedance of the drive circuit during the current-based drive operation, and also, regardless of the change of both of the resistance of the coil assemblies 42 a and 42 b and the contact resistance between the contact feeler 14 a and 14 b and the electrode means 15 a and 15 b resulting from the change in ambient temperature, the supply of the drive current (control signal) of the predetermined value can be effected in a stable manner to the drive unit 4 a.

It is to be noted that, in order to minimize the difference between the gain during the current-based drive operation and that during the voltage-based drive operation, the following circuit constants are employed.

The gain of the differential amplifier 69 a = (Resistance Value of Coil Assembly 42 a)/(Resistance Value of Resistor 68 a) The gain of the differential amplifier 69 b = (Resistance Value of Coil Assembly 42 b)/(Resistance Value of Resistor 68 b) As hereinbefore described, by making the gain during the current-based drive relative to a load resistance at a frequency about the fundamental drive frequency which is low coincide substantially with the gain during the voltage-based drive at a frequency about the mechanical resonant frequency of the drive unit which is high, the voltage to be applied to each of the electrodes 15 a and 15 b can be maintained to exhibit a characteristic curve shown by the solid line in Fig 6 (a) so that the linearity of the control signal can be attained.

Where, by way of example, the gain during the current-based drive and that during the voltage-based drive differ from each other, the output from each cyclic signal generator 61 a or 61 b which provides an input to the associated drive circuit, that is, the voltage applied to the associated electrode 15 a or 15 b which voltage corresponds to a linear portion of the cyclic signal shown in Fig 6 (a), will fluctuate as shown by the phantom line in Fig 6 (b) and a loss of linearity may occur in the control signal.

In practice, however, it is not easy to make the gains during the current-based and voltage-based drives equal to each other in view of any possible change in resistance of one or both of the coil assemblies 42 a and 42 b and also the circuit constants A-series of experiments have proven that the coincidence of the gains 35.

within a tolerance of + 3 d B is effective to obtain the practically acceptable linearity of the control signal.

Although in the foregoing description no reference has been made to the angle of inclination of the head gap in each of the transducing heads, that is, the azimuth of the head gap, it is well known that the respective head gaps of the paired transducing heads according to the VHS recording and reproducing system are inclined in opposite directions relative to each other so that one of the transducing heads cannot trace the track formed by the other of the transducing heads In such case, since the level of a signal tends to be lowered for each recorded track as shown in Fig 7 (b), the employment of the technique to displace the transducing heads such as in the present invention is more effective.

Although the present invention has fully been described in connection with the preferred embodiment thereof with reference to the accompanying drawings used only for the purpose of illustration, those skilled in the art will readily conceive numerous changes and modifications within the framework of obviousness upon the reading of the specification herein presented of the present invention.

For example, instead of the use -of the leaf spring 41 supported in a cantilever fashion, either a torsion spring or a parallel spring may be employed, or a rigid support plate may be employed Where the rigid support plate is used, the rigid support plate must be supported for movement in a direction close towards and away from the permanent magnet 43.

Also, the extension arm 45 a shown as forming an integral extension of the ferromagnetic plate 45 may be a non-magnetizeable member separate from the ferromagnetic plate 45 and connected to the plate 45, or may be a member either connected to or formed integrally with the yoke member 44.

The use of the yoke member 44 used to connect the associated drive unit to the upper drum half 3 may not be essential in the practice of the present invention.

Instead of the use of the yoke member 44, the ferromagnetic plate 45 may be so shaped and so configured as to enable the associated drive unit to be secured therethrough to the upper drum half 3.

Similarly, the use of the support stock 8 shown and described as rigidly mounted on the drive shaft 1 may not be always essential in the practice of the present invention and may therefore be dispensed with, in which case the upper drum half 3 may be mounted direct on the drive shaft.

Yet, other than the three adjustment holes Sla 317 to 51 c, one or more extra adjustment holes may be formed in the annular top wall of the upper drum half 3 The bearing hole 49 a for the passage of the screw member 49 which has been shown as formed at a location on one side of the permanent magnet 43 close towards the drive shaft 1 may be defined at a location on the opposite side of the permanent magnet 43 close towards the associated transducing head 5.

Accordingly, such changes and modifications are, unless they depart from the spirit and scope of the present invention as delivered from the claims annexed hereto, to be construed as included therein.

Claims (1)

1 A head-carrier drum control apparatus which comprises:

a rotatably supported, generally cylindrical head-carrier drum assembly; a drive unit accommodated in the head-carrier drum assembly and including a coil means, disposed in a magnetic circuit, and at least one positionable magnetic transducing head cooperable with the coil means to dis- place in a direction generally widthwise of a length of magnetic tape; a contact feeler through which a control signal is supplied to the drive unit; an electrode means held in contact with the con- tact feeler and connected electrically with the coil means; and a drive circuit for supplying an electric cur- rent required to drive the drive unit at a frequency associated with the cycle of rotation of the head-carrier drum assembly and at a frequency at least about a fun- damental drive frequency through a current-based drive operation to cause a trace of movement of the transducing head depicted during information reproduction to properly follow a track recorded by a magnetic transducing head on the length of magnetic tape during information recording.

2 The apparatus as claimed in Claim 1, wherein the drive circuit is comprised of an amplifier having a cur- rent feedback loop.

3 A head-carrier drum control apparatus which comprises:

a rotatably supported, generally cylindrical head-carrier drum assembly; a drive unit accommodated in the head-carrier drum assembly and including a coil means, disposed in a magnetic circuit, and at least one positionable magnetic transducing head cooperable with the coil means to dis- place in a direction generally widthwise of a length of magnetic tape; a contact feeler through which a control signal is supplied to the drive unit; an electrode means held in contact with the con- tact feeler and connected electrically with the coil means; and a drive circuit for supplying an electric volt- age required to drive the drive unit at a frequency associated with the cycle of rotation of the head-carrier drum assembly and at a frequency at least about a mechani- cal resonant frequency of the drive unit through a voltage-based drive operation to cause a trace of movement of the transducing head depicted during information reproduction to properly follow a track recorded by a mag- netic transducing head on the length of magnetic tape during information recording.

4 The apparatus as claimed in Claim 3, wherein-the drive circuit is comprised of an amplifier having a volt- age feedback loop.

A head-carrier drum control apparatus which comprises:

a rotatably supported, generally cylindrical head-carrier drum assembly; a drive unit accommodated in the head-carrier drum assembly and including a coil means, disposed in a magnetic circuit, and at least one positionable magnetic transducing head cooperable with the coil means to dis- place in a direction generally widthwise of a length of magnetic tape; a contact feeler through which a control signal is supplied to the drive unit; an electrode means held in contact with the con- tact feeler and connected electrically with the coil means; and a first drive circuit for supplying an electric current required to drive the drive unit at a frequency associated with the cycle of rotation of the head-carrier drum assembly and at a frequency at least about a fun- damental drive frequency through a current-based drive operation and a second drive circuit for supplying an electric voltage required to drive the drive unit at a frequency about at least a mechanical resonant frequency of the drive unit through a voltage-based drive operation, thereby to cause a trace of movement of the transducing head depicted during information reproduction to properly follow a track recorded by a magnetic transducing head on the length of magnetic tape during information recording.

6 The apparatus as claimed in Claim 5, wherein the drive circuit is comprised of an amplifier having a cur- rent feedback loop and a voltage feedback loop connected parallel to the current feedback loop.

7 The apparatus as claimed in Claim 5, wherein the gain relative to a load resistance during the current- based drive operation is made equal to that during the voltage-based drive operation.

8 The apparatus as claimed in Claim 1, wherein said drive unit comprises a leaf spring made of non- magnetizeable material, and the magnetic circuit is formed by a magnet member capable of emanating a magnetic force counteracting with a magnetic force which may be developed by the coil means, and a ferromagnetic member forming a part of the magnetic circuit together with the magnet mem- ber, and wherein said positionable magnetic transducing head is mounted on one end of the leaf spring, and wherein said coil means has a generally cylindrical shape and is mounted on the leaf spring with the longitudinal axis of said coil means oriented in a vertical direction substan- tially perpendicular to the leaf spring, said coil means being positioned inside an annular gap defined between the ferromagnetic member and the magnet member.

9 The apparatus as claimed in Claim 8, wherein the ferromagnetic member has an extension arm extending there- from in a direction away from the transducing head, said leaf spring having the opposite end secured to the exten- sion arm.

The apparatus as claimed in Claim 3, wherein said drive unit comprises a leaf spring made of non- magnetizeable material, and the magnetic circuit is formed by a magnet member capable of emanating a magnetic force counteracting with a magnetic force which may be developed by the coil means, and a ferromagnetic member forming a part of the magnetic circuit together with the magnet mem- ber, and wherein said positionable magnetic transducing head is mounted on one end of the leaf spring, and wherein said coil means has a generally cylindrical shape and is mounted on the leaf spring with the longitudinal axis of said coil means oriented in a vertical direction substan- tially perpendicular to the leaf spring, said coil means being positioned inside an annular gap defined between the ferromagnetic member and the magnet member.

11 The apparatus as claimed in Claim 10, wherein the ferromagnetic member has an extension arm extending 2 al therefrom in a direction away from the transducing head, said leaf spring having the opposite end secured to the extension arm.

12 The apparatus as claimed in Claim 5, wherein said drive unit comprises a leaf spring made of non- magnetizeable material, and the magnetic circuit is formed by a magnet member capable of emanating a magnetic force counteracting with a magnetic force which may be developed by the coil means, and a ferromagnetic member forming a part of the magnetic circuit together with the magnet mem- ber, and wherein said positionable magnetic transducing head is mounted on one end of the leaf spring, and wherein said coil means has a generally cylindrical shape and is mounted on the leaf spring with the longitudinal axis of said coil means oriented in a vertical direction substan- tially perpendicular to the leaf spring, said coil means being positioned inside an annular gap defined between the ferromagnetic member and the magnet member.

13 The apparatus as claimed in Claim 12, wherein the ferromagnetic member has an extension arm extending therefrom in a direction away from the transducing head, said leaf spring having the opposite end secured to the extension arm.

14 A head-carrier drum control apparatus substantially as herein described with reference to and as illustrated in Figs 1 to 6, 8 and 9 of the accompanying drawings.

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