GB2067339A - Disc reproducing system for compensating mechanical imperfections in the disc - Google Patents

Abstract

A servo-mechanism operates upon a tone arm (actuator 89) and hence on the cartridge, in response to an error signal in order to compensate for vertical deviations of the disc. The servo has an outer closed loop 201 including a sensor 82 which senses vertical deviations of the disc relative to the cartridge as well as an inner closed loop 200 including a sensor 80 which senses vertical deviations of the cartridge (tone arm). By this means resonance effects are eliminated from the sensed reference path information provided by sensor 82. <IMAGE>

Description

1

SPECIFICATION

Disc reproducing system for compensating mechanical imperfections so GB 2 067 339 A 1 Commercially manufactured disc phonograph records exhibit various mechanical imperfections, and further system i ' mperfections resuitfrom the mechanical means employed to reproduce the record. The present invention is concerned with one category of disc reproducing system imperfections: spurious vertical deviations of the record groove, primarily resulting from record warp and rumble, including record pressing noise. Such imperfections can cause significant degradation of the reproduced signal.

A general discussion of record warp is included in the following published paper: "Record Warps and 10 System Playback Performance% Larry Happ and Frank Karlov,Journalof theAudio Engineering Society, vol 24, No. 8, October 1976, pp 630-638. The authors found warp frequencies in the range of about 1/2 Hz (the once around frequency at 33-1/3 rpm) to beyond 10 Hz, with 95% of the warps below 8 Hz. Peak physical amplitude height of the warps was greatest at low frequencies at about 0. 6 mm maximum and decreased with increasing frequency.

Various problems are caused by record warp: the tone arm may bounce or sway with respect to the record surface due to the vertical and, to some extent, lateral, forces which result as the stylus attempts to track the varying record height. This may cause not only variations in tracking force but bottoming of the cartridge or complete loss of contact between the stylus and groove. Such variations in tracking force from optimum will often affect the reproduced signal at audible frequencies. In addition to causing stylus and arm tracking 20 problems, excessive stylus excursions result in geometrically related distortions and electromechanical non-linearity of the cartridge. Moreover, sub-audible warp signals can cause distortion by amplifier overload in electronic systems passing such low frequencies and, if applied to the speaker system, can cause substantial woofer movement that can result in extraneous noises and the distortion of higher frequency audible signals, including droppler distortion. Further, the geometrical relationship of the stylus and record 25 groove is such that a warp results in a forward and backward oscillation of the stylus tip over the recorded groove information, which frequency modulates (advances and delays) the reproduced signal causing "wow". Wow may also result from variations in rotational speed as the stylus load on the record groove varies.

The requirement to track warped phonograph records satisfactorily has resulted, in prior art systems, in 30 the necessity to consider ton earm/cartridge/styl us/record geometry very carefully and to seek the best combination, usually a compromise of such factors as stylus and tone arm mass, tone arm damping, stylus compliance and damping, and tracking force so as to provide a controlled tone arm resonance above the commonly encountered warp frequencies, yet below the frequency of the lowest recorded groove information. An arm resonance of 10 Hz has been advocated by several designers: Keisuka Ikegarni and 35 Susumu Hoshimi, "Advance in Turntable and Tone-Arm Design% Journal of theAudio Engineering Society, May 1976, Vol 24, No 4, pp 276-280 and Peter Rother, "The Aspects of Lowinertia Tone-Arm Design% Journal of the Audio Engineering Society, September 1977, Vol 25, No 9, pp 550-559.

Although in principle the proper selection of tone arm and cartridge parameters may make possible the tracking of warped records, the matching of arms and cartridges is often complicated in practice because of 40 the wide variation in tone arms and cartridges available. Further, even atthe design stage, the selection of optimum tone arm and cartridge parameters for warp tracking may not be optimum for tracking higher frequency groove information. Even when the record is properly tracked, the problem of geometric and motor wow from warps still remains.

Various passive devices for tracking warped records are known. These devices typically employ an element riding on the record surface and fixed or coupled to the pickup cartridge or the tone arm in the vicinity of the cartridge. Such devices include both damped elements and undamped or fixed elements.

Exemplary prior art damped element devices are disclosed in U.S. Patents Nos. 2,572,712 (spring loaded plunger), and 2,328,862 (elastically mounted auxiliary stylus). Fixed elements are disclosed in U.S. Patents Nos 3,228,700 (felt pad at end of tone arm with cartridge pivoted in tone arm) and 3,830,505 (air bearing, 50 adjacent cartridge). It is known also to employ a dash pot or a brush adjacent the cartridge to damp oscillations and assist in tracking warps. Further, proposals for a relatively rigid element coupling the tone arm to the record surface are known. It has also been suggested that the record be clamped or weighted at its periphery and/or centre in order to eliminate warp.

An active prior art system for treating record warp is described in the following paper: "Overcoming 55

Record Warps and Low-Frequency Turntable Rumble in Phonographs% Kenneth Clunis and Michael J Kelly, Journal of theAudio Engineering Society, July/August 1975, Vol 23, No 6, pp 450-458. In this system the cartridge output is used to servo control the vertical tone arm position to assist in tracking the record warp.

Similar systems are disclosed in U.S. Patents Nos 3,623,734 and 3,830,505. It is also known to provide a closed loop around the tonearm movements only, in order to improve arm/cartridge damping. It is an object 60 of the present invention significantly to improve the performance of these prior art tone arm systems.

According to the present invention, there is provided a gramophone disc playback apparatus wherein an actuator operates upon a cartridge in response to an error signal to compensate for vertical deviations of the disc, characterised in that the error signal is provided by a vertically fixed sensor or differentially by a first sensor sensing vertical deviation of the cartridge and a second sensor sensing vertical deviations of the disc 65 2 GB 2 067 339 A 2 relative to the cartridge, whereby, in either alternative, the error signal represents vertical deviations of the disc relative to a reference plane.

The present invention is based on observations taken from the situation shown in Figure 1 a and Figure 1 b.

Figure 1 a represents a sectional side view of the top half of a hypothetical record master on which silent grooves have been recorded. The groove depth "a" is a constant and represents the instantaneous vertical signal modulation with respect to a perfect reference path or surface. The reference surface may be the flat lacquer master disc surface, but in accordance with Figure 2, the reference surface may optionally be defined in the cutting process, preferably by a flat secondary cutting stylus following the main cutting stylus and arranged to smooth and dimensionally define the land between the grooves. In some cases, in the frequency range in which there is little vertical information recorded on the disc (e.g. below 30 Hz) the groove itself may 10 be used as the reference path.

Figure 1 b represents the situation after making a record pressing of the master. The vertical groove position is no longer constant but contains irregularities. In the case of warp, these are dimensionally correlated on the two sides of the record (the thickness remains substantially constant), because they arise simply from thermal and handling related distortions during and after removal of the record from the press. 15 Higher frequency mould grain noises, however, are not correlated on the two sides of the record, since different dies and stampers are used; the disc thus contains local variations of thickness. Such imperfections are caused by the pressure transmittal of dimensional irregularities from the back to the front of the stamper during the pressing operation. The back surface irregularities may include metallic crystals arising from the replication process, patterns resulting from grinding operations to smooth the back surface, dirt and dust trapped between the stamper and the die of the record press, and surface irregularities of the disc.

As the stamper thickness is some 0.18 mm to 0.25 mm the rigidity or stiffness of the material will limit the shortest wavelengths which can be transmitted through localized bending and distortion of the stamper.

Thus, such wavelengths might be of the order of 0.50 mm. This results in a highest frequency of mould grain noise at the outer diameter of a twelve inch disc (groove velocity about 50 cm per second) of the order of 1 kHz.

Further sources of low frequency noise on the record itself may include non-homogeneity of the pressing material and geometric distortions due to differential cooling effects resulting from rapid and uneven temperature changes in the die face. Moreover, as discussed previously, noises are also contributed by the reproducing system - namely, turntable and environmental rumble and acoustically transmitted vibrations 30 of the turntable and disc.

Thus, in a conventional reproducing system, the reproduced quantity "b" is obtained, employing the tone arm position as a reference. The quantity "b" thus includes undesired low frequency noise components.

Closer considerations of this matter shows that the low frequency noise components from all the sources mentioned above are not inextricably mixed with the original signal modulations. Rather, the recorded 35 signal quantity "a" remains intact and unharmed by the pressing and reproducing process and by mechanical imperfections in the reproducing system. Thus, the quantity "a" can be recovered if the distorted reference path at the point of stylus contact is used as the reference point during reproduction.

This application and other applications were divided out of my application 48108178 and reference may be made to the published application Serial No 2013957 for a complete discussion of the present and related 40 inventions which are all versions of or related to Vertical Noise Compensation (VNC) briefly characterised as follows:

Application No. Ser No 45 48108178 2013957 Disc VNC 00418178 Cartridge and pre-amp VNC 00419178 Tone arm VNC 00420178 00421178 Second cutting stylus 50 00422178 Reference plane sensing Briefly disc VNC involves compensation by using the disc, cartridge and pre-amp VNC involve mechanical or electrical cancellation of rumble information in the cartridge or preamplifier, tone arm VNC involves compensation by servo control of the tone arm, "second cutting stylus" relates to the subject matter of Figure 2 herein and "reference plane sensing" refers to the concept explained below in conjunction with Figure 3B.

In prior arttone arm VNC systems, the error signals used include armcartridge resonance components or other misleading information. In the present invention, measurements avoiding these defects are made and processed to control the tone arm; namely, reference path information is obtained and employed to control 60 the tone arm and optionally to perform additional corrections via the other embodiments. Low frequency components can be employed in a turntable VNC for warp compensation and higher frequency components can be used in a cartridge VNC or pre-amp VNC for rumble and mould grain noise reduction.

For optimum mechanical and acoustical performance, it is preferable to combine the tone arm VNC method with the cartridge VNC andlor the pre-amp VNC method. For example, warp and rumble effects may65 2 r 3 GB 2 067 339 A 3 be compensated up to, say, 20 Hz using a tone arm VNC, with frequencies above this being treated by a cartridge VNC or pre-amp VNC.

Compatibility characterizes all the embodiments of the invention. Conventional records may be played on reproducers including the invention; conversely, records produced with the optional defined reference 5 surface may be played on conventional reproducers.

The reduction of noise and tracking problems effected by the invention may permit a lower modulation level and a higher groove density to be employed, leading to longer playing times and/or smaller record diameters.

The fact that the invention solves the problem of low frequency noise leads to the further possibility that higher frequency components of the signal may be recorded on the disc in electronic noise reduction encoded form, such as by the system known as "Dolby B". This system, which treats only those signals above about 1 kHz, produces a compressed signal which has a proven history of being sufficiently compatible to permit the single inventory manufacturing and distribution of cassette tapes. Such acceptance in the case of encoded discs would be much more difficult, if not impossible, to achieve on a commercial basis if it were necessary to treat the low frequency signals as well. The encoded discs would, of course, preferably be played back using a noise reduction decoder for reduction of high frequency record pressing noise and low level ticks and pops.

Thus the present invention can make a significant overall contribution to the current performance and future possibilities of the conventional analogue disc record system.

The invention will be described in more detail, by way of example, with reference to the accompanying 20 drawings in which:

Figure 1A is a sectional side view of the top half of a hypothetical record master on which silent grooves have been recorded.

Figure 18 is a sectional side view of a hypothetical record pressing made from the master disc of Figure 1A.

Figure 2 is a partly sectional side view of a master disc during the cutting process, using a conventional signal cutting stylus and a secondary reference plane cutting stylus.

Figure 3A is a partially block generalized representation of direct reference path information sensing.

Figure 38 is a partially block generalized representation of indirect reference path information sensing.

Figure 4A is a partially cut away perspective view of one type of direct reference path information sensing. 30 Figure 48 is a partially cut away perspective view of a further type of direct reference path information sensing.

Figure 5 is a partially cut away side view of one type of reference patharm sensing.

Figure 6 is a partially cut-away perspective view of one type of reference path-arm sensor.

Figure 7 is a partially cut away perspective view of yet a further type of reference path-arm sensor. 35 Figure 8 is a partially cut away perspective view of still another type of reference path-arm sensor.

Figure 9 is a partially cut away perspective view of the styli portion of another type of reference path-arm sensor.

Figure 10A is a block diagram of prior art electrical tone arm damping arrangement.

Figure 108 is a block diagram of a prior art tone arm servo arrangement.

Figure 11 is a block diagram of a arm VNC system embodying the present invention.

In the embodiment to be described, only the essential inventive features will be shown or discussed in detail. Thus, except where otherwise specified, amplifiers, attenuators, equalizers, differentiators, integra tors, feedback loop compensators, gain controls and the like are used as ordinarily required in electronic technology. Likewise, except as otherwise discussed, the detailed design of styli, sensor transducers, actuator transducers, and the mechanical and electro-mechanical aspects of discs, cartridges, tone arms, drive motors, and the like will not be treated.

The vertical position of an unmodulated portion of the record (reference path) is sensed at or in close proximity to the signal pickup means, typically a stylus. An important element of the invention is the recognition that close proximity, high resolution sensing is useful for reducing mould grain noise. It follows 50 however, that it is necessary for the reference path to be as unblemished as possible. For example, it should be free of scratches. Moreover, the groove "horns" or ridges of material at the groove edges projecting into the land area should preferably be removed during the disc manufacturing process.

Polishing of the metal mould is a known method for removing groove horns. Another method is shown in Figure 2, which shows a further reference path cutting stylus 4 following the groove cutting stylus 6. The 55 substantially flat bottom edge of the reference path stylus not only removes the groove horns but cuts away residual rumble modulations on the lacquer master 2 and compensates for any vertical rumble introduced by the recording lathe. A perfectly quiet reference surface is thereby defined for use with the reproducing embodiments of the invention. In one embodiment, (Figure 6), a pilot groove provides the reference path. In this case, the reference path cutting stylus cuts an unmodulated groove adjacent the signal groove.

Reference path information sensing (ie sensing of the distorted undulating reference path) is a key element of the various embodiments. A generalized representation of reference path sensing is shown in Figures 3A and 3B. Referring to Figure 3A, the reference path information can be obtained directly, by means of a sensor which follows the signal stylus laterally but is vertically independent. Sensor 8 is attached to a reference plane. In a conventional turntable the attachment will typically be to the tone arm mounting surface. In 65 1 so is 4 GB 2 067 339 A 4 theory, the attaching surface can be any suitable reference surface, including astable surface apart from the turntable itself. A movable member 'I l, forming a portion of sensor 8Jol lows the surface undulations of the disc surface. In practice both disc contacting and non- contacting sensors are usable, as described below in connection with Figures 4A and 4B.

In a simple embodiment the reference path stylus is situated on the cartridge in the manner of Figures 6-9, hereina - fter described. In this case the stylus is relatively stiffly coupled to the cartridge body and tone arm, which results in a relatively high vertical resonant frequency of the tone arm. The stylus may be used for rumble and mould grain noise reduction as well as being used to provide reference path information in a tone arm VNC.

In the Figure 4A version, the sensor section of the arm may be verticallyfixed and the vertical displacement sensing transducer may comprise non-mechanical means to sense the disc surface 9, such as by ultrasonic or capacitive means or by a light beam and detector (e.g. light emitting diode and photodiode).

A light beam focused preferably at the point of contact of the stylus, but with a beam diameter encompassing at least one land area, may be angularly directed at the surface; vertical variations are then manifested as lateral variations, which are sensed by one or more photodetectors. This technique has the 15 advantage of providing a relatively wideband error signal without any attendant mechanical resonances.

Warp, rumble and mould grain noises at least up to several hundred Hz can thereby be compensated.

In the example of Figure 4A, the tone arm 10, which is vertically fixed, butfree to move laterally, has a U-shaped end 12 in which a cartridge 14 is pivoted on crosswise pin 16. A light source 18 and detectors 20,22 arrangement similarto that of the sQnsorversion of Figure 5, described hereinafter, generate the sensor 20 signal.

A further mechanical version of reference path sensing, shown in Figure 4B, employs a separate stylus laterally coupled to, but vertically independent of, the signal pickup cartridge. An arm 24, which can be pivoted for lateral (horizontal) movement only, has a first lateral support member 26 on which a tone arm section 28 carrying cartridge 14 is pivoted at 30. A second lateral support member and pivot enclosed within a housing 32 has a shank 34 carrying secondary stylus 36. A transducer at the pivot within housing 32 functions as a sensor of the secondary stylus 38 vertical movement. The secondary stylus is arranged to lift from the record whenever the signal stylus is lifted. Preferably, the stylus 36 is dimensioned to contact the land areas adjacent the signal stylus 38 of cartridge 14.

Secondary stylis may be constructed of any of various long wearing materials compatible with disc 30 surfaces and resistant to grooving effects, such as sapphire or diamond. Sensor transducers may be any of various types known in the art, including, but not limited to: electromagnetic, photoelectric, Hall effect, magneto-diode, potentiometric, or variable resistance, capacitance or inductance. The untreated output of the transducer may represent position, velocity, acceleration, orforce (as with a pressure responsive transducer).

The mechanical characteristics of the reference path sensor assembly can be optimized for the vertical sensing function only. The frequency of vertical resonance (sensor f lexing and mass) should be placed substantially above the highest warp frequency, and indeed well into the audio band, in order to extend the highest frequency of correction upwards, thereby to reduce audible rumble and mould grain noise. The secondary stylus must be situated very close to the primary stylus - e.g. within 1 mm for correction to about 40 Hz. Even closer spacing of about 0.1 mm for correction to about 500 Hz is preferable for reduction of mid-range mould grain noise.

The secondary stylus or sensor means may be positioned slightly in advance of the primary (signal) stylus to generate an anticipatory error signal. This is useful for relaxing the gain and phase requirements of electro-mechanical servo loops or for ensuring optimal error cancellation where mechanical or electrical phase shifts are present, as for example with low pass filtering of the reference path information.

Other embodiments of the invention employ reference path-arm information; this is the signal obtained by sensing the distance between the reference path and arm (ie cartridge). This signal will necessarily include tone arm movements and arm/cartridge resonance effects. A first sensor version simply employs the vertical % component information from the pickup cartridge, as is known in the prior art. This method provides useful 50 information above the frequency of arm/cartridge resonance, but is limited to cases and to the frequency range in which channel separation is deliberately reduced during disc cutting (eg below 100 Hz).

In orderto obtain reference path-arm information up to higher frequencies it is necessaryto provide a land sensor which is independent of the signal stylus. Non-mechanical sensing means such as those mentioned previously in connection with Figure 4A may be used, however being fixed to the cartridge holding arm or 55 cartridge ratherthan to a verticallyfixed arm. An example of such a sensor is shown in Figure 5. A stereophonic pickup cartridge 40 has a conventional cantilevered shank 42 and stylus tip 44 shown in engagement with a phonograph disc 9. A light source 46, such as a light emitting diode (LED) or diode laser, for example, generates a beam of light to cause an area of the record in the order of a millimeter in diameter, or smaller, to be illuminated. The reflected light is received atone or more photo receptors 48 and 50, such as 60 photo diodes, in the same manner as that of the description of Figure 4A. The light illumination location and diameter are preferably chosen to illuminate the area in which the stylus tip 44 is located and the adjacent land areas so that the reflected light is responsive primarily to local variations in the land at or just preceding the stylus, which variations are representative of the rumble and mould grain noise at that point. The output 65 of receptors 48 and 50 maybe fed to a differential emiplifierto provide an indication of local land variations; a 65 i j.

GB 2 067 339 A 5 suitable circuit arrangement may be made responsive only to vertical land position variations and not to the total light reflected, which will depend upon groove modulations. Such techniques are used in automatic slide focusing mechanisms, for example.

Examples of mechanical versions of reference path-arm sensors are shown in Figures 6-9. In each of the embodiments, a dual stylus pickup cartridge is provided in which a conventional stylus tracks the groove information content and the secondary stylus senses warp and rumble information. A third cartridge to record surface contact device may optionally be used, such as a brush or damper of the prior art warp tracking devices mentioned above.

An ideal reference path sensing method is shown in Figure 6, in which a shallow unmodulated pilot groove 52 is provided adjacent the main information carrying groove 54 in a phonograph disc 9a. The tip 56 of secondary stylus 58 of cartridge 60 rides in the pilot groove and senses both vertical and lateral warp and rumble frequencies. The embodiments of the invention are then adapted to employ both the vertical and lateral information provided. Fortunately, lateral warp and rumble are not serious problems and it is sufficient in a practical system to deal with vertical components only.

Referring to Figure 7, in an arrangement suited to conventional commercial phonograph records, the 15 cartridge 62 has a main stylus having a shank 42 and a stylus tip 44 tracking an information carrying groove 54 of a phonograph record 9. The secondary shank 64 and stylus tip 66 are situated on one or both sides of the main stylus and may fully or partially encircle it. The contact area may be biased towards the outside of the record, if desired, so that pre-echo effects are minimized in the reference path information. The secondary stylus tip has a substantially flat bottom with operative dimensions sufficiently large (e.g. some 20 fraction of a millimeter) so that it rides reliably on at least one land area and is thereby substantially unresponsive to lateral information and to any information content of the groove, responding only to the land height variations which are a measure of the warp and rumble. The tip 66 is guided laterally by the main stylus, and may be held in place by a compliant coupling 68 which generally maintains the relative positions of the two styli but does not interfere with the stylus movements.

Another view of the styli is shown in Figure 8. An elongated block shaped tip 70 for secondary stylus 72 spanning the land between several grooves is located ahead of and to the outside of the main stylus 42. As mentioned previously, the secondary stylus tip may optionally have a U- shape (shown by way of example as element 74 in Figure 9) or an 0-shape, surrounding the main stylus. The contact area may be biased towards the outside of the record, if desired, so as to decrease pre-echo effects. A further practical matter is that the 30 stylus arrangement should preferably not trap dust but should deflect it away.

For most tone arm and cartridge configurations, the design of the reference path-arm information sensors should be such that the tracking force of the main stylus should preferably comprise the main portion of the overall cartridge tracking force in order to avoid affecting the side thrust forces on the cartridge and to avoid reducing the main stylus force available for tracking warps and large amplitude signals. A secondary stylus 35 tracking force a small fraction that of the main stylus; e.g. 114, 1110, or even less - is adequate to sense the relatively low amplitude and low frequency rumble components. The secondary stylus is preferably compliantly connected to the cartridge body, the compliance preferably being substantially greater than that of the main stylus. The above compliance and tracking force considerations apply primarily to offsettone arm systems in which warp is not compensated. (in some systems the secondary stylus shank may be 40 relatively stiffly coupled to the cartridge body). Further, the effective mass of the secondary stylus and related moving parts, together with the flexing or stiffness properties ofthe shank, should produce a high frequency resonance well above the highest mould of grain noise components of interest; thus, a resonant frequency of at least 1-2 kHz would be suitable for the reference path sensor. As with the design of conventional signal cartridges, suitable mechanical damping can be applied to the secondary stylus. A low 45 pass mechanical filter may be incorporated if desired, so that the information provided by the secondary stylus is band limited for reduced sensitivity to dust and surface scratches.

Information from the reference path-arm sensor is used in one way or another to cancel corresponding vertical information from the signal stylus. Wholly mechanical cancellation arrangements may be used, as hereinafter described. In the simplest arrangement, with a non-compliant secondary stylus shank, the vertical movements of the cartridge body subtract from the corresponding movements of the primary stylus.

Alternatively, secondary stylus information may interact in the magnetic or electromechanical arrangements of the main signal transducer in such a way as to cancel error information. In some arrangements, a separate or coordinated transducer may be provided forthe secondary stylus. The combination of signals, as by interconnected coils, may be accomplished within the cartridge itself or the signals may be brought out for 55 external combination. The signals may be used internally and also brought out for use in other embodiments of the invention. For example, the high frequency components from the reference path sensor may be utilized internally or brought out to the preamplifier for mould grain noise reduction (cartridge VNC or pre-amp VNC), and the low frequency components may be brought out for dealing with warp via actuation of the turntable or tone arm (turntable VNC or tone arm VNC).

As discussed previously, accurate reference path information sensing may be achieved directly, by means of a verticallyfixed sensor. In an approximation, the vertical position of the tone arm may be used, by the methods known in the prior art. Atransducer 89 (Figure 413) is mounted between the arm and vertical pivot so as to give an output related to the vertical position or angle of the tone arm and cartridge. This method of sensing is useful for providing reference path information below the armcartridge resonant frequency (eg 65 6 GB 2 067 339 A 6 Hz). It is possible to employ a relatively stiffly mounted secondary stylus to raise the resonant frequency of the tone arm. The reference path-arm sensors described previously in connection with Figures 5-9 are primarily useful above the resonant frequency. At or near the resonant frequency, phase and amplitude errors will be introduced into the reference path information by both of these methods. Hence, these methods are useful only at warp or rumble frequencies somewhat removed from the arm-cartridge resonant frequency - that is, normally below about 5 Hz and above about 20 Hz.

A substantially error-free warp and rumble sensing signal can be derived, enabling the corrective action to be applied over the whole frequency range of interest (e.g. 0.5 Hz up to several hundred Hz) without interference from arm/cartridge resonance. The method employs a combination of the first and second sensing methods described in the previous paragraph above (ie, tone arm sensing and reference path-arm sensing) in a manner shown schematically in Figure 3B. This approach is based on the recognition that both signals contain the same error signals (from arm-cartridge interaction effects), but in complementary form, whereby they can be cancelled to leave a difference signal accurately indicative of the warp and rumble. The vertical arm position signal x is an indication of the arm 76 to reference plane distance, whereas the reference path-arm signal y is an indication of the cartridge 78 to disc distance; the difference z is the warp 15 and rumble amplitude; that is, z = x - y. The arm/cartridge resonance error signals contained in x and y signals from the tone arm sensor 80 and reference path-arm sensor 82 are cancelled in combiner 84, providing a substantially error free reference path information signal 86. The y signal can be derived by the various means which have been discussed, using either the signal stylus 78A, in some cases providing useful information up to about 30 Hz, or the secondary stylus 7813, by which useful information is obtained 20 up to about several hundred Hz.

Prior art feedback tone arm systems are laid out in the manner of Figures 1 OA and 1 OB. The tone arm vertical actuator is an electromechanical transducer so arranged to apply a force to the tone arm or cartridge in a direction normal to the disc surface, in response to an electrical signal from the sensor and amplifier. A further motor unit can be employed to perform similarly on a horizontal basis (or 450/45% In one prior art version shown in Figure 10A, a tone arm sensor is arranged to monitorthe vertical velocity of the tone arm; the negative feedback loop thereby acts to provide damping forthe tone arm. The damping depends on loop gain, which must not be so high as to interfere with the tracking of warps, which are tracked passively. The overall result is that the tone arm/cartridge resonance effects are reduced but that the feedback loop does not directly enter into tracking of the disc surface.

In other prior art versions, shown in Figure 10, the attempt is to employ a servo loop to track the warps actively. The distance between the tone arm and disc surface is sensed either by a separate transducer or via the cartridge output signal. This signal includes the arm/cartridge resonance characteristics; the uncertain and rapid changes of loop phase and gain in the region of resonance makes compensation difficult, limits the loop gain which can be used, and seriously interferes with the effectiveness of the servo action. The prior art 35 feedback tone arm techniques are thus only partially effective in dealing with the problems of record warp.

The embodiment of Figure 11 employs a double closed loop in such a manner that the sensed reference path-arm information is caused to be substantially the same as reference path information, thereby eliminating resonance effects. Both vertical and lateral operation (or 4501450) are possible, but for simplicity the discussion will be confined to vertical operation only.

Figure 11 shows a version of a tone arm VNC in accordance with the invention in which the warp and rumble error signal itself is enclosed within a negative feedback loop. The loop automatically provides tone arm movements which correspond to those of the disc surface. It is essential that the correct error signal should be employed - ie reference path information without interference from arm/cartridge resonance effects.

Consider the operation of the system of Figure 11 if the outer loop 201 were broken at point A. The arm would remain stationary throughout the frequency range of concern (0.5 Hz - 20 Hz), provided that sufficient gain is employed in the vertical actuator servo loop (inner loop 200); the gains of amplifiers 202 and 206, for example, may be set to ensure this condition without fear of oscillation, as there are no unusual oscillation provoking elements within the inner loop. Under this condition the output of the reference path-arm sensor is pure reference path information. If the outer loop is then closed at point A the arm will follow the reference path but at an amplitude depending on the overall outer loop gain. The gain of amplifier 218 may be set to provide the desired factor of reduction in output from the reference path-arm sensor, which, it should be noted, provides pure reference path information but at a reduced amplitude. Thus, the invention remedies the problems of the prior art tone arm feedback systems (Figure 1013) in effectively eliminating the troublesome arm/cartridge transfer characteristics from the feedback loop, whereby the arm is actuated by and follows the reference path.

r z so r 1 7 GB 2 067 339 A 7

Claims (1)

1. CLAIM

   A gramophone disc playback apparatus wherein a servo mechanism operates upon a cartridge in response to an error signal to compensate for vertical deviations of the disc, characterised in that the servo mechanism comprises an outer closed loop including a sensor which senses vertical deviations of the disc relative to the cartridge, amplifying means and an actuator moving the cartridge vertically, and an inner. closed loop including a second sensor which senses vertical deviations of the cartridge, amplifying means and the actuator.

   Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1981. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.

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