GB2067820A - Disc Reproducing System for Compensating Mechanical Imperfections - Google Patents

Abstract

A tone arm 76 is vertically servo- controlled to cause a reference stylus 78b to follow a reference path, such as a reference land between the disc grooves. The signal stylus 78a therefore provides a pure signal representing the modulating information. The error signal controlling the servo is a Reference Path Information signal z (output 86). z is formed by subtracting y, the signal provided by a sensor 82 reponsive to the stylus 78b, from x, the signal provided by a tone arm sensor 80. In an alternative the signal z is provided directly by a vertically fixed sensor which tracks with the cartridge. <IMAGE>

Description

1 GB 2 067 820 A 1

SPECIFICATION

Disc Reproducing System for Compensating Mechanical Imperfections Commercially manufactured disc phonograph records exhibit various mechanical imperfections, and further system imperfections result from 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 80 in the following published paper: "Record Warps and System Playback Performance", Larry Happ and Frank Karlov, Journal of the Audio 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 problems, excessive stylus excursions result in geometrically related distortions and electro-mechanical nonlinearity 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 doppler distortion. Further, the geometrical relationship of the stylus and record 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 the necessity to consider tonearm/cartridge/stylus/record geometry very carefully and to seek the best combination, usually a compromise of such factors as stylus and tone arm mass, tone arm dampling, 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 Ikegami and Susumu Hoshimi, "Advance in Turntable aInd Tone-Arm Design", Journal of theAudio Engineering Society, May 1976, Vol 24, No. 4, pp 27 6-280 and Peter Rother, "The Aspects of Low-inertia 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 the wide variation in tone arms and cartridges available. Further, even at the 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, 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 Record VVarps 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 of the present invention significantly to improve the performance of these prior art tone arm systems.

According to the present invention, there is 2 GB 2 067 820 A 2 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 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 Fig. 1 a and Fig.

1 b. Fig. 1 a represents a sectional side view of the top half of a hypothetical record master on which 80 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 85 disc surface but in accordance with Fig. 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 90 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 be used as the reference path.

Fig. 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. Higher frequency mould grain noises, however, are not correlated on the two sides of the record, since 105 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 110 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 t 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 mold 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 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 -bthus includes undesired low frequency noise components.

Closer consideration 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 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 48108/78 and reference may be made to the published application Serial No. 2013957 fora complete discussion of the present and related inventions which are all versions of or related to Vertical Noise Compensation (VNC) briefly characterised as follows:

Application No. 48108/78 100 00418/81 00419/81 00420/81) 00421/81 00422/81 Ser. No. 2013957 Disc VNC Cartridge and pre-amp VNC Tone arm VNC Second cutting stylus 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 Fig. 2 herein and "reference plane sensing" refers to the concept explained below in conjunction with Fig. 3B.

In prior art tone 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 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 t.

3 GB 2 067 820 A 3 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 and/or 70 the pre-amp VNC method. For example, warp and rumble effects may be compensated up to, say, Hz using a tone arm VNC, with frequencies above this being treated by a cartridge VNC or pre-ampVNC.

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 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 least 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 1()0 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, 110 by way of example, with reference to the accompanying drawings in which:

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

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

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 313 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 130 information sensing.

Figure 4B 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. Figure 8 is a partially cut away perspective view of still another type of reference path-arm sensor. 80 Figure 9 is a partially cut away perspective view of the styli portion of another type of reference path-arm sensor. Figure 1 OA is a block diagram of a prior art electrical tone arm damping arrangement. 85 Figure 1 OB is a block diagram of a prior art tone arm servo arrangement. Figure 11 is a block diagram of a tone arm VNC system embodying the present invention employing direct reference path sensing. 90 Figure 12 is a block diagram of a further type of tone arm VNC system embodying the present invention in which indirect reference path sensing is employed. In all of the embodiments to be described, only the essential inventive features will be shown or discussed in detail. Thus, except where otherwise specified, amplifiers, attenuators, equalizers, differentiators, integrators, 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 tranducers, and the mechanical and electro-mechanical aspects of discs, cartridges, tone arms, drive motors, and the like will not be treated.

In the several embodiments, 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 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 or removing groove horns. Another method is shown in Fig. 2, which shows a further reference path cutting stylus 4 following the groove cutting stylus 6. The 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 4 GB 2 067 820 A 4 surface is thereby defined for use with the reproducing embodiments of the invention. In one embodiment, (Fig. 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 (i.e. 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 Figs. 3A and 3B. Referring to Fig. 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 so to a reference plane. In a conventional turntable the attachment will typically be to the tone arm mounting surface. In theory, the attaching surface can be any suitable reference surface, including a stable surface apart from the turntable itself. A movable member 11, forming a portion of sensor 8, follows the surface undulations of the disc surface. In practice both disc contacting and noncontacting 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, hereinafter 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 Fig. 4A version, the sensor section of the arm may be vertically fixed and the vertical displacement sensing transducer may comprise nonmechanical means to sense the disc surface 9, such as by ultrasonic or capacitive means or by 105 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 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, but free to move laterally, 120 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 similar to that of the sensor version of Figure 5, described hereinafter, generate the sensor signal.

A further mechanical version of reference path sensing, shown in Figure 413, employs a separate stylus laterally coupled to, but vertically independent of, the signal pickup cartridge. An arm 24, which can be pivoted for lateral 130 (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, theg stylus 36 is dimensioned to contact the land areas adjacent the signal stylus 38 of cartridge 14.

Secondary styli may be constructed of any of various long wearing materials compatible with disc 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 lintreated output of the transducer may represent position, velocity, acceleration, or force (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 flexing 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 50 Hz. Even closer spacing of about 0. 1 mm for correction to about 500 Hz is preferably 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 (i.e. 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 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 (e.g. below 100 Hz).

In order to obtain reference path-arm information up to higher frequencies it is necessary to provide a land sensor which is independent of the signal stylus. Non-mechanical c sensing means such as those mentioned previously in connection with Fig. 4A may be used, however being fixed to the cartridge holding arm or cartridge rather than to a vertically fixed arm. An example of such a sensor is shown in Fig. 70 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 at one or more photo receptors 48 and 50, such as photo diodes, in the same manner as that of the description of Fig. 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 of receptors 48 and 50 may be fed to a differential amplifier to provide an indication of local land variations; a 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 Figs. 6-9. In each of the embodiments, a dual stylus pickup 100 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 105 brush or damper of the prior art warp tracking devices mentioned above.

An ideal reference path sensing method is shown in Fig. 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 Fig. 7, in an arrangement suited to conventional commercial phonograph records, the 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 GB 2 067 820 A 5 the reference path information. The secondary stylus tip has a substantially flat bottom with operative dimensions sufficiently large (e.g. some 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 9tylus movements.

Another vievu 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 O-shape, surrounding the main stylus. The contact area may be biased towards the outside of the record, if desired, so as to go decrease pre-echo effects. A further practical matter is that the stylus arrangement should preferably not trap dust but should deflect it away.

For most tone arm and cartridge configurations, the design of the reference patharm 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 tracking force a small fraction that of the main stylus- e.g. 1/4, 1/10, 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 offset tone arm systems in which warp is not compensated. (In some systems the secondary stylus shank may be relatively stiffly coupled to the cartridgebody). Further, the effective mass of the secondary stylus and related moving parts, together with the flexing or stiffness properties of the 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 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 6 GB 2 067 820 A 6 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 for the 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 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 lower 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 vertically fixed sensor. In an approximation, the vertical position of the tone arm may be used, by the methods known in the prior art. A transducer 89 (Figure 413) is mounted between the arm and vertical pivot so as to give 100 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 arm-cartridge resonant frequency (e.g. 10 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 Figs. 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- 115 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 120 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 (i.e. tone arm sensing and reference path arm sensing) in a manner shown schematically in Fig. 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 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 up to about several hundred Hz.

Prior art feedback tone arm systems are laiq out in the manner of Figs. 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 0).

In one prior art version shown in Fig. 1 OA, a tone arm sensor is arranged ty monitor the vertical velocity of the tone arm; the negative feedback loop thereby acts to provide damping for the 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 Fig. 1 OB, 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/6artridge 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 feedback tone arm techniques are thus only partially effective in dealing with the problems of record warp.

Improved tone arm systems in accordance with the present invention are shown in Figs. 11 and 12, utilizing the improved direct and indirect error sensing methods, shown in Figs. 3A and 313, respectively, which effectively eliminate the effects of arm/cartridge mechanical resonance from the error signal.

The embodiment shown in Fig. 11 employs an open loop and direct sensing of the reference path via a transducer 8 which is independent of vertical movements of the cartridge; this type of z v 7 GB 2 067 820 A 7 sensor is illustrated in Figs. 4A and 413. The reference path information is amplified in 55 amplifier 196 and applied to the tone arm (cartridge) vertical actuator 89, which may be a moving coil motor unit as in the feedback tone arms of the prior art. The signal polarity and gain are set to provide a vertical drive to the cartridge body corresponding to the warp and rumble undulations of the disc surface, a condition which will result in zero, or at least a minimum output from the signal cartridge at least in the warp frequency range. Depending on the gain setting, the system may also undercorrect or overcorrect vertical errors. When the gain is optimally set by gain control 198, the system will be able to effect correction through and above the arm/cartridge resonant frequency region only if the arm 70 displacements are positively related to the reference path information signal. That is, the arm must be damped, either mechanically or electrically, so as to eliminate the arm/cartridge resonance and obtain a positive displacement effect. The actuator damping and/or servo considerations discussed previously in relation to the turntable VNC embodiments are relevant here. An exemplary servo loop 200 around the tonearm vertical actuator is shown at the right hand portion of Fig. 11 and includes a tonearm vertical movement sensor 80 (the transducer is of the type described in connection with Fig. 413, however arranged to sense tonearm vertical movement) amplifier 202, adder/subtractor 204 and actuator drive amplifier 206. The gain and other characteristics of the loop 200 are set to obtain good overall warp and rumble compensation performance up to, say 20 Hz, with higher frequencies of reference path information being tapped off at 208 and utilized more conveniently in a pre-amp VNC, to be described. It should be noted that the actuator servo 200 is isolated and used only to obtain a positive displacement effect in the present invention, so that much higher values of loop 200 gain may be employed than in the closed loop arrangements of the prior art. The first prior art embodiment mentioned uses the actuator loop only to damp the tone arm and not as part of an overall servo system. The other prior art loops include the arm/cartridge transfer characteristic, severely limiting the usable gain before oscillation.

Switch 212 permits opening of loop 200 to deactivate the system for test or demonstration purposes. Display means 108 allows the reference path information to be observed.

Tone arm VNC embodiments may be AC coupled, DC coupled, or a combination thereof. A bias control (such as 210 in Fig. 11) can be used to set stylus force and/or to raise and lower the stylus. In an AC-only system the tracking force can be mechanically determined, as with conventional tonearms; a bias control may, however, optionally be employed to override the error signal for raising and lowering purposes.

Fig. 12 shows a further open loop tonearm VNC in which a reference path information deviation system of the indirect type shown in Fig. 313 is employed. The gains of amplifiers 214 and 214a are set to conform to the conditions shown in Fig. 313, whereby substantially pure reference path information is obtained, unadulterated by arm/cartridge resonance effects. The operation of the system is essentially the same as that of the open loop system of Fig. 11. Under, over, or optimal correction can be obtained, depending on the setting of the gain control. As in the system of Fig. 11, there are no particular constraints on the gain or other properties of the tonearm actuator servo loop 200. Actuator damping is required only to ensure effective warp and rumble compensation, not to prevent oscillators. Thus, the loop 200 gain is vertical actuator, in order to yield a good warp and rumble compensation in the frequency range of interest (e.g. up to 20 Hz).

For economy, tonearm sensors 80 and 80a may be the same sensor. Likewise, amplifiers 202 and 214 may be the same amplifier, with attenuation as required to provide appropriate levels (gains) at the inputs of the combining networks 216 and 204.

Claims (2)

Claims

1. 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 relative to the cartridge, whereby, in either alternative, the error signal represents vertical deviation of the disc relative to a reference plane.

2. Apparatus according to claim 1, wherein the second sensor comprises a second stylus on the cartridge sensing an unmodulated part of the disc.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1981. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.