CA1147268A - Disc reproducing system for compensating mechanical imperfections - Google Patents

Abstract

Abstract of the Disclosure The invention disclosed is primarily concerned with reducing the effect of spurious vertical deviations in the groove of a phonograph disc which result principally from record warp. Such imperfections can cause significant degradation of the reproduced signal. The playback apparatus of the invention comprises a turntable and cartridge and a sensor which senses vertical deviations of a disc on the turntable. The sensor controls the operation of an actuator which moves the disc in a direction to eliminate vertical deviation of the disc at the cartridge. According to a preferred arrangement the sensor senses vertical deviation of the disc in close proximity to the cartridge and with respect to a vertically fixed reference.

Description

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BACKGROUND OF THE INVENTION
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.

WARP
A general discussion of record warp is included 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 1 95~ of the warps below 8 llz. Peak physical amplitude heig~t oE

2 the warps was greatest at low frequencies at about 0.025 in.
maximum and decreased with increasing frequency.
4 Various problems are caused by record warp: the tone arm may bounce or sway with respect to the record surace due to the 6 vertical and, to some extent, lateral, forces which result as the 7 stylus attempts to track the varying record height. This may 8 cause not only variations in tracking force but bottoming of the 9 cartridge or complete loss of contact between the stylus and 10 groove. Such variations in tracking force from optimum will 11 often affect the reproduced signal at audible frequencies. In 12 addition to causing stylus and arm tracking problems, excessive 13 stylus excursions result in geometrically related distortions and 14 electro-mechanical non-linearity of the cartridge. Moreover, sub-audible warp signals can cause distortion by amplifiér overload 16 in electronic systems passing such low frequencies and, if 17 applied to the speaker system, can cause substantial woofer 18 movement that can result in extraneous noises and the distortion 19 of higher frequency audible signals, including doppler distortion. Further, the geometrical relationship of the stylus 21 and record groove is such that a warp results in a forward and 22 backward oscillation of the stylus tip over the recorded groove 23 information, which frequency modulates (advances and delays) the 2 reproduced signal causing "wow". Wow may also result from 2 variations in rotational speed as the stylus load on the record 2 groove varies.
2 The requirement to track warped phonograph records 2 satisfactorily has resulted, in prior art systems, in the 2 necessity to consider tonearm/cartridge/stylus/record geometry

3 very carefully and to seek the best combination, usually a 11~7Z68 compromise, of such factors as stylus and tone arm mass, ~one a~m b 2 damping, stylus compliance and damping, and tracking force so as _ ;
3 to provide a controlled tone arm resonance above the commonly

4 encountered warp fre~uencies, yet below the frequency oE the lowest recorded groove information. ~n arm resonance of lO H%
6 has been advocated by several designers: Keisuka Ikegami and Susumu Hoshimi, "~dvance in Turntable and Tone-Arm ~esign", 8 Journal of the Audio Engineering Society, May 1976, Vol. 24, No.
4, pp. 276-280 an~ Peter Rother, "The Aspects of Low-Inertia Tone-Arm Design", Journal of the ~udio Engineering Society, September 11 1977, Vol. 25, No. 9, pp. 550-559.
12 Although in principle the proper selection of tone arm and 13 car~ridge parameters may make possible the tracking of warped 14 records, the matching of arms and cartridges is often complicated in practice because of the wide variation in tone arms and 16 cartridges available. Further, even at the design stage, the 17 selection of optimum tone arm and cartridge parameters for warp 18 tracking may not be optimum for tracking higher Erequency groove 19 information. Even when the record is properly tracked, the problem of geometric and motor wow from warps still remains.
21 Various passive devices for tracking warped records are -22 known. These devices typically employ an element riding the 23 record surface and fixed or coupled to the pickup cartridge or 24 the tone arm in the vicinity of the cartridge. Such devices 2~ include both damped elements and undamped or fixed elements.
26 Exemplary prior art damped element devices are disclosed in ~.S.
27 Patents Nos. 2,572,712 to Fisher (spring loaded plunger), and 28 2,328,862 to Thompson (elastically mounted auxiliary stylus).
29 Fixed elements are disclosed in ~.S. Patents Nos. 3,228,700 to Andrews et al (felt pad at end of tone arm with cartridge --1 3~7Z6~

1 ¦pivoted in tone arm) and 3,830,505 to Rabinow (air bearing, ~
adjacent cartridge). It is known also to employ a dash pot or a 3 1 brush adjacent the cartridge to damp oscillations and assist in 4 tracking warps. Further, proposals for a relatively rigid element coupling the tone arm to the record surface are known.
6 It has also been suggested that the record be clamped or 7 weighted at its periphery and/or center in order to eliminate 8 warp.
9 An active prior art system for treating record warp is described in the following paper: "Overcoming 11 Record Warps and Low-Frequency Turntable Rumble in Phonographs", 12 Kenneth Clunis and Michael J. Kelly, Journal of the Audio 13 Engineering 5Ociety, July/~ugust 1975, Vol. 23, No. 6, pp. 450-14 458. In this system the cartridge output is used to servo control the vertical tone arm position to assist in tracking the 16 record warp. Similar systems are disclosed in U.S. Patents Nos.
17 3,623,734 to Sakamoto et al and 3,830,505 to Rabinow. It is 18 also known to provide a closed loop around the tonearm movements 19 only, in order to improve arm/cartridge dam~ing. Aspects of the present invention can significantly improve the performance of 21 these prior art tone arm systems.

2 Turntable rumble may result from turntable bearings, motor 2 drive systems, and environmental vibrations. Considerable 2 efforts are made by turntable manufacturers to eliminate rumble 2 from these sources.
2 Other turntable related disturbances are caused by acoustic 2 feedback (sonic and infra-sonic) from the loudspeakers, whereby the turntable and/or record may act as a receptor for the 3 vibrations, resulting in tonal coloration or even howling.

1 ¦Devices for reducing these effects include a fluid filled 2 ¦ turntable mat disclosed in U.S. Patent No. 3,997,174 to ¦Kawashima, and flexible turntable support cups in U.S. Patent 4 No. 4,054,291 to Maeda, both for providing a conforming damped support under warped records.
6 Notwithstanding these efforts, the main source of low 7 frequency annoyance is record pressing rumble or mold grain noise 8 from the disc itself. The spectrum of record pressing noise is 9 discussed by John Eargle, "Performance Characteristics of the Commercial Stereo Disc",Journal of the Audio Engineering 11 Society, August 1969, vol. 17, No. 4, pp. 416-422. Mold yrain 12 noise may extend generally to several hundred Hz.
13 Record pressing rumble and turntable rumble are reduced 14 conventionally by means of high-pass filters in the signal paths.
Optimum tonearm/cartridge resonance characteristics are also 16 useful in reducing low frequency rumble effects.

19 Prior art approaches to dealing with warp are directed primarily to the symptoms of warp. For example, the passive 21 tone arm to record surface contact devices and the closed loop 22 tone arm systems act chiefly as means to enhance the ability of 23 the cartridge and tone arm simply to track warps. Consequently, 24 such approaches may fail to correct other effects of warp and may degrade tracking ability and signal quality at non-warp 26 frequencies. With regard to rumble, prior art techniques have 27 been directed primarily to electrical filtering rather than to 2390 ¦dealing with e rumble mech~nism itself.

~1~7~6~3 It is the object of the present invention to reduce the .. effects of warp and rumble without in any way degrading the bandwidth or other performance characteristics of the signal channels themselves.
The invention will now be exp].ained in greater detail with reference to the accompanying drawings, in which:
Figure lA is a sectional side view of the top half of a - hypothetical record master on which silent grooves have been recorded;
Figure lB is a sectional side view of a hypothetical record pressing made from the master disc of Figure lA;
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 in accordance with one aspect of the present invention;
Figure 3A is a partially block generalized representation of direct reference path information sensing in accordance with one aspect of the present invention;
Figure 3B is a partially block generalized representation of indirect reference path information sensing in accordance with a ..
further aspect of the present invention;
Figure 4A is a partially cut away perspective view of one type of direct reference path 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 path-arm 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;

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Figure 9 is a partially cut away perspective view of the styli portion of another type of reference path-arm sensor;
Figure 10 is a block diagram of a turntable VNC (vertical noise compensator) system in accordance with the invention;
Figure ll is a cross sectional side view of one type of vertical actuator usable in a turntable VNC system;
Figure 12 is a cross sectional side view of a further - type of vertical actuator usable in a turntable VNC;
Figure 13 is a cross sectional side view of yet a further type of vertical actuator usable in a turntable VNC system;
Figure 14A is a pzrtly cross sectional side view of a turntable employing a vertical actuator;
. Figure 14B is a cut away partly cross sectional side view ~ of a further turntable employing a vertical actuator;
.. Figure 15A is a cross sectional side view of a retrofit type vertical actuator for use with a conventional turntable;
Figure 15B is a partly cross sectional side view of a : further type of retrofit vertical actuator for use with a conventional turntable;
i 20 Figure 16A is a block diagram of a prior art electrical tone arm damping arrangement;
Figure 16B is a block diagram of a prior art tone arm servo arrangement;
Figure 17 is a block diagram of a tone arm VNC system according to the present invention employing direct reference path sensing;
. Figure 18 is a block diagram of a further type of tone arm VNC system in accordance with the present invention in which indirect reference path sensing is employed;
Figure 19 is a block diagram of a further type of tone arm VNC system according to the present invention in which the error 1~7~6~3 signal is included in a negative feedback loop;
Figure 20 is a functional block diagram of a cartridge VNC according to the present invention;
Figure 21A is a block diagram showing exemplary com-bination interconnections between transducers in a cartridge VNC
according to the present invention;
Figure 21B is a block diagram showing an alternative exemplary combination interconnection between transducers in a cartridge VNC according to the present invention;
Figure 22 is a perspective view of a portion of a cartridge VNC;
Figure 23 is a perspective view of a portion of a further cartridge VNC; and Figure 24 is a block diagram showing a pre-amp VNC.
The present invention is based on observations taken from the situation shown in Fig. la and Fig. lb. Fig. la 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 an aspect of the invention, shown in Fig. 2, the reference surface may optionally be defined in the outting 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 be used as the reference path.
Fig. lb represents the situation after making a record pressing of the master. The vertical groove position is no longer con-stant but contains irregularities. In the case of warp, these are ~ ._ 1~ 47~

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 mold 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 1~7~6~3 1 dimensional irregularities from the back to the front of the 2 stamper during the pressing operatlon. The back surface -3 irregularities may include metallic crystals arising from the 4 replication process, patterns resulting from grinding operations to smooth the back surface, dirt and dust trapped between the 6 stamper and the die of the record press, and surface 7 irregularities of the die.
8 As the stamper thickness is some .007"to .OlO" the 9 rigidity or stiffness of the material will limit the shortest wavelengths which can be transmitted through localized bending 11 and distortion of the stamper. Thus, such wavelengths might be of the order of .020". This results in a highest frequency of 13 mold grain noise at the outer diameter of a twelve inch disc 14 (groove velocity about 20 inches per second) of the order of l k~lz.
Further sources of low frequency noise on the record 16 itself may include non-homogeneity of the pressing material and 17 geometric distortions due to differential cooling effects 18 resulting from rapid and uneven temperature changes in the die 19 face. Moreover, as discussed previously, noises are also contributed by the reproducing system -- namely, turntable and 21 environmental rumble and acoustically transmitted vibrations 22 of the turntable and disc.
23 Thus, in a conventional reproducing system, the reproducec 24 quantity "b" is obtained, employing the tone arm position as a reference. The quantity "b" thus includes undesired low frequency 26 noise components.
27 Closer consideration of this matter shows that the low 2a frequency noise components from all the sources mentioned above ar~
29 not inextric y mixed with the oriqinal signal modulations.

32 ~C) ~7-7~6~3 RatherJ the recorded signal quantity "a" remains intact and unharmed by the pressing and reproducing process and by mechanical imperfections in the re-producing 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. Preferably, the reproducer system acts to remove spurious deviations of the reference point so that again the disc surface is in effect flat (i.e., effectively vertically stable) in the vicinity of the stylus. Alternatively, the undulating reference point is used in determining the true signal quantity "a".
According to a broad aspect of the present invention, there is provided a gramophone disc playback apparatus comprising a turntable and a cartridge, sensor means arranged to sense vertical deviation of a disc on the turntable and an actuator responsive to a signal from the sensor means so to move the disc as to tend to eliminate vertical deviation of the disc at the cartridge.
In some embodiments the sensor means senses vertical deviation of the disc in close proximity to the cartridge and with respect to a vertically ` fixed reference.
In the context of the invention sensing, "in close proximity"
means within a small fractional part (e.g., less than about one-tenth) of the shortest wavelength which it is desired to correct. If the mold grain noises have wavelengths as short as about .020 inches, this implies sensing within about .002 inches - i.e., on an immediately adjacent land area. This would represent the limit of the technology of the invention. For lower frequency noises and warp it is, of course, unnecessary to sense in such close proximity.

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m ere are four m~in embodlments of the invention, which for conven-ien oe may be referred to as a Vertical Noise Ccmpensator (VNC). m e embodi-ments may be used separately or in combination.
In all embodlments, the vertical position of an unmodulated por-tion of the record is sensed at or in close proximity to the stylus. The information so derived may be referred to as reference path information. In some embodlments, the reference path information is sensed with respect to the arm or cartridge position; such information may be referred to as refer-ence path-arm information. Reference path information or reference path-arm information is preferably obtained via sensing means arranged to sense the land position adjacent the signal groove and in close proximity to the signal stylus. It is important that the vertical sensor responds substant-ially only to vertical information; in disc recording technology the term "vertical" conventionally has the meaning of perpendicular to the disc sur-face, or in an axial direction. A less desirable alternative, for the treat-ment of warp and very lcw frequency rumble effects only, is to sense the groove depth itself.
In the first embodbment, reference path information is obtained and used in a closed loop serv~ system including an actuator which moves the disc substantially vertically, at least in the vicinity of the pickup stylus.
Ideally, the result is that at least all vertical disc movement in the vicinity of the pickup stylus is removed, thereby allcwing the stylus effect-ively to track a warp-free and rumble-free record. m is embcdlment may be referred to as a disc VNC (or a turntable VNC, inasmuch as vertical actuation of the disc is most readily accomplished via mechanisms associated with the turntable).
A second embcdiment, which may be called a tone arm VNC, is an improvement of the closed loop tone arm techniques of the 1 prior art. In prior art systems, the error signals used incluc1e 2 arm-cartridge resonance components or other misleading information.
3 In the present invention, measurements avoiding these defects are 4 made and processed to control the tone arm; namely, reference path information is obtained and employed to control the tone arm and 6 optiona]ly to perform additional corrections via the other 7 embodiments. Low frequency components can be employed in a 8 turntable VNC for warp compensation and higher frequency 9 components can be used in a cartridge VNC or pre-amp VNC for rumble and mold grain noise reduction.
11 In a third embodiment, which may be referred to as a 12 cartridge VNC, reference path-arm information is obtained and 13 used to effect correction within or following the cartridge. The 14 reference path-arm information is cancelled either ele;ctrically or mechanically from the information provided by the signal 16 stylus. The reference path-arm information may optionally be 17 brought out of the cartridge in order to perform additional 18 corrections via the other embodiments.
19 In a fourth embodiment, which may be called a pre-amp VNC, the reference path-arm information is obtained and cancelled from 21 he audio output electronically in the pre-amplifier. This 22 rrangement can be used to reduce mold grain noise, rumble and 23 ome of the effects of warp.
24 For optimum mechanical and acoustical performance, it is referable to combine the turntable VNC or tone arm VNC methods 26 ith the cartridge VNC and/or the pre-amp VNC methods. ~or 27 xample, warp and rumble effects may be compensated up to, say, 28 0 Hz using a turntable VNC or a tone arm VNC, with frequencies 29 bove this being treated by a cartridge VNC or pre-amp VNC.
Compatibility characterizes all the embodiments of the 31 nvention. Conventional records may be played on reproducers f3 ,~

11~7~68 1 ¦ including the invention; conversely, records produced with the ~
2 ¦ optional deflned reference surface of the invention may be playe~
3 on conventional reproducers.
4 I The reduction of noise and tracking problems effected by the invention may permit a lower modulation 1evel and a higher groove density to be employed, leading to longer playing times 7 and/or smaller record diameters.
8 The fact that the invention solves the problem of low 9 frequency noise leads -to the further possibility that higher frequency components of the signal may be recorded on the disc in 11 electronic noise reduction encoded form, such as by the system 12 known as "Dolby B". This system, which treats only those signals 13 above about l kHz, produces a compressed signal which has a prove 14 history of being sufficiently compatible to permit the single inventory manufacturing and distribution of cassette tapes. Such 16 acceptance in the case of encoded discs would be much more 17 difficult, if not impossible, to achieve on a commercial basis if 18 it were necessary to treat the low frequency signals as well.
19 The encoded discs would, of course, preferably be played back using a noise reduction decoder for reduction of high frequency 21 record pressing noise and low level ticks and pops.
22 Thus the present invention can make a significant overal 23 contribution to the current performance and future--possibilities 24 of the conventional analogue disc record system.
These and other features of the present invention will b 26 appreciated as the following detailed description is read in 27 ¦¦ conne~tion w h the drawlngs.

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1 ~ DESCRIPTION Ol; TIIE EMBODIMENTS
2 1 In all of the embodiments to be described, only the essential inventive features will be shown or discussed in detail.
4 Thus, except where otherwise specified, amplifiers, attenuators, equalizers, differentiators, integrators, feedback loop 6 compensators, gain controls and the like are used as ordinarily required in electronic technology. Likewise, except as 8 otherwise discussed, the detailed design of styli, sensor transducers, actuator transducers, and the mechanical and electro-mechanical aspects of discs, cartridges, tone arms, drive 11 motors, and the like will not be treated.

REFERENCE PATH

14 In the several embodiments, the vertical position of an unmodulated portion of the record (reference path) is sensed at 16 or in close proximity to the signal pickup means, typically a 17 stylus. An important element ofthe invention is the recognition 18 that close proximity, high resolution sensing is useful for 19 educing mold grain noise. It follows however, that it is ecessary for the reference path to be as unblemished as possible.
21 or example, it should be free of scratches. Moreover, the groove 22 "horns" or ridges of material at the groove edges projecting into 23 he land area should preferably be removed during the disc 24 anufacturing process.
Polishing of the metal mold is a known method or removing 26 roove horns. Another method is shown in Fig. 2, which shows a 27 urther reference path cutting stylus ~ following the groove 28 utting stylus 6. The substantially flat bottom edge of the 29 eference path stylus not only removes the groove horns but cuts way residual rumble modulations on the lacquer master 2 and com-31 ensates for any vertical rumble introduced by the recording lathe.

32 perfectly quiet reference surface is thereby defined for use with he reproducing embodiments of the invention. In one embodiment, -l5-7~68 (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. Fig. 3B is described below under the heading "Reference Path Sensing By Indirect Means." Referring to Fig. 3A, the reference path information can be obtained directly, by means of a sensor which follows the signal stylus and cartridge laterally but is vertically independent, the main purpose being to avoid the problems of arm/cartridge vertical resonance.
Sensor 8 is attached 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 non-contacting sensors are usable, as described below in connection with Figs. 4A and 4B.
In the Fig. 4A version, the sensor section of the arm may be vertically fixed and the vertical displacement sensing transducer may com-prise non-mechanical means to sense the disc surface 9, such as by ultra-sonic or capacitive means or by a light beam and detector (e.g. light emit-ting 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 11~7~6~

then manifested as lateral variations, which are sensed by one or more photodetectors. This technique has the advantage of providing a relative-ly wideband error signal without any attendant mechanical resonances. Warp, rumble and mold grain noises at least up to several hundred }Iz can thereby be compensated.
In the example of Figure 4A, the ~one arm 10, which is vertically fixed, but free 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 similar to that of the sensor version of Figure 5, described hereinafter, generate the sensor signal.
In the simplest mechanical sensing embodiments 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 alone for rumble and mold grain noise reduction. It may also be used with a tone arm vertical position sensor to provide reference path information in a turntable VNC
or a tone arm VNC.
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 move-ment. 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 styli may be constructed of any of various long wearing materials compatible with disc surfaces and resistant to grooving effects, 1~726~
. .
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 trans-ducer may represent position, velocity, acceleration, or force (as with a pressure responding transducer~.

..

-17a-1147~68 The mechanical characteristics of the reference path senso r assembly can be optimized for the vertical sensing function -- _ 3 only. The frequency of vertical resonance (sensor flexing and 4 mass) should be placed substantially above the highest warp

5 ¦frequency, and indeed well into the audio band, in order to lextend the highest frequency of correction upwards, thereby to 7 Ireduce audible rumble and mold grain noise. The secondary styluc 8 ¦must be situated very close to the primary stylus -- e. g. within 9 ¦l mm for correction to about 50 HZ. Even closer spacing of about 10 ¦0.l mm for correction to about 500 Hz is preferable for reduction 11 ¦of mid-range mold grain noise.
12 ¦ The secondary stylus or sensor means may be positioned 13 ¦slightly in advance of the primary (signal) stylus to generate 14 ¦an anticipatory error signal. This is useful for relaxing the 15 Igain and phase requirements of electro-mechanical servo loops or 16 ¦for ensuring optimal error cancellation where mechanical or 17 ¦electrical phase shifts are present, as for example with low pass 18 ¦filtering of the reference path information.
19 l 20 ¦ REFERENCE PATH-ARM INFORMATION SENSING
21 ¦ Other embodiments of the invention employ reference path-22 arm information; this is the signal obtained by sensing the 23 distance between the reference path and arm (i.e. cartridge).
24 This signal will necessarily include tone arm movements and arm/cartridge resonance effec~s. A first sensor version simply 26 employs the vertical component information from the pickup 27 cartridge, as is known in the prior art. This method provides 28 useful information above the frequency of arm/cartridge resonance, 29 but is limited to cases and to the frequency range in which channel separation is deliberately reduced during disc cutting 31 (e.g. below l00 Hz).

~ ~ 1147Z68 1 In order to obtain reference path-arm information up to _ 2 igher frequencies it is necessary to provide a land sensor which--3 is independent of the signal stylus. Non-mechanical sensing 4 means such as those mentioned previously in connection with Fig. 4A
ay be used, however being fixed to the cartridge holding arm or

6 cartridge rather than to a vertically fixed arm. An example of

7 such a sensor is shown in Fig. 5. A stereophonic pickup cartridge

8 40 has a conventional cantilevered shank 42 and stylus tip 44

9 shown in engagement with a phonograph disc 9. A light source 46, such as a light emitting diode (LED) or diode laser, for example, 11 generates a beam of light to cause an area of the record in the 12 order of a millimeter in diameter, or smaller, to be illuminated.
13 The reflected light is received at one or more photo receptors 14 48 and 50, such as photo diodes, in the same manner as that of the descrip-tion of Fig. 4A. The light illumination location and 16 diameter are preferably chosen to illuminate the area in which the 17 stylus tip 44 is located and the adjacent land areas so that 18 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 mold grain noise at that point.
21 The output of receptors 48 and 50 may be fed to a differential 22 amplifier to provide an indication of local land variations; a 23 suitable circuit arrangement may be made responsive only to vertica L
24 land position variations and not to the total light reflected, whic 25 will depend upon groove modulations. Such techniques are used 26 in automatic slide focusing mechanisms, for example.
27 Examples of mechanical versions of reference path-arm 28 sensors are shown in Figs. 6-9. In each of the embodiments, a 29 dual stylus pickup cartridge is provided in which a conventional stylus tracks the groove informationcontent and the secondary 31 stylus senses warp and rumble information. In the cartridge -~NC

32 le diment o~ the invention, such a dual contact com~ination has 1 ¦the potential of improving both warp and rumble performance _ 2 ¦within a unitary, self-contained pickup cargridge. A third -- :
~: cartridge to record surface contact device may optionally be used, 4 such as a brush or damper of the prior art warp tracking devices ; 5 mentioned above.
6 An ideal reference path sensing method is shown in Fig. 6, 7 in which a shallow unmodulated pilot groove 52 is provided 8 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 11 warp and rumble frequencies. The embodiments of the invention 12 are then adapted to employ both the vertical and lateral 13 information provided. Fortunately, lateral warp and rumble are 14 not serious problems and it is sufficlent in a practical system to deal with vertical components only.
16 Referring to Fig. 7, in an arrangement suited to 17 conventional commercial phonograph records, the cartridge 62 has 18 a main stylus having a shank 42 and a stylus tip 44 tracking an 19 information carrying groove 54 of a phonograph record 9. The secondary shank 64 and stylus tip 66 are situated on one or both 21 sides of the main stylus and may fully or partially encircle it.
22 The contact area may be biased towards the outside of the record, 23 if desired, so that pre-echo effects are minimized in the 24 reference path information. The secondary stylus tip has a substantially flat bottom with operative dimensions sufficiently 26 large (e.g. some fraction of a millimeter) so that it rides 27 reliably on at least one land area and is thereby s~bstantially 28 unresponsive to lateral information and to any information content 29 of the groove, responding only to the land height variations which are a measure of the warp and rumble. The tip 66 is guided 31 laterally by the main s-tylus, and may be held in place by a 32 compliant coupling 68 which generally maintains the relative ~1~7~
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 (shcwn by way of exa~ple as element 74 in Figure 9) or an O-shape, surroundlng the main stylus. In Figure 8, the contact area is shcwn biased towards the outside of the record so as to decrease pre-echo effects. A
further practical matter is that the stylus arrange~ent 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 foroe in order to avoid affecting the side thrust force on the cartridge and to avoid reducing the main stylus force available for tracking warps and large amplitude signals. A secondary stylus tracking foroe a small fraction that of the main stylus -- e.g. 1/4, 1/lO, or even less -- is adequate to sense the relatively low amplitude and low frequency rumble components. m e secondary stylus is preferably csmpliantly connected to the cartridge body, the compliance preferably being substantially greater than that of the main stylus. The above complian oe and tracking foroe con-siderations apply primarily to off-set tone arm systems in which warp is not compensated. (In some systems the secondary stylus shank may relatively stiffly couple the stylus to the cartridge body). Further, the effective mass of the secondary stylus and related moving parts, together with the flex-; ing or stiffness pr~perties of the shank, should produ oe a high frequency resonance well above the highest mold of grain noise components of interest;
thus, a resonant frequency of at least 1-2 kHz would be suitable for the referen oe path sensor~ As with the design of conventional signal , ~

~1~7Z61 3 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 sensoT is used in one way or another to cancel corresponding vertical information from the signal stylus. Wholly mechanical cancellation arrangements may be used, as herein-after 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 arrange-ments of the main signal transducer in such a way as to cancel error informa-tion. In some arrangements, a separate or coordinated transducer may be pro-vided for the secondary stylus. The combination of signals, as by inter-connected coils, may be accomplished within the cartridge itself or the signals may be brought out for external combination. The signals may be used internal-ly 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 mold grain noise - 20 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).
REFERENCE PAT~ INFORMATION SENSING BY INDIRECT MEANS
As discussed previously, accurate reference path information sensing may be achieved directly, by means of a vertically fixed sensor. In an ap-proximation, the vertical position of the tone arm may be used, by the methods known in the prior art. A transducer of the types previously mentioned in connection with Figure 4B 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 1~7~6~3 .
and cartridge. This method of sensing is useful for providing reference path information below the arm-cartridge resonent frequency (e.g. 10 Hz).
It is possible to employ a relatively stiffly mounted secondary stylus to raise the 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.
In an improved method, 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 ~i.e., 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 ; 20 complementary form, whereby they can be cancelled to leave a differencesignal accurately indicative of the warp and rumble. The vertical arm position signal x is an indication of the arm 76 to reference plane dis-tance, 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 sub-stantially error free reference path information signal 86. The y signal can be derived by the various means which have been 11D~7Z68 1 discussed, using either the signal stylus 78A, in some cas~s - -_ 2 providing useful information up to about 30 Hz, or the secondary 3 stylus 78B, by which useful information ls obtained up to about 4 several hundred Hz.

7 Referring to Fig. lO, a bloclc diagram i.s shown of a 8 turntable VNC (vertical noise compensator) embodiment of the 9 invention, in which vertical posi-tion or displacemcnt errors in the vicinity of the pickup transducer are sensed to control the 11 vertical displacement of the disc in a closéd loop 12 servomechanism system. In effect, a reference plane for the 13 disc is set by the bias displacement level YO applied to 14 adder/substractor 90. The reference plane may optionally be made variable by means of control 92. An error signal on line 94 16 is developed by subtracting the amplified reference path 17 information signal on line 96 from the bias signal. The error 18 signal is applied to an amplifier 98 and a vertical actuator lO0 19 that controls the positioning of the disc 9 in a direction substantially normal to the disc surface, at least in the region 21 of the signal stylus. A signal ln2 relating to spurious 22 vertical displacement of the disc in the vicinity of the pickup 23 transducer is generated by the reference path sensor 104 and 24 applied to the amplifier 106.
For reduction of warp, the feedback system should be 26 effective at least in the frequency region of about 0.5 Hz 27 through about lO ~z. The system may be AC coupled, DC coupled 28 or a combination thereof. For reduction of rumble and mold grain 29 noise, the bandwidth of the correction action is extended into the audio range (e.g. up to several hundred Hz). The correction 31 may be wholly mechanical, via the vertical actuator.
32 Alternatively, the higher rumble frequencies (e.g. above 50 ~fz) 11~7Z68 l ¦ may be compensated via a cartridge VNC embodiment or a pre~amp ~
2 ¦ VNC. If desired, a crossover network can be used to divide I the treated frequency ranges appropriately. The overall system ¦ can thus make a signi~icant contribution both to reducing warp 5 I and its side cffects, and to reducing audible noise.
6 ¦ The spurious vertical displacement dealt with by the 7 I system may optionally be exhibited by display means ]08. Switch 8 110 may optionally be provided to break the loop in order to 9 switch off the correction action. Blocks 112 and 114 are described in connection with Fig. 11, below.
ll Means are required to control the disc position in 12 response to sensed variations in the vertical disc position in 13 the vicinity of the pickup transducer. The disc height may be 14 varied uniformly across its surface or only in the vicinity of th pickup transducer. The necessary linear or angular motions may 16 be accomplished via the disc, the turntable, the drive motor, 17 or the whole motorboard assembly.
18 A representative translational vertical actuator, 19 resembling a woofer loudspeaker with a flat cone, is shown in Fig. 11. A metal platter 116 is supported by drive shaft 118 21 in bearing 120. Platter 116 is provided primarily for 22 rotational mass, in order to reduce wow and flutter, and may be 23 reduced in mass or even eliminated in some designs. The upper 24 surface of platter 116 is conical to receive a light weight phono-graph record supporting turntable 122, which includes index 26 pin 124. Turntable 122 may be partly hollowed out, with a cover 27 member 126, to minimize weight. One or more "spiders", or 28 annular sets of corrugated material 128 and 130, suspend 29 turntable 122 from the platter 116. The corrugated material may be a light-weight fiber material of the type commonly used in 31 loudspeaker suspensions. A push rod 132 passing through the 32 center of d e shaft 118 ridiDg on ball bearing 134 is driven ~ --25-11~7~68 1 by a moving coil 136 which forms part of a moving coil mot~r -2 assembly 138, used here as a motion transducer for push rod 132. ~_ 3 Assembly 138 includes permanent magnet 140 and a tubular core _ 4 142 suspended from the magnet 140 to carry winding 136 in proper relation to the magnet. ~ pJa~ir of leads 144 and 146 are driven 6 by amplifier 98 (Fig. 10). Springs or other such support means 7 may be provided to balance the weight of the turntable assembly 8 and to vertically center the motor unit 138.
Turntable 122 is preferably of very light-weight material such as foamed plastic, to minimize the mass required to be 11 moved by the transducer 138. Also, the push rod to drive shaf-t 12 interface friction is reduced as low as practicable by using a 13 nylon or other low friction bearing, for example. Such bearings 14 may be dispensed with in some designs, particularly if an additional spider is used at the bottom of the pushrod 132.
16 Other types of mechanical coupling, such as hydraulic and 17 pneumatic, may also be used.
18 The motor assembly 138 preferably provides a positive 19 displacement output for a given electrical signal input; this avoids mechanical resonances of the system. This type of 21 performance may, for example, be provided by enclosing the motor 22 unit itself within its own servo loop, preferably including a 23 motor position sensor such as 112 in Fig. 10 (and Fig. 11) and a 24 suitable amplifier 114. Alternatively, if the motor unit simply provides an essentially undamped force, then it is necessary 2 to provide an appropriate amount of passive damping; this, 27 however, requires a large amount of drive amplifier power. A
28 more practical method is to ensure that the masses and compliance 2 of the turntable assembly result in a resonant frequency either 3 well below or well above the frequency range of interest. The 31 resonant frequency might be placed at about 50 Hz, for example, 3 to provide a well controlled behavior up to about 20 Hz. Passive ¦ mechanical damping using viscous materials may be employed -2 ¦ as required. These considerations also apply to the further -3 turntable VNC embodiments to be described. Such passive 4 displacement control methods do not interfere with operation of 5 ¦ the record player when the VNC is swi-tched off; this is not the 6 case, however, with the tone arm VNC embodiments to be described 7 in which electronic servos are preferred so that the arm may be 8 ¦ handled manually and can track normally when the VNC is switched of .
9 ~ In an alternative embodiment of the vertical actuator,

10 I shown in Fig. 12, the transducer 138 is located in a cylindrical

11 ¦ cut out 148 in modified platter 116a. Thus, the transducer 138

12 I rotates along with platter 116a. The push rod is thus

13 ¦ eliminated, along with its mass and friction. ~lowever, in order

14 ¦ to power the motor assembly 138, a pair of slip rings 150 or

15 ¦ other electrical transmission means is provided.

16 ¦ Alternatives to the vertical translation mechanisms which

17 ¦ have been described are directed to controlling the disc height

18 ¦ only in the vicinity of the pickup transducer. Such a method

19 ¦ may controllably rock or tilt the turntable so as to provide

20 ¦ vertical movement along the line traced by the pickup cartridge.

21 ¦ In the example of FIG. 13, a tiltable phonograph supportina

22 ¦ turntable 122a ls spaced above a metal platter 116b. Platter

23 ~ b is provided only for rotational mass, in order to reduce

24 ¦ wow and flutter if necessary, and may be dispensed with in some

25 ¦ designs. Turntable 122a has a downward conical annular portion

26 1 152 that is coupled to platter 116b for rotary motion but

27 ¦ permits rocking or tilting of the turntable. Drive shaft 118a

28 ¦ for platter 116b, powered by a suitable rotational drive motor,

29 ¦ is seated in bearings 120a. A hollow center is provided in

30 ¦ drive shaft 118a for tilt rod 132a that terminates in turntable

31 ¦ 122a and index pin 124. Tilt rod 132a is coupled by a rotary

32 joint 154 and rod 156 to a moving coil motor assembly 138, of I

11~7Z68 1 the type described above, located to provide lateral moti~on to~
2 the bottom end of rod 132a and hence, to tilt turntable 122a.
Alternatively, the entire turntable and drive motor 4 assembly can be tilted relative to the tohe arm and cartridge, in the manner of the embodiment of FIG. 14~. A turntable 158 6 driven by a motor 160 through drive shaft 162 is supported by 7 sub-base 164 suspended by compression springs 166, 168 from a base 170, to which the arm and cartridge assembly 172 is mounted.
9 A moving coil transducer assembly 138 controllably moves one end of the sub-base 164 to tilt the turntable relative to the arm/
11 cartridge assembly.
12 In a variation of the arrangement of Fig. 14A a 13 displacement transducer is located so as to move vertically the 14 turntable and drive shaft, possibly including the drive motor.
15 For economy, the displacement transducer may be made:a part of 16 the drive motor. Fig. 14B shows an arrangement in which the 17 entire drive motor and turntable are moved vertically. A
18 corrugated annulus 130a, similar to material 130, suspends the 19 motor 160 from an annular support 171 from the motorboard 164 to 20 permit vertical movement.
21 Further tilting embodiments useful as retrofits for 22 existing turntable structures are shown in Figs. 15A and 15B. A
23 conventional turntable 164a, driven by shaft 162a, has a tilting 24 surface assembly 174 resting on its top surface. Assembly 174 25 includes a tilting turntable member 176 generally coextensive 26 with the size and shape of the underlying existing turntable 27 164a and having an extended periphery wlth downward depending 28 edges. Turntable 176 has a downward depending conical annular 29 portion that contacts a cone shaped member 177 that slips over th 30 index pin 179 on the underlying turntables 164a and spaces the 31 extending portions of turntable 176 above the underlying turntabl 32 to permit tilting. In order to provide rotational coupling betwei ~n -~8-~ 7~;~d6~3 1 the turntables, an annulus 178, of sufficient weight and surface _ 2 friction to couple securely to the underlying turntable surface, = _ is coupled to a hollowed out under portion of the turntable 176 b~
4 means 180 that are rotationally rigid yet yieldable to rocking S motion. Corrugated material such as described above in connectio - with Figs. 11 and 12 is suitable. The downward depending edges 7 182 of the upper turntable are metallic so that an electromagnet 8 184 arranged to controllably pull on the edge at one location 9 controls the turntable tilt.
In Figure 15B, an alternative rocking mechanism 183 11 engages the top of a record 185, being placed in position after 12 the record is put on the turntable. A linear motor 138, as 13 described in relation to Fig. 13, provides the re~uired rocking 14 action through rotary joint 154.
For simplicity in retrofitting such warp compensation 16 devices, the warps themselves may be sensed in an approximate 17 fashion at the record edge, at a position substantially where 18 the pickup cartridge intersects the record, using a lamp and 19 photocell assembly 192. This sensing approach is most accurate at the record edges where warp is greatest. Other sensing means 21 as described above, can be used if greater accuracy is desired.
22 An advantage of a disc reproducing system employing a 23 turntable VNC, particularly of the vertical translation types of 24 Figs. 11, 12 and 14B, is the possible use of a tone arm essential~ Y
fixed vertically and mounted only for lateral movement relative 26 to the record. This simplifies reference path sensing, which 27 may be done directly, as in Figs. 4A and 4B. It also follows 28 that if a pivot is used either for accommodating any residual 29 vertical motion of the disc or for placing the stylus on the disc, the pivot can be situated very close to the cartridge 31 without fear of introducing warp wow. If desired, the vertical 32 ~ actuator may be controlled so as to effect engagement and 1.1~7Z68 1 disengagement of the record and the cartridge. The vertic,al - _ -2 actuator can thus eliminate the need for an automatic lifting _ 8 mechanism associated with the tone arm.
4 The substantial elimination of record warp by turntable S VNC reproducers is accompanied by a corresponding rcduction in 6 the many problems previously mentioned in relation to warp.
7 Moreover, the effective existence o~ "w~rp-free" records makes 8 possible the design of tone arms, pickup transducers, and signal 9 styli taking this operating condition into account. For example, it will be easier to optimize the design of an offse-t 11 tone arm which is required to move in a substantially lateral 12 direction only. The reduced tracking pressure and maximum 13 excursion of the stylus result in a different set of 14 electromechanical parameters for cartridge design. The ]atter observation applies also ~o the tone arm VNC reprodu~ers to be 16 described below.

18 Prior art feedback tone arm systems are laid out in the 19 manner of Figs. 16A and 16B. The tone arm vertical actuator is an electromechanical transducer so arranged to apply a force to 21 the tone arm or cartridge in a direction normal to the disc 2 surface, in response to an electrical signal from the sensor and amplifier. A further motor unit can be employed to perform 24 similarly on a horizontal basis (or 45/45).
In one prior art version shown in Fig. 16A, a tone arm 2 sensor is arranged to monitor the vertical velocity of the 2 tone arm; the negative feedback loop thereby acts to provide 2 damping for the tone arm. The damping depends on loop gain, 3 which must not be so high as to interfere with the tracking of warps, which are tracked passivelv. The overall result is that 31 the tone arm/cartridge resonance effects are reduced but that the 32 feedback loop does not directly enter into tracking of the disc surface. I

~7~8 In other prior art versions, shown in Figure 16B, 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 feedback tone arm techniques are thus only partially effective in dealing with the problems of record warp.
Improved tone arm systems in accordance wi~h the present invention are shown in Figures 17, 18 and 19. Figures 17 and 18 utilize the improved direct and indirect error sensing methods, shown in Figures 3A and 3B, respectively, whic.h effectively eliminate the effects of arm/cartridge mechanical resonance from the error signal. The embodiment of Figure 19 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. In some of the embodiments, both vertical and lateral operation ~or 45/45) are possible, but for simplicity the discussion will be confined to vertical operation only.
The embodiment shown in Figure 17 employs an open loop and direct sensing of the reference path via a transducer 8 which is in-dependent of vertical movements of the cartridge; this tvpe of sensor is illustrated in Figures 4A and 4B. The reference path information is amplified in 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 ~ 1~7~68 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 displacements are positively related to the reference path in-formation signal. That is, the arm must be damped, either mechanical-ly or electrically, so as to eliminate the arm/cartrige resonance and obtain a positive displacement effect. The actuator dampling and/or servo considerations discussed previously in relation to the turn-table VNC embodiments are relevant here. An exemplary servo loop 200 around the tonearm vertical actuator is shown at the right-hand portion of Figure 17 and includes a tonearm vertical movement sensor 80 ~the transducer is of the type described in connection with Figure 4B, 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 informa-tion 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 displace-ment 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 over-all servo system. The other prior art loops include the arm/cartridge transfer characteristic, severely limiting the usable gain before oscillation.

7~68 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 Figure 17) 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.
Figure 18 shows a further open loop tonearm VNC in which a reference path information derivation system of the indirect type shown in Figure 3B is employed. The gains of amplifiers 214 and 214a are set to conform to the conditions shown in Figure 3B, whereby sub-stantially 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 Figure 17.
Under, over, or optimal correction can be obtained, depending on the . setting of the gain control. As in the system of Figure 17, there are : 20 no particular constraints on the gain or other properties of the tone-arm actuator servo loop 200. Actuator damping is required only to ensure effective warp and rumble compensation, not to prevent oscilla-tions. Thus, the loop 200 gain is set to provide adequate damping or a positive displacement effect of the 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.

11~7268 1 1 Fig. l9 shows a version of a tone arm VNC in accoraance _ 2 with the invention in which the warp and rumble error signal ~_ 3 ~ itself is enclosed within a negative feedback loop. The loop 4 automatically provides tone arm movements which correspond to S those of the disc surface. As in the previous embodiments, it 6 is essential that the correct error signal should be employed --7 i.e. reference path information without interference from 8 arm/cartridge resonance effects.
9 Consider the operation of the system of Fig. l9 if the outer loop 201 were broken at point A. The arm would remain 11 stationary throughout the frequency range of concern (0.5 Hz - 20 12 Hz), provided that sufficient gain is employed in the vertical 13 actuator servo loop (inner loop 200); the gains of amplifiers 14 202 and 206, for example, may be set to ensure this condition without fear of oscillation, as there are no unusual oscillation 1 provoking elements within the inner loop. Under this condition 17 the output of the reference path-arm sensor is pure reference 18 path information. If the outer loop is then closed at point A
19 the arm will follow the reference path but at an amplitude depending on the overall outer loop gain. The gain of amplifier 21 218 may be set to provide the desired factor of reduction in 2 output from the reference path-arm sensor, which,it should be 23 noted, provides pure reference path information but at a reduced 2 amplitude. Thus, the invention remedies the problems of the 2 prior art tone arm feedback systems (Fig. 16B) in effectively 26 eliminating the troublesome arm/cartridge transfer characteristics 27 from the feedback loop, whereby the arm is actuated by and 2 follows the reference path.

CARTRIDGE VNC
3 Fig. 20 shows a functional black diagram of cartridge VNC
3 embodiments of the invention. Warp, rumble, and mold grain ~7~68 noise from the main stylus ~groove-arm sensor 220) are cancelled from the output signal in one way or another by arrangement 222, which may, for example, be mechanical, magnetic or electrical in nature, using the error information provided by the reference path-arm sensor 224.
This is preferably accomplished entirely within a unitary dual stylus ; cartridge assembly. The operating parameters can thus be fixed and pre-set by the cartridge manufacturer, whereby installation either - on an original or replacement basis is a simple matter.
The reference path-arm sensor 224 may be of the types pre-viously discussed and shown in Figures 5 - 9. Two independent sets of transducers 226 and 228 ~e.g. magnets and coils) may be provided within the cartridge, as illustrated schematically in Figures 21A
and 21B, which show exemplary combination interconnections. The transducer outputs may be further divided to accommodate the left and right signals. A low pass electrical or mechanical filter may be provided (e.g. 300 Hz low pass), with external control of the characteristics, if desired, to reduce any non-rumble or non-mold grain noise components sensed by the secondary stylus, such as might be caused by scratches or rough groove edges. Reference path informa-tion terminals, shown in Figures 20 and 21 may be provided, whereby low frequency components may be utilized by turntable VNC or tone arm VNC embodiments.
In a further version, shown by way of example in Figure 22, the secondary stylus shank 72, instead of coupling to its own trans-ducer, is coupled to the stator components of the signal transducer, so that rumble information sensed by the secondary stylus 70 can-cels out rumble information sensed by the primary stylus 44.
That is, for rumble frequencies there is no relative movement between e.g. the iron or magnet 230 and the coils 232. Various other analogous cancellation arrangements may be employed, using ~726~
moving coils, moving iron, moving magnets and the like. Two like armature ` elements may share the same stator environment, such as two coils, each associated with its stylus, sharing a common magnetic field.
The aforementioned stator components of the signal transducer may include the cartridge body itself. However, for optimum noise cancellation, the mass to be displaced by the secondary stylus should be as low as possible.
Thus the signal stylus and the secondary stylus preferably have independent compliant connections 231 and 233 to the cartridge body. For optimum tracking, the secondary stylus preferably accounts for the lesser portion of the overall cartridge tracking force and the lesser portion of the overall vertical stiffness.
Further mechanically coupled arrangements are possible in which the movements of the secondary stylus cancel corresponding movements of the principal stylus. One example is shown in the embodiment of Figure 23, where-in a U-shaped stylus tip 74 partially surrounds the main stylus tip 44. An A-shaped shank 234 connects tip 74 to first and second members 274 and 276 that are fixed relative to the cartridge. The main stylus shank 42 is attached to the cross piece of shank 234 at point 278. The distal end of shank 42 is connected to a conventional magnet or iron piece 242 which co-operates with coils 244. In operation, secondary stylus 74, riding on more than one land area adjacent the groove engaged by main stylus tip 44, responds to rumble frequency components and cancels movements of magnet 242 due to corresponding responses of the main stylus.
PRE-AMP VNC
The reference path-arm signal may be combined with the main stylus signal in the electronic manner shown in Figure 24. This embodiment functions in essentially the same manner as the cartridge VNC described previously and is primarily applicable to sensors of the types shown in Figures 4A and 4B, and 5, or to separate transducer versions of Figures 6 -9. Each sensor has a respective pre-amplifier 244 and 246. ~he null control j 1 is set for optimum noise cancellation in combiner 248. If ~ - _ ~
2 desired, electrical filtering, signal delay correctors or other _ _ processing may be inserted into either or both signal chains in 4 jorder to optimize the noise reduction effect under all operating 5 conditions. For example, a transient noise suppressor 252 can be 6 1 employed to reduce the effect of "pops" and "clicks" caused by 7 scratches on the record and the like.
8 The signal on the disc being reproduced can be encoded in 9 ~ electronic noise reduction form, for example using high frequency 10 ¦ compression in order to reduce high frequency noise when played 11 back, using high frequency expansion by means of the system known 12 as 'IDolby s". Such high frequency noise reduction will be most 13 effective psycho-acoustically when combined with low frequency 14 disc noise reduction as provided by the various embodiments of the invention. The system of Fig. 24 can provide for playback 16 of a Dolby B encoded disc by optionally providing a Dolby B type 17 decoder 256, preferably following subtractor 248 or, less 18 desirably, in the signal chain of blocks 220 and 244 before ~ subtractor 2 29l 32~ _37-

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A gramophone disc playback apparatus comprising a turntable and a cartridge, sensor means arranged to sense vertical deviation of a disc on the turntable and an actuator responsive to a signal from the sensor means so to move the disc as to tend to eliminate vertical deviation of the disc at the cartridge.

2. Apparatus according to claim 1, wherein the actuator moves the disc bodily in the vertical direction.

3. Apparatus according to claim 1, wherein the actuator tilts the axis of rotation of the disc to move the disc substantially vertically at the cartridge.

4. Apparatus according to claim 1, 2 or 3, wherein the sensor means senses vertical deviation of the disc in close proximity to the cartridge and with respect to a vertically fixed reference.

5. Apparatus according to claim 1, wherein the sensor means comprises a sensor sensing vertical deviation of the cartridge with respect to a fixed reference, a cartridge sensor sensing vertical deviation of the cartridge relative to the disc, and means combining signals from the two sensors in opposition to provide a signal representing vertical deviation of the disc at the cartridge.

6. Apparatus according to claim 5, wherein the cartridge is carried by a tone arm and the said sensor is an arm sensor sensing vertical deviation of the tone arm.

7. Apparatus according to claim 1, 2 or 3, wherein the cartridge is vertically fixed during reproduction from the disc.

8. Apparatus according to claim 1, 2 or 3, wherein the sensor means include means sensing vertical deviation of the disc relative to the cartridge and wherein a compensating mechanical or electrical coupling at least partially removes from the audio signal provided by the cartridge the effects of vertical deviation of the discs relative to the cartridge at frequencies higher than those at which the actuator operates to end to eliminate vertical deviation of the disc at the cartridge.