GB2068627A - Disc reproducing system for compensating mechanical imperfections in disc - Google Patents

Abstract

A cartridge has a main stylus 42, 44 tracking a modulated disc groove and a secondary stylus 70, 72 tracking unmodulated disc lands 9. The main stylus vibrates a moving iron armature 230 while the secondary stylus vibrates coils 232, whereby rumble information sensed by both styli cancels out. The styli have independent mountings 231, 233 such that the secondary stylus is the more compliant. The cancelling action may be achieved in various other ways such as mechanically coupling the styli or electrically cancelling the signals from transducers individual to the styli. <IMAGE>

Description

1 GB 2 068 627 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, including record pressing rumble. Such imperfections can cause significant degradation of the reproduced signal.

Turntable rumble may resultfrorn turntable bearings, motor drive systems, and environmental vibrations. 10 Considerable efforts are made by turntable manufacturers to eliminate rumble from these sources.

Otherturntable related disturbances are caused by acoustic feedback (sonic and infra-sonic) from the loudspeakers, whereby the turntable and/or record may act as a receptorfor the vibrations, resulting in tonal colouration or even howling. Devices for reducing these effects include a fluid filled turntable mat disclosed in U.S. Patent No. 3,997,174 and flexible turntable support cups in U.S. Patent No. 4,054,291, both for 15 providing a conforming damped support under warped records.

Notwithstanding these efforts, the main source of low frequency annoyance is record pressing rumble or mould grain noise from the disc itself. The spectrum of record pressing noise is discussed by John Eargle, "Performance Characteristics of the Commercial Stereo Di9c", Journal of theAudio Engineering Society, August 1969, Vol 17, No 4, pp 416-422. Mould grain noise may extend generally to several hundred Hz.

Record pressing rumble and turntable rumble are reduced conventionally by means of high-pass filters in the signal paths. Optimum tone arm/cartridge resonance characteristics are also useful in reducing low frequency rumble effects. Filtering does not deal with the rumble mechanism itself.

It is the object of the present invention to reduce the effects of rumble without in any way degrading the bandwidth or other performance characteristics of the signal channels themselves.

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

Figure 1 b represents the situation after making a record pressing of the master. The vertical groove 35 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 different dies and stampers are used; the disc thus contains local variations of thickness. Such imperfections 40 are caused by the pressure transmittal of dimensional irregularities from the back to the front of the stamper during the pressing operation. The back surface irregularities may include metallic crystals arising from the replication process, patterns resulting from grinding operations to smooth the back surface, dirt and dust trapped between the stamper and the die of the record press, and surface irregularities of the die.

As the stamper thickness is some 0.18 mrn to 0.25 mm the rigidity or stiffness of the material will limit the 45 shortest wavelengths which can be transmitted through localized bending and distortion of the stamper.

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

Further sources of low frequency noise on the record itself may include non-homogenity of the pressing 50 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 55 arm position as a reference. The quantity "b" thus 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 60 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 0e, effectively vertically stable) in the vicinity of the stylus. Alternatively, the 11 11 undulating reference point is used in determining the true signal quantity a.

Thus, in accordance with the present invention, there is provided a gramophone cartridge with a stylus for 65 2 GB 2 068 627 A 2 providing an audio signal representing the groove modulation of a disc, and means carried by the cartridge for sensing an unmodulated part of the disc in close proximity to the stylus, the sensing means being substantially more compliant than the stylus.

Further according to the invention there is provided a gramophone disc playback apparatus comprising a cartridge with a stylus for providing an audio signal representing the groove modulation of a disc being played back, means carried by the cartridge for sensing an unmodulated part of the disc in close proximity to the stylus, the sensing means being substantially more compliaritthan the stylus, and a compensating mechanical or electrical coupling between the stylus and sensing means wherebythe sensing means at least partially remove from the audio signal the effects of vertical deviations sensed by the sensing means.

In the context of the invention sensing "in close proximity" means within a small fractional part (e.g. less 10 than about one-tenth) of the shortest wavelength which it is desired to correct. if the mould grain noises have wavelengths as short as about 0. 50 mm, this implies sensing within about 0.05 mm - 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.

In all embodiments, the vertical position of an unmodulated portion of the record is sensed at or in close 15 proximity to the stylus. The information so derived may be referred to as reference path information, which 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 substantially only to vertical information; in disc recording technology the term'vertical" conventionally has the meaning of perpendicular to the disc surface, or in an axial direction. A less desirable alternative, for the treatment of 20 warp and very low frequency rumble effects only, is to sense the groove depth itself.

The reference path-arm information is cancelled either electrically or mechanically from the information provided by the signal stylus. The reference path-arm information may optionally be brought out of-the cartridge in order to perform additional corrections via the mechanisms of related inventions (see below).

In another embodiment, the reference path information is cancelled from the audio output electronically in 25 the pre-amplifier. This arrangement can be used to reduce mould grain noise, rumble and some of the effects of warp.

This application and other applications were divided out of my application 48108/78 and reference may be made to the published application Serial No 2 013 957 for a complete discussion of the present and related inventions which are all versions of or related to Vertical Noise Compensation (VNC) briefly characterised as 30 follows:

Application No. - Serial No.

48108178 2013957 Disc VNC 00418/81 Cartridge and pre-amp VNC 00419/81 Tone arm VNC 40 00420/81 Tone arm VNC 00421/81 Second cutting stylus 45 00422/81 Reference plane sensing Briefly disc VNC involves compensation by moving the disc, cartridge and pre-amp VNC involve mechanical or electrical cancellation of rumble information in the cartridge or pre-amplifier, tone arm VNC 50 involves compensation by servo control of the tone arm, -second cutting stylus" relates to the subject matter of Figure 2 herein and "reference plane sensing" refers to the concept explained below in conjunction with Figure 3B.

For optimum mechanical and acoustical performance, it is preferable to combine the turntable VNC or tone arm VNC methods with the cartridge VNC and/or the pre-amp VNC methods. For example, warp and 55 rumble effects may be compensated up to, say, 20 Hz using a turntable VNC or a tone arm VNC, with frequencies above this being treated by a cartridge VNC or pre-amp VNC.

Compatibility characterises 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 of Figure 2 herein may be played on conventional reproducers.

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

The factthatthe invention solves the problem of lowfrequency noise leads to the further possibility that higherfrequency 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 treat only those signals above65 4 3 GB 2 068 627 A 3 about 1 kHz, produces a compressed signal which has a proven history of being sufficiently compatible to permit the single inventory manufacturing and distribution of cassette tapes. Such acceptance in the case of encoded discs would be much more difficult, if not impossible, to achieve on a commercial basis if it were necessary to treat the low frequency signals as well. The encoded discs would, of course, preferably be played back using a noise reduction decoder for reduction of high frequency record pressing noise and low level ticks and pops.

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

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

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

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

Figure 2 is a partly sectional side view of a master disc during the cutting process, using a conventional 15 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 3B is a partially block generalized representation of indirect reference path information sensing.

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

Figure 4B is a partially cut away perspective view of a further type of a direct reference path information 20 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. 25 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 functional block diagram of a cartridge VNC according to the present invention.

Figure 1 1A is a block diagram showing exemplary combination interconnections between transducers in a cartridge VNC according to the present invention.

Figure 1 1B is a block diagram showing an alternative exemplary combination interconnection between transducers in a cartridge VNC according to the present invention.

Figure 12 is a perspective view of a portion of a cartridge VNC.

Figure 13 is a perspective view of a portion of a further cartridge VNC.

Figure 14 is a block diagram showing a pre-amp VNC.

1 In all of the embodiments to be described, onJy the essential inventive features will be shown or discussed in detail. Thus, except where otherwise specified, 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 transducers, and the mechanical and electro-mechanical aspects of discs, cartridges, tone arms, 40 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 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 45 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 Figure 2, which shows a further reference path cutting stylus 4following the groove cutting stylus 6. The substantially flat bottom edge of the reference path stylus not only removes the groove horns but cuts away 50 residual rumble modulations on the lacquer master 2 and compensates for any vertical rumble introduced by the recording lathe. A perfectly quiet reference surface is thereby defined for use with the reproducing embodiments of the invention. In one embodiment, (Figure 6), a pilot groove provides the reference path. In this case, the reference path cutting stylus cuts an unmodulated groove adjacent the signal groove.

Reference path information sensing (i.e. sensing of the distorted undulating reference path) is a key 55 element of the various embodiments. A generalized representation of reference path sensing is shown in Figures 3A and 3B. Referring to Figure 3A, the reference path information can be obtained directly, by means of a sensor which follows the signal stylus laterally but is vertically independent. Sensor 8 is attached to a reference plane. In a conventional turn-table 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 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, 65 4 GB 2 068 627 A 4 which results in a relatively high vertical resonant frequency of the tone arm. The stylus maybe used alone for rumble and mould grain noise reduction. It may additionally be used with a tone arm vertical positions sensorto provide reference path information in a turntable VNC or a tone arm VNC.

In the Figure 4A version, the sensor section of the arm may be vertically fixed and the vertical displacement sensing transducer may comprise non-mechanical means to sense the disc surface 9, such as 5 by ultrasonic or capacitive means or by a light beam and detector (e.g. light emitting diode and photodiode).

A light beam focused preferably at the point of contact of the stylus, but with a beam diameter encompassing at least one land area, may be angularly directed atthe 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. 10 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, has a U-shaped end 12 in which a cartridge 14 is pivoted on crosswise pin 16. Alight source 18 and detectors 20,22 arrangement similar to that of the sensor version of Figure 5, described hereinafter, generate the sensor signal " Afurther mechanical version of reference path sensing, shown in Figure 413, employs a separate stylus laterally coupled to, butvertically 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 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 pivotwithin housing 32 functions as a 20 sensor of the secondary stylus 38 vertical movement. The secondary stylus is arranged to lift from the record whenever the signal stylus is lifted. Preferably, the stylus 36 is dimensioned to contact the land areas adjacentthe signal stylus 38 of cartridge 14.

Secondary styli may be constructed of any of various long wearing materials compatible with disc surfaces and resistaritto grooving effects, such as sapphire or diamond. Sensor transducers may be any of 25 various types known in the art, including, but not limited to: electromagnetic, photoelectric, Hall effect, magneto-diode, potentiometric, or variable resistance, capacitance. The untreated 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 highestwarp 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 gain noise. The secondary stylus must be situated very close to the primary stylus -- e.g. within 1 mm for correction to about Hz. Even closer spacing of about 0.1 mm for correction to about 500 Hz is preferable for reduction of mid-range mould grain noise.

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

Other embodiments of the invention employ reference path-arm information; this is the signal obtained by 40 sensing the distance between the reference path and arm (ie cartridge). This signal will necessarily include tone arm movements and arm/cartridge resonance effects. Afirst 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 sensing means such as those mentioned previously in connection with Figure 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 Figure 5. A stereophonic pickup cartridge 40 has a conventional cantilevered shank 42 and stylus tip 44 shown in 50 engagement with a phonograph disc 9. A light source 46, such as a light emitting diode (LED) of diode laser, for example, generates a beam of light to cause an area of the record in the order 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 Figure 4A. The light illumination location and diameter are preferably chosen to illuminate the area in which the stylus tip 44 is located and the adjacent 55 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 amplifierto 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 60 slide focusing mechanisms, for example.

Examples of mechanical versions of reference path-arm sensors are shown in Figures 6-9. In each of the embodiments, a dual stylus pickup cartridge is provided in which a conventional stylus tracks the groove information content and the secondary stylus senses warp and rumble information. In the cartridge VNC embodiment of the invention, such a dual contact combination has the potential of improving both warp and 65 GB 2 068 627 A 5 rumble performance within a unitary, self-contained pickup cartridge. A third cartridge to record surface contact device may optionally be used, such as a brush or damper of the prior art warp tracking devices mentioned above.

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

Referring to Figure 7, in an arrangement suited to conventional commercial phonograph records, the 10 cartridge 62 has a main stylus having a shank 42 and a stylus tip 44 tracking an information carrying groove 54 of a,phonograph record 9. The secondary shank 64 and stylus tip 66 are situated on one or both sides of the main stylus and may fully or partially encircle it. The contact area may be biased towards the outside of the record, if desired, so that pre-echo effects are minimized in the reference path information. The secondary stylus tip has a substantially large (e.g. some fraction of a millimeter) so that it rides reliably on at 15 least one land area and is thereby substantially unresponsive to lateral information and to any information content of the groove, responding only to the land height variations which are a measure of the warp and rumble. The tip 66 is guided laterally by the main stylus, and may be held in place by a compliant coupling 68 which generally maintains the relative positions of the two styli but does not interfere with the stylus movements.

Another view of the styli is shown in Figure 8. An elongated block shaped tip 70 for secondary sty)u -72 spanning the land between several grooves is located ahead ol 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 decrease pre-echo effects. A further practical matter is that the 25 stylus arrangement should preferably not trap dust but should deflect it away.

For most tone arm and cartridge configurations, the design of the reference path-arm information sensors should be such that the tracking force of the main stylus should preferably comprise the main portion of the overall cartridge tracking force in order to avoid affecting the side thrust forces on the cartridge and to avoid reducing the main stylus force available fortracking warps and large amplitude signals. A secondary stylus 30 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 compensat ed. (In some systems the secondary stylus shank may be relatively stiff ly coupled to the cartridge body). Further, the effective mass of the secondary stylus and related moving parts, together with the flexing or stiff ness 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 40 pass mechanical filter may be incorporated if desired, so that information provided by the secondary stylus is band limited for reduced sensitivity to dust and surface scratches.

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

Alternatively, secondary stylus information may interact in the magneticor 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 50 external combination. The signals may be used internally and also brought outfor use in other embodiments of the invention. For example, the high frequency components from the reference path sensor may be utilized internally or brought outto the preamplifierfor mould grain noise reduction (cartridge NINC or pre-amp VNC), and the lowfrequency components may be brought out for dealing additionally with warp via actuation of the turntable ortone arm (turntable VNC ortone 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, bythe methods known in the prior art. Atransducer 89 is mounted between the arm and vertical pivot so as to give an output related to the vertical position or angle of the tone arm and cartridge. This method of sensing is useful for providing reference path information below the arm-cartridge resonant frequency (e.g. 10 Hz). It is 60 possible to employ a relatively stiff ly mounted secondary stylus to raise the resonant frequency of the tone arm. The reference path-arm sensors described previously in connection with Figures 5-9 are primarily useful above the resonant frequency. At or near the resonant frequency, phase and amplitude errors will be introduced into the reference path information by both of these methods. Hence, these methods are useful only at warp or rumble frequencies somewhat removed from the arm- cartridge resonant frequency --that is, 65 6 GB 2 068 627 A 6 normally below about 5 Hz and above 20 Hz.

In an improved metod, a substantially error-free warp and rumble sensing signal can be derived, enabling the corrective action to be applied over the whole frequency range of interest (e.g. 0.5 Hz up to several hundred Hz) without interference from arm/cartridge resonance. The method employs a combination of the first and second sensing methods described in the previous paragraph above (ie, tone arm sensing and reference path-arm sensing) in a manner shown schematically in Figure 3B. This approach is based on the recognition that both signals contain the same error signals (from arm- cartridge interaction effects), but in complementary form, whereby they can be cancelled to leave a difference signal accurately indicative of the warp and rumble. The vertical arm position signal x is an indication of the arm 76 to reference plane distance, whereas the reference path-arm signal y is an indication of the cartridge 78 to disc distance; the difference z is the warp 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 78B, by which useful information is obtained up to 15 about several hundred Hz.

Figure 10 shows a functional block diagram of cartridge VNC embodiments of the invention. Warp, rumble -7 and mould grain 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 bythe 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 intallation either on an original or replacement basis. is a simple matter.

The reference path-arm sensor 224 may be of the types previously 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 11 A and 11 B, which show exemplary combination interconnections. The transducer outputs may be further divided to accommodate the left anf 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-mould grain noise pomponents sensed by the secondary stylus, such as might be caused by scratches or rough groove edges. Reference path information 30 terminals, shown in Figures 10 and 1 1A 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 12, the secondary stylus shank 72, instead of coupling to its own transducer, is coupled to the stator components of the signal transducer, so that rumble information sensed by the secondary stylus 70 cancels out rumble information sensed by the primary stylus 35 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 moving coils, moving iron, moving magents 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. 40 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 13, wherein 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 50 connected to a conventional magnet or iron piece 242 which cooperates 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.

The reference path-arm signal may be combined with the main stylus signal in the electronic manner shown in Figure 14. This embodiment functions in essentially the same manner as the cartridge VNC described previously and is primarily applicable to sensors orthe 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.

The null control 250 is setfor optimum noise cancellation in combiner 248. If, desired, electrical filtering, signal delay correctors or other processing may be inserted into either or both signal chains in order to optimize the noise reduction effect under all operating conditions. For example, a transient noise suppressor 252 can be employed to reduce the effect of "pops" and "clicks" caused by scratches on the record and the like.

The signal on the disc being reproduced can be encoded in electronic noise reduction form, for example using high frequency compression in order to reduce high frequency noise when played back, using high 65 p Z - 7 GB 2 068 627 A 7 frequency expansion by means of the system known as "Dolby B". Such high frequency noise reduction will -be most effective psycho-acoustically when combined with lowfrequency disc noise reduction as provided by the various embodiments of the invention. The system of Figure 14 can provide for playback of a Dolby B encoded disc by optionally providing a Dolby B type decoder 256, preferably following subtractor 248 or, less 5 desirably in the signal chain of blocks 220 and 244 before subtractor 248.

Claims (17)

1. A gramophone cartridge with a stylus for providing an audio signal representing the groove modulation of a disc, and means carried by the cartridge for sensing an unmodulated part of the disc in close 10 proximity to the stylus, the sensing means being substantially more compliant than the stylus.

2. A gramophone cartridge according to claim 1, wherein the sensing means is a second stylus.

3. A gramophone cartridge according to claim 2, wherein the second stylus is mechanically coupled to the first stylus.

4. A gramophone cartridge according to claim 2, wherein the two styli are mechanically coupled to two 15 parts of a transducer so that the signal provided by the transducer is varied differentially by the two styli.

5. A gramophone cartridge according to claim 1, wherein the sensing means is a photo-electric sensor.

6. A gramophone cartridge according to any of claims 1 to 5, wherein the sensing means is so mounted on the cartridge as to sense lands of the disc between the grooves thereof.

7. A gramophone cartridge according to claim 2, 3 or4, wherein the second stylus has a tip shaped to 20 ride on the lands of the disc.

8. A gramophone cartridge according to claim 2, 3 or 4, wherein the second stylus has a tip shaped to ride in a pilot groove formed in the lands of the disc.

9. A gramophone cartridge according to any of claim 1 to 8, wherein the sensing means senses only one or both of the land areas immediately flanking the groove in which the stylus is engaged.

10. A gramophone cartridge according to any of claims 1 to 8, wherein the sensing means senses one or more of the land areas radially outside the groove in which the stylus is engaged.

11. A gramophone cartridge according to any of claims 1 to 10, wherein the sensing means senses at a circumferentially leading point relative to the stylus.

12. A gramophone disc playback apparatus comprising a cartridge with a stylus for providing an audio 30 signal representing the groove modulation of a disc being played back, means carried by the cartridge for sensing an unmodulated part of the disc in close proximity to the stylus, the sensing means being substantially more compliant than the stylus, and a compensating mechanical or electrical coupling between the stylus and sensing means whereby the sensing means at least partially remove from the audio signal the effects of vertical deviations sensed by the sensing means.

13. Apparatus according to claim 12, wherein the sensing means comprises a second stylus operative to sense an unmoduiated part of the disc.

14. Apparatus according to claim 13, wherein the second stylus is mechanically coupled to the first stylus.

15. Apparatus according to claim 13, wherein the two styli are mechanically coupled to two parts of a 40 transducer so that the signal provided by the transducer is varied differentially by the two styli.

16. Apparatus according to claim 13, wherein the two styli are coupled to respective transducers whose output signals are combined in opposition.

17. Apparatus according to claim 12, wherein the sensing means is a photoelectric sensor providing a compensating electrical signal for the audio signal.

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