GB2067340A - Gramophone disc system for compensating mechanical imperfections - Google Patents

Abstract

In cutting a master disc the signal cutting stylus 6 is followed by a reference plane cutting stylus 4 which removes the groove horns left by the stylus 6 and provides a perfectly quiet reference land which is tracked, in reproduction, by a reference stylus relative to which the signal stylus provides the playback signal. <IMAGE>

Description

1 - 10 GB 2 067 340 A 1

SPECIFICATION

Gramophone disc system for compensating mechanical imperfections Commercially manufactured disc photograph records exhibit various mechanical imperfections, and further 5 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, namely spurious vertical deviations of the record groove. Such imperfections can cause significant degradation of the reproduced signal.

Turntable rumble may resuitfrom turntable bearings, motor drive systems, and environmental vibrations. 10 Considerable efforts are made byturntable manufactures 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 receptorforthe vibrations, resulting in tonal coloration or even howling. Devices for reducing these effects include a fluid filled turntable mat disclosed in U.S. Patent No. 3,997,174 to Kawashima, and flexible turntable support cups in U.S. Patent No. 4,054,291 to 15 Maeda, both for 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 Disc", Journalof theAudio Engineering Society, August 1969, voi 17, No 4, pp 416-422. Mould grain noise may extend generallyto 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.

The present invention is concerned with the provision of a reference surface which can be used in achieving improved reproduction and the invention provides a gramophone disc system as defined in the 25 claims below.

The invention will now be described in more detail, byway 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 18 is a sectional side view of a hypothetical record pressing made from the master disc of Figure 1A.

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

Figure 3A is a partially block generalized representation of direct reference path information sensing in 35 accordance with one embodiment of the present invention.

Figure 38 is a partially block generalized representation of indirect reference path information sensing in accordance with a further embodiment of the present invention.

Figure 4to 6 are partially cut away perspective views of three types of reference path-arm sensor.

The present invention is based on observations taken from the situation shown in Figure 1A and Figure 1 B. 40 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 'V' is a constant and represents the instantaneous vertical signal modulation with respect to a perfect reference path or surface. In accordance with the invention, as shown in Figure 2, the reference surface is defined in the cutting process, by a preferably flat secondary cutting stylus following the main cutting stylus and arranged to smooth the dimensionally define the land 45 between the grooves.

Figure 1 b represents the situation after making a record pressing of the master. The vertical groove position is no longer constant but contains irregularities. In the case of warp, these are dimensionally correlated on the two sides of the record (the thickness remains substantially constant), because they arise simply from thermal and handling related distortions during and after removal of the record from the press. 50 Higher frequency mould grain noises, however, are not correlated on the two sides of the record, since different dies and stampers are used; the disc thus contains local variations of thickness. Such imperfections are caused by the pressure transmittal of dimensional irregularities from the back to the front of the stamper during the pressing operation. The back surface irregularities may include metallic crystals arising from the replication process, patterns resulting from grinding operations to smooth the back surface, dirt and dust 55 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 mm to 0.25 mm the rigidity or stiffness of the material will limit the shortest wavelengths which can be transmitted through localized bending and distortion of the stamper.

Thus, such wavelengths might be of the order of 0.50 mm. This results in a highest frequency of mould grain noise at the outer diameter of a twelve inch disc (groove velocity about 50 cm per second) of the order of 60 1 kHz.

Further sources of low frequency noise on the record itself may include non-homogeneity of the pressing material and geometric distortions due to differential cooling effects resulting from rapid and uneven temperature changes in the die face. Moreover, as discussed previously, noises are also contributed bythe reproducing system -- namely, turntable and environmental rumble and acoustically transmitted vibrations 65 GB 2 067 340 A of the turntable and disc.

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

Closer consideration of this matter shows that the low frequency noise components from all the sources mentioned above are not inextricably mixed with the original signal modulations. Rather, the recorded signal quantity -a- remains intact and unharmed by the pressing and reproducing process and by mechanical imperfections in the reproducing system. Thus, the quantity "a" can be recovered if the distorted reference path at the point of stylus contact is used as the reference point during reproduction. Preferably, the reproducer system acts to remove spurious deviations of the reference point so that again the disc surface is in effect flat (ie effectively vertically stable) in the vicinity of the stylus. Alternatively, the undulating 10 reference point is used in determining the true signal quantity "a".

Thus, it is possible to compenate for mechanical imperfections in the disc by measuring an appropriate quantity to produce an error signal and by utilizing this signal to effect an appropriate correction, mechanically andlor electrically. A sensing means generates a signal responsive to deviations in the reference path at or in close proximity to the pickup or stylus and that signal is processed to control the system in order to minimize the effect on the disc playback signal caused by the deviations.

In the context of the invention sensing "in close proximity" means within a small fractional part (eg less 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 - ie on an immediately adjacent land area. This would representthe limit of the technology of the invention.

This application and other applications were divided out of my application 48108/78 and reference may be made to the published application Serial No 2013957 for a complete discussion of the present and related inventions which are all versions of or related to Vertical Noise Compensation (VNC) brief ly characterised as follows:

Application No Ser No.

48108178 2013957 Disc VNC 30 00418181 Cartridge and pre-amp VNC 00419181 00420181 Tone arm VNC 35 00421181 Second cutting stylus 00422/81 Reference plane sensing 40 Briefly disc VNC involves compensation by using the disc, cartridge and pre-amp VNC involve mechanical or electrical cancellation of rumble information in the cartridge or preamplifier, tone arm VNC involves compensation by servo control of the tone arm, "second cutting stylus" relates to the subject matter of Figure 2 herein and "reference plane sensing- refers to the concept explained below in conjunction with Figure 3B.

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

In the several embodiments, the vertical position of an unmodulated portion of the record (reference path) is sensed at or in close proximityto the signal pickup means, typically a stylus. Close proximity, high resolution sensing is useful for reducing mould grain noise. It follows however, that it is necessary for the 55 reference path to be as unblemished as possible. For example, it should be free of scratches. Moreover, the groove "horns" or ridges of material at the groove edges projecting into the land area are removed during the disc manufacturing process. - Polishing of the metal mould is a known method of removing groove horns. Another method is shown in Figure 2, which shows the further reference path cutting stylus 4 following the groove cutting stylus 6. The 60 substantially flat bottom edge of the reference path stylus not only removes the groove horns but cuts away residual rumble modulations on the lacquer master 2 and compensates for any vertical rumble introduced by the recording lath. A perfectly quiet reference surface is thereby defined for use with the reproducing embodiments described below.

A generalized representation of reference path sensing is shown in Figures 3A and 3B. Referring to Figure 65 A f Z 3 GB 2 067 340 A 3 3A, the reference path information ca be obtained directly, by means of a sensor which follows the signal stylus laterally but is vertically independent. Sensor 8 is attached to a reference plane. In a conventional turntable the attachment will typically be to the tone arm mounting surface. In 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.

In a simple embodimentthe references path stylus is situated on the cartridge. 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 mould grain noise reduction. It may also be used with a tone arm vertical positions sensor to provide reference path information in a turntable VNC or a tone arm VNC.

Secondary styli may be constructed of any of various long wearing materials compatible with disc surfaces and resistant to grooving effects, such as sapphire or diamond. Sensor transducers may be any of various types known in the art, including, but not limited to: electromagnetic, photoelectric, Hall effect, magneto-diode, potentiometric, or variable resistance, capacitance or inductance. The untreated output of 15 the transducer may represent position, velocity, acceleration. or force (as with a pressure responding transducer).

The mechanical characteristics of the reference path sensor assembly can be optimized for the vertical sensing function only. The frequency of vertical resonance (sensor flexing and mass) should be placed substantially above the highest warp frequency, and indeed well into the audio band, in order to extend the 20 highest frequency of correction upwards, thereby to reduce audible rumble and mould grain noise. The secondary stylus must be situated very close to the primary stylus - - eg within 1 mm for correction to about Hz. Even closer spacing of about 0.1 mm for correction to about 500 Hz is preferably for reduction of mid-range mould grain noise.

The secondary stylus or sensor means may be positioned slightly in advance of the primary (signal) stylus 25 to generate an anticipatory error signal. This is useful for relaxing the gain and phase requirements of electro-mechanical servo loops or for ensuring optimal error cancellation where mechanical or electrical phase shifts are present, as for example with low pass filtering of the reference path information.

Other embodiments of the invention employ reference path-arm information; this is the signal obtained by sensing the distance between the reference path and arm (le cartridge). This signal will necessarily include 30 tone arm movements and arm/cartridge resonance effects. A first sensor version simply employs the vertical component information from the pickup cartridge, as is known in the prior art. This method provides useful information above the frequency of arm/cartridge resonance, but is limited to cases and to the frequency range in which channel separation is deliberately reduced during disc cutting (eg below 100 Hz).

In order to obtain reference path-arm information up to higher frequencies it is neccessary to provide a 35 land sensor which is independent of the signal stylus. Non-mechanical sensing means such as those mentioned previously may be used, however being fixed to the cartridge holding arm or cartridge rather than to a vertically fixed arm.

Referring to Figure 4, in an arrangement suited to conventional commercial phonograph records, the cartridge 62 has a main stylus having a shank 42 and a stylus tip 44 tracking an information carrying groove 40 54 of a photograph record 9. The secondary shank 54 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 substantially flat bottom with operative dimensions sufficiently large (eg some fraction of a miffirneter) so that it rides reliably on at least one land area and is thereby substantially unresponsive to later 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 intefere with the stylus movements.

Another view of the styli is shown in Figure 5. An elongated block shaped tip 70 for secondary styus 72 50 spanning the land between several grooves is located ahead of and to the outside of the main stylus 42. As mentioned previously, the secondary stylus tip may optionally have a U- shape (shown byway of example as element 74 in Figure 6) or an 0-shape, surrounding the main stylus. The contact area may be biased towards the outside of the record, if desired, so as to decrease pre-echo effects. A further practical matter is that the stylus arrangement should preferably not trap dust but should deflect it away.

For most tone arm and cartridge configurations, the design of the reference path-arm information sensors should be such that the tracking force of the main stylus should preferably comprise the main portion of the overall cartridge tracking force in order to avoid affecting the side thrust forces on the cartridge and to avoid reducing the main stylus force available for tracking warps and large amplitude signals. A secondary stylus tracking force a small fraction that of the main stylus - - eg 1/4, Vlo, or even less - - is adequate to sense the relatively low amplitude and low frequency rumble components. The secondary stylus is preferably compliantly connected to the cartridge body, the compliance preferably being substantially greater than that of the main stylus. The above compliance and tracking force considerations apply primarily to offset tone arm systems in which warp is not compensated. (in some systems the secondary stylus shank may be relatively stiffly coupled to the cartridge body). Further, the effective mass of the secondary stylus and 65 4 GB 2 067 340 A 4 related moving parts, together with the flexing or stiffness properties of the shank, should produce a high frequency resonance well above the highest mould of grain noise components of interest; thus, a resonant frequency of at least 1-2 kHz would be suitable for the reference path sensor. As with the design of conventional signal cartridges, suitable mechanical damping can be applied to the secondary stylus. A low pass mechanical filter may be incorporated if desired, so that the information provided by the secondary stylus is band limited for reduced sensitivity to dust and surface scratches.

Information from the reference path-arm sensor is used in one way or anotherto cancel corresponding vertical information from the signal stylus. Wholly mechanical cancellation arrangements may be used. In the simplest arrangement, with a non-compliant secondary stylus shank, the vertical movements of the cartridge body subtract from the corresponding movements of the primary stylus. Alternatively, secondary stylus information may interact in the magnetic or electromechanical arrangements of the main signal transducer in such a way as to cancel error informaton. In some arrangements, a separate or coordinated transducer may be provided for the secondary stylus. The combination of signals, as by interconnected coils, may be accomplished within the cartridge itself or the signals may be brought out for external combination.

The signals may be used internally and also brought out for use in other embodiments of the invention. For 15 example, the high frequency components from the reference path sensor may be utilized internally or brought out to the preamplifier for mould grain noise reduction (cartridge VNC or pre-amp VNC), and the low frequency components may be brought out for dealing with warp via actuation of the turntable or tone arm (turntable VNC or tone arm VNC).

As discussed previously, accurate reference path information sensing may be achieved directly, by means 20 of a vertically fixed sensor. In an approximation, the vertical position of the tone arm may be used by the methods known in the prior art. Atransducer 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 belowthe arm-cartridge resonant frequency (eg 10 Hz). It is possible to employ a relatively stiffly mounted secondary stylus to raise the resonant frequency of the tone 25 arm. The reference path-arm sensors described previously in connection with Figures 4 to 6 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 (eg 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 35 recognition that both signals contain the same error signals (from arm- cartridge interaction effects), but in complementary form, wherebythey can be cancellecd 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 79 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 40 and V 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 V signal can be derived by means of a sensor responsive to the tone arm position.

Claims (2)

1. Agramaphone disc system comprising cutting stylus means for impressing on a disc a position modulated signal groove in relation to a vertical reference position, second cutting stylus means for impressing a defined land surface on the disc representative of the vertical reference position, and means for reading out the modulated groove in relation to a vertical reference defined by the land surface.

2. A gramophone disc system, comprising cutting stylus means for impressing on a disc a position modulated signal groove in relation to a vertical position, the signal groove having adjacent uncut land areas, the cutting stylus means causing groove horns projecting into the land area at the groove edges, and signal-free second cutting stylus means following the signal groove cutting stylus means for impressing a defined land surface on said disc representative of said vertical position and for removing the groove horn.55 Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1981. Published by The Patent Office, 25 Southampton Buildings. London, WC2A lAY, from which copies may be obtained.

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