CA1180663A - Disc reproducing system for compensating mechanical imperfections - Google Patents

Abstract

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 having a stylus 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. The sensor is substantially more compliant than the stylus and is arranged to sense an unmodulated part of the disc.
The sensor may be formed as a second stylus. 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

This i5 a divisional of copendin~ Canadian application serial No. 317,805 filed December 12, 1978 by Ray Milton Dolby.
BACKGROUND OF THE INVENTION

_ Commercially manufactured disc phono~3raph records exhibit various mechanica:l imperfcctions, and urther system impe.rEect.ions result :Exom the mecIlan.ical n~eans employed to reproctuce the .record. rrlle present .invelltion is concerneil t~ith olle cate~ory o:E disc xep~oduc~ system imperfeo~.ions: SpUl`iOUS
vert.ical deviatioIls o.E the recor~ ~roove, primarily resultinr.3 from record warp and rumble, inc:Ludin~3 record pressin~ noise. ~uch imperfections can cause si~3nificant degradation of the reproduced signal.
WARP
A general discussion of record warp is included in the following published paper: "Record Warps and System Playback Performances", 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 ran~e of about 1/2 Hz (the once around frequency at 33-1/3 rpm) to beyond 10 H3, with 95~ of the warps below 8 FI2. Pea~ physical amplitude hei~ht of the warps was c3reatest at lo~ fre~ueIlcies at about 0.0~5 in.
maximum and decreased witll in~rcasin~3 ~reqlleIlcy.
Var.ious problems are cauxed bv record warp. the tone arm may bounce or st~ay witll respect to the l`eCOrd sur~ace due to the vertical and, to some e~tent~ lateral, forces which result as the stylus attempts to trac~ the varyin~ record hei~3ht. This may cause not only variations in tr~c~ing force but bottoming of the cartridge or complete loss of contact between the stylus and groove. Such variations in txackinc3 force from optimum will often affect the reproduced sic~nal at audible frequencies, In addition to causiny stylus and arm trackin~ problems, e~cess.ive stylus excurs.-ons result Jal c~eome trically related dis tOI`tiOllS allc~
electro-mecllanical no~ learity cf the cartxicl-~e. ~loreover, sub-audibl~ wa.rp ~ nals call cause. ~.i.stor~ioll Ly amplifier o~olr~
:Load in e:L~ctrollic sys~om~Y. pass:itlc,l ~ucl~ .low frecluetlc.ies alld, i~
appliec~ to the speaker systetn~ can cause su~stantia.'L ~oafer movemellt that can result in e~traneous noises ~nd the c~istort.ioll of hic3her frequency audible si~nals, includinc3 dopplel- clistorticn.
Further, the c~eometrical relationship of the stylus and recorc~
~roove is such that a warp results in a forwa~d and bac~ward oscillation of the stylus tip over the recorded groOtJe informa-tion, which frequency modulates ~advances and delays~ the reproduced signal causing "wow". Wow may also result ~rom variations in rotational speed as the stylus load Oll the record groove varies.
The requirement to trac~ ~arped phono~rapll records satisfactorily has resulted, in prior art systen~s, in the necessity to consider tonearn~/cartrid~e~stylus~recorcl c3eometry very carefully and to see~ the best co~ atYLo~ sucal.ly a compromisel of such factors~ as styllls an-t tOlle arm mass, tone arm damping, stylus compliance allc~ damp:in~, and tracl~in~ Eorce 90 ;IS
to provide a controlled tone arm resonance a~ove the conullonly encountered t~arp frequencies, yet belo~ the freq~lency of ~he lot~est recorded c~roove informatioll, ~n arm resonance of 10 H2 has been advocated by several desic3ners: Keisuka Ikec3ami and Susumu Hoshimi, "Advance in Turntable and Tone-Arm Desi~n", Journal of the Audio EncJineerinc3 Society, May 1976, Vol. 2~, ___ _ _ NoO 4, pp. 276-280l and Peter Rother, "The Aspects of l,o~-Inertia Tone-~rm Desi~n", J_urna.L of t.he t~udlo_En~ leer.Lnq_S .~tv, September 1~77, Vo:L. 25, No. ~, pp~ 55n-~5~.
~ ltllo~ p~inc.ip~ p~ r ~ n~
and cart:ridqe parameters may make possib.Le the trackillct c~f ~arped .records, the matchin~ of a.rms and cnrtridc~es is o~ell complicated in praetiee because of the wide variation ln tolle a.rms allcl cartridges available. Further, even at the desi~n sta~e, the selection of optimum tone arm and cartridc3e parameters for ~arp traeking may not be optimum for traekinc3 hic3her frec~uency ~roove information. Even when the record is properly tracked, the problem of geometric and motor wo~ from ~arps still remains.
Various passive devices for trackinc~ ~arped records are known. These devices typically employ an element ridin~ the record surfaee and fi~ed or eoupled to ~le pic~up car~rid~e or the tone arm in the vi.cinity of the cartridc3e. S~lcll devices inelude both damped elements and ulldamped or ~i~ed elements.
Exemplary prior art ~amped element devices a.re dlsclosed in United States Patents Nos. 2,572,71~ to Fis}ler (sprin~ loa(led plullqer), and 2,325,862 to Thompsoll ~elastica~lly mounted au~illary styl~ls).
Fi~ed elements are disclosed iIl United S~ates Patents Nos.
3,228,70~ to Alldrews et al (felt pa~ at end of tone arm ~ith cartridc3e pivoted in tone arm~ and 3,S30,505 to ~abinow (air bearing adjacent cartridc3e). It is ~nown also to employ a dash 6~i3 pot or a brush adjacent the cartridge to damp oscillations and asslst in trackin~ warps. Further, proposals for a relatively rigid element coupling the tone arm to the record surface are known. It has also been sug~ested that the record be clampe~ or weigllted at its periphery and/or c~nter in or~er to eliminate warp.
An ~ctive priox alAt system or treatin~ 1~eCO1~C~ ~arP .iS
described in the Eollowin~ paper: "Ov~xcomin~ ~eco~d ~arps aI1d L,ow-Fre~uency Turntab:Lc Rumble in Phc~no~Jraphs", I;eI~neth C1~111iS
and Michael J. KelLy, Journal o~ the ~udio En~illeerin~ Societv, July/Au~ust 1975, Vol. 23, ~o~ 6, pp.450-~58. In this system the cartridge output is used to servo control the vertical tone arm position to assist in trackin~ the record warp. Similar systems are disclosed in United States Patents Nos. 3,623,734 to Sakamoto et al, and 3t830,505 to Rabinow. It is also known to provide a closed loop around the tone arm movements only, in order to improve arm/cartridge dampin~. Aspects of ~he present invention can significantly improve the performance of these prior art tone arm systems.
RUMBLE
Turntable rw~le may result f1-om turntable bearin~s, motor drive systems, and ~nvironmelltal vibrations. Considerable eorts ar^e made by turntable n~an~acturers to eli~ late rumble ~rom these sources.
Other turntable related disturballces are caused by acoustic feedback ~sonic and in~ra-sonic) from tlle loudspea~ers, ~hereby the turntable and/or record may act as a receptor for the 6~

vibrations, resulting in tonal colouration or even howlin~.
Devices for reducing these effects include a fluid filled turn-table mat disclosed in United States Patent No. 3,997,17~ to Kawashima, and fle~.ible turntable support cups in Unitecl ~tates Patent No. 4,054,29l to Maecla, both for prov.idin~ a collormin~
clamped suppo:r:t unde?r. warpe~ records.
Notwitllst~ tll~s~ s, t~le~A m~ l.ow ;Ere?~lenCY ~:111nOYal1Ce? iS reCC~Xd pressin~J .rul)lbl~ or mold ~3rain noiSe from the d.isc i~selE. The spectrum o'c recol-d pressin~ noisc? is :l0 diseussed by John .~ar~le, "Performallee Chal^acteristies o the Commercial Stereo Disc", Journal of the Audio En~ineelin~3 Society, August 1969, Vol. 17, No. 4, pp. ~16-~22. ~1old ~rain noise may extend generally to several hundred Hz.
Reeord pressing rumble and turntable r~le are reduced conventionally by means o high-pass filters in the si~nal paths.
Optimum tone arm/cartridge resonance eharacteristics are also useful in reducin~ low frequency rumble effeets.
SU~RY OF THE INVENTION
Prior art approaches to dealin~3 wit11 warp are direeted primarily to the symptoms o:~ warp. FOI~ e~amp.le, th.? passive tone arm to record surfacee contact ~eviees alld the? clo5ed loop tone arm systems act ehiefly as mealls to ellhallce the aL~i.lity Q
the cartrid~e a1ld tone arm sim~ly t~ trac~ ~arps. Collst?quelltly~
sueh approaches may ail to c~.rrect other e~ects o wa.rp ancl may d~rade trackill~ abilitv and sicJnal q~lalitv at n~n-warp ~requencies. With re~ard to run~le, prior art techniques have been directed primarily to electrical filterin~ rather than to 6~i3 dealing with the rumble mechanism itself.
It is the object of the present invention to reduce the effects of warp and rumble without :in any way de~radin~ the band-width or other performance characteristics oE the si~nal chanllels themselves.
rrhe invention will now be e~pla.ine~ in ~.reater ~etail w.ith re:Eerence to the accomp~llyin~ drawin~s, .in which:
Fi~ure 1~ a ~ectiorlal s.ide vi.ew o khe top hal ot a hypoth~tica.l..l-ecord mas-ter on whicll silellt ~xooves have been recorded-Figure lB is a sectional side view of a hypothetical record pressin~ made from the master disc of Fi~ure lA;
Figure 2 is a partly sectional side view of a master disc during the cutting process, usin~ a conventional signal cutting stylus and a secondary reference plane cuttin~ stylus in accordance with one aspect of the present invention;
Fi~ure 3A is a partially bloc~ ~enerali~ed representa-tion of direct reference path information sensin~ in accordance with one aspect of the present invention;
Fi~ure 3B is a partially bloc~ ~enexali~ed representa-tion of indirect reerence path information sensill~ in accordance wi~l a furthel- aspect of the presellt invention;
Fi~ure ~A is a partially c~lt away persp~ctive view of one type of direct reerence pat.h .information sensiny;
Fi<~ure IB is a partia.~ly CU~ away perspective view of a further type of direct reference patll information sensin~;
Fi~ure 5 is a pa.rtially cut away side view of one type of reference path-arm sellsin~;

6~i3 Figure 6 is a partially cut away perspective vie~ of one type of reference path-arm sensor;
Figure 7 is a partially cut away perspective vie~ of yet a further type of reference path-arm sensor;
Fi~ure 8 is a par-tially cut a~ay perspective v.ie~ of still anothe.r type o;~ l~efe~ence pa-tl~-arm sensox;
Fi~u:re 9 .is ~ pa.rtial.l.y cut away pe.rspective view o t.he styl:i port:ioll of another ky~e o reference path-al-m sensor;
Fi~ure 10 is ~ b:loc~ dia~ram o a turn-table ~C
(verti.cal noise compensator) system in accordance ~ith the invent.ion;
Figure 11 is a cross sectional side view of one type of vertical actuator usable in a turntable ~NC 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 viet~ of yet a further type of vertical actuator usable in a turntable V~C
system;
Fi~ure l~A i5 a partly Cl-OSS sectional si~e vie~ of a turntable employin~ a vertical actuator;
Fi~ure l~B is a cut at~ay partly Cl`OSS sectional si.le vie~ of a further turntable emplovLn~ a vertica.l actuator;
Fi~ure 15A is a cross sectiollal side vie~ of a retrofit type vertical actuator for use ~ith a collventional turntable, Fi~ure lSB is a partly Cl'OSS sectional side viet~ of a further type of retrofit vertical actuator for use ~ith a conventional turntable;

i3 Figure 16A is a block diagram of a prior art electrical tone arm damping arrangement;
Fi~ure 16B is a block dia~ram of a prior art tone arm servo arran~ement;
Figure 1'7 is a block diagram of ~ tone arm VNC system according to the present lnvention ~mp.~oy.itlg d.irect re~erence path sellsin~;
~ u~ locl; ~ J-~ h~ t~p~ .o axm VNC s,vstem .in accordallce with the presel~t, inve~lltion in wl~i ch :in~ixect .reference path sensin~ is employed;
Figure 19 is a block diagram of a further type of tone arm VNC system accordin~ to the present invention in which the e.rror signal is included in a ne~ative feedback loop;
Figure 20 is a functional bloc~ diagram of a cartridge VNC according to the present invention;
Figure 21A is a block dia~ram showinq e~emplary combination interconnections between transducers in a cartrid~e VNC according to the present invention;
Fi~ure 21B is a block dia~ram sho~in~ an alternative exemplary combination interconnection bet~eell transducers in a cartrid~e VNC according to the present invelltioll;
Fi~ure 22 is a perspective vie~.~ of a portion of a cartridge VNC;
Fi~ure 23 is a perspec~ive vie~ o~ a portioll of a Eurther cartridge ~C; and Fi~ure 2~ is a bloc~ diagram showin~ a pre-amp VNC, The presellt invention is ~ased on observations ta~en 6~3 Erom the si-tuation shown in Figure lA and Figure ls. Figure lA
represents a sectional side view of the top half of a hypothetical record master on which silent c~rooves have been recorded. The c3roove depth "a" is a constant and repxesents the instantaneous vertical si~nal modu~atlon with respect to a perect reEerence path or SUl-faCe . 'I`he re~er^cnce sur;~ace may be the flat lacc~uer master disc su~ ce, ~)llt in accord~nce wikh an aspect of the :invention, showll in I~ ure 2, the reference s~lr~ace mav optioIlally be deined in the cutcinc~ ~roce3s, pre~ra~Ly bv ~
:10 ~lat secondary cuttinc~ styl~ls Eollo~inc~ the main cuttin~ stylus and arranged to smooth and dimensionally define the land bet~een the grooves. In some cases, in the fre~uency ranc3e in which there is little vertical information recorded on the disc ~e.g., below 30 Hz) the c3roove itself may be used as -the reference path.
Figure lB represents the situation after makin~ a record pressinc~ of the master. The vertical groove position is no loncJer 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 handlin~ related distortions durin~ and after removal o~ the record from the pl-eSS. ~lic~her trequency mold ~rain noises, ho~ever, are not correlated on the two sides o the record, since di~erellt dies and stampers are u5ed; the dise thus contains local variations of thickness. Such imperections are caused by the pressure transmittal of dimensional irre~ular^ities from the back to the front of the stamper during the pressill~ operation. The b~ck surface irregularities may include metallic crystals arisiny from tne replication process, patterns resulting from ~rindin~ operations to smooth the back surface, dirt and dust trapped between the stamper and the die oE the record press, and surface .irregularities of the dle.
~ s the s-t~mpelA th:i.c~ness :is some .007" to .010" t.he ric~idity or^ st.i.Eflless of tl~e matex.ial w.il.l. limit tlle sllol-test.
wavelcn~ths which Call be ~ansmitted throu~ll .loca;~ ed ~endinc~
~nd clistol:tion o tlle stampc~. Thlls, such wavelell~Jth~ ~lic~l~t be o:E th~ oxder of .020". This results in a hi~hest frequency of mold ~rain noise at the outex diameter o.f a twelve inch disc t~roove velocity about 20 inches per second~ o the or~er of 1 kHz.
Further sources of low frequency noise on the record itself may include non-homogeneity of the pressin~ material and geometric distortions due to differential coolin~ effects result-ing from rapid and uneven temperature chan~es in the die face.
Moreover, as discussed previously, noises are also contributed by the reproducing system -- namely, turntable and environmental rumble and acoustically transmitted vibratiolls of the t~lrntable and disc.
Thus r in a conventiollal reproducill~ system, the reprodllced quantit.y "b" i.s obtaillec~, employin~ the tone arm pOSi`tiOIl as a reerellce. ~he qu~ntity "b" thus includes undesired low frequellcy noise components.
Closer consideration of this matter shows that the low frequency noise components from all the sources mentioned above 6~ii3 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 reproducin~ system. Thus, the quantity `'a"
can be recovered iE -the distorted reference path at ~he point o.E
stylus contact is used as the refexence pOillt dllr.in~ r~prod~lct.ion.
Pre:Eexab:Ly, the reproducer system ~cts to rem~ve ~pllr.ious c~ev:iat.ion~ of -the r~E~retlce po.int so th~t a-~aln the ~;is~ su~ ce :is .in ef~ect flelt (i..~ E~ct:.ively verticaL;Ly stable) in the lo ViCill.ity O e the stylus. ~ltern~tively, the und~llatillg reference point is used in determining the true si~nal quantity "a".
~ hus, in accordance with the teachin~s of the present invention, the problems of the prior art are solved by a disc record reproducing system in ~hich mechanical imperfections in the disc or in the system, causing deviations in the tracked disc path, are compensated by means of measurin~ an appropriate quantity to produce an error signal and by means of utili~ing this signal to effect an appropriate correction, mechanically and/or electrically. A sensing means ~enerates a si~nal responsive to deviations in a reference path at or in close pro~imity to the pickup or stylus and that signal is processed to control the system in order ~o minimi~e tl~e effect on the di5c playback si~nal caus~d by the deviations.
Briefly stated, ~Iccording to a broad aspect o the present invention, there is provided a gramophone cartrid~e ~ith a stylus for p~r~vidin~ an audio signal representin~ the groove modulation of a disc, and means carried by the cartridge for sensing an unmodulated part of the disc in close pro~imity to the stylus, the sensing means being substantially more compliant than the stylus.
In the context of -the invention sensin~, "in close proximity" means within a small fractional part (e.~., less than about one--tentll) oE -the shortest wavelen~th wllicll it is desired t:o coxxect~ If the mold ~rain noiscs hav~ wavelen~3ths as sllort as about .0~.0 inches, thLs implies Sel15ill~ wi~h~in ahout .00~
:Lnches ~ e~, on an immc~:iately a~ljncellt Land area. Tllis would :l0 represQIlt the limit oE the technoloyy of the inventioll. F'or lower frequency noises and warp it is, of course, unnecessal-y to sense in SUC}l close proximity.
There are four main embodiments of the invention, ~hich or convenience may be referred to as a Vertical Noise Compensator (VNC). The embodiments may be used separately or in combination.
In all embodiments, the vertical position of an unmodulated portion of the record is sensed at or in close proximity to the stylus. The information so derived may be reerred to as reerence path information. In some embodiments, the reference path inormation is sensed with respect to the arm or cartric~e position; such information ma~y be referred to as reerence path-arm in~orma~ion. R~fer~llce path in~ormation or reerence path-arm information is preferab:ly obtained via sensin~
means arran~ed to sense the land position adjacent the si~nal ~roove and in close pro~imity to the si~nal stylus. It is important that the vertical sensor responds substantially only to vertlcal information; in disc recordin~ technology the term "vertlcAl" conventionally has the meanin~ of perpendicular to the disc surface, or in an axial directlon. A less desirable alternati~e, Eor ~he treatment of warp and very low frequency rumble efEects only, is to sense the ~roove deptll itselÇ.
In the irs-t embodimetlt, referellce patll informa~ioll is obta:ined and used in a elosecl loop selvo syskom includin~ a actuator which moves the disc subst.~llt;ially ve~tically, ak le<ls~
in thc v:iclnity o~E the pickup stylils. ~dea`l:Ly, the result is that at least all vertical disc movemellt in the vicinity of the pickup stylus is removed, thereby allowin~ the stylus effectively to track a warp-free and rumble-free record. This en~bodiment 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 embodiment, ~hich may be called a tone arm VNC, is an improvement of the closed loop tone arm techniques of the prior art. In prior art systems, the error si~nals used include arm-cartridge resonance components or other misleadin~
information. In the present inventioll, measurements avoidin~
these deects are made and processed to control the tone al-m;
namely, reerellce path information is obtained alld elllployed to control the tone ann all~i optiollally to perform adiitional corrections via the other ell~bodiments. Lo~ frequency components can be employed in a turntable VNC for ~arp compensatioll and hi~her- frequellcy components can be used in a cartrid~e VNC or pre-amp VNC for run~ble and mold ~rain noise reduction.

ii3 In a third embodiment, which may be referred to as a cartridge VNC, reference path-arm information is obtained and used to effect correction within or followin~ the cartrid~e. The reference path-arm information is cancelled either electrically or mecha.nically from the in:Eormation provided by the signal stylus. The .re:Eerence pa-tll-arm in:Eo.rmation may optionally be brought out of tlle cartrldge .in o;rder to pe:r.~orm additional corrcctions via the otller en~bodimenks.
In a Eourtll embod.imellt~, which may be cal.led a pxe-amp VNC~ tlle reEerence path-arm informat.ioll .is obt;lined and callcel:Led :Erom the audio output electron.ically in the pre-amplifier. Tilis arrangement can be used to reduce mold grain noise, l-umble and some of the effects o warp.
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 rumble effects may be compensated up to, say, 20 Hz usin~ a turntable VNC or a tone arm VNC, ~ith frequencies above this being treated by a cartrid~e VNC or pre-amp VNC.
Compatibility characteri~es all the embodimellts of the invention. Conventional records may be played on reproclucers includin~ the inventioll; conversely, records produced ~ith the optional definecl reerence surface of the invention may be played on conventional reproducers.
The reduction of tlOiSe and tl~ac~in~ problems effected by the invention may permit a lo~Ter modulation level and a hi~her ~roove density to be employed, leadin~ to longel- playing Ei3 times and/or smaller record diameters.
The fact that the invention solves the problem of low :Erequency noise leads to the further possibility -that hi~her :Erequency components o:E the sl~nal may be racorded Oll the disc in electronic noise reductl.on encoded Eorm, such as by the system ]cnown as "Dolby ~". This system, whicll treats onl~ tllose s.i~nals above about 1 ];llz, produce~. a eompressed si~nal wllicll has a proven histoxy o:l. be~ suf~:iciently compatibl~ to pertnit th~ sin~le illveIltol.y manu:Eacturirl~ and distribution of cassette :l0 tapes. Such acceptance in the case of encoded discs would ba much more difficult, iE 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 si~nificant overall contribution to the current performance and future possibilities of the conventional analo~ue disc record system.
These and other eatures of the pr~sant inVentiOII will be appreGiated as the Eollowin~ deta.iled description is read in connection with the drawin~s.
DESCR.IPTION OF TIIE E~IRODI~lENTS
. ... .
In all of the elllbodimants to be desclibed, only tl~e essential inventive featul-es will ~e ShOWIl or discussed in detail. Thus, e~cept whare otherwise specified, ampli:Eiers, attenuators, aqualizers, differentiators, inte~rators, feedbac~

loop compensators, gain controls and the like are used as ordinarily required in electronic technology. Likewise, e~cept as otherwise discussed, the detailed design of styli, sensor transducers, actuator transducers, and the mechanical and electro-mechanical aspects oE discs, cartridges, tone ~rms, drive motors, and the li~e will not be trcate~.
l~r,~`~R~NC~ P~TTI
In the several emboclilllents, the vertical posi~ion o an ~Inmo~ul~te~ portlon oE tlle ;recor~ (reE~rence p~tll) is ~ense~
at or in close p:L^o~.imity to the sictnal pic~up means, typically a stylus. ~n important element oE ~e invention is the recognition that close proximity, hi~h resolution sensing is useful for reducing mold grain noise. It follows however, that it is necessary for the reference path to be as unblemished as possible. For example, it should be free of scratches. ~Iore-over, the groove "horns" or ridges of material at the groove edges projecting into the land area should preferably be removed during the disc manufacturin~ process.
Polishing of the metal mold is a ~nown method of removin~ groove horns. Another method is shown in Fi~ure ?, which shows a further reference p~th cuttin~ stylus ~ following the groove Cut~ stylus 6. The substantially fl~t bottom edge o the reference path s-tylus not only removes the groove horns but cuts away resid~lal run~le mo~llations on the lac~uer master 2 and compellsates or any vertical r~mlble introduced by the recordin~ 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 re~erence path. In this case, the reference pa-th cutting stylus cuts an ~mmodulated groove adjacent the si~nal groove.
Reference path in:~ormation sensin~ (i.e., sensing o the distor-ted undulating refere1lce path) .is a key eleme1~t of the various embod.iments. ~ ~ener~-t:l.ixed ~representat.io1l o~ re~erence path sens:in~ is show11 in Figu.1.^es 3~ all~ 31~. F`.i~3u1i~e 3~ is de-A~c:1.^ibed below u1lde.L~ the headin~3 "Reference Path Sensin~ By :LndLrect Mealls~ Refer.r.ing to Fiqure 3~, the reference pat11 in~ormation can be obtained directly, by means of a sensor which follows the signal stylus and cartridge laterally but is vertically independent, ~he main purpose being to avoid the problems of arm/
cartridge vertical resonance. Sensor 8 is attached to a reference pla.ne. In a conventional turntable tl-te attachment will typically be to the tone arm mounting surface. In theory, the attaching surface can be any suitable reerence surface, including a stable surface apart from the turntable itsel. ~ movable member ll, orming a portion of sensor S, follot~s the surface undulations of the disc surface. In practice both disc contacting and non-contacting sensors are usct~le, as described belot~ in connection with Figures ~ and 4B.
In the Figure ~ ve.rsion, the sensor sectio1l o~ the arm may be vertically ~i~ed and the vertical displacement sensi1lg transducer may comprise non-mechanical mea1ls to sense the disc surface 9, such as by ultrasonic or capacitive means or by a li~ht beam and detector (e.~., li~ht emitti1l~ diode and photodiode). A light beam focused preferably at the point of contact of the stylus, but wi-th a beam diameter encompassin~ at least one land area, may be an~ularly directed at the surface;
ver-tical variations are then maniEested as la-teral variations, which are sensed by one or more photodetectors~ This tecllni~ue has the advanta~e o:E provid:i.n~ a relatively wl~eb;lnd e~^ror Si~
WitllOUt any atten(la.ll`t meCIlall:l.Ca.~ reSC\IlallC:~5. ~al`p, rUlnLle alld mo:ld c~raill no.ises at least ~Ip ~.c~ several hulldl^ed ~ can thereby b~ COlllp~llS~ t~d.
:L0 In the e~ample o.E FicJure 4~, the tone a.rm 10, wl~ich is vertically f.ixed, but free to move laterally, has a U-sllaped end 12 in which a cart.ridge 14 is pivoted on cross~ise 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 si~nal.
In the simplest mechanical sensin~ embodiments the reference path stylus is situated on the cartrid~e in the manner of Figures 6 - 9, hereinafter described. In this case the stylus is relatively stiffly coupled to the cartrid~e body and tone arm, which results in a relatively hi~ll vertical resonallt frequellcy of the tone arm. The stvlus may be used alone :~or rumble and mold ~rain noise reduction. I~ may also ~e ust~d Wi th a tone alm verti.cal positioll sellsor to provide referellce path in~O~matiQIl in a turnta~le VNC or a tone arm VNC.
~ further mechanical version of reference path sensill~, shown in Figure ~B, employs a separate st~rlus laterally coupled to, but vertically indepelldent of, the si~nal pic~up cartrid~e.

-- lg --8~ 3 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 housin~ 3~ has a shank 34 carryin~ secondary stylus 36. ~ trallsducer at the pivot wi.thin hous.in~ 32 Eunctions as a SellSOr 0~ ~he seconda.ry stylu~ ~8 vert.ica.l movement. The s~Gollda.ry stylus is axl~an~ed ko ~ t :Erom the record whellcver th@ s.it~nal sty.lus .is l.i~te~.
Pre:Eerably, the sty:Lus 36 .is dimensioned tc~ conkact the land areas :L0 adjacent the si~nal stylus 38 of cartrid~e 14.
Secondary styli may be constructed of any of various long wearing materials compatible with disc surfaces and resistant to grooving effects, such as sapphire or diamond. Sensor transducers may be any of various types ~nown in the art, including, but not limited to: electroma~netic, photoelectric, Hall effect, magneto~diode, potentiometric, or variable resistance, capacitance or inductance. The untreated output of the transducer may represent position, velocity, acceleration, or force (as with a pressure respondin~ trallsducer).
The mechanical char-actelistics of the re~erence path sensor assembly can be optimi2ed fnl the vertical sensin~
Eunction only. The frequellcy of vertlcal resollance (sensor 1exin~ and mass) should be placed subs~.antially above the hi~hest warp frequency, and indeed ~ell illtQ tlle audio band, in order -to e~tend the hi~hest frequency o.f corL-ection upwards, tllereby to reduce audible rumble and mold ~rain noise. The secondary stylus must be situated very close to the primary stylus -- e.~ ithin 1 mm for correction -to about 50 Elz. Even closer spacin~ of about 0.1 mm for correction to about 500 H~ is preferab~e for reduction of mid-range mold ~rain noise.
The secondary stylus or sensor means may be positioned slightly in advance oE the primary (si~nal) stylus to ~ellerat~ an anticipator~ error si~nal. rllis :is useful Eor r~la~:in3 the cla.in ~nd pll~s~ r~ .L~J~-m~cll~ v~ z~p~ c~l~ t~
~nsu~ opt:ima.L e~xor cance:llat:ioll Whel`e mechc~nicll O~A
o:l@ctrica~ phas~ sh:i.ts a~e p~esent, as Eor e~ample with low p;lSS
:L0 ~ilt~r.in~ oE th~ reEerence path inEormation~
REFERENCE PA_H~ARM INFOR~TION SENSIN~.
Other en~odiments of the invention employ reference path-arm information this is the si~nal obtained by sensin~ the distance between the reference path and arm (i e., cartrid~e).
This signal will necessarily include tone arm movements and arm/cartridge resonance effects. A first sensor version simply employs the vertical component information from the pic~up cartridge as is known in the prior art. This method provides useful information above the frequency of arm~cartrid~e resonance but is limited to cases and to tlle re-~uency ran-~e in ~hicll channel separation is deli~erately d~lrin~ disc cuttin~ (e.~.
below 100 H~).
In order to obtain reterellce pa~ ann in~ormatioTl up to hi~her frequencies it is necessary to provide a land sensor which is independent o the si~nal stylus. Non-mecllanic~l sensin-~means such as those mentioned previously in connection with Fi~ure 4A may ~e used ho~ever beill-~ ed to the cartrid~e -- _O

6~3~

holdin~ arm or cartrid~e rather than to a vertically fi~ed arm.
An example of such a sensor is shown in Fi~ure 5. A stereophonic pickup cartrid~e 40 has a conventional cantilevered shan~ 42 and stylus tip 44 shown in en~a~ement with a phono~rapll disc 9.
light source 46, such as a .li~ht emitt.ill<J diode (I,ED) or diode laser, :Eor ~mpl~ x~lt~s ~ lk to ~h~ .r~ox~ i.n l.ll~ o}~ mi:l:Li~ m~
t:o be iL1uminat:ed. 'L`ho l~ellacte~ ht~ is ~^eceived at on;~ r,x more phot.o receptors 4~ alld 50, such as photo ~.iode~" in the same :L0 manner as that of the descriptloll of Fi~ure ~l~. Tl~e li~llt illumination location and diameter are preferably chosell to illuminate the area in which the stylus tip ~4 is located and the adjacent land areas so that the reflected li~ht is responsive primarily to local variations in the land at or just precedin~
the stylus, which variations are representative of the rumble and mold ~rain noise at that point~ The output of receptors ~S and 50 may be fed to a differential amplifier to provide an indication of local land variations; a suitable circuit arran~ement may be made responsive only to vertical lalld posi tiOIl variations and not to the total li~ht reflected, which will depelld UpOIl ~l'OOVe modulations. Such techlli~ues are used in a~ltomatic sl.ide focusin-~mechanisms, for e~ample~
E~amples of IlleChalliCal versions of reference path-clrm sensors are shown in Fi~Jures 6 - 9~ In eaCII of the en~odiments, a dual stylus pic~up cartrid~e is provided in which a conventiollal stylus trac~s the ~roove inormation content and the secondary stylus senses ~arp and run~le information. In the cartrid~e VNC

embodiment of the invention, such a dual contact combination has the potential of improving both warp and rumble performance within a unitary, self-contained pickup cartrid~e. A third caxtrid~e to record surface contact device may optionally be used, such as a brush or damper oE the prior ar-t warp trackin~J devices mentiolled above.
~ n ideal re~E~xellce patll Sensill~J metllod is sllown in F:i~ul^e 6, in ~hich a sllallow unmo(iulakeci pilot ~xoov~ 52 is provided adjacent the main information carryin~ ~roove 5-1 in a phonog^aph disc 9a. rrhe tip 56 o~ secolldary stylus 58 of cartrid~e 60 rides in the pilot ~roove 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 run~le are not serious problems and it is sufficient in a practical system to deal with vertical components only.
Refer~ing to Figure 7, in an arran~ement suited to conventional commercial phono~raph records, the cartrid~e 6~ has a main stylus havin~ a shank 4~ and a stvlus tip 4~1 trac~in~ an information carryin~ ~roove 5~ of a phono~rapll recol-d ~. The secondary shank 64 and stylus tip ~i are situated on one or both sides of the main stylus and mcly fully or partially encixcle it.
The contact area may be biased towards ~he o~ltside ot the recor~, i~ desired, so that pre-ecllQ effects are minimi~ed in the refexel~ce path inforll~ation. The secondary stylus tip has a substantially flat bottom with operative diMensions sufficiently lar~e ~e.~., some fraction of a millimeter) so that it rides 6~i3 reliably on at least one land area and is thereby substantially unresponsive to lateral information and to any information content of the groove, respondin~ only to the land hei~3ht variations t~hicl are a measure of the warp and rumble. The tip 66 is yui~e~
laterally by the main s-tylus, and may be held in place b,y a compllant coupl.in~ 6S wllicll ~enera.l.l,y mailltaills t:he relative positlons o:E the two styl.i bllt doe5 not .inter;fe.re t~ith ~he st,y:L~Is movements.
~nothcr view of the styli is showll in Fi~3ure 8. ~n elon~ated block shaped tip 70 for secondary stylus 72 Spallllill~J
the land between several c3rooves is located ahead of an~ to the outside of the main stylus 42. As mentioned previously, the secondary stylus tip may optionally have a U-silape (sho~n by way of example as element 74 in Figure 9) or an O-shape, surroundin~
the main stylus. In Fic3ure 8, the contact area is shot~n biased towards the outside of the record so as to decrease pre-echo effects. A further practical matter is that the stylus arran~e-ment should preferably not trap dust but should deflect it at~ay.
For most tone arm and cartrid~e con~i~3llrations, the design of the referellce path-arm in.~o.rmatioll sensors should be such that the trac~in~ force o the main stylus should preferablv comprise the. main portion o:~ the overal:l cartrid-,~e tl`aC~ force in order to avoid a.Efectin~ the ~ e thrust ;~crce on the cartrid-3e aIId to avoid redUCill~ the maill stylus ~Ol`Ce availa~le for trac~ 3 ~arps and lar~e amplitude si~3nals. .~ secolldarv stylus trac~in~3 orce a small ~raction that o:E the m~in stvlus -- e.~., 1/~, 1/10, or even less ~- is adequate to sense the relatively lot~l amplitude - ~3 -and low frequency rumble components. The secondary stylus is preferably compliantly connected to the cartridge body, the compliance preferably bein~ substantially greater than that of the main stylus. The above compliance and tracking force considerations apply primaxil~r to ofE-set tone alm systems in wllich warp :is not compensatec~. (Cn some sy5tem5 tl~e seconda~y stylus sllc~]~ Mc~y ~ iv~ t~ y co~lp.l~ s kyl~ t~
cartr.ic~cJe body.) Fu~l~ther, ihe ~c~ect.i.ve m~nss of~ tl~e secon~ar stylus an~ xe:lated movin~ pclrts, ~.ogetller with thc f-le~ or st.ii~i~ness prope.rties o:E the shank, should produce a hi~ fxequellcv resonance we:ll above the hi~hest mold of grain noise componellts oE interest; thus, a resonant frequency of at least 1 - 2 ~Hz 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. ~ low pass mechanical filter may be incorporated if desired, so th~t 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 Ol^ another to cancel co~respondill~ vertical infol-mation from the si~nal stylus. Wholly mecllanical callcellatioll arrall~e-ments may be used, as hereinafter described. In the siinplest arran~ement, with a non-compliallt secondary stylussllan~, the vert-ical movemellts of the c~rtridge body subtrac t from the corresponcl-ing movements of the primary stylus. ~lternatively, secondary stylus information may interact in the maglletic or electromechanical arrangements of the main signal transducel- in such a ~ay as to cancel error information. In some arrangements, a separate or coordinated transducer may be provided for the secondary stylus.
The combination of si~nals, as by interconllected coils, may be accomplished within the cartrid~e itself or the si~nals mav ~e brought out for extel-nal combination. Tlle sic~nals may be ~Ised lnternally and also brouqllt Ollt for ~Ise in o~:llex~ elllb~diments o~
the invent:ioll. For example, the l~ h ~r~quellcy compollellts ~ro thc re~erence patll s~nsol May be util;zcd intelrnally o~ broll~311l out to the pr~mpli;Eier For mol(l ~ra:in noise reducl-ioll (ca~ id~e VNC or pre-amp VNC), and the lot~ Ere(luency components may be brouc3ht out for dealin(3 with warp via actuation of the turntable or tone arm (turntable VNC or tone arm VNC).

REFERENCE PATH INFORMATION SENSING BY INDIRECT ~ S_ AS discussed previously, accurate reference path infor-mation sensing may be achieved directly, by means of a vertically fixed sensor. In an approximation, the vertical pOSitiOIl of the tone arm may be used, by the methods knot~n in the prior art. A
transducer of the types previously mentioned in connection ~ith Figure 4B is mounted bett~een the arm and vertical pivot so as 2Q to ~ive an output related to tlle vertical position or an91e of the tone arm and cartrid~e. Tllls metllQd of sensill~ is use~ul for providin~ reference path informatioll belot~ the arm-cartrid~e resonant frequency (e.~, 10 M~ t is possible to elllploy a relatively stiEfly mounted secondary stylus to raise the frequency of the tone arm~ Tlle referellce path-arm sensors described previously in connection t~ith Fi~ures 5 to 9 aLe pLimarily useful above the resonallt frequency. At or neaL the LeSOnant frequency, - ~5 -ii3 phase and amplitude errors will be introduced into the reference pa-th inEormation by both oE these methods. Hence, these methods are useEul only at warp or rumble frequencies somewhat removed Erom the arm-cartrid~e resonant fre~uellcy -- that is, normally be:low about 5 Hz and abc)ve abo~lt ~0 Mz.
rl~ ~t~ impl~ov~ m~?tllo~ bs~llt~ ly ~ r~
~ r~bl~ s~rls~ t~ S~ riv~ n~ o~ ti~
actio~ to be applie~c~ e~ve~^ the whole ~re~lency l'all~e o;E illtel^~st (e.~., 0.5 llz up to several hundred ~1~) without inter~er*nce from arm/cartrid~ resonance. The method employs a con~ination of the Eirst and second sensin~ methods described in the previous para~raph above (i.e., tone arm sensin~ and reference path-arm sensing) in a manner shown schematically in Figure 3B. This approach is based on the recognition that both si~nals contain the same error signals (from arm-cartridge interaction effects), but in complementary form, whereby they can be cancelled to leave a difference si~nal accurately indicative of the warp and run~le.
The vertical arm position si~nal ~ is an indication oc the arm 76 to reference plane distance, wllereas the reference path-arn) si~nal y is an indication oE the cartrid~e 7~ to disc distance;
the diference ~ is the warp and rumbl* amplitude; that is, z = ~ - y. q~he arm/'cartrid~c resc-ll;lnce e~rol~ si~,~nals contained in ~ and y si~nals from the tone arm sellsor S0 and referellce path-arm sensor S2 are cancelleA in combiner S~, pro~ridill~ a substantially error free re~erence patll informatioll si~nal 86.
The y signal can be derived by ~le ~rarious mealls ~hich ha~e been discussed, usin~ either the si~nal st~Jlus 7S~, in son~e cases providing useful information up to about 30 Hz, or the secondary stylus 78B, by which useful information is obtained up to about several hundred Hz.
TURNTABLE VNC
ReEerrillg to Figul-e 10, a block diagxclm is sllown of a -turntable VNC (vert.i.cal nolse compellsator) ~ml~odiment o.~ the .inven~tion~ .in wh.icll ver~.ic:al ~)Os.i~.ic~n 01` d.isp:lacelnQnt e~ ol~s ill-~.h~ V~ .ll.i t ,V oE -~ picl~p ~ s~r~ ntr~
ve:rtical displacement o:E ~he disc .in a c.losed :loop servomecilanlsm systeM~ In eEfect, a re:Eerellce plalle ~or the d.isc is set by the bias displacenent level YO appl.ied to adder/subtractox ~0. The reference plane may optionally be made variable by means of control 92. An error signal on line 94 is developed by subtract-ing the amplified reference path information signal on line 96 from the bias signal. The error signal is applied to an amplifier 98 and a vertical actuator 100 that controls the positioning of the disc 9 in a direction substantially nor~.al to the disc surface, at least in the re~ion of the sigllal stylus.
A signal 102 relating to spurious vertical displacemellt of the disc in the vicinity o the pic~up transducer is generated by the reference path sensor 104 and applied to tlle ampliier 106.
For reductioll o:E ~Yarpt ~he feedbac~ system should be effective at least. in the frequellcy re~ion of about 0.5 ~
throu~h about 10 ~l~. The system may be ~ co~lp.led, DC coupled or a combination thereof. Fo.r .reduction of rumble and mold grain noise, the bandwidth of the cor.rectioll actioll is e~tended into the audio range (e.g., up to several hundred H~.). The correction 6~

may be wholly mechanical, via the vertical actuator. Alternative~
ly, the higher rumble frequencies (e.~., about 50 H~) may be compensated via a cartrid~e VNC embodiment or a pre-amp ~C. If desired, a crossover network can be used to divide the treated Ere~uency ran~es appropriately. Th~ overall xystem can thlls ma~e a si~niEicant con~ribut:ion bo~ o recluc.illcJ w~rp ~Ind its side e~E~ects, and to .rCdUCi.ll-~ audible noise.
'rlle spurious ve^tical dis~lacemel-lt deal.t witll ~y t~le system may optionallv le e~lliblted by display mealls lOS. Switch :lO 110 may optionally be provided to brea~ the loop in ordel~ to switch off the correction action. Blocks 112 and 114 are described in connection with Fi-3ure 11, below.
Means are required to control the disc position in response to sensed variations in the vertical disc position in the vicinity of the pickup transducer. The disc hei~ht may be varied uniformly across its surface or only in the vicinity of the pickup transducer. The necessary linear or an~ular motions may be accomplished via the disc, the turntable, the drive motor, or the whole motorboard assembly.
A representative translatlonal vertical aCtUatOl`, resemblin~ a woofer loudspe~er wit.ll a flat COlle, iS sho~ in Fi~3ure 11. A metal platter 116 is supporte(l by d.r.ive sha ~t llS
in bearin~ 1~0. Platter 116 is provided primarily or rotational mass, in order to reduce wo~ and flutter, and may be reduced in mass or even eliminated in some d~si~3ns. The upper surface of platter 116 is conical to receive a li-3ht-wei~ht phono~raph record supportin-3 turntable 122, which includes inde~ pin 1~4.

-- ~S --6~ii3 Turntable 122 may be partly hollowed out, with a cover me~ber 126, to minimize weight. One or more "spiders", or annular sets of corrugated material 128 and 130, suspend turntable 1~2 fxom the platter 116. The corru~ated material may be a li~ht~~ei~ht :Eibre materia:L o:E the type comMo~ scd .in loudspea~er suspens.ions. ~ push rod .l32 passin-~ th.ro~l~h the cen~e of c~r.ive shaft l:lS rid:in~ Oll ba.ll beal-in~ 13~ driven b~y a movin~3 coil .1.36 wh:ich forms pa.rt o;E a mov:in~ coil motor assembly :l33, ~Ised l~ere as a motion trallsducer fo.r pusll .rod 132. ~ssembly 13S
includes permanent ma~net 140 and a tubular core 14~ suspended from the magnet 140 to carry windin~ 136 in proper .relation to the ma~net. Supply leads 144 and 146 are driven by ampliier 98 (Figure 10). Springs or other such support means may be provided to balance the weight of the turntable assembly and to vertically centre the motor unit 138.
Turntable 122 is preferably of very light-weight material, such as foamed plastic, to minimize the mass required to be moved by the transducer 133. ~lso, the push rod to drive shaft interface friction is reduced as low as practicable by usin~ a nylon or other low friction bearin~, for e~ample. Such bearin~s may be dispansed with in son~e desi~ s, pa.Lticularly if an additional spidex is used at tlle bottom O.t the push.rod 13~.
Other types o mec}lanical coupl.in~, sucll as hydrau.lic ancl pneumatic, may also be used.
The motor assembl~ 13S pxeferably provides a positive displacement output for a ~iven electrical si~nal input; this avoids mechanical resonances of the systeM. This type of ~9 ~ ~8~ 3 performance may, for example, be provided by enclosin~ the motor unit itselE within its own servo loop, preferably includiny a motor position sensor such as 112 in Fi~ure 10 (and Fi~ure 11) and a suitable amplifier 114. A:Lternatively, if the motor unit simply provides an essentially ulldamped force, th.?ll it is necessary to pl-ovide an approp~ e amoull~. oE pas~;ive dampil~
th.is, howevex, re~uires a lar~le amoun~ of drive a~ l;iEie~ ower.
mo~? p~ ct.i~l m(?t.ll~d is t~ ~115~ t ~ m~s~s ~
compl:iances oE -the turl~tab:le asselllbly result ln a r~?SOllant Ere~uency eitller well below or we:LI abovt? the Erequellcy ran~e of interest. The resonant frequency mi~ht be placed at about 50 ll~, for example, to provide a well controlled behaviour up to about 20 H~. Passive mechanical dampin~ usin~ viscous materials may be employed as required. These considerations also apply to the further turntable VNC en~odiments to be described. Such passive displacement control methods do not interfere t~ith operation of the record player when the VNC is switched off; tllis is not the case, however, with the tone arm VNC embodimellts to be described in which electronic servos are preferLed so that the arm mav be handled manually and can trac~ norlllall~v t~hell thc? ~C is st~itched ofE.
In an alterllative eml)odimel-t of the vel~tical actllatort shown in Fi~ure 1~, the transd~lcc?r 13S is located in a cylin~rical cut out l~S in modified platter 116a. Thus, the transc~ucer 13S rotates alon~ ith platter 116a. The push rod is thus eliminated, alon~ with its m~ss and frictioll. However, in order to power the motor assen~lv 13S, a pair of slip rin~s ~L~8~63 150 or other electrical transmission means is provided.
Alternatives to the vertical translation mechanisms which have been described are directed to controllin~ the disc height only in the vicinity o.E the plckup transducer. Such a method may controllably rock or -tilt the ~urntable so as to p.rovide vert.ical movement a.l.oll~ the .linc t.raced by the pic~up cart.rid~e.
In the e~amp.l.e of l~i~u.re 13, a ti:ltable pllollo~lrapl suppo.rtin-~ tu.rn-table 122a is spaced abov~ a metal p.latter 1161 ~:La~ter 1:l6b is provided only for rotational mass, in o.rder to reduce wow and flutter if necessary, and may be dispensed with in some designs. Turntable 122a has a downward conical annular portion 152 that is coupled to platter 116b for rotary motion but permits rocking or tilting of the turntable. Drive shaft 118a for platter 116b, powered by a suitable rotational drive motor, is seated in bearin~s 120a. A hollow centre is provided in drive shaft 118a for tilt rod 132a that ternina-tes in turn-table 122a and index pin 124, Tilt rod 132a is coupled by a rotary joint 154 and rod 156 to a movin~ coil motor assembly 138, of the type described above, located to provide .lateral motion to the bottom end o:E rod 132a .llld l1C~nCe, to ti.lt turnt~ble 122a.
Alternatively, the elltire tul^ntable and drive motor asseMbly can be tilted relative to the tonP arm and cartrid~e, in the mannel- of ~he ell~odiment of Fi~ure 1~ turntable 15S
driven by a motor 160 throu~h drive shat 162 is supported by sub-base 164 suspended by compression sprin~s 166, 16S from a ~8~ i3 base 170, to which the arm and cartridge assembly 172 is mounted.
A moving coil transduce.r assembly 138 controllably moves one end o~ the sub-base 164 to tilt the turntable relative to the arm/
cartridge assembly.
In a va.iation o:E the arrall~el)lent of Fi~ure llA a displacement trallsducer :is loca-ted so as to move vext.ieally ~he -turntable alld drive sha~t, possib~y incluelin~ the ~k`.iVe nlOtOl`.
Fo.r ecollollW, the d:isplacemerl~ t.rallsducel may be made a pa~ of l:he drive motor. Fi~u.re 14B shows an a.rran~3eMent in wllich tlle entire drive motor and turntable are moved vertically. A
corru~ated annulus 130a, similar to material 130, suspends t!le motor 160 from an annular support 171 from the motorboard 164 to permit vertical movement.
Further tilting embodiments useful as retrofits for existing turntable structures are shown in Figures 15A and 15B.
A conventional turntable 164a, driven by shaft 162a, has a tilting surface assembly 174 resting on its top surface.
Assembly 174 includes a tiltin~ turntable member 176 ~enerally coextensive with the size and shape of the underlyin~ e~istin~
turntable 164a and havin~ an extellded periphery witll downward dependin~ ed~es. Turntable 17G has a downward dependill~ conical annular portion that contacts a cone shaped member 177 that slips over the index pin 179 Oll the underlviII~ turntables 16~a and spaces the extendin~ portions or turntable 176 above the underlyin~ turntable to permit tiltin~. In order to provide rotational couplin~ between the turntables, an annulus 17S, o sufficient wei~ht and surace friction to couple securely to the ~L8~3 underlying turntable surface, is coupled to a hollowed out under portion of the turntable 176 by means 180 tha-t are rotationally ri~.id yet yieldable to rockin~ motion. Corru~ated material such as described above in connection wlth Fi~ures 11 and 12 is suitab:Le. The downward depelldill~ ed~es 1~ o~ tl~e upper turntab:Le are metal:l.ic so ~.hat a~ electron~a~net 1~;l axran~ed to corltrol.lably pull Oll ~lle ed~Je at one locatlo contro:l.s th~ turll-tab:Le `tilt.
Ill Fi~ure 1.5.13, an alterllative rockin~ mecllanism 183 en~a~es the top of a record 185, beinc~ placed in position after the record is put on the turntable. ~ linear motor 138 r as described in relation to Figure 13, provides the required roc~-ing action throu~h rotary ~oint 154.
For simplicity in retrofittin~ such warp compensation devices, the warps themselves may be sensed iII an appro~imate fashion at the record ed~e, at a position substantially ~here the pickup cartridge intersects the record, usin~ a lamp and photocell assembly 192. This sensin~ approach is most accurate at the record ed~es where ~arp is ~reatest. Otilel- sensin~ means as described above, can be used iE ~reater accuracy is desire~.
An advallta~e oE a disc repxoducin~ system elllployin-~ a turntable VNC, particularly of the vertical trallslatioll types o Fi~ures 11, 1~ alld l~B, is the possible use of a tone a~
essentially i~ed vertically and moullted only for :lateral move-ment relative to the record. This simplifies reference patll sensin~ ich may be done directly, as in Fi~ures ~A and ~B.
It also ollo~s that if a pivot is used either for accommodatin~

any residual vertical mo-tion of the disc or for placin~ ~he stylus on the disc, the pivot can be situated very close to the cartridge without :Eear o:E lntroducin~ warp wow. If desired, the vert:ical actuator may be controlled so as -to e~EEect c?ll~a~le-ment a.lld clisen~a~emen-t o:E the recorcl ancl the cartrid~e. Tlle vertlcal actuator C`all thus el:im.i.lla~ tlle ll~?ed for an autolllat.:ic l.i :E tillC~J ll;l~?Ch~llli~3M aSSOC.i~,l t.~ W.i tll tll~? tC 11~ m .
'rll~ S~ S l:a~ n ~ C~ t ~r~ b~ lt.~
VNC reproduce.rs is accompan:ied by a cor.respolld.i~ reducticn in th~ man~ p:roblems previously mcntioned in relation to W~l`p.
Moreove.r, the e:EEective existence of "warp-free" reco.rds makes possible the design of tone arms, pickup transducers, and si~nal styli taking this operatin~ condition into account. For example, it will be easier to optimize the desi~n of an offset tone arm which is required to move in a substantially lateral direction only. The reduced tracking pressure and ma~imur.l excursion of the stylus result in a difEerent set of electro-mechanical parameters for cartrid~e desi~n. The latter observation applies also to -the tone arm V~C reproducers to be 2n described below.
TON~ VNC
Prior art feed~ack tPIli? al.`m sys~ems are laid out ill the manner of Fi(lures l6f~ and 16~. ~rl.e tOI`le arm ve~tica.l act~lator is an electromechanical trallsducer so arran~ed to apply a orce to the ~.one arm or cartrid~P in a direction noLmal to the disc surface, in response to an electrical si-~nal from the sensor and amplifier. A furtlle1 motor unit can be employed to perorm similarly on a horizontal basis (or 45/45).
In one prior art version shown in Fic3ure 16A, a tone arm sensor is arran~ed to monitor the vertical velocity of the tone arm; the ne~ative Eeedback loop thereby acts to provide dampinc3 Eor the tone arm. The dampln~ depends Oll loop ~T~in~
wll.icll must not be so h.i~h a5 to :inter~exe w.itll tlle tracki~ of Wal`pS, which are traclied passively. Tlle avexall result is tlla~
t}~ tonc ~rm/~rt~ J~ s~ nc~ t~ r~cl~ t ~ t the Eeedb~lck :Ioop does not ~irectly ~nter i~nko t~ ckill~ of the t~isc surEace.
In other prior art versions, shown in Fi~ure 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 cartrid~e output si~nal, This si~nal includes the arm/cartrid~e 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 tlle servo actioll. The prior art eedback tone arm techniques are thus only paltially effective in dealin~ with the problems of l`eCOXd wa.rp.
Improved tone arm systems in accordance Witil the present invention are showll in Fi~ures 17, lS alld 19. Fic3ures 17 and 18 utili2e the improved direct and indirect error sensin~
n~ethods, shown in Fi~ures 3~ and 3B, res~ectively, which effectively eliminate the effects ot arm/cartl-id~e mechanical resonance from the error si~nal. The en~odiment of Fi~ure 19 employs a doukle closed loop in such a manner that the sensed reference path-arm information is caused to be substantially the same as reference path inEormation, thereby eliminatin~
resonance eEfects. In some oE the embodiments, both vertical and lateral operation (or 45/45) are possib:le, but ~or simplicity the discussioll will be con~ined to velt.ical operati~n ol~
~ w~ Fi~ 7 ~Illpl~y~, ~n ~ n ~op and direct sensill~ Q.~ t.he re~Ee~ellCe path v.ia a trilllSd~lCer ~
which is indapen~ant of vertical movements o~ the cartrid~le;
tllis t~ype of se~llsor is illustrateA in Fi~ures ~ and 4B. The reference path information is amplified in amplifier 196 and applied to the tone arm (cartrid~e) vertical actuator 89, ~hich may be a moving coil motor unit as in the feedback tone arms of the prior art. The si~nal polarity and ~ain are set to provide a vertical drive to the cartridge body correspondin~ to the warp and rumble undulations of the disc surface, a condition which will result in ~ero, or at least a minimum ou-tput from the si~nal cartridge at least in the warp frequency ran~e. Dependin~ on the ~ain settin~, the system may also undelcorrect or overcorrect vertical errors. When the ~ain is optimally set bv ~ain control 198~ the system will ba able to effect correction throu~h and abov~ the arm/cartridqa reson~ant fre~uency le~tiOn o~ly if the arm displacements are positively relat2d to the referellce path inormation si~nal. That is, the arm must be damped, either mechanically or electrically, so as to eliminate the arm~cartrid~e resonance and obtain a positive displacement effect. The ac-t~lator 6~3 damping and/or servo considerations discussed previously in relation to the turntable VNC embodiments are relevant here.
An exemplary servo loop 200 around the tone arm vertical actuator is shown at the ri~ht-hand portion of Fi~ure 17 and includes a tone arm vertical movement sensor 80 (the transducer is of the type describec~ in connection with ~ ure ~B, howevex alla~ ed to sense tOlle arm vertical moveme~ ) ampliEier 20~, a~c~c?l~
~ubtrac-tor 204 alld ackuator drive amp.li.ier 206. Tlle ~ain ancl oth~ character.ist.ics of the loop 200 are set ~.o obtai~ ood 10 overall warp a~nd rumble compensatiol~ performanc~e up to, say, 20 Mz, with hic~her frequencies of reference patll info~mation bein~
tapped off at 208 and utilized more conveniently in a pre-amp VNC, to be described~ It should be noted that the actuator servo 200 is isolated and used only to obtain a positive displacement effect in the present invention, so that much hi~her values of loop 200 gain may be employed than in the closed loop arran~ements of the prior art. The first prior art embodiment mentioned uses the actuator loop only to damp the tone arm and not as part of an overall servo system. The other prior art loops include the arm/cartrid~e trallsfer characteristic, severelv limitin~ the usable ~ain beEore oscillation.
Switch 21~ per^mits openin~ o~ loop )00 to deactivate the s~st.em for test or demonstra~ion purposes. Display means 108 allows the refel-ence path inform~tion to be observed.
Tone arm V~C embodimellts may be AC coupled, DC coupled, or a combinatioll thereof. ~ bias control tsucll as 210 in Fi~ure 17) can be used to set stylus force andfor to raise and lower i3 the stylus. In an AC-onl.y system the tracking force can be mechanically determined/ as with conventional tone arms; a bias control may, however, optionally be employed to override the error sic~nal :Eox raisin~ and lowerin~ purposes.
Fi~ure 18 shows a .Eurther open loop kone arm ~NC .in which a re.Eerence patll .infol..mation derivat:ion systeM o~ the incl.i.rect t~pe showll in ;L~ ure 3~ .is employe~. The ~ains of ~mpll:Eiers 2.1.~l an~ 2:l~1a a.re sek to col~orm to khe collclitions shown in ~ u~re 313, wh~reby substantlaLl~y pure re~er~ellce pa~l~
in:Eormatioll is obtailled, unadulte.rated by arm~cartrid~e resonance efects. The operatioll of the system is essentially the same as that of the open loop system of Fi~ure 17. Under, over, or optimal correction can be obtained; dependin~ on the settin~ of the gain control. As in the system of Figure 17, there are 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 oscillations. Thus, the loop 200 ~ain is set to provi~e adequate damping or a positive displacemellt effect of the vertical actuator, in order to yield a ~ood warp an~ r~lmble COltlpeIlSatiOIl iII the frequency rall~e o~ interest (e-~ lp to 20 H~).
For econom~, tone arm SellSO.L`S 80 and 80~ may be the same sensor. T.ikewise, amplif:iers ~02 an~ may be tile same amplifier, with attenuation as req~lired to provide appropriate levels (~ains) at the inputs of the combinin~ networ~s _16 and 20~.

Figure 19 shows a version of a tone arm VNC in accord-ance with the inven-tion in which the warp and rumble error si~nal itself is enclosed within a ne~ative feedback loop. The loop automatically provides tone arm movements which co.r-respond to those o:E the d.isc sul-Eace. ~s ln the prev:Lous embodiments, it is essential that the COl-l-eCt errOI` .".i~ sho-lld be emp.l~ed --~ e~lc~ p~ m~t~ n wlt~ t int~ r~m ll~m~
cartrid~e :reson~nce efec~.s.
Consi~er thc operat.ion vf thc system of Fi~u.re 19 if 10 the outer loop 201 wer^e broken at point ~. The a.rm would remain stationary throu~hout the :Frequency ran~e of concern (0.5 H~ -20 Hz), provided that sufficient ~ain is employed in the vertical actuator servo loop (inner loop 200); the gains of amplif~ers 202 and 206, for e~ample, may be set to ensure this condition without fear of oscillation, as there are no unusual oscillation provoking elements within the inner loop. Under this condition the output of the reference path-arm sensor is pure reference path information. If the outer loop is then closed at point A
the arm will follo~ the reference path but at an amplitude depe.ndin~ on the overall outer loop ~ain. The ~ai.ll o amplifier 218 may be set to provide t.he desired faCtOl` of reduction in output from the reference path-arm SellSOr, which, it should be noted, provides pUl-e referellce pa-th information but at a reduced amplitude. Thus, the invelltioll remedies the problems of the prior art tone arm eed~ac~ systems (Fi~ure 16B~ in efectively eliminatin~ the troublesome armJcartrid~e transer characteristics from the feedback loop, wller~by the arm is act~lated by and follows the reference path.

6~i3 _ RTRIDGE VNC
Figure ~0 shows a :Eunctional block dia~3ram of cartrid~e VNC embodiments of the invention~ Warp, rumble, and mold ~l-ain noise from the main stylus (~3roove-arm sensol 220) are cancelled :Erom the output si~3na.l in one way or another by axra~ elllellt 2~2, whicll may, fox e~amp.1.e, bc mecllallica:l, ma~netic or elect.lical i.n ~ tu.r~, u~.in~ rlno.~ E(~ p~ Vi~ b~ ~h~ ~
path~a:l^m SellSOr 2~ rh.is .is pre;Eere~b ly accompli.shed ent.ilely w.ithin a unltaxy dual sty.lus cartl^id~e assen~ly. ~.rhe operatin~
pa.rameters can thus be fixecl and pre-set by the cartrid~3e manufacturer, whereby installation either on an ori~3inal 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.~3., magnets and coils) may be provided within the cartrid~e, as illustrated schematically in Figures 21A and 21B, which show e~emplary combination inter-connections. The transducer outputs may be further divided to accommodate the left and ri~ht si~3nals. A lo~ pass electrical or mechanical filter may be prov.ided (e.-3., 3QQ ~1~ lo~ pass), with e~ternal control of the charact~l~istics, .if desired, to reduce any non-rumble or non-mold ~3rain llOiSe COmpOllell-tS SellSed by the secondary stylus, sucll as mi~3ht be caused by sclatches or rou~h ~roove ed~es~ Reference path ln~ormation termlllals, sllow in Fi~ures 20 and 21 may be provided, ~helehy low frequencv components may be utili2ed by turntable VNC or tone arm VNC
embodiments.

i3 In a further version, shown by way of e~ample in Figure 22, the secondary stylus shank 72, instead of couplin(3 to its own transducer, is coupled to the stator components oE
the si(3nal transduee.r, so that rult~le information sensed ~y the seeondary stylus 70 cancels out rumble .in~c~rmat.ion sense~ ~y t:he primary sty.Lus ~l4. rrhat .is, fol rumb.l.e Erecluenclc?s there is 1~0 rc31ative movement ~ecweell e.~., the i~-on c-r mac~net. 230 a~d ~.he eo:i:ls 232. Var.ious c~the.r ana:Loc~cus eane~llat:ioll arrall~emelltS
mrly be emp:l.oyed, usinc~ movin~ coils, movi.n~1 iroll, movi.n~ ma~nets ~nd the l.ike. Two like armature elelllellts may share the same stator environment, such as two coils, each associatec~ ~ith its stylus, sharinc3 a common mac3netic field.
The aforementioned stator components of the si~nal transducer may include the cartridc3e body itselE. However, for optimum noise cancellation, the mass to ~e displaced by the secondary stylus should be as lo~ as possible.
Thus the si~nal stylus and the secondary stylus preferably have independent compliant comlections 231 and 233 to the cartridc3e ~ody. For optimum trackill~, the secondary stylus pre:Eerably accoun-ts for the lesser portion o:E the overall eartridc3e trackinc~ foxee ancl the lesser pQrtioll of the ovelall vertieal sti:E~ness.
Further mechanically c~ollpled arrall~Jement3 are poss:ible in ~hich the movements o:E the secolldary sty:lus eancel corres-pondinc3 movemellts of the principal stylus. One e~ample is sho~rn in the en~o~imellt of Fi~ure 23, ~hereill a U-shaped stylus tip 7 partially surrounds the main stylus tip ~. An A-shaped shank -- ~1 --234 connects tip 74 to first and second members 274 and 276 that are fixed relative to the cartrid~e. The main stylus shank ~
is attached to the cross piece of shank 234 at point 27S. The distal end of shank 42 is connected to a conve1ltional ma~net or iron piece 242 which coopera-tes wi-th coi.',.s 24~t. In operation, secondary stylus 74, r.idin~ O11 more tha1l Ollc land area a~jacen~
the ~.roove er~a~ed by Mclin st:y.lus tip 4~i, I`eSpOnC~S to rumble :Ercquencv components and cancels move1llellts of ma~Jll~t 2~12 ~ue to COrreSpO:tldill~ reSpOllSeS o.E tlle mai1l sty:Lus.

.1.0 .rRE-~MP VNC
The reference path-arm si~nal may be con~ined with the main stylus siynal in the electronic manner shown in Fi~ure 2~.
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 Fi~ures 4A and 4B, and 5, or to separate transducer versions of Fi~ures 6 - 9. Each sensor has a respective pre-amplifier 244 and 246. The null control 250 is set for optimum noise cancellation in combiner 2~S. If desired, electrical filterin~, si~nal delay correctors or other processin~ may be inserted into ei~ller or both si~nal ChaillS in order to optimi2e the noise l`edUCtiOIl effect. Ullder all operatin-~conditions~ For e~ample t a trallSiellt noise suppressor 252 ca be employe~ to re~uce the effect of i'pops`' alld "cl.ic~s" caused by scratches on the recoxd a1ld the l.i~e.
The si~nal on the disc bein~ reproduced can be encoded in electronic noise reductlon form, for e~ample usin~ hi~h frequency compression in order to reduce hi~h frequency noise - ~12 -6~3 when played back, using high frequency e~pansion by means of the system known as "Dolby s". Such higll frequency noise reduction will be most effective psycho-acoustically when combined ~ith low :Erequency clisc noise reduction as provided by the vario~s embodiments o:E the invention. The system of Fi~ure ~4 can provide :Eor playback o:~ a Dolby ~3 encod~ disc b~y optiollally prov.idi.n~ a Dolby B type decode.r ~56, pr.e:Eer~b.1.y followin~
subt.ractor 24S or, Less des.irab:Ly, .in tlle si~nal c~h;lin o~ bloc~s 220 and ~44 be:Eore subt.racto.r 248.

- 4~ -

Claims (17)

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

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 vertical deviations of an unmodulated part of the disc in close proximity to the stylus, the sensing means being substantially more compliant than the stylus and serving to detect mode grain noise affecting the said audio signal.

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 parts of a trans-ducer 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 claim 1 2 or 3 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 1 2 or 3, wherein the second stylus has a tip shaped to ride on the lands of the disc.

8. A gramophone cartridge according to claim 1, 2 or 3, wherein the second s stylus has a tip shaped to ride in a pilot groove formed in the lands of the disc.

9. A gramophone cartridge according to claim 1. 2 or 3, wherein the sensing means senses only one or both of the land areas immaculately flanking the groove in which the stylus is engaged.

10. A gramophone cartridge according to claim 1, 2 or 3, 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 claim 1, 2 or 3, 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 signal represent-ing 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 unmodulated 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 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 photo-electric sensor providing a compensating elec-trical signal for the audio signal.