GB2140191A - Phonograph - Google Patents

Abstract

Apparatus for correcting or compensating for the tracking error of the pivotally mounted (1) tone arm (23) of a phonograph includes a first intermediate member (9) to which said tone arm (23) is pivotally (1) connected, a second intermediate member (12) which is rotatably connected to the first intermediate member (9) by a belt (18) and pulleys (13) which are sized in such a way that a photodiode source (21) and a photodiode detector (22) both carried by the second intermediate member (12) will tend to be indirectly moved by a motor to maintain them substantially constantly in register with a point on the tone arm (23). A slotted mask (24) carried by the tone arm (23) is interposed between the source (21) and the detector (22) and its slot defines a central "null" position in which said motor will not be driven when the required relationship exists. Deviation in either direction will cause the motor to be driven to re-establish this relationship. An insignificant maximum tracking error below 0.1 DEG can readily be attained. Other constructions in which the second intermediate member (12) is omitted and in which the means to control operation of the motor are differently constructed and arranged are described and illustrated. <IMAGE>

Description

SPECIFICATION Phonograph The present invention relates to phonographs for playing analogue, mechanically-tracked, records, as opposed to digital, laser-read, records.

In the manufacture of a phonograph record, a cutting tool, which vibrates in accordance with the signal to be recorded, is traversed radially in a straight line across the surface of a rotating master record disc from the edge towards the centre, thus cutting a spiral groove which is modulated by the recorded signal. This master disc is then used to form vinyl records for playback, by a series of replicating processes. In the playback process, a pickup cartridge, which contains the replay stylus, is carried across the 300 mm or other diameter rotating vinyl record at the end of a pivotted arm, typically 200 mm long. The locus of the replay stylus is therefore a circular arc of 200 mm radius, unlike the linear radial path of the original cutting stylus, as shown in Fig. 1 of the accompanying drawings.It has been shown by Baerwald, Stephenson and others that, by arranging the layout of the replay arm with respect to the centre of rotation of the vinyl record so that optimised values of "overhang" and "offset" are achieved, the "tracking error" can be minimised. "Overhang" is the linear distance by which the length of the arm from its pivot to the stylus tip exceeds the linear distance between the arm pivot and the centre of rotation of the record disc. "Offset" is the angle between the fore-and-aft axis of the pickup cartridge and a line drawn from the stylus tip to the arm pivot.

"Tracking error" is the angular error between the tangent to the groove of the vinyl record measured at the stylus tip, and the fore-and-aft axis of the pickup cartridge. This relationship is also shown in Fig. 1 of the drawings. Experts differ in opinion over the best formulae and values to use for optimising the values of overhang and offset to minimise tracking error. For example, a decision must be made as to what are the outer (R2) and inner (R1) recorded radii on the recorded disc, between which the values of tracking distortion will be minimised. These recorded radii are very likely to vary from record to record, and so a compromise must be made.

Formulae for calculating offset angle (z) and overhang (H), which are accepted by many experts, have been quoted by Kessler and Pisha (Kessler, M. D. and Pisha, B. V.) in "Tonearm Geometry and Setup Demystified", Audi, Jan 1980 (U.S.A.) in dependence upon arm length L:

Therefore (H) overhang=L-A These formulae may be used to derive values of overhang and offset which minimise tracking error. To use these formulae, the stages are: i) Decide on the maximum and minimum recorded radii between which the tracking is to be optimised. The arm length, from pivot to stylus, must also be known.

ii) Calculate the two radii for zero tracking error.

iii) Substitute these values to calculate the values of overhang and offset.

Even when these (or similar) formulae are used, it will be seen from Fig. 2, which illustrates typical loci of the tracking error across a record that, apart from the two radii of zero error, there is a finite value of error across the record, and that the distortion resulting from this maximum value cannot be reduced below about 1%. This residual tracking error is audible on very high quality record playing systems, in the use of which the two radii of zero error can be distinguished aurally from the remainder of the record disc, where tonal distortion and lack of coherency of audio images due to tracking error become apparent.

According to the present invention, there is provided apparatus for correcting or compensating for the tracking error of the pivotally mounted tone arm of a phonograph, said correction/compensation apparatus including a first intermediate to which said tone arm is pivotally connected, means constructed and arranged to sense the instantaneous tracking radius of the playing stylus of said tone arm when a phonograph provided with said apparatus is in use, and a motor drivingly connected to said first intermediate member, the motor being operable under the control of said sensing means to displace the intermediate member in a direction tending to maintain overhang (as defined in this document) at the optimum value to ensure minimum tracking error.

The present invention stems from the knowledge that, as evidenced by Figs. 1 and 2 and the formulae, for given values of arm length and offset it is possible to obtain reduced or zero tracking error across substantially an entire record disc by altering the value of overhang, for example, by altering the distance between the arm pivot and the centre of the record, according to the instantaneous tracking radius of the stylus.

The invention may be implemented by moving the pivotal axis and/or by varying the arm length, although the former is preferred, inter alia because it is then possible to design embodiments of the invention as a separate device which can be used to modify an existing phonograph.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:~ Figure 1 diagrammatically illustrates the problem of tracking error and the ways in which it may be reduced or eliminated, Figure 2 graphically illustrates the effect of tracking error when compensation therefor is provided based upon known formulae, Figure 3 illustrates the principle involved in one embodiment of the invention, Figure 4 comprises a plan view and two relatively perpendicular horizontally viewed elevations showing a practical embodiment corresponding to Figure 3, Figure 5 is a graph showing the relationship of a variable value M to a variable value R in regard to the embodiment of Figures 3 and 4, Figure 6 is a graph showing the relationship of value M to a variable value w in regard to the same embodiment, Figure 7 is a block diagram showing how a motor is fiducially controlled to effect tracking error correction/compensation in the embodiment of Figures 3 to 6, Figure 8 is similar to Figure 3 but illustrates the principle involved in a second embodiment of the invention, Figure 9 comprises a plan view and two relatively perpendicular horizontally viewed elevations showing a practical embodiment corresponding to Figure 8, Figure 10 is a graph showing the relationship of the variable value R to the variable value w in regard to the embodiment of Figures 8 and 9, Figure 1 1 is a graph showing the relationship of the value w to a variable value t in regard to the same embodiment, Figure 12 is a diagram illustrating the required geometry of the embodiment of Figures 8 to 1 1 of the drawings when w=o, Figure 1 3 corresponds to Figure 12 but shows the position when a servomotor of the system has caused an intermediate member to move into the fourth quadrant that is shown in Figure 8, Figure 14 is a graph showing a linear relationship of value w to value t for a preferred third embodiment, Figure 15 comprises a side elevation and two plan views illustrating one practical construction of the embodiment of Figure 14 of the drawings, Figure 16 is a plan view, to an enlarged scale, showing a preferred construction for some parts corresponding to Figure 15, Figure 17 is a side elevation corresponding to Figure 16 but omitting a motor whilst including an elevation showing the formation of a slotted mask, and Figure 18 is a graph showing the relationship between tracking error and instantaneous record groove radius for both a conventional record arm and one mounted in accordance with the embodiment of Figures 14 to 1 7 of the drawings.

Referring to the accompanying drawings, there now follows a Glossary of the Terms that are used in the following description and in those drawings, Linear Dimensions R=lnstantaneous radius of groove being traced (variable) L=Effective arm length, i.e. from pivot to stylus tip (fixed) M=lnstantaneous distance between platter centre and arm pivot for linear tracking using straight line motion (variable) P=Radius of intermediate link (eccentric member) (fixed) C=Distance from platter centre to eccentric fixed pivot (fixed) D=lnstantaneous distance from platter centre to arm pivot (variable) R,=lnner modulated groove (fixed) R2=Outer modulated groove (fixed) N1=Small null radius (fixed) N2=Large null radius (fixed) A=Mounting centre for optimum arm geometry (fixed) H=Overhang for optimum arm geometry (fixed) Angles z=offset angle (fixed)-the angle between the groove tangent and centre-line of tone-arm.

x=90-z for linear tracking t=angle between arm centre-line and intermediate member u=angle between arm centre-line and the line from the platter centre to the arm pivot (length D) w=angle of rotation of intermediate member measured from the centre of rotation of this member to (I) the arm pivot and (II) the platter centre. In the absence of an intermediate member, w=u.

y=angle between intermediate member and the line from the platter centre to the arm pivot (length D).

This specification describes a number of different ways in which the position of the arm pivot may be adjusted automatically as the replay pickup tracks the record from outer to inner recorded radii, so as to ensure very significantly reduced or zero tracking error.

Three basic embodiments will be described. In the first embodiment, the arm pivot is mounted on an intermediate member capable of being moved linearly along the line joining it to the centre of rotation of the record, i.e. to increase or reduce the overhang, according to the correct relationship with the radius being tracked by the stylus, so as to reduce tracking error substantially to zero. In the second embodiment, the arm pivot is mounted eccentrically on an intermediate member capable of being rotated, so that again the overhang may be increased or reduced according to the radius being tracked by the stylus so as substantially to eliminate tracking error.It will be shown that, although the dimensions may be chosen so as to make the rotation of the intermediate member follow closely the angular displacement of the arm as the stylus tracks the record, it is not possible to eliminate tracking error by following the arm itself. The third, and preferred, embodiment also employs an intermediate member capable of being rotated, with the arm pivot mounted eccentrically on this intermediate member, but also employs a second intermediate member which rotates with the pivoting arm as it allows the pickup to track the record groove. By employing a simple following servo system to maintain a constant angular relationship between the auxiliary member and the arm, it is possible substantially to eliminate the tracking error.

First Embodiment Elimination of Tracking Error by Linear Movement of the Pivot Towards the Record Centre Figure 3 shows the geometrical relationships by means of a line diagram, and Figure 4 shows a possible mechanical arrangement. From Figure 3, applying the cosine rule, the following relationships hold (1) M2=R2+L2-2.R.L. cos x (2) R2-M2+L2-2.M..L. cos w For zero tracking error, (Z+x) must equal 900, i.e. the pickup axis must always lie on the tangent to the groove being tracked. Therefore from equation 1 the required value of M is given by

N.B. L and x are fixed: only R varies as the record groove is tracked.

This relationship between M and R may be expressed as a relationship between M and w, using equation (2), M2+L2-R2 (4) whence w=cos~' 2ML Figures 5 and 6 show graphs of M vs R and M vs w, illustrating these nonlinear relationships.

Figure 4 shows a mechanical arrangement for this embodiment, in which an arm pivot 1 is mounted on an intermediate member 2 which latter moves linearly in a guideway 3 under the actuation of a stepping or stepper motor 4 driven via a rack-and-pinion 5. The block diagram of Figure 7 shows that the motor 4 is driven by an amplifier 6 controlled by digital signals from a ROM (Readonly-memory) 7 holding values of M corresponding to values of w, according to Figure 6, and accessed via values of w derived from an angular digitizer 8. The position of the digitiser 8 and of its reading head 8A are shown in Figure 4.

It should be noted that the point of minimum value M in the curve (5) M2-R2+L2-2.R.L. cos x will occur when dM =0 dR Differentiating dM (6) 2M------=2R~2L 2M =2R~2L cos x dR Whence dM R-Lcosx (7) dR M for a minimum value of M R-L cos x (8) -O M R L cos x = M M (9) .'. R=L cos x The turning point on the graph will occur when the value of R=L cos x. At this point, the direction of motion of the intermediate member 2 must be reversed.

Second Embodiment Figure 8 shows the geometrical relationships and Figure 9 a mechanical construction for this embodiment, which embodiment varies the distance between the arm pivot and the record centre by rotation of the intermediate member about a centre eccentric to the arm pivot.

From Figure 8, applying the cosine rule, the following relationships hold.

(10) D2=C2+P2-2CP cos a;-R2+L2-2RL cos x R2+L2-2RL cos x-C2-P2 (11) #cos w= -2CP also from the cosine rule R2~L2~D2 (12) cos u=~ -2DL C2~D2~p2 (13) cos y= -2DL Figure 10 shows a graph of R vs W. This graph shows a turning point. Therefore the servomotor must reverse to hold the correct value of w.

The turning point of the curve R2+L2-2RL cos x-C2-P2 (14) cos w= -2PC will occur when dw =0 dR Differentiating dw 2R 2L cos x sin w = + dR -2PC 2PC dw 2R 2L cos x (15) .. = + dR +2PC sin w ~2PC sin w for turning point dw =O dR 2R 2Lcosx (16) 2PC sin w 2PC sin w .-. R=L cos x Thus, the turning point will occur when the value of R=L cos x.

Figure 11 shows a graph of w vs t.

Figure 9 shows a mechanical arrangement for this embodiment in which the arm pivot 1 is mounted on an intermediate member 9 which can rotate in ball or other low friction bearings about a centre 10 under the action of a motor 1 1 which may drive via a worm and wheel 1 1 A. The stepper motor 1 1 is controlled by digital signals from a ROM holding values according to Figure 1 1 and is accessed via values of input from an angular digitiser 1 1 B. The position of the digitiser 1 1 B and its reading head are shown in Figure 9. Alternatively, an analogue motor may be used, preferably in a close-loop configuration using analogue signals derived either from conversion of digital signals from the ROM or from a cam.

For the intermediate member to continue through w=o position into the 4th quadrant (Fig. 8), the pivot must pass through a line from the turntable/platter centre to the eccentric fixed pivot (length C).

The geometry of this position is shown in Figure 12. For linear tracking to be maintainable, the value of C-B must coincide with a value of M, (from the analysis of Figure 3) and the value of R must be common to the two cases analysed.

It has been shown that, at the turning point of the two curves, R (17) R=L cos x. cos x= L Referring to Figure 12, cos x=R/L can only occur when the triangle R, L, C-P is a right angled triangle, where v=9O0. Therefore, for linear tracking at the w=o position, (18) C-P=L.sin x.

If the arrangement shown in Figure 8 has the geometry where the mounting centres, C=L.sin x+P, the servomotor can continue to drive through the line C, and into the 4th quadrant, therefore not reversing. The geometry for the system in the 4th quadrant is shown in Figure 13.

Where D2-C2+P2-2CP cos w-R2+L2-2RL cos x R2+L2-2RL cos x-C2-P2 (19) ,'.cosw= -2CP also R2~L2~D2 (20) cos u= -2DL C2-D2-P2 (21) cosy= -2DL The same formulae as for the 1 st quadrant case.

Third (Preferred) Embodiment The geometry of Figure 8 shows that the angle t=u+y-1 80, similarly from Figure 12 angle t=1 80-(y-u).

Figure 14 gives the graph of w vs t showing a linear relationship between w and t.

Therefore, a point on a second intermediate member which is constrained to rotate below the arm pivot, but geared to rotate so that its relationship to w is the linear relationship between w and t, will track a point on the arm, and therefore the complete system may be controlled by "slaving" a point on this second intermediate member to follow a point on the arm, thus eliminating the need for a ROM or cam system to hold non-linear relationships.

A mechanical arrangement for this embodiment is shown in Figure 1 5. The arm pivot 1 is mounted on the first intermediate member 9 which can rotate in ball or other low-friction bearings about the centre 10 under the action of the motor 11 which may again drive via the worm and pinion gearing 11 A. A second intermediate member 12 is rotatable about a vertical bearing 12 in alignment with the arm pivot 1, and is connected by a pulley/belt arrangement 18 to a spindle 13 which is fixed to a base at the centre 10. The drive ratio between the second intermediate member 12 and the spindle 13, e.g. the ratio of pulley diameters, is set to be equal to the slope of the linear relationship between w and t, as shown in Figure 14. A tracking photodiode source 21 and detector 22 are mounted on the member 9 and the tone arm, respectively.

Figures 1 6 and 1 7 of the drawings show a modification of the construction illustrated in Figure 15 in which modification the arm pivot 1 is again mounted on the first intermediate member 9 which latter is rotatable about the centre 10 by way of ball or other low-friction bearings and a vertical shaft which physically embodies that centre 10. The required rotation is effected by the servomotor 11 through the intermediary of a worm and pinion arrangement 16 that powers displacement of the member 9, the servomotor 11 being releaseably secured to a base 19 by a bracket 17. The previously mentioned second intermediate member 12 is rotatable about a ball or other low-friction bearing defining a pivotal axis which is in alignment with that defined by the arm pivot 1.The second intermediate member 12 is retained in its operative position by a plate 20 and is connected by the pulley/belt arrangement 18 to the spindle 13 which acts as a fixed pulley that is fastened to the base 19 and whose central axis coincides with the centre 10. The fixed spindle or pulley 13 is of a tubular formation containing the vertical shaft which physically affords the centre 10 and the bearings which enable that shaft to be angularly displaceable relative to the spindle/pulley 13.

The drive ratio between the pulley-like second intermediate member 12 and the spindle/pulley 13 depends, of course, upon the relative diameters thereof and this relationship between those diameters is set, by appropriate sizing, to equal the linear relationship between the values w and t that is shown in Figure 14. In this modified construction, both the photodiode source 21 and the photodiode detector 22 are mounted on the second intermediate member substantially beneath the tone arm which is indicated by the reference 23 in Figures 16 and 17, said source 21 and detector 22 being in axial register and closely spaced apart relationship with one another. A slotted mask 24 depends from a bracket carried by the tone arm 23 and has a regularly cylindrically curved configuration whose axis of curvature coincides with that of the arm pivot 1.A detail in Figure 1 7 shows one possible configuration of the slot in the mask 24, the "servo-null" position being at the centre of that slot which is of symmetrically divergent-edged formation in opposite directions from the centre. Alternatively, the slot in the mask 24 may take the form of two horizontal rectangular slits, displaced one from the other in the vertical plane, and the photo-diode source 21 and the detector 22 may each be duplicated so that each source/detector pair operates with one of the slits in the mask 24.The advantage of the arrangement shown in Figures 1 6 and 17 of the drawings is that, when the tone arm 23 is moved rapidly at, for example, the commencement or completion of a record-playing operation, or when a user wishes to locate a particular portion of the groove without -having to hear the remainder of the record, the aligned reference position relationship between the source 21 and the detector 22 is not temporarily lost and the increased illumination of the detector 22 by the source 21 through a "wider" portion of the slot in the mask 24 causes the fiducial servomotor 11 immediately to operate and quickly regain a minimum tracking error position of the system in which the centre of the slot in the mask 24 will lie between the source 21 and the detector 22. A similar advantage is given by the described alternative arrangement.

Figure 18 of the drawings illustrates graphically the very considerable reduction in tracking error that results from employing either version of the third embodiment of the invention. Curve A shows the variation in tracking error in degrees that occurs for a conventionally pivoted tone arm, when correctly set up, playing a 12" (305 mm.) record from its outer recorded radius of 146 mms. to its inner recorded radius of 58 mms. It will be seen that the tracking error at the outer radius of 146 mms. commences at 20 but reduces to 0 degrees at a radius of 119 mms. The error rapidly builds up again to 1.20 at a 90 mm. radius and rapidly reduces again to 0 at a 67 mm. radius. However, the error builds up again to 130 by the time that the inner radius of 58 mms. is reached.

Curve B on the graph of Figure 18 shows the situation when the tone arm is mounted to correct, or compensate for, tracking error in accordance with either Figure 1 5 or Figures 1 6 and 17 of the drawings. The curve B shows a tracking error significantly below 0.1 throughout the entire recorded area.

Each whole degree of tracking error corresponds to approximately 1 % second harmonic distortion where the tracking error is relatively low and, with a maximum tracking error of less than 0.1 on the curve B of Figure 18 of the drawings, it will be seen that this distortion is so low as to be insignificant for all practical purposes. This very low tracking error, and its relatively slow rate of increase and subsequent decrease during the playing of a record, result in an improved and consequently more pleasing sound quality which more faithfully reproduces the originally recorded music, speech or other sound material. Naturally, the rate of wear of both records and styli is significantly reduced as compared with the use of a tone arm which is not provided with tracking error correction/compensation in accordance with the invention. The embodiments of Figures 15 to 17 of the drawings and the graphic representation of Figure 18 relate to a tone arm having an effective length of 220 mms with an offset of 250 and an overhang of 18.9 mms.

Claims (14)

1. Apparatus for correcting or compensating for the tracking error of the pivotally mounted tone arm of a phonograph, said correction/compensation apparatus including a first intermediate member to which said tone arm is pivotally connected, means constructed and arranged to sense the instantaneous tracking radius of the playing stylus of said tone arm when a phonograph provided with said apparatus is in use, and a motor drivingly connected to said first intermediate member, the motor being operable under the control of said sensing means to displace the intermediate member in a direction tending to maintain overhang (as defined in this document) at the optimum value to ensure minimum tracking error.

2. Apparatus as claimed in claim 1 , wherein the first intermediate member to which said tone arm is pivotally connected is arranged so as to be displaceable rectilinearly in a direction which extends radially with respect to the axis about which the platter/turntable of a phonograph provided with said apparatus will rotate in the use of the latter.

3. Apparatus as claimed in claim 1, wherein the first intermediate member is rotatable about a centre and has said tone arm pivotally connected to it at a location eccentrically spaced from said centre.

4. Apparatus according to claim 2 or claim 3, wherein the means which senses the instantaneous tracking radius of the playing stylus of said tone arm is arranged to control the operation of the motor by way of an angular digitiser, a read-only-memory and a drive amplifier.

5. Apparatus as claimed in claim 1, wherein a second intermediate member is provided and carries at least part of said means constructed and arranged to sense the instantaneous tracking radius of the playing stylus, the second intermediate member being rotatably connected to the first intermediate member by parts which are sized to produce a transmission ratio therebetween which is such that, in use, a point on the second intermediate member will be moved to tend to remain constantly in substantial register with a point on said tone arm.

6. Apparatus as claimed in claim 5, wherein said parts comprise fixed and rotatable pulleys sized to establish said transmission ratio and a belt or like member drivingly interconnecting them.

7. Apparatus as claimed in claim 5 or 6, wherein the second intermediate member carries both a source of ene'rgy and a detector of energy derived from that source, said source and detector being arranged in aligned register but with a slotted mask carried by said tone arm interposed between them, and wherein the openings in said mask are so formed as to define a substantially central position in which insufficient energy will be received from said source by said detector to cause operation of the motor.

8. Apparatus as claimed in claim 7, wherein said openings comprise two horizontal rectangular slits displaced from one another in the vertical plane, said source and detector being duplicated with each source/detector pair co-operating with a corresponding one of said slits.

9. Apparatus as claimed in claim 7, wherein the slot in said mask is shaped to define two substantially symmetrical triangles of relatively small base length and apex angle, the apices of the two triangles coinciding in position substantially centrally of the slot and both triangles diverging away from that substantially central position towards their respective bases.

10. Apparatus as claimed in any preceding claim, wherein said means constructed and arranged to sense the instantaneous tracking radius of the playing stylus comprises a photodiode source and a photodiode detector.

11. Apparatus as claimed in any one of claims 1 to 10, wherein the motor is a servomotor.

12. Apparatus as claimed in any one of claims 1 to 10, wherein the motor is a stepper/stepping motor.

13. Apparatus for correcting or compensating for the tracking error of the pivotally mounted tone arm of a phonograph substantially as hereinbefore described with reference to any of the embodiments that are illustrated in Figures 3 to 1 8 of the accompanying drawings.

14. A phonograph when provided with apparatus as claimed in any preceding claim.