GB2077961A - Turntable speed control - Google Patents

Abstract

An annular magnet (23) is secured to the underside of the turntable of a record player (13) so as to rotate therewith and a flat signal generating coil (25) is mounted below the turntable so as to have a signal induced therein as the magnet (23) rotates. Comparator means (34) compares a voltage derived from the induced signal with a standard reference signal, the speed of a driving motor (17) being adjusted in response to the comparator (34) output, to achieve a constant desired turntable speed. <IMAGE>

Description

SPECIFICATION Record player turntable speed control This invention relates to a record player turntable speed control and more particularly to a record turntable speed control for an indirectly driven record player, that is a record player having a turntable, and an electric motor, the turntable not being directly mounted on the spindle of the motor but being driven from the motor via a drive transmission means.

For example, the drive transmission means may comprise an endless belt which is engaged around a drum provided on the underside of the turntable and around the spindle of the electric driving motor, or the drive transmission means may comprise a jockey wheel which engages both a depending flange of the turntable and the spindle of the driving motor to transmit drive to the turntable.

In such a record player which reproduces sound from a disc record placed upon the turntable, it is desirable to rotate the turntable at a precise speed corresponding to the type of record being played, different types of record requiring different speeds of rotation e.g. 331/3 r.p.m. or 45 r.p.m. This is because if there is variation in the speed of rotation of the record, a reproduced signal from the record will be frequency modulated, resulting in what is known as wow/flutter.

It is particularly desirable to ensure accurate speed of rotation of turntables in high quality record players which aim at "high fidelity" reproduction.

There have been proposals to control the rotational speed of the driving motor to ensure that it operates at a precise speed of rotation, by sensing the speed of rotation of the motor driving spindle.

However, such proposals do not take into account variations in speed of the turntable which occur by virtue of the indirect drive transmission means such as the endless belt or jockey wheel in the examples described above, but only variations in the speed of rotation of the motor spindle due to, for example, modulations in the power supply to the motor.

Controlling the speed of rotation of the motor by sensing the speed of the motor does not therefore guarantee accurate control of the speed of rotation of the turntable because of the nature of the drive transmission means.

In a belt drive for example, the belt usually comprises a rubber or other resilient band which can stretch and thus permit slipping to occur between the belt and the spindle and/or drum.

In a jockey wheel drive, slip may occur between the jockey wheel and the spindle and/or depending flange, especially after the parts have become worn.

The object of the invention is to provide a new or improved record turntable speed control for controlling the rotational speed of the turntable in an indirectly driven record player.

According to the invention we provide a record player comprising a turntable mounted for rotation about a vertical axis, an electric driving motor, drive transmission means indirectly transmitting drive from the spindle of the motor to the turntable, control means for controlling the speed of rotation of the turntable, said control means comprising a magnet secured to the turntable so as to rotate with the turntable, a signal generating coil mounted in a fixed position so as to be within the magnetic field of the magnet as the turntable rotates, the sequential changes in the magnetic flux produced by the rotating magnet inducing an alternating electrical signal in the signal generating coil, said signal corresponding to the rotational speed of the turntable, a control circuit including electrical comparator means for comparing said induced signal with a standard reference signal which corresponds to the exact desired speed of rotation of the turntable, means for increasing or decreasing the rotational speed of the driving motor in accordance with an output signal from the comparator means which is dependent upon the difference between the induced and reference signals, to achieve the exact desired rotational speed of the turntable.

Thus, it will be appreciated that the control of the rotational speed of the driving motor is governed by the output signal from the comparator means, which is representative of the actual rotational speed of the turntable. Thus, the driving motor is responsive to any variation in rotational speed of the turntable to adjust the speed of the turntable so that it rotates at the exact desired speed. Any variation in the speed of the turntable which may arise from the drive transmission means between the driving motor and the turntable is thus corrected.

Preferably, the magnet is of disc-shaped annular configuration, the central axis of the magnet being arranged to be coincident with the axis of rotation of the turntable.

With this arrangement, there is no necessity to increase the overall depth of the turntable to accommodate the magnet and there is no adverse effect on the dynamic balance of the turntable.

A mounting means such as a post, or spindle receiving means may be provided which may extend through the central hole of the magnet and the magnet may conveniently be secured such as by adhesive bonding to the underside of the turntable.

The windings of the signal generating coil may conveniently be laid flat, for example as a printed circuit on a flat insulated plate, which flat coil can readily be mounted below the turntable. This arrangement involves the minimum of interference with the normal construction of the record player.

The flat coil may be circular and the central axis of the coil may be coincident with the central axis of the magnet and the axis of rotation of the turntable.

However, other arrangements may alternatively be provided.

It will be appreciated that the signal which is induced in the signal generating coil during operation of the record player will have a frequency which is proportional to the rotational speed of the magnet and thus proportional to the rotational speed of the turntable. However, the amplitude of the induced signal may be small in which case the signal may be fed to an amplifier of the control circuit before being fed to said comparator means.

Said comparator means may comprise any known form of comparator.

In one embodiment, the comparator means may compare the voltage of the reference signal with a voltage obtained from the induced signal, the induced signal being fed to a sub-circuit which converts the frequency thereof into a representative voltage which is compared with the reference signal.

Preferably, the induced signal is fed, after amplification, to a sub-circuit or circuits which forms the alternating signal into a square wave, prior to conversion to facilitate said conversion. The voltage obtained from the square wave may be rectified to produce a direct voltage, and the reference signal may comprise a direct voltage which is dependent upon the selected speed of rotation of the turntable.

The reference signal may thus be manually selected.

It has been found that such a circuit as described above wherein the reference signal comprises a manually selected direct voltage which is compared with a direct voltage obtained from the alternating induced signal, provides an approximate constant selected turntable speed.

However, where more accurate control is required, for high fidelity reproduction, an oscillator such as a quartz crystal or other accurate oscillator, may be included in the control circuit. The oscillator may provide a constantfrequencywhich is compared with the frequency of the induced signal to provide an output which is, together with the selected direct voltage, compared with the direct voltage obtained from the induced signal.

Preferably, the output of the frequency comparator is fed to a sub-circuit or circuits which convert said output into a direct voltage which direct voltage is added to said selected direct voltage to obtain the reference signal.

Thus both the frequency of, and voltage derived from, the induced signal, are compared with the constant frequency signal from the oscillator and the manually selected voltage respectively, to provide said output signal from the comparator to increase or decrease the speed of rotation of said driving motor.

In a preferred embodiment, switch means are provided to enable said oscillator and frequency comparator to be disengaged from the control circuit, when great speed accuracy is not required, for example when changing speed.

The invention will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view of the underside of a turntable for use in a record player in accordancewith the invention; Figure 2 is a plan view of part of a deck plate of a record player in accordance with the invention, with the turntable removed for clarity; Figure 3 is a block circuit diagram showing one arrangement of a circuit for a control means in accordance with the invention.

Referring to Figures 1 and 2, a record player 10 comprises a deck plate 11 which is provided with an upstanding mounting post 12 on which a turntable 13 may be mounted for rotation, the turntable 13 having a central aperture 14 in which the post 12 is received, and the deck plate 11 having thrust bearing means (not shown) on which a downwardly depending boss 16 of the tu rntable 13 rests.

Mounted below the deck plate 11 is an electric driving motor 17 having a spindle 18 which extends upwardly from the deck plate 11 and drives the turntable 13 by an indirect drive means.

The turntable has a drum 20, the central axis of which is coincident with the axis of rotation C of the turntable 13, the drum providing a pulley about which an endless belt (not shown) is engaged, which belt is also engaged about the spindle 18 of the motor 17 to provide an indirect belt drive.

In the turntable, two diametrically opposite apertures 22 are provided, one to facilitate assembling the belt drive and the other to dynamically balance the turntable.

The record player described above is well known and it will be appreciated that the invention is applicable to other indirect drive record players, such as a record player having a jockey wheel type drive wherein a jockey wheel is engaged with the spindle 18 and a peripheral rim or other depending flange of the turntable, or any other indirectly driven record player.

Referring again to the drawings, adhesively secured to the underside of the turntable 13 is an annular disc-shaped magnet 23 of a control means which controls the speed of rotation of the turntable.

The central axis of the magnet 23 is also coincident with the axis of rotation C of the turntable 13.

As the magnet 23 is disc like, it is not necessary to increase the depth of the turntable to accommodate the magnet, or to further dynamically balance the turntable.

Secured to the deck plate 11, directly beneath the position which the magnet 23 will occupy when the turntable 13 is mounted on the deck plate 11, is a signal generating coil 25 of the control means, the windings of the coil being laid out flat and provided as a printed circuit on a flat insulated plate. The coil 25 is substantially circular and the central axis of the coil is again coincident with the axis of rotation C, i.e.

the axis of the mounting post 12.

The coil 25 is thus in the magnetic field of the magnet 23, when the turntable 13 is mounted for rotation. As the turntable 13 rotates, the magnet 23 will also rotate and a sequentially changing magne tic flux will induce an alternating signal in the coil 25, the frequency of the signal being dependent upon the speed of rotation of the magnet 23 and thus the speed of rotation of the turntable 13.

It will be apparent to those skilled in the art that it is not necessary for the windings of the coil 25 to be laid out flat in the manner described, although by providing this arrangement it has been found that assembly of the remainder of the record player is not interfered with. Further, it is not necessary for the magnet 23 to be annular although this provides the above described advantages. Many other arrangements are possible.

The two ends of the coil 25 are each connected to a terminal 28, 29, which terminals are connected to an electrical control circuit which is shown diagramma ticallyin Figure 3.

The induced signal from the coil 25 is fed to an amplifier 30 which amplifies the induced signal because the amplitude is small.

The alternating amplified signal is then fed to a sub-circuit 31 which forms the signal into a square wave. The thus formed square wave is then fed to a further sub-circuit 32 which converts the square wave signal into a voltage signal, the value of the voltage being dependent upon the frequency of the induced signal, and a still further sub-circuit 33 integrates the voltage signal with respect to time, to provide a substantially direct voltage V1 which is fed to a comparator 34.

A reference signal V2 which is compared with V1 by the comparator 34 is obtained as follows.

The record player has a manually controlled input circuit 35 conveniently provided as a microprocessor, having a number of manual inputs a to fat least some of the inputs a to operating the input circuit 35 to provide a direct voltage signal the value of which is dependent upon the selected speed of rotation of the turntable 13.

For example, input a may produce a direct voltage signal corresponding to 331/3 r.p.m., for large records, whilst input b may provide a direct voltage signal corresponding to 45 r.p.m. for smaller records. Other inputs cto fmay be arranged to increase or decrease the selected rotational speed by a desired amount, as required.

The direct voltage signal thereby produced is amplified in amplifier 36 to thereby provide a reference signal V4.

Where great accuracy is not required, the signal V4 may be fed directly to comparator 34 as V2, and compared with the voltage signal V1 derived from the induced signal.

The output from the comparator 34 is fed to a further sub-circuit 38 where it is translated into a voltage to control the motor 17, when switch S1 is closed, when it is desired to rotate the turntable 13.

The voltage supplied to the motor 17 thus varies in response to variation in speed of the turntable 13, and compensates for both variations in speed due to fluctuations in speed of rotation of the motor 17, and due to variations in speed due to the indirect drive means described above.

Where very accurate speed control is required, a quartz crystal or other oscillator 40 may be included in the circuit which generates a constant reference frequency f2.

The frequency fl of the induced signal, obtained after the square wave forming stage 31 of the circuit, is compared with the reference frequency f2 in a frequency comparator 41. The output from the comparator 41 is integrated with respect to time by a sub-circuit 42, to provide a substantially direct voltage V3. The voltage V3 is added to voltage V4 obtained from the manual input 35 at an adder 39 to provide a reference signal V2 which is fed to comparator 34. The circuit then operates as described above.

A switch S2 is provided to enable the oscillator 40, frequency comparator 41 and integrator 42 to be disengaged from the circuit where very accurate control of the speed of the turntable is not required, for example during speed changing.

It will be appreciated by those skilled in the art that the amplifiers 30,36 and sub-circuits 31 to 33, and 42, comparators 34 and 39 may take any known reform, for example the amplifiers and sub-circuits may comprise standard integrated circuits.

The inputs a to fof the circuit 35 may be push-button type inputs or of any other desired type.

Other control circuits may alternatively be provided, for example, a control circuit in which only the frequency of the induced and reference signals are compared may be provided, and the motor may comprise an alternating current motor wherein the speed is controlled by the compared frequency.

In any case, as the control means senses the speed of rotation of the turntable 13, any variation in the speed of the turntable due to, for example, slipping between the engaging parts of the direct drive means, is corrected, as the circuit controls the speed of the driving motor.

Claims (18)

1. A record player comprising a turntable mounted for rotation about a vertical axis, an electric driving motor, drive transmission means indirectly transmitting drive from a spindle of the motor to the turntable, control means for controlling the speed of rotation of the turntable, said control means comprising a magnet secured to the turntable so as to rotate with the turntable, a signal generating coil mounted in a fixed position so as to be within the magnetic field of the magnet as the turntable rotates, the sequential changes in the magnetic flux produced by the rotating magnet inducing an alternating electrical signal in the signal generating coil, said signal corresponding to the rotational speed of the turntable, a control circuit including electrical comparator means for comparing said induced signal with the standard reference signal which corresponds to the exact desired speed of rotation of the turntable, means for increasing or decreasing the rotational speed of the driving motor in accordance with an output signal from the comparator means which is dependent upon the difference between the induced and reference signals, to achieve the exact desired rotational speed of the turntable.

2. A record player according to Claim 1 wherein the magnet is of disc-shaped annular configuration, the central axis of the magnet being arranged to be coincident with the axis of rotation of the turntable.

3. A record player according to Claim 2 wherein a mounting means are provided which extend through the central hole of the magnet.

4. A record player according to any one of the preceding claims wherein the magnet is conveniently secured to the underside of the turntable.

5. A record player according to any one of the preceding claims wherein the windings of the signal generating coil are laid flat.

6. A record player according to Claim 5 wherein the signal generating coil is provided as a printed circuit on a flat insulated plate.

7. A record player according to Claim 5 or Claim 6 where dependant upon Claim 2 wherein the flat coil is circular and the central axis of the coil is coincident with the central axis of the magnet and the axis of rotation of the turntable.

8. A record player according to any one of the preceding claims wherein the induced signal is fed to an amplifier of the control circuit before being fed to said comparator means.

9. A record player according to any one ofthe preceding claims wherein the comparator means compare the voltage of the reference signal with a voltage obtained from the induced signal.

10. A record player according to Claim 9 wherein the induced signal is fed to a sub-circuit which converts the frequency thereof into a representative voltage which is compared with the reference signal.

11. A record player according to Claim 10 wherein the induced signal is fed to a sub-circuit or circuits which form the alternating signal into a square wave prior to conversion.

12. A record player according to Claim 11 wherein the voltage obtained from the square wave is rectified to produce a direct voltage, and the reference signal comprises a direct voltage which is dependent upon the selected speed of rotation of the turntable.

13. A record player according to any one of the preceding claims wherein the control circuit includes an oscillator.

14. A record player according to Claim 13 where dependent upon Claim 12 wherein the oscillator provides a constant frequency which is compared with the frequency of the induced signal to provide an output which is, together with the selected direct voltage, compared with the direct voltage obtained from the induced signal.

15. A record player according to Claim 14wherein the output of the frequency comparator is fed to a sub-circuit or circuits which convert said output into a direct voltage which direct voltage is added to said selected direct voltage to obtain the reference signal.

16. A record player according to Claim 14 or Claim 15 wherein switch means are provided to enable said oscillator and frequency comparator to be disengaged from the control circuit.

17. A record player substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

18. Any novel feature or novel combination of features hereinbefore described and/or shown in the accompanying drawings.