US3267196A

US3267196A - Electronic tremolo device

Info

Publication number: US3267196A
Application number: US326692A
Authority: US
Inventors: Alan B Welsh; Mathew A Slaats
Original assignee: Jasper Electronics Manufacturing Corp
Current assignee: Jasper Electronics Manufacturing Corp
Priority date: 1963-11-29
Filing date: 1963-11-29
Publication date: 1966-08-16
Anticipated expiration: 1983-08-16

Description

1966 A. B. WELSH ETAL 3,267,196

ELECTRONIC TREMOLO DEVICE Filed NOV. 29, 1965 5 Sheets-Sheet 1 ORGAN zj fgia'fgtl Fl TTOENE Y5 6, 1966 A. B. WELSH ETAL 3,267,196

ELECTRONIC TREMOLO DEVICE 5 Sheets-Sheet 2 Filed Nov. 29, 1963 HTTO ENE Y5 A g 1966 A. B. WELSH ETAL ELECTRONIC TREMOLO DEVICE 3 Sheets-Sheet 5 Filed Nov. 29, 1965 TO V I I5 EHTO TKHNSFOEMEE VIBRHTO DEPTH 5-H 260 VIBRHTO ,q OENEYS FIG-l0 United States Patent 3,267,196 ELECTRONIC TREMOLO DEVICE Alan B. Welsh, Huntingburg, and Mathew A. Slaats, Jasper, Ind., assignors to Jasper Electronic Mfg. Corp., Jasper, Ind., a corporation of Indiana Filed Nov. 29, 1963, Ser. No. 326,692 10 Claims. (Cl. 841.18)

This invention relates to musical instruments and is particularly concerned with electric organs and the like which are adapted for utilizing a plurality of speakers.

A well known attachment for an electric organ is a rotary baffle that is placed in front of a speaker and which is rotated to provide a spatial effect, which is somewhat like a tremolo. The spatial effect produced in this manner is popular and many of these devices are in use. The production of a spatial effect, in this manner, however, has certain rather severe limitations that prevent its adaptation in many circumstances. Only a limited number of speakers can be treated in this manner, the range of effects is limited, the device is expensive, and it may become noisy because of bearing wear. With the foregoing in mind, it is a primary object of the present invention to provide a novel arrangement for producing spatial or tremolo effects in speakers of musical instruments and in particular for electric organs.

Another object of this invention is the provision of an arrangement whereby any number of speakers can be controlled at the same time and wherein the individual speakers can be under selective control.

Still another object of this invention is the provision of a control arrangement of the nature referred to which will be fully adjustable as to speed of change and total change between high and low volume output of the speakers and which is not sensitive to the frequency of the various sounds being produced in the speaker.

The objects of the present invention are accomplished, in general, by introducing a variable impedance in the circuit to each of the speakers to be controlled and then controlling the impedance to produce the rise and fall in the volume of the individual speakers according to whatever pattern is desired. Preferably, the impedances are controlled by utilizing a light sensitive element in circuit therewith and varying the amount of light supplied to the light sensitive element whereby the resulting variations in the impedance are smooth and free of sharp changes thereby avoiding scratching and bumps and pops in the sound issuing from the loudspeaker.

The nature of the present invention will be more fully understood upon reference to the following specification taken in connection with the accompanying drawings, in which:

FIGURE 1 is a schematic representation of an electric organ having a plurality of speakers associated therewith and adapted for being controlled in accordance with the present invention;

FIGURE 2 is a schematic view showing how three or more speakers could be connected to be supplied by the output from the wire and with the speakers being provided with control means for controlling the respective volumes thereof;

FIGURE 3 is a schematic view showing one particular manner of controlling the impedance in a speaker circuit wherein a disc having apertures is placed to rotate between a source of light and a light sensitive element;

FIGURE 4 is a view looking in at the face of a rotating disc of FIGURE 3 showing the light source behind the disc;

FIGURE 5 is a schematic view showing a neon oscillator for providing the variable illumination for a light sensitive element in circuit with the impedance.

FIGURE 6 is a schematic View similar to that of FIG- 3,267,l96 Patented August 16, 1966 URE 5 but showing how the neon oscillator could supply its output of a filament lamp which, in turn, illuminates the light sensitive element;

FIGURE 7 is a schematic view showing the use of a transistor as the impedance in the speaker circuit and which transistor is under the control of a light sensitive element variable in accordance with a neon oscillator;

FIGURE 8 is a schematic view showing the use of potentiometer in the impedance circuit with the potentiometer being motor driven to provide for variations in the impedance;

FIGURE 9 is a fragmentary view showing a typical installation of the present invention in an organ circuit, and

FIGURE 10 is a view showing one form of the device which has been used with success.

Referring to the drawings somewhat more in detail, FIGURE 1 shows an electric organ 10 which may be substantially conventional and the output of which organ is supplied to

lines

12 and 14 leading to

speakers

16, 18 and 20. These speakers may be embodied directly in the organ cabinet or'they may include some or all remote from the organ. One of the lines 12 may be grounded as at 22 and the other line :14 includes in circuit with each individual speaker the

impedances

24, 26, 28, respectively. By controlling the several impedances the respective outputs of the speakers can be controlled and, in this man ner, many different effects can be created, further, the impedances are so arranged that they can be controlled as to magnitude and at any desired frequency and in any sequence.

Then, any desired rise and fall of signal power to the speakers can be obtained and the sequence of the rise and fall of the signal power to the individual speakers can be controlled according to substantially any pattern whereby there can be produced, for example, the sensation of a rotating source of sound; an alternating pattern of the source of sound; a simultaneous rise and fall of sound in the several speakers; or any other desired control of the sound from any one or all of the speakers. The rotating source of sound and the alternating pattern of the source of sound referred to will produce a tremolo effect somewhat similar to that known with respect to organs.

The simultaneous rise and fall of sound from the several speakers would, on the other hand, produce somewhat of a repeater effect. In any case, substantially any desired effect can be produced and there is substantially no limit to the modifications in the effects that can be produced.

A particular advantage of this invention is that there is no limit to the number of speakers that can easily be controlled in accordance with the present invention and, furthermore, the type of rise and fall of the signal, or wave form thereof, is easily controllable so that the rate of attack and decay of each speaker in reproducing the signal power thereto can readily be adjusted. Still further, the control of the signal power to the speakers according to the present invention is improved over heretofore known devices in that it is not limited to low frequency response because of the directionality characteristics of the bass notes, but the tremolo and repeater effects are also quite pronounced for the lower frequencies as well as for the medium and upper range of frequencies.

Referring now to FIGURE 2, a representative control circuit arrangement is shown somewhat more in detail than in FIGURE 1. In FIGURE 2 the output of the organ will be seen to comprise the secondary 11 of the output transformer of the amplifier of the organ. The line 14 leading from the secondary 11 to the

several speakers

16, 18 and 20 passes through the

respective impedances

24, 26, and 28 referred to and each of which 40 that is controlled at the will of the player.

vcoil 30 in the speaker circuit and a secondary coil 32,

both mounted on a coupling magnetic core 34. The primary 30, for example, may represent a load of 8 ohms and the secondary might be 8000 ohms. Thus, when no impedance is reflected to the primary coil from the secondary coil, the transformer has substantially not efiect on the signal delivered to the respective speaker.

According to this invention, secondary coil 32 is in circuit with a current controlling device such as light sensitive element 36 which may, for example, be a cadmium sulphide photocell which, as is known, provides a light sensitive resistance. When the cell is not illuminated the resistance is extremely high, and when the cell is illuminated the resistance is reduced in accordance with the amount of illumination while, furthermore, a substantial amount of power can be transmitted through such a cell. It will be apparent, therefore that by controlling the illumination of cell 36, the current flow in secondary coil 32 can be controlled and this will, in turn, control the impedance otfered in primary coil 30 to the signal being delivered to the respective speaker.

FIGURE 2 shows that cell 36 can be illuminated by a source of light 38 supplied from variable voltage source by a control element 42. Numerous ways of varying the illumination supplied to cell 36 are possible, and for this reason, FIGURE 2 merely shows schematically a variable source of illumination for this purpose.

FIGURE 2 also shows the switches 16S, 18S and 208 could be provided in the speaker circuits making the speakers selectively effective if so desired. One or more of these switches could be under the control of the knee lever 44 shown on the organ in FIGURE 1 so that any one or more of the speakers could be brought in and out Further, control elements 42 could be grouped at or adjacent the organ so as to be controllable by the player from his playing position.

One of the preferred manners of varying the impedance in the speaker circuit is shown in FIGURES 3 and 4. In these figures the speaker is indicated at 50 and the transformer in circuit therewith the light sensitive resistance element 54 which may be shunted by resistor 56 which provides means for effecting a further control of the variation of the impedance in the circuit to speaker 50.

Light sensitive resistor 54 receives illumination from a lamp 58 which is on the opposite side of a rotary disc 60 from resistor 54. This disc has apertures 62 and the disc is mounted on a rotating-shaft 64 adapted for being driven in rotation by motor 66 which is preferably variable in speed by a control element such as rheostat 68. Lamp 58 is supplied with power from secondary 70 of a supply transformer 72 which is also connected to supply the lamps pertaining to other speakers which can be controlled together with speaker 50. The illumination of the lamp is preferably under the control of rheostat 74.

As will be seen in FIGURE 4, there are three of the lamps 58, indicated at 58A, 58B and 58C, each pertaining to a ditferentlight sensitive resistor and each resistor being in circuit with the variable impedance of a different speaker of the musical instrument. The apertures 62 in the disc are so arranged that they successively cover the 'lamps 58A, 58B and 58C and successively uncover the lamps at a speed depending'on the speed of rotation of the disc and at a sequence depending upon the relative positioning of the lamps behind the disc. There could be more or fewer of the lamps if so desired and the apertures 62 could be more or fewer than shown and furthermore could be shaped in any desired manner which would provide a ready means for controlling the rate of attack and 'decay occurring at the controlled speakers. Still further, the lamps and cells could be staggered radially to provide for more or less individual control of the cells if so desired.

FIGURE shows schematically an arrangement wherein the light sensitive cell 80 pertaining to impedance 82 of the speaker 84 is illuminated by a neon lamp 86 in an oscillator circuit that includes rheostat 88 in the supply line, and capacitor 90 flashing a neon lamp 86 at the frequency determined by rheostat 88 and capacitor 90 illuminates cell 80. As before, there could, of course, be as many controlled speakers as desired under individual or multiple control to obtain the same range of effects as referred to above.

In FIGURE 6 the impedance for speaker 102 is under the control of light sensitive element 104 which is illuminated by a filament lamp 106, the supply of power to which is under the control of neon oscillator circuit 108 conforming substantially to the neon oscillator circuit of FIGURE 5. In FIGURE 6, the starting and stopping of each cycle more gradual than with the circuit of FIGURE 5.

In FIGURE 7 it is shown how a transistor 110 could be employed as the impedance in the circuit to speaker 112. In FIGURE 7 the emitter 114 of the transistor supplied from secondary 11 of the output transformer of the organ and collector 116 of the transistor is connected to the speaker 112. The base 118 of the transistor is supplied from a source of voltage, the B- terminal, for example, by way of light sensitive resistor element 120 which is variably illuminated by the neon oscillator circuit generally indicated at 122.

It will be understood that the emitter-base current for A transistor 110 could be ondary 134 and having a rotating arm 140 connected to the other side of secondary 134, said arm being driven in rotation by motor 142 that can be varied in speed by speed control element such as rheostat 144. A rheostat 146 can also be provided across the terminals of secondary 134 to control the response thereof to the rotation of arm 140. As arm 140 rotates, the impedance of element will vary.

The light sensitive element for controlling the impedance is preferred because an element of this type is inherently scratch free and is easily controllable, but the potentiometer arrangement of FIGURE 8 is fully operable and is capable of all of the adjustability of any of the arrangements using light sensitive elements.

By the use of an impedance arrangement of the nature disclosed and described power ratios of greater than twenty decibels are possible, which is more than sutficient to produce the desired effect. When utilizing a transistor according to FIGURE 8, power ratios as high as on the order of forty decibles are possible which will sufiice for even the most severe and exacting requirements.

Reference to FIGURE 9 will show a typical actual installation in an organ employing the arrangement of the present invention. In FIGURE 9 the output transformer of the amplifier of the organ is indicated at 200 and it includes the secondary 202 which supplies the speakers of the organ. These speakers include the two main speakers 204 that are connected in parallel and two

auxiliary speakers

206 and 208. The line from one side of secondary 202 is grounded at 210 and the other side of the secondary is connected to one side of each of the aforementioned speakers. The other side of speakers 204 is connected by a line 212 to one side of the primary coil 214 of the transformer forming the variable impedance for speakers 204.

The primary side 216 of the transformer is in circuit with a resistance 218 and photocell 220. The photocell is periodically and variably illuminated throughaperture means 222 of rotary disc 224 by a light source 226 supplied from a voltage source such as the power transformer of illumination would be somewhat cludes a blade 254 secondary 228. A drive motor 230 is provided for rotating disc 224.

Speaker 206 similarly has the side thereof opposite its connection to the organ amplifier connected to one end of the primary 232 of the impedance associated therewith which is similar to the impedance in circuit with speakers 204, and which is also arranged for being controlled in the same manner through another photocell and light source under the control of disc 224.

Speaker 208 is likewise connected with one end of the primary 234 of its variable impedance which forms a part of a unit similar to with speakers 204.

The end of primary 214 of the impedance for speakers 204 is connected to ground, whereas the ends of the pri maries 232 and 234 of the impedances for

speakers

206 and 208 are connected through a capacitor C to a line 236 leading to a terminal 238 of the tremolo tab switch generally indicated at 240. The aforementioned line 212 leading from the speakers 204 to one end of primary 214 also extends to the tab switch 240 and is connected to terminal 242 thereof. This tab switch comprises a third terminal 244.

The tab switch has

blades

246 and 248, both of which have their stationary end connected to ground and which are movable from the position shown wherein blade 246 engages terminal 244 and blade 248 engages terminal 242 into position where blade 248 blade 246 is open-circuited with the switch adjusted into its first-mentioned position. The tremolo is off and at that already described in connection this time line 236 is open circuited, thus making

ineffective speakers

206 and 208, whereas primary 214 of the impedance for speakers 204 is short-circuited making this impedance ineflFective. At this time no tremolo effect will be had.

The tab switch 240 in its other position however, line 236 is connected with ground thus completing the circuit through primary 232 and 234 whereupon these impedances are effective and likewise the

speakers

206 and 208 are also effective. Similarly, the ground connection for cutting the tremolo in and out.

With reference to terminal 244 of taib switch 240, this terminal is connected with the olf terminal 250 of tab switch 252 pertaining to the vibrato. This tab switch inwhich engages terminal 250 when the tab switch is in position so that the line 256 leading to grid 250 of oscillator tube 260 of the vibrato circuit is grounded, thereby preventing the tube from oscillating. Movement of either blade 254 of the tab switch 252 into its on position for movement of the blades of tab 240 into their on position will be effective for commencing tube 260 to oscillate to make the vibrato effective.

FIGURE 10 shows the manner in which the photocell .and illuminating lamps, scanning disc and motor of the tremolo device of the present invention can be arranged in a practical unit. In FIGURE 10, 300 is a supporting base and it carries a bear-ing302 for shaft 304 on which disc 306 is mounted. Disc 306 has a rim 308 engaged by resilient wheel 310 on the output shaft of drive motor 31 2. Drive motor 312 is mounted on a cover 314 clamped to ring 316 by screws 318 so as to form an enclosure for disc 306.

Cover 312 carries holders 320 for illuminating lamps 3 2 2, the light from which passes through apertures 324 in disc 306 to photocell 326 carried in base 300.

As mentioned before, the individual photocells could be arranged at various radial distances from the axis of rotation of the disc, and the pertaining illuminating lamps and apertures could be similarly arranged so that any particular pattern or control of the several photocells could be had. It will be apparent that FIGURES 9 and 10 show one practical embodiment of the principles of 6 the present invention which have been somewhat more broadly disclosed in FIGURES 1 through 9.

It will be understood that this invention is susceptible to modifications in order to adapt it to different usages and conditions; and accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

What we claim is:

1. In combination; a musical instrument having an electrical output, a plurality of speakers, a circuit connecting each speaker to said instrument to receive said electrical output and to convert the same to sound output, impedance means in each circuit, and means for individually controlling said impedance means for varying the sound output of the respective speakers, said impedance means comprising a transformer having a primary coil in the pertaining circuit and also having a secondary coil, variable resistance means in parallel with each said secondary coil, and means for varying each variable resistance means during spaced periods of time, each variable resistance means during a said period of time decreasing smoothly to a predetermined minimum amount and thereafter increasing to a predetermined maximum amount, and means for efiecting said varying of the respective variable resistance means so the said periods thereof are in sequential overlapping relation 2. In combination; a musical instrument having an electrical output, a plurality of speakers connected to receive said output and to convert the same to sound output, impedance means in circuit with each of said speakers, and means for individually controlling said impedance means for varying the sound output of the several speakers, said impedance means comprising a transformer having a primary coil in the circuit to the respective speaker and also having a secondary coil, and variable resistance mean-s in circuit with said secondary coil, and means for varying each variable resistance means during spaced periods of time, each variable resistance means during a said period of time decreasing smoothly to a predetermined minimum amount and thereafter increasing to a predetermined maximum amount, and means for effecting said varying of the respective variable resistance means so the said periods thereof are in sequential overlapping relation, said variable resistance means comprising a light sensitive resistance element and a controlled source of light associated with said element for the variable illumination thereof.

3. In combination; an electrical musical instrument ha ing an output transformer with terminals, a plurality of speakers connected to said terminals to receive the output from said transformer and convert it into sound, an impedance in circuit with each speaker, each impedance comprising a transformer having a primary coil in series with the speaker and having a secondary coil, a resistance element connected in parallel with each said secondary coil, each resistance element comprising a light sensitive resistance cell, a source of illumination adjacent each cell,

each source directing its beam toward its respective cell,

means for varying the intensity of each said source of illumination, said cells and their pertaining sources being distributed circumierentially about a predetermined axis, disc means interposed between said sources of illumination and their pertaining cells and rotatable on said axis and circumferentially spaced apertures in said disc means adapted successively to register with each said source and its pertaining cell during rotation of said disc means thereby to bring about periodic illumination of each cell by its pertaining source of illumination, and means for driving said disc means in rotation.

4. In combination; an electrical musical instrument having an output transformer with terminals, a plurality of speakers connected to said terminals to receive the output from said transformer and convert it into sound, an impedance in circuit with each speaker, each impedance comprising a transformer having a primary coil in series with the speaker and having a secondary coil, a

resistance element connected in parallel with each said secondary coil, each resistance element comprising a light sensitive resistance cell, a son-nee of illumination adjacent each cell, means for varying the intensity of each said source of illumination, said cells and their pertaining sources being distributed circumferentially about a predetermined axis, disc means interposed between said sources of illumination and their pertaining cells and rotatable on said axis and circu-mferentially spaced apertures in said disc mean-s adapted successively to register with each said source and its pertaining cell during rotation of said disc mean-s thereby to bring about period-icillnm in ation of each cell by its pertaining source of illumination, said means'for driving said disc means comprising a variable speed drive motor drivingly connected with said disc means.

5. In combination; an electrical musical instrument having an output transformer, a plurality of speakers, a circuit connecting each speaker to said transformer to receive the output from said transformer an-d convert it into sound, an impedance in each circuit, each impedance comprising a transformer having a primary coil in the ertaining circuit and having a secondary coil, -a resistance element connected across each said secondary coil, each resistance element comprising a light sensitive resistance cell, a source of illuminationfor each cell, and means for varying the intensity of each said source of illumination, each source of illumination comprising a neon lamp, and circuit means connected to each neon lamp for causing periodic illumination thereof.

6. In combination; an electrical musical instrument having an output transformer, a plurality of speakers, a circuit connecting each speaker to said transformer to receive the output from said transformer and convert it into sound, an impedance in circuit with each speaker, each impedance comprising a transformer having a primary coil in the speaker circuit and having a secondary coil, a resistance element connected across each said secondary, each resistance element comprising a light sensitive resistance cell, a source of illumination for each cell, means for varying the intensity of each said source of illumination, each said source of illumination comprising a filament lamp, a neon tube in series with each lamp, and circuit means connected to said lamp and tube and including a condenser connected in parallel with said tube and lamp, and means for supplying direct current to said circuit whereby said condenser will receive a charge and will then discharge and cause said neon tube periodically to become conductive for illuminating said filament lamp.

7. In combination; an electrical musical instrument having an output transformer, a plurality of speakers, circuits connecting said speaker to said transformer to receive the output from said transformer and convert it into sound, an impedance in each circuit, each impedance comprising a transformer having a primary coil in the pertaining circuit and having a secondary coil, a resistance element connected across each said secondary coil, said resistance element comprising a circular resistance unit, one point of said circular resistance unit being connecte-d 'to one side of said secondary coil, and an arm connected to the other side of said secondary coil and arranged for contacting said resistance element and sweeping over said circular resistance element when driven in rotation.

8. An electrical musical instrument having an output transformer, speaker means, a circuit connecting said speaker means to said transformer to receive the output from said transformer and to convert it into sound, and variable impedance means in said circuit, said variable impedance means comprising a transistor having the collector-emitter circuit in the said circuit and also having a base, and means for supplying a cyclically varying voltage signal to said base for varying the impedance offered in the speaker circuit by the said emitter-collector circuit of the transistor.

9. In an electric organ; an output transformer having a secondary, a plurality of speaker means, a circuit connecting each speaker means to said secondary to receive a signal from said secondary and convert it into a sound output, a variable impedance in each circuit, each variable impedance comprising a transformer having a first coilin the pertaining circuit and having a second coil magnetically coupled to said first coil, means for automatically and controllably varying the resistance across the terminals of said second coils to vary the impedance of the respective circuits to the respective speaker means, means for open-circuiting some of said first coils to make the respective speaker means ineffective, and means for short circuiting at least one of said first coils to make the pertaining impedance means ineffective in the circuit of the respective speaker means.

10. In an electric organ; an output transformer having a secondary with terminals, a plurality of speaker means connected to the terminals of said secondary to receive a signal therefrom and convert it into a sound output, a variable impedance in circuit with each said speaker means, each variable impedance comprising a transformer having a first coil in series with the pertaining speaker means and having a second coil magnetically coupledjto said first coil, means for automatically and controllably varying the resistance across theterminals of said second coils, means for open circuiting some of said first coils to make the respective speaker means ineffective, and means for short circuiting at leastone of said first coils to make the impedance thereof zero, said means for varying the resistance across the terminals of said second coils comprising a light sensitive resistance cell connected across the terminals of each of said second coils, a frame in which said cells are mounted in circumferentially spaced relation about a predetermined axis, a light source mounted in said frame in opposition to each said cell, a disc rotatably mounted in said frame on said axis and interposed between said light sources and said cells, a motor for driving said disc in rotation, and apertures formed in said disc in circumferentially spaced relation and located to register sequentially with said light sources and the respective cells so that during rotation of said disc said cells are variably illuminated for periodically varying the resistance thereof.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Shields: Novel Uses For Photoconductive Photocells,

Electronics World (mag), August 1961, pages 46, 47.

ARTHUR GAUSS, Primary Examiner. D. D. FQRRE ASSir mnt Examiner.

Claims

4. IN COMBINATION; AN ELETRICAL MUSICAL INSTRUMENT HAVING AN OUTPUT TRANSFORMER WITH TERMNALS, A PLURALITY OF SPEAKERS CONNECTED TO SAID TERMINALS TO RECEIVE THE OUTPUT FROM SAID TRANSFORMER AND CONVERT IT INTO SOUND, AN IMPEDANCE IN CIRCUIT WITH EACH SPEAKER, EACH IMPEDANCE COMPRISING A TRANSFORMER HAVING A PRIMARY COIL IN SERIES WITH THE SPEAKER AND HAVING A SECONDARY COIL, A RESISTANCE ELEMENT CONNECTED IN PARALLEL WITH EACH SAID SECONDARY COIL, EACH RESISTANCE ELEMENT COMPRISING ALIGHT SENSTIVE RESISTANCE CELL, A SOURCE OF ILLUMINATION ADJACENT EACH CELL, MEANS FOR VARYING THE INTENSITY OF EACH SAID SOURCE OF ILLUMINATION, SAID CELLS AND THEIR PERTAINING SOURCES BEING DISTRIBUTED CIRCUMFERENTIALLY ABOUT A PREDETERMINED AXIS, DISC MEANS INTERPOSED BETWEEEN SAID SOURCES OF ILLUMINATION AND THEIR PERTAAINING CELLS AND ROTATABLE ON SAID AXIS AND CIRCUMFERENTIALLY SPACED APERTURES IN SAID DISC MEANS ADAPTED SUCCESSIVELY TO REGISTER WITH EACH SAID SOURCE AND ITS PERTAINING CELL DURING ROTATION OF SAID DISC MEANS THEREBY TO BRING ABOUT PERIODIC ILLUMINATION OF EACH CELL BY ITS PERTAINING SOURCE FOR ILLUMINATION, SAIDMEANS FOR DRIVING SAID DISC MEANS COMPRISING A VARIABLE SPEED DRIVE MOTOR DRIVINGLY CONNECTED WITH SAID DISC MEANS.