US2323905A

US2323905A - Television

Info

Publication number: US2323905A
Application number: US416232A
Authority: US
Inventors: Peter C Goldmark
Original assignee: Columbia Broadcasting System Inc
Current assignee: CBS Broadcasting Inc
Priority date: 1941-10-23
Filing date: 1941-10-23
Publication date: 1943-07-13
Anticipated expiration: 1960-07-13

Description

my 1943., P. c. Gomis/mmm ZBBESB TELEVISION Filed Oct. 23, 194]. 2 Sheets-Sheet l K Oca? ff f4`l @j Agg; f1/27. /aye pia/Je /0 (ane) I (F/e//f/' Jj J4 197m?? una //nfw Juiy 139 1943. P. C. @0mm/ARK www@ TELEVISION Filed OCJC. 25, 194]. 2 SheeS-Sheeb 2 Fi G B l? G 5 @QW/Q, MV @M ATTORNEYS Patented July 13, 1943 TELEVISION Peter C. Goldmark, New Canaan, Conn., asslgnor to Columbia Broadcasting System, Inc., New York, N. Y., a corporation of New York Application October 23, 1941, Serial No. 416,232

22 Claims.

This invention relates primarily to television, especially to color television apparatus employing a rotatable color nlter element. The inven- .tion particularly contemplates the provision of a compact, efficient combined local generator and friction brake for use in effecting synchronism of the rotatable color filter element, and a circuit adapted to effectively utilize the combined generator and brake. Although especially designed for use in television, in their broader aspects the synchronizing circuit and combined generatorbrake could be used in other fields.

In onetype of color television system Which has been successfully operated, successive images corresponding to different color aspects of an object eld are transmitted to a receiver and successively reproduced in the corresponding colors. For example, the object field may be scanned once for the red aspect, a second time for the green aspect, and a third time for the blue aspect. At the receiver the successive field scansions are reproduced and exhibited in the respective colors. Commonly a rotating color lter element having segments of the selected primary colors is employed at the transmitter, and a similar rotatable color filter element is employed at the receiver. The color lter element may be in the form of a disk, a drum, or other form, as desired.

In order to reproduce successive images in the colors to which they correspond it is necessary to provide means for synchronizing the rotation of the receiver color filter element with the successively reproduced images. Suitable apparatus for effecting synchronism is disclosed by the applicant in co-pending application Serial No. 373,713, filed January 9, 1941, for Television As there pointed out, it is not only necessary that the disk or other form ofelement rotate in synchronism with the successive images, but it is also necessary that the element be properly phased with respect to the images so that :filter segments of different color are associated with their respective images. In this prior application the specific embodiment discloses an asynchronous motor for supplying most of the driving power, and a phonic motor for effecting synchronization.

The present application discloses apparatus in which a friction brake is employed to effect synchronization, rather than a phonic motor. It has been found that the friction brake is more eili-` cient than the phonic motor, even when the latter is employed as a, brake, and is simpler in construction. Furthermore, the operation ofthe friction brake is quieter than that of the phonic motor, which is an important consideration in a receiver where quiet operation is essential.

In accordance with the invention, a generator is mounted on the shaft of the rotatable color filter element, or is directly coupled thereto, so as to generate a frequency which is directly proportional to the frequency at which the lter segments traverse the image reproducing area. A synchronizing Wave is derived from the received color television signal and the generator is preferably constructed so that the frequency of the locally generated Wave Will be equal to that of the control synchronizing wave when the apparatus is in synchronism. The phase of the two waves is compared in a suitable circuit employing an electronic tube and the output of the phasecomparing circuit is supplied to the friction brake. The phase-comparing circuit is designed, by suitably biasing the electronic tube or otherwise, to yield a rectified output current Whose value varies with the phase relationship of the two waves, the rectified output current being at a minimum when the Waves are out of phase and at a maximum when the waves are in phase. Advantageously, the bias or other circuit constants are selected so that the minimum current for the out of phase relationship is very small. The friction brake is electromagnetically operated by supplying the rectied output current to a suitably positioned operating coil. It is further advantageous to supply the brake operating coil with only the direct current component of the rectified synchronizing current, as by shunting a condenser of fairly large capacitance across the operating coil so as to by-pass the alternating current components.

When the apparatus has been started and synchronism obtained, it may be found that a reproduced image corresponding to the red aspect of the object field is'being reproduced through a lter segment of the Wrong color, say green. In such case, correct phasing can readily be obtainedy by placing a push-button switch in the condenser circuit. The switch is designed to normally close the condenser circuit and, when momentarily opened, the synchronizing action of the brake is interrupted. The filter element will thereupon be driven at an asynchronous speed by the asynchronous motor and, when the proper color phase has been reached, the push-button may be released to restore the synchronizing action of the brake.

In accordance with another aspect of the invention, the local generator and friction brake are combined in a single structure so as to provide a compact, efficient unit which can be readily assembled in a receiver cabinet. In this structure, suitable means are provided for preventing current in the brake operating coil from affecting the operation of the generator, and vice versa. Advantageously, the pulley which is coupled to the asynchronous motor is employed as the brake rotor. The stator is mounted to permit at least slight axial movement to vary the frictional braking force with the brake rotor. The stator is normally restrained from rotating about its axis,but the, orientation of the stator about its axis is preferably adjustable so as to permit correct phasing of the color filter element with respect to the scanning area. The combined generator and brake described hereinafter also contains novel features which maintain the generator rotor in proper position with respect to the stator at all times. Provision is also made for preventing the generator current from producing considerable friction between generator rotor and the stator.

The invention will be more fully understood by reference to the specific -embodiment illustrated in the drawings vand the following description thereof. In the drawings:

Fig. l is a diagram illustrating the general arrangement of the apparatus;

Fig. 2 is a face view of a suitable color :filter disk;

Fig. 3 `is a section of the combined generator and friction brake, taken along Athe axis thereof;

Fig. 4 is an end view of the combined generator and friction brake of Fig. 3;

Fig. 5 is a diagram illustrating representative field synchronizing pulses and corresponding saw-tooth scanning and synchronizing waves derived therefrom;

Fig. 6 is a .circuit diagram of a specific embodiment of the invention; and

Fig. 7 is a diagram illustrating the manner` in which a rectified output current may be obtained whose magnitude varies with the phase relationship of the local and control synchronizing Waves. A

Referring to Fig. 1, the cathode ray receiver tube I0 is provided with a luminescent image reproducing surface II on which images are successively reproduced by an electron beam from gun I2. The electron beam is deflected repeatedly over substantially the same area of the reproducing surface II bydeflecting plates I3, energized at line scanning frequency by a suitable saw-tooth wave generator I4, and by vertical deflecting plates I5, energized by a suitable saw-tooth generator I6 of field scanning frequency. These generators may be synchronized to reproduce images from a received color television video signal in any desired manner. Of course, magnetic defiecting coils may be emchanged as desired, the invention in no way depending upon the particular selection illustrated. Furthermore, other shapes of rotatable filter elements may be employed if desired.

Filter disk I1 is attached to shaft I8 which rotates in tube I9 lined with suitable bearing material. Tube I9 is secured to the chassis of the receiver by suitable stanchions 2|. Attached to shaft IB is an adjustable pulley 22 driven by asynchronous motor 23 through belt 265. It is foundadvantageous to employ an induction motor for the asynchronous motor and to select a motor somewhat larger than is necessary to drive the disk, in order to minimize speed variations caused by variations in line voltage. It is preferred to mount the pulley on the same shaft as the color lter disk, but it could be otherwise coupled thereto if desired.

In the embodiment here specifically illustrated, the pulley 22 also functions as the brake rotor of the electromagnetically operated friction brake. Pulley 22, being driven through a belt, would be prevented from free axial movement even if it vwere free on shaft I8. Hence in the specific embodiment here shown the stator is constructed to allow vat least slight axial movement so as to permit varying the braking force. The stator is restrained from rotating by means of a cord 26 which is Wrapped around the stator and secured thereto by pin 2l. Cord 26 is also wrapped around a cylinder`26 mounted at` one ployed instead of the electrostotic deflecting of segments andfshape of the segments may be end of an adjusting screw 29. The adjusting screw is mounted in a stanchion 3d, attached to the chassis, and is held from turning by a suitable amount of friction. This type of mounting not only permits the stator to move in an axial direction, but permits the orientation of the stator to be changed by rotating the adjusting screw 29. This orientation adjustment is advantageous to permit thev accurate phasing of the color filter disk with the successive images. It will be understood that in normal operation the stator does not move about its axis, although it can move along its axis by the amount necessary to change the force with which it engages the brake rotor.

The generator rotor 3| of magnetic material is mounted on the shaft I8 at the end of the stator opposite the brake rotor. The manner in which the generator rotor is mounted will be explained in connection with Fig, 3.

The output of the local generator is fed through line 32 to a phase-comparing circuit 33. The latter circuit is also supplied with a control synchronizing wave 34 derived from the received color television signal. The output of the phase-comparing circuit is supplied through line 35 to the actuating coil of the brake.

Referring now to Fig. 3, the adjustable pulley 22 is shown as comprising a hub section 4I integral with the inner flange 42 of the pulley. 'I'he hub section 4I is secured to shaft I8 by set screw 43. The hub section is threaded on the outside and carries the correspondingly threaded outer ange M of the pulley. By rotating the outer flange on the hub it may be moved toward or away from the inner flange 42, thereby vary- 1ng the effective diameter of the pulley and consequently Varying the ratio between pulley 22 and the driving pulley of motor 23. When the correct ratio is obtained, the outer flange M is secured to the hub section 4I by set screw 45, It will be understoodthat the particular construction of the adjustable pulley here shown forms no part of the present invention and any suitable form of pulley, non-adjustable if desired, may bc selected.

The stator has a core of magnetic material comprising a hub section provided with a bearing 4T of brass or other suitable material Within which the shaft I8 rotates. Encircling the hub section 46 and spaced therefrom is a cylindrical section 48 having on one end thereof an annular ring 49 and at the other end thereof suitably shaped, axially extending teeth 5I for the generator'. The hub section 46 and encircling section 48 are joined between the ends thereof by a radially extending section 52, thereby deining two annular spaces in which the brake coil 53 and generator coil 54 are placed. The stator magnetic circuit ior the brake is completed through the pulley 22 which is of magnetic material such as steel. This brake magnetic circuit comprises the radial section 52, the pulley 22, and the portions of the hub 45 and encircling cylinder 48 therebetween. the rnnular plate 49 being considered as part of the encircling cylinder 48.

In Fig. 3 the radial section 52 is shown integral with the hub section 4S, and the encircling section 48 and plate 49 are shown as separate pieces secured together by bolts. It will be understood that this is for convenience in construc-ting the stator core, and that the sectiens can be integral or sub-divided as desired, the assembly forming an integrated core however. The term section as employed herein and in the claims does not require the element to be non-integral with other sections.

A thin fiber disk is advantageously placed between the stator and the pulley. This ber disk is advantageously not secured to either stator or pulley and serves, in effect, as a brake lining. If desired, it could of course be secured to either the stator or the pulley. The disk could of course be dispensed with if desired, but the employment of such a disk of smooth hard material is found helpful in securing a smooth braking action and in avoiding noise. 'I'he disk, being of non-magnetic material. also provides a small air gap in the brake magnetic circuit which is found advantageous. Ii the disk were eliminated, an air gap could still be provided by, for example, cuttingr down the hub section 4E so that it is out of contact with the pulley.

It will be understood that the pulley or brake rotor may be said to frictionally engage with the stator, even though the engagement is through the disk as an intermediary.

The generator rotor 3| is provided on its periphery with teeth 51 which cooperate with the stator teeth 5I so that as the rotor rotates the reluctance oi the generator magnetic circuit varies so as to produce an alternating current in the generator winding 54. Since the stator is allowed to move axially, at least to a slight extent, the generator rotor 3i is also mounted so as to be free to move axially and maintain its position with respect to the stator. To this end, generator rotor 3| is not secured to the shaft i8, but is restrained from rotating with respect to the shaft by a pin 58 mounted in a collar 59 which is secured'to shaft I8 by set screw 5i. A flat spring 62 is placed on the shaft I8 between collar 59 and generator rotor 3l as a biasing means. This spring continuously presses the generator rotor 3i against thestator, and the stator against the pulley 22. By keeping these three elements in contact at all times, the

f erator.

braking lorce is made smooth. Fluctuations in the braking action might result in excessive hunting of the color ltcr disk and hence the use of spring 82 to bias the two rotors and stator together is distinctly advantageous. It is preierable to make spring 62 just strong enough to maintain the elements in contact, without introducing excessive friction.

In the combined generator and brake structure of the invention it is important that current, in the generator coil 54 be prevented from appreciablyv affecting the brake action, and it is likewise important that current in the brake coil 53 be prevented from appreciably affecting the gen- In the structure of Fig. 3 the radially extending section 52 provides a common path of low reluctance for both brake and generator magnetic circuits which prevents current in either coil from producing substantial iiux in the magnetic circuit of the other coil. Considering the generator coil 54, the magnetic path between the ends of section 52 is of much lower reluctance through section 52 than through the portions of the hub section 46 and encircling section 48 to the left of radial section 52 and brake rotor 22. This is particularly true when there is a small air gap between brake rotor and stator core. Hence, most of the flux produced by coil 54 will iiow through the radial section 52 and will not pass through the brake rotor 22. Similarly, considering coil 53 with respect to the generator magnetic circuit, the radial section 52 provides a path of low reluctance compared to the path through the teeth 5i and 57 and the generator rotor. Hence, most of the iiux produced by coil 53 will pass through the radial section 52 and will not affect the generator voltage.

Inasmuch as the generator rotor 3l is in the eld of the stator 25, a certain amount of friction would normally result in an axial direction. To prevent this, the generator rotor 3l is provided with an integral hub section 63 which eX- tends within the hub section 46 of the stator, thereby overlapping the end of the stator core. The clearance between these two sections is made small in order to provide a radial path of low reluctance for the iiux in the generator circuit. A washer 64 of brass or other non-magnetic material is inserted between the generator rotor and the stator to prevent substantial axial ux at this point. Similarly, the clearance between the generator rotor and the end 65 of the stator hub is made sufficiently large so that the reluctance in the axial direction is fairly high.

It is also advantageous to design the rotor and stator teeth so that the rotor teeth are radially within the stator teeth. When so designed, the force produced by varying flux between the teeth is substantially in the radial direction'and does not substantially change the friction between generator rotor and stator. Moreover, such a construction permits slight axial displacement of the rotor teeth without appreciably aiecting the gap between rotor and stator teeth, thus providing a desirable tolerance.

In operation, the generator coil 54 may be en-,

nizlng signals in a manner hereinafter described.

In the brake, a varying current in coil 53 produces a corresponding varying uX in the brake magnetic circuit which includes the pulley 22 functioning as a brake rotor. The Varying ux produces a varying force of attraction between the pulley and the adjacent end of the stator. The resulting friction varies with the force of attraction and increases and decreases the braking force. Fiber disk 55 acts as a buffer between the two metal surfaces to produce a smooth braking action, and also insures that the reluctance between the stator and the pulley is high compared to that of the radial section 52, so that current in the generator coil cannot appreciably affect the braking action.

Referring now to Fig. 5, a series of pulses is shown at a which represent the eld synchronizing signals customarily transmitted as part f the television signal. Successive field scansions are here shown as representing red, green and blue aspects of the object field. The eld scansions may be interlaced or non-interlaced, as desired. The synchronizing pulses are employed in a conventional manner to generate, or control the generation of, a saw-tooth scanning Wave of field scanning frequency such as is shown in Fig. 5b. The line scanning saw-tooth wave is generated in a conventional manner and hence is not shown. For the specific embodiment here shown, the pulses recur at the rate of 120 per second.

The local 'generator shown in Fig. 3 may be designed so as to generate substantially a pure sine wave. In comparing the phase of the locally generated wave with the control synchronizing wave it is advantageous, although not essential, to have the control synchronizing wave also substantially sinusoidal, Such a sine wave is shown in Fig. 5c. The sine wave of Fig. 5c can be derived directly from the field synchronizing signals of Fig. 5a by suitable amplification and filtering. However, since the eld saw-tooth scanning wave of Fig. 5b is customarily generated under the control of the eld synchronizing signals, it is more convenient to use this wave to generate the sine wave of Fig. 5c. This may be done by simple amplification and filtering. It will be lunderstood that no attempt has been made in Fig. 5 to show the exact phase relationship between the saw-tooth wave and the corresponding sine wave, since the particular phase relationship can be taken care of by suitably orienting the color lter disk on its shaft.

Referring to Fig. 6, the saw-tooth wave of Fig. 5b is supplied through coupling condenser 7| to the grid l2 of the lefthand elements in electronic tube 13. The tube 'I3 is in eiect two tubes in'a single envelope, a tube of the 6F8G type having been employed with success. The lefthand elements act simply as an amplifier, and a suitable input resistance 14 and cathode resistance I5 are employed. The resulting plate current passes through a filter circuit composed of a choke coil 16 and condensers 'l'i to yield a resulting wave which isA substantially sinusoidal. This voltage is supplied to grid 'i8 of the righthand elements of tube 13. Resistance 9i and 92 and condenser 93 are provided in the output circuit of the lefthand elements df tube 13 to act as a filter for the plate supply. Any desired ltered supply may of course be employed. K y

The locally generated voltage from generator` in frequency to that of the control synchronizing wave when the disk is rotating synchronously. Hence the number of teeth on the generator rotor and stator are selected to give a 1Z0-cycle per second voltage for the specic case given.

The generator coil 54 is energized froma source of constant voltage at 8|, which is conveniently the B+ supply, the circuit being completed through resistance 82 to ground. The magnitude of resistance 82 is selected with respect to the voltage supplied at 8| to yield a suitable value of energizing current. The Ibias for grid 'i8 is supplied from a potentiometer 83 through grid resistance 80. For the 6F8G tube, a C-supply of *50 volts and a resistance 84 of one-half megohm has been employed with success. In order to properly adjust the apparatus it is found advantageous to also provide a variable cathode resistance 85 in order to conveniently change the amplication factor of the righthand elements of tube '13. A variable resistance of 1000 ohms has been employed with success. A jack 86 is provided in order to insert conveniently an ammeter for measuring the brake current. The output current of the righthand elements is fed through line 81 to th actuating coil 53 of the friction brake. A condenser 88 of relatively large capacitance is shunted across the brake coil in order to by-pass the alternating frequency components of the output wave. While the capacitance can be selected over a wide range, and depends on the constants of the brake coil, etc., a 2 mfd. condenser has been employed with success. Although shunting the brake coil is a very satisfactory way of supplying only the direct current component to the coil, other means for accomplishing the result may be employed if desired.

A normally-closed pushbutton switch 89 is inserted in the condenser circuit to permit convenient color phasing of the color filter disk. The voltage applied at 8l is also utilized as the plate voltage supply for the righthand elements of tube T3 through brake coil 53. A B+ supply of about 375 volts has been employed with success.

The preferred operation of the apparatus of Fig. 6 is illustrated in Fig. 7. The characteristic of the right-hand elements of tube 13 is shown as line 94. The tube is biased substantially to `cut-off by suitable adjusting potentiometer 83. As illustrated, if the locally generated voltage and control synchronizing voltage are equal in magnitude and out of phase, as shown at A, the two waves cancel and the tube remains substantially at cut-olf, yielding a very small output brake current as shown at A'. If the two waves are o'f equal magnitude and 120 out of phase, the waves combine in the well known manner to yield a resultant voltage of the same amplitude as either of the two waves, illustrated at B. Due to the bias on the grid 18, a resulting rectified Wave B' is obtained. Wave B' has an average value corresponding to the direct current component represented by the dotted line B. When the two waves are of the same m'agnitude and in phase, the resultant is a wave of twice the amplitude of either, as shown at C. Due to the rectifying action of the tube, the resultant output current wave is as shown at C. The average direct current value of this wave is shown by the dotted line C".

It will therefore be seen that as the phase between the locally generated wave and the control synchronizing wave changes, the average direct current component of the output wave 7 change in brake current with change in phase with a minimum steady current through the brake, it being realized that the steady currentresults in heating the brake. It will be understood that the voltage of the power mains commonly available at a receiver will usually fluctuate somewhat. Therefore allowance must be made to take care of such fluctuations. I-Lence it is advisable to design the brake and circuit components and adjust the circuit s that at the normal operating voltage the disk will be prop'- erly synchronized when the brake current is intermediate'its maximum and minimum values.

The braking effect is usually not directly proportional 'to the difference in phase so that a suitable normal operating point is when the two Waves are approximately 90 out of phase. The local wave will lag the incoming wave in the stable operating condition. Then, if the disk should be running somewhat below synchronous speed, the locally generated wave will move somewhat farther out of phase with respect to the incoming synchronizing signal, thereby decreasing the brake current and permitting the asynchronous motor to drive the disk faster. On the other hand, if the disk should tend to run above synchronous speed, the two Waves will shift more into phase, thereby increasing the brake current and causing the disk to slow down. A change in the voltage of the power mains will commonly atleet somewhat the speed of the asynchronous motor and cause the phase-comparing circuit `and brake to establish synchronism with the disk at a slightly different phase with respect to the reproduced images. However, by suitably selecting the asynchronous motor, as by employing an induction motor of somewhat greater power than is necessary to drive the disk, and by designing the color illter segments to permit a slight amount of phase variation, correct operation may still be obtained. l

If, when the receiver is initially started, images representing certain colors are exhibited through lters of din'erent colors, the pushbutton 89 may be depressed until proper color relationship is obtained, and. then released to establish synchronism in the proper color phase. Depressing pushbutton 89 opens the circuit of condenser 88. While the exact manner in which this operates to remove the synchronizing effect is not fully understood, it is believed that the opening of the circuit of condenser 88 alters the plate impedance of the righthand elements of tube 13 to such an extent that the resulting brake current is markedly decreased, thereby preventing the brake from fully counteracting the accelerating effect of the asynchronous motor. The brake coil, being wound on a magnetic core such as iron, of course has a fairly high inductance, much like a choke coil. This theory is not insisted upon, but in any event it is found that the described operation can be obtained.

Ii desired, other means for momentarily removing the synchronizing action of the brake can be employed, for example, actually interrupting the brake coil circuit or momentarily biasing grid 'I8 beyond cut-ofi.

Although locally generated waves and waves derived from the incoming synchronizing signals of approximately'sinusoidal form and of equal magnitude have been found satisfactory, other forms and non-equal magnitudes can be employed i-i' desired. Also, instead of using the customary field synchronizing signals, special transmitted signals could be employed.

While the combined generator-brake construction of Fig, 3 and the circuit of Figj are especially suited to each other, it will be understood that the combined generator-brake may be used with other types of synchronizing circuits and that the circuit of Fig. 6 can be employed with any desired form of local generator and brake. If desired, the combined generatorbrake and synchronizing circuit can be employed jointly or separately to synchronize apparatus other than a color filter element.

It will be understood that neither the combined generator-brake nor the synchronizing circuit of the invention is limited to the mere i details of construction and design and arrangement of parts disclosed, since many modifications may be made by those in the art without depmting from the spirit and scope of the invention.

I claim:

1. In a color television receiver for receiving a color video signal successive portions of which represent different color aspects 0f an Object eld and accompanying synchronizing signals, said receiver having a rotatable color filter element for presenting images corresponding to said successive portions in their respective colors, apparatus for synchronizing the rotation of the color filter element with successive images which comprises an asynchronous motor coupled to said rotatable illter element and tending to drive the element over synchronous speed, a friction brake rotor of magnetic material coupled to said rotatable illter element, a stator mounted coaxially with said friction brake rotor and adapted to frictionally engage therewith, said stator having a magneticcircuit completed through the brake rotor and a brake energizing coil positioned to produce iiux therein, a generator rotor of mag'- netic material axially spaced from the brake rotor, said stator having a magnetic circuit completed through the generator rotor and a generator coil positioned to encircle flux therein, said brake and generator magnetic circuits having a common path of low reluctance positioned to prevent the production of substantial ilux between stator and brake rotor by the generator coil and between the stator and generator rotor by the brake coil, said generator rotor and stator being adapted to generate a local synchronizing wave in the generator coil of the frequency of said synchronizing signals when the iilter element rotates at synchronous speed, means for comparing the phases of said local synchronizing wave and said received synchronizing signals and yielding a rectified output current whose magnitude varies in accordance with the difference in said phases, said brake energizing coil being connected to receive the direct current component of said rectified output current, and means for substantially preventing the alternating current comronent of said rectified output current from flowing in said coil.

2. In a color television receiver for receiving a color video signal successive portions of which represent different color aspects of an; object field and accompanying synchronizing signals,

said receiver having a rotatable color lter element for presenting images corresponding to said successive portions in their respective colors, apparatus for synchronizing the rotation of the color filter element with successive images which comprises an asynchronous motor coupled to said rotatable filter element and tending to drive the element over synchronous speed, a stator having a core of magnetic material, a shaft extending within said stator and coupled to said rotatable `filter element, a friction brake rotor of magnetic material attached to said shaft at one end of said stator and adapted to frictionally engage therewith, said stator having a magnetic circuit completed through the brake rotor and a braise energizing coil positioned to produce flux therein, means for restr y. the stator from substantial rotational movement and 7allowing at least slight axial movement thereof to vary the frictonal braking force with the brake rotor, a generator rotor of magnetic material on said shaft at the other end of the stator and mounted to allow axial but V'not substantial rotational movement relative to the shaft, said stator having a magnetic circuit completed through the generator rotor and a generator coil positioned to encircle flux therein, said generator rotor and stator being adapted to generate a local synchronizing wave in the generator coil of the frequency of said synchronizing signals when the filter element rotates at synchronous speed, biasing means for pressing the generator rotor toward the stator and the stator toward the brake rotor, means for applying said local synchronizing wave and a wave corresponding to said synchronizing signals to control the ow oi current in an electronic tube, said electronic tube being connected to yield a rectified output current which varies in accordance with the difference in phase of said waves, said brake energizing coil being connected to receive the direct current component of said rectified output current, and a condenser connected across said energizing coil to by-pass the alternating current component of the rectied output current and cause substantially only the direct current component to iiow in the coil. l

3. In a color television receiver, a combined local generator and brake for synchronizing a rotatable color filter element which comprises a friction brake rotor of magnetic material coupled to said rotatable lter element, a stator mounted coaxially with said friction brake rotor and adapted to frictionally engage therewith, said stator having a magnetic circuit completed through the brake rotor and a brake energizing coil positioned to produce ux therein, a generator rotor of magnetic material axially spaced from the brake rotor, said stator having a magnetic circuit completed through the generator rotor and a generator coil positioned to encircle ux therein, said brake and generator lmagnetic circuits having a common path of low reluctance positioned to prevent the production of substantial ux between stator and brake rotor by the generator coil and Abetween the stator and generator rotor by the brake coil.

4. In acolor television receiver, a combined local generator and brake for synchronizing a rotatable color filter element which comprises a stator having a core of magnetic material, a friction brake rotor of magnetic material coupled to said rotatable filter element and mounted coaxially with the stator to frictionally engage with one end thereof, said stator having a magnetic circuit completed through the brake rotor and a brake energizing coil positioned to produce flux therein, a generator rotor of magnetic material positioned coaxially with the stator at the other end thereof, said stator having a' magnetic circuit completed through the generator rotor and a generator coil positioned to encircle ux therein, said brake and generator magnetic circuits having a common path of low reluctance between the respective coils to prevent current in either coil from producing substantial flux in the circuit associated with the other coil.

5. In a color television receiver, a combined local generator and brake for synchronizing a rotatable color filter element which comprises a stator having a core of magnetic material comprising a hub section, a section encircling the hub section and spaced therefrom, and a radially extending'section of low reluctance joining the hub and encircling sections between the ends thereof to define two annular spaces, a friction brake rotor of magnetic material coupled to said rotatable lter element and mounted coaxially with the stator to frictionally engage with one end thereof and complete a brake magnetic circuit between the adjacent. ends of the encircling and hub sections of the core, a brake energizing coil positioned in the annular space between the brake rotor and said radially extending section, a generator rotor of magnetic material mounted coaxially with the stator at the other end thereof, the generator rotor and said otherl end of the stator having cooperating teeth to complete a magnetic circuit between the encircling and hub sections of the core Whose reluctance varies as the rotor rotates, and a generator coil-positioned in the other annular space of the stator between the generator rotor and said radially extending section.

6. In a color television receiver, a combined local generator and brakefor synchronizing a rotatable color lter element which comprises a stator having a core of magnetic material, a shaft extending within said stator and coupled to said rotatable lter element. a friction brake rotor of magnetic material attached to said shaft at one end of said stator and adapted to frictionally engage therewith, said stator having a magnetic circuit completed through the brake rotor and a brake energizing coil positioned to produce flux therein, means for restraining the stator from substantial rotational movement and allowing at least slight axial movement thereof to vary the frictional braking force with the brake rotor, al generator rotor of magnetic ma- L terial on said shaft at the other end of the stator 2,323,905 rotor of magnetic material attached to said shaft' l at one end of said stator and adapted to frictionally engage therewith, said stator having a magnetic circuit completed through the brake rotor and a brake energizing coil positioned to produce ux therein, means for restraining the stator from substantial rotational movement and allowing at least slight axial movement thereof to vary the frictional braking force with the brake rotor, a generator rotor of magnetic material on said shaft at the other end of the stator and mounted to allow axial but not substantial rotational movement relative tov the shaft, said stator having a magnetic `circuit completed through the generator rotor and a generator coil positioned to encircle flux therein, said generator rotor partially overlapping the end of the stator core to provide a substantially radial flux path of relatively low reluctance, the axial flux path between generator rotor and stator being of relatively high reluctance to thereby prevent flux in the generator magnetic circuit from producing excessive friction between generator rotor and stator.

8. In a color television receiver, a combined local generator and brake for synchronizing a rotatable color filter element which comprises a stator having a core of magnetic material, a shaft extending within said stator and coupled to said rotata'ble filter element, a friction brake rotor of magnetic material attached to said shaft at one end of said stator and adapted to frictionally engage therewith, said stator having a magnetic circuit completed through the brake rotor and a brake energizing coil positioned to produce flux therein, means for restraining the stator from substantial rotational movement and allowing at least slight axial movement thereof to vary the frictional braking force with the brake rotor, a generator rotor of magnetic material on said shaft at the other end of the stator and mounted to allow axial but not substantial rotational movement relative to the shaft, the periphery of the generator rotor and the adjacent portion of the stator having cooperating teeth to complete a generator magnetic circuit with the stator whose reluctance varies as the rotor rotates, the teeth of the rotor being radially within the teeth of the stator, a generator coil positioned to produce flux in said generator magnetic circuit, and biasing means for pressing the generator rotor tooward the stator and the stator toward the brake rotor.

9. In a color television receiver, a. combined local generator and brake for synchronizing a rotatable color filter element which comprises a stator having a core of magnetic material comprising a hub section, a section encircling the hub section and spaced therefrom, and a radially extending section of low reluctance joining the hub and encircling sections between the ends thereof to define two annular spaces, a shaft extending within said stator and coupled to said rotatable filter element, a friction brake rotor of magnetic material attached to said shaft at one end of the stator to frictionally engage therewith and complete a brake magnetic circuit between the adjacent ends of the encircling and hub sections of the core, a brake energizing coil positioned in the annular space between the brake rotor and said radially extending section, means for restraining the stator from substantial rotational movement and allowing at least slight axial movement thereof to vary the frictional braking force with the brake rotor, a generator rotor of magnetic material on said shaft at the other end of the stator and mounted to allow axia1 but not substantial rotational movement relative to the shaft, said generator rotor having a hub extending within the stator hub to provide a substantially radial flux path of relatively low reluctance, the axial flux path between generator rotor and stator being of relatively high reluctance to prevent flux in the generator magnetic circuit from producing excessive friction between generator rotor and stator, the periphery of the generator rotor and the adjacent end of the stator having cooperating teeth to complete a generator magnetic circuit with the stator whose reluctance varies as the rotor rotates, the teeth of the rotor being radially within the teeth of the stator, a generator coil positioned in the other annular space of the stator between the generator rotor and said radially extending section, and rbiasing means for pressing the generator rotor continuously against the stator and the stator continuously against the brake rotor.

10. In a color television receiver having a rotatable color filter element driven by an asynchronous motor, a combined local generator and brake for synchronizing said rotatable color lter elementwhich comprises a stator having a core of magnetic material comprising a hub section, a cylindrical section encircling the hub section and spaced therefrom, and a radially extending section joining the hub and cylindrical sections between the ends thereof to define two annular spaces, a shaft extending through the stator and coupled to said rotatable lter element, a pulley of magnetic material attached to said shaft at one end of the stator to frictionally engage therewith and complete a brake magnetic circuit between the adjacent ends of the hub and cylindrical sections of the core, said pulley being coupled to said asynchronous motor, a brake energizing coil positioned in the annular space of the stator between the pulley and said radially extending section, means for restraining the stator from substantial rotational movement and allowing at least slight axial movement thereof to vary the frictional braking force with the pulley, a generator rotor of magnetic material on said shaft at the other end of the stator and mounted to allow axial but not substantial rotational movement relative to the shaft, the magnetic reluctance between the inner and outer ends of the radially extending section through the section being low relative to the reluctance of the path through the pulley and of the path through the generator rotor, a generator coil positioned in the other annular space of the stator between the generator rotor and said radially extending section, and biasing means for pressing the generator rotor continuously toward the stator and the stator continuously toward the pulley.

1l. In a color television receiver having a rotatable color filter element driven by an asynchronous motor, a combined local generator and brake for synchronizing said rotatable color filter element which comprises a stator having a core of magnetic material comprising a hub section, a cylindrical section encircling the hub section and spaced therefrom and a radially extending section joining the hub and cylindrical sections between the ends thereof to define two annular spaces, a shaft extending through the stator and coupled to said rotatable lter element, a pulley of magnetic material attached to said shaft at one end of the stator to frictionally engage therewith and complete a brake magnetic circuit between the adjacent ends of the hub and cylindrical sections of the core, said pulley being coupled to said asynchronous motor, a thin disk of non-magnetic material between the pulley and the stator core, a brake energizing coil positioned in the annular space` of the stator between the pulley and said radially extending section, means for restraining the stator from substantial rotational movement and allowing at least slight axial movement thereof to vary the frictional braking force with the pulley, a generator rotor of magnetic material on said shaft at the other end of the stator and mounted to allow axial but not substantial rotational movement relative to the shaft, said generator rotor having a hub extending within the stator hub to provide a substantially radia1 flux path of relatively low reluctance, the axial flux path between generator rotor and stator being of relatively high reluctance to prevent ux in the generator magnetic circuit from producing excessive friction between generator rotor and stator, the periphery of the generator rotor and the adjacent end of the stator having cooperating teeth to complete a generator magnetic circuit whose reluctance varies as the rotor rotates, the teeth of the rotor being radially within the teeth of the stator, the magnetic reluctance between the inner and outer ends of the radially extending section through the section being low relative to the reluctance of the path through the pulley and low relative to the reluctance of the path through the generator rotor, a generator coillpositioned in the other annular space of the stator between the generator rotor and said radially extending section, and biasing means for pressing the `generator rotor continuously against the stator and the stator continuously against the pulley.

12. Apparatus for synchronizing a. rotatable element which comprises a friction brake rotor of magnetic material coupled to said rotatable element, a stator mounted co-axially with said friction brake rotor and adapted to, frictionally engage therewith, said stator having a magnetic circuit completed through the brake rotor and a brake energizingcoil positioned to produce flux therein, a generator rotor of magnetic material axially spaced from the brake rotor, said stator having a magnetic circuit completed through the generator rotor and a generator coil positioned to encircle flux therein, said brake and generator magnetic circuits having a common path of low reluctance positioned to prevent the production of substantial flux between stator and brake rotor by the generator coil and between the stator and generator rotor by the brake coil.

13. Apparatus for synchronizing a rotatable element which comprises a stator having a core of magnetic material comprising a hub section, a section encircling the hub section and spaced therefrom, and a radially extending section of low reluctance joining the hub and encircling sections between the ends thereof to dei-lne two annular spaces, a friction brake rotor of magnetic material coupled to said rotatable element and mounted co-axially with the stator to frictionally engage with one end thereof and complete a brake magnetic circuit between the adjacent ends of the encircling and hub sections of the core, a brake to complete a magnetic circuit between the encircling and hub sections of the core whose reductance varies as the rotor rotates, Jthe magnetic reluctance between the inner and outer ends of the radially extending section through the section being low relative to the reluctance of the path through the brake rotor and of the path through the generator rotor, and a generator coil positioned in the other annular space of the stator between the generator rotor and said radially extending section.

14. Apparatus for synchronizing a rotatable element which comprises a stator having a core of magnetic material comprising a hub section. a cylindrical section encircling the hub section and spaced therefrom, and a radially extending section joining the hub and cylindrical sections between the ends thereof to denne two annular spaces, a shaft extending through the stator and coupled to said rotatable element, a friction brake rotor of magnetic material attached to said shaft at one end of the stator to frictionally engage therewith and complete a brake magnetic circuit between the adjacent ends of the hub and cylindrical sections of the core, a brake energizing coil positioned in the annularspace between the brake` rotor and said radially extending section, means for restraining the stator from substantial rotational movement and allowing at least slight axial movement thereof to vary the frictional braking force with the brake rotor, a generator rotor of magnetic material on said shaft at the other end of the stator and mounted to allow axial but not substantial rotational movement relative to the shaft, said generator rotor completing a generator magnetic circuit between the adjacent ends of the hub and cylindrical sections of the core and partially overlapping said ends toprovide a substantially radial flux path of relatively low reluctance, the axial flux path between generator rotor and stator being of relatively high reluctance to prevent ux in the generator magnetic circuit from producing excessive friction between generator rotor and stator, the magnetic reluctance between the inner and outer ends of the radially extending section through the section being low compared to the reluctance of the path through the brake rotor and of the path throdgh the generator rotor, a generator coil positioned in the other annular space of the stator between the generatbr rotor and said raing section, a generator rotor of magnetic mateother end of the stator having cooperating teethV is dially extendingsection, and biasing means for pressing the generator rotor continuously toward the` stator and the stator continuously toward the brake rotor.

15. In a color television receiver for receiving a color video signal `successive portions of which represent different color aspects of an object eld and accompanying synchronizing signals, said receiver having a rotatable color filter element 'for presenting images corresponding to said successive portions in their respective colors, apparatus for synchronizing the rotation of the color lter element with successive images which comprises an asynchronous motor coupled to said rotatable iilter element and tending to drive the element over synchronous speed, a local generator coupled to the rotatable filter element and adapted to generate a local synchronizing wave of the frequency of said synchronizing signals when the filter element rotates at synchronous speed, means for comparing the phases of said local synchronizing wave and said received synchronizing signals and yielding a rectified output current whose magnitude varies in accordance with the difference in said phases, a friction brake having an electro magnetic operating coil and coupled to brake the rotatable filter element, said operating coil being connected to receive the direct current component of said rectified output current, and means for substantially preventing the alternating current component of said rectified output current from flowing in said coil.

16. In a color television receiver for receiving a color videosignal successive portions of which represent different color aspects of an object field and accompanying synchronizing signals, said receiver having a rotatable color filter element for presenting images corresponding to said successive portions in their respective colors, apparatus for synchronizing the rotation of the color filter element with successive images which comprises an asynchronous motor coupled to said rotatable filter element and tending to drive the element over synchronous speed, alocal generator coupled to the rotatable filter element and adapted to generate a local synchronizing wave of the frequency of said synchronizing signals when the filter element rotates at synchronous speed, means for applying said local synchronizing wave and a wave corresponding to said synchronizing signals to the control means of an electronic tube, said control means being biased to yield a rectified output current, whereby said rectified output current varies in accordance with the difference in phase of said waves, a friction brake having an electromagnetic operating coil and coupled to brake the rotatable filter element, said operating coil being connected to receive the direct current component of said rectified output current, and means for substantially preventing the alterating current component of said rectified output current from fiowing in said coil.

' 17. In a color television receiver for receiving a color video signal successive portions of which represent different color aspects of an object fleld and accompanying synchronizing signals, said receiver having a rotatable color filter element for presenting images corresponding to said successive portions in their respective colors, apparatus for synchronizing the rotation of the color filter element with successive images which comprises an asynchronous motor coupled to said rotatable filter element and tending to drive the element over synchronous speed, a local generator coupled to the rotatable filter element and adapted to generate a local synchronizing wave of the frequency of said synchronizing signals when the filter element rotates at synchronous speed, means for applying said local synchronizing wave and a wave corresponding to said synchronizing signals to control the flow of current in an electronic tube, said electronic tube being connected to yield a rectified output current which varies in accordance with the difference in phase of said waves, a friction brake having an electromagnetic operating coil and coupled to brake the rotatable filter element, said operating coil being connected to receive the direct current component of said rectified output current, and a condenser connected across said operating coil to by-pass the alternating current component vof the rectified output current and cause substantially only the direct current component to flow in the coil.

. 18. In a color television receiver for receiving a color video signal successive portions of which represent different color aspects of an object field and accompanying synchronizing signals,

said receiver having a rotatable color filter element for presenting images corresponding to said successive portions in their respective colors, apparatus for synchronizing the rotation of the color filter element with successive images which comprises an asynchronous motor coupled to said rotatable filter element and tending to drive the element over synchronous speed, a local generator coupled to the rotatable filter element and adapted to generate a local synchronizing wave of the frequency of said synchronizing signals when the filter element rotates at synchronous speed, a phase-comparing circuit comprising an electronic vacuum tube having control grid means, means for applying said local synchronizing wave and a Wave of similar wave-shape derived from said received synchronizing signals to said control grid means, said control grid means being biased to yield a rectified output current which varies in accordance with the difference in phase of said waves, said bias being sufiicient to yield a relatively low plate current in said tube when said waves are out o! phase, a friction brake having an electromagnetic operating coil and coupled to brake the rotatable filter element, said operating coil having a relatively high inductance and being connected in the plate circuit of said tube, and a condenser connected across said operating coil to by-pass the alternating current component of the rectifled output current and cause substantially only the direct current component to flow in the coil.

19. In a color television receiver for receiving a color video signal successive portions of which represent different color aspects of an object field and accompanying synchronizing signals, said receiver having a rotatable color filter element for presenting images corresponding to said successive portions in their respective colors, apparatus for synchronizing the rotation of the color filter element with successive images which comprises an asynchronous motor coupled to said rotatable filter element and tending to drive the element over synchronous speed, a local generator coupled to the rotatable filter element and adapted to generate a local synchronizing wave of the frequency of said synchronizing signals when the filter element rotates at synchronous Speed, a phase-comparing circuit comprising an electronic vacuum tube having control grid means, means for applying said local synchronizing wave and a wave of similar wave-shape derived from said received synchronizing signals to said control grid means, said control grid means being biased to yield a rectified output current which varies in accordance with the difference in phase of said waves, a friction brake having an electromagnetic operating coil and coupled to brake the rotatable filter element, said operating coil having a relatively high inductance and being connected in the plate circuit of said tube, a condenser in shunt with said operating coil to by-pass the alternating currentl component of the rectified output current and cause substantially only the direct current component to flow in the coil, and a switch in said shunt circuit biased to establish said circuit and operable to break the circuit to remove the synchronizing action of the brake and thereby change the colors in which successive images are presented.

20. In a color television receiver for receiving a color video signal successive portions of which represent different color aspects of an object eld and accompanying synchronizing signals, said receiver having a rotatable color filter element for presenting images corresponding to said successive portions in their respective colors, apparatus for synchronizing the rotation of the color iilter element with successive images which comgrid means being biased to yield a rectied output current which varies in accordance with the difference in phase of said waves, said bias being suiiicient to yield a .relatively low plate current in said tube when said waves are 180 out of phase, an impedance in the cathode circuit of said tube, a friction brake having an electromagnetic operating coil and coupled to brake the rotatable i'llter element, said operating coil having a relatively high inductance and being connected in the plate circuit of said tube,I a condenser in shunt with said operating coil to by-pass the alternating current component of the rectified output current and cause substantially only the direct current component to flow in the coil, and a switch in said shunt circuit biased to establish said circuit and operable to break the circuit to remove the synchronizing action of the brake and thereby change the colors in which successive images are presented.

21. Apparatus for synchronizing a rotatable element with control synchronizing signals which comprises an asynchronous motor coupled to said rotatable element and tending to drive the element over synchronous speed, a local generator \coupled to the rotatable element and adapted to generate a local synchronizing wave of the frequency of said synchronizing signals when the rotatable element rotates at synchronous speed, means for comparing the phases of said local synchronizing wave and said control synchronizing signals and yielding a rectied output current whose magnitude varies in accordance with the dlierence in said phases, a friction brake having an electromagnetic operating coil and coupled to brake the rotatable element, said operating coil being connected to receive the direct current component of said rectified output current, and means for substantially preventing the alternating current component of said rectified output current from iiowing in said coil.

22. 'I'he method of synchronizing a rotatable element with control synchronizing signals which comprises driving said rotatable element with an asynchronous motor tending to drive the element over synchronous speed, locally generating asynchronizing wave of frequency proportional to the speed of rotation of said element and equal to the frequency of said control synchronizing signals when the element rotates at synchronous speed, comparing the phases of the local synchronizing wave and said'control synchronizing signals and deriving a rectified output current whose magnitude varies in accordance with the difierence in said phases, and supplying substantially only the direct current component of said rectied output wave to the operating coll of an electromagnetic friction brake coupled to brake said rotatable element.

PETER C. GOLDMARK.