US1923757A - Sound recording system - Google Patents

Description

Aug. 22, 1933. I c, 5|LENT 1,923,757

SOUND RECORDING SYSTEM Filed June 19, 1951 2 Sheets-Sheet l N lllln IIIIH R\ llHll mm INVENTOR H. C. SILENT By mm ur.

A T TORNE V 2, 1933- H. c. SILENT SOUND RECORDING SYSTEM T M M h I l. S? M A MW /c. W H 2 V,

m2 KTEQ k A TTOR/VEV Patented Aug. 22, 1933 UNITED STATES PATENT FFICE SOUND RECORDING SYSTEM Application June 19, 1931. Serial No. 545,455

23 Claims.

This invention relates to the recording of sound on a photographic film and particularly to the recording of sound when the impression produced by the recording device is not in the center of the available characteristic of the recording medium.

The object of this invention is a system for controlling the operation of a film sound recording system which is simple and rugged in construction and easily applied to the recording channels at present in commercial use.

A feature of this invention is a method of supplying power from the control circuitto the recording device in which the effect of this power on the circuit supplying sound modulated power to the recording device is neutralized. Another feature of this invention is the provision of means for controlling the input of sound modulated power to the controlling circuit and the recording device to prevent the impression produced by the recording device from exceeding the allowable limits of the characteristic of the photographic medium. A further feature of the invention resides in the use of a detecting device in the control circuit which has an impedance of the same order as the impedance of the recording'device and the provision of means whereby the impedance of the detecting device is maintainedat a value less than the normal value.

Experiments have shown that the noise currents produced during the reproduction of a film sound record are approximately proportional to the average transmission of the positive sound record when the transmission is defined as the ratio of the light transmitted through the positive record divided by the light incident on the record. The average transmission of the sound record may be considered to be equal to the transmission of the unmodulated portions of the record. Assuming that the phototgraphic processes are controlled so that no overall distortion is produced, the transmission of the positive sound record will be proportional to the exposure of the negative sound record when this exposure lies within the limits of the straight portion of the characteristic of the emulsion used. The exposure is proportional to the product of the intensity of the li ht incident on the film multiplied by the time during which the negative is subjected to the light. Thus, if the average exposure of the negative is as small as possible, the sound record on a positive print of the negative will have a small value of transmission and the noise produced during the reproduction of a positive sound record printed from the negative will be a minimum.

It may be shown, however, that the volume of the reproduced sound is proportional to the product of the average transmission of the positive sound record multiplied by the percentage modulation of this average due to the record of the sound. Thus, in order to avoid distortion of the volume of the sound reproduced, when the average transmission of the positive sound record is reduced, the percentage modulation of this transmission must be inversely increased. In the present invention, the percentage modulation of the transmission of the positive sound record for all volumes of sound is large, and the average transmission is varied according to the volume of sound and is correspondingly small. To produce a positive record having a small average transmission, the exposure of the negative must be correspondingly small. Thus, in the 7 present invention the time of exposure of the negative is small for small volumes of sound and is increased in accordance with the increase in the average volume of the sound.

In a preferred embodiment of this invention, a light valve of the type disclosed in United States Patent 1,638,555, granted August 9, 1927 to E. CL Wente, may be employed though it will be obvious that this invention is not restricted to this specific form of light valve but may be readily applied to many other well known types of light valves. In recording sound by means of a light valve of this type, the intensity of the light incident on the photographic medium is constant but the time of exposure of the medium to the light varies with the height of the slot (lengthwise of the film) defined by the ribbons of the light valve. The average time of exposure of the negative sound record will be defined by the mean height of the slot formed by the ribbons, that is, by the mean position about which the ribbons vibrate.

In accordance with the present invention, the; intensity of the recording light is adjusted so that when no current is flowing in the light valve ribbons the exposure produced will be in the center of the allowable range of exposures for the photographic emulsion used. The gain of the amplifiers supplying the recording device is then adjusted so that the loudest sound to be recorded will cause the ribbons to vibrate until their maximum movement will just close the recording slot. A constant unidirectional cur-. rent, which for convenience may be termed a static bias, is applied to the ribbons in such direction and of such magnitude that in the absence of sound, the ribbons are drawn close together and the height of the slot defined by the ribbons is small. A portion of the sound modulated currents is tapped oil the output of the recording amplifier and supplied to a full wave detector. The output of the detector will be a pulsating unidirectional current whose magnitude varies in accordance with the volume of the sound. This current, which may be termed a dynamic bias, affects the ribbons of the light valve in such direction as to oppose the effect of the static bias and cause the ribbons to move and increase the height of the slot. When the effect of the dynamic bias becomes equal in magnitude to the effect of the static bias, the ribbons will be under the control of the mechanical adjustment of the valve only and the slot will be of normal height. With the regular adjustment of the recording light, the average exposures produced under these conditions are in the center of the allowable range of exposures for the emulsion used and complete modulation of this average exposure will cover the whole allowable range of exposures. If the eiiect of the dynamic bias were permitted to increase beyond this value, the ribbons of the light valvewould be forced apart and the slot would be of abnormal height. Even partially complete modulation of the slot when opened to the abnormal height would cause the exposure to exceed the allowable range of exposures. For example, if the slot be opened to twice its normal height a photographic overload would be produced by any modulation whatsoever and the sound record would be distorted. To avoid the occurrence of this form of overload, the invention contemplates the use of power limiting amplifiers in the sound recording or in the controlling circuits to limit the maximum power supplied to the light valve ribbons to less than the value that will cause overload. An alternative method applicable to the controlling circuit only contemplates the use of a rectifier element which will prevent the resultant current due to the static bias and the dynamic bias in opposition from reversing in direction. In accordance with the invention, the sound modulated currents are supplied to the ribbons of the lightvalve through a transformer having a tapped secondary winding. The light valve ribbons are connected in series with a non-inductive resistance equal to the motional impedance or, if desired, the static resistance of the ribbons and the combination is connected across the whole secondary winding of the transformer. The current resulting from the static bias and dynamic bias in oppositionis suppliedto the tap oi the transformer and flows through the two portions of the wind ings inopposite directions, then through the light Valve ribbons and the resistance back to the control circuit. Themagnetic efiect of this current flowing in the secondary winding of the transformer is thusneutralized. The sound modulated currents from the transformer will flow through the light valve ribbons and the resistance inseries.

Fig.1 represents in schematic form an embodiment'of' the invention including power limiting amplifiers; f

Fig.2 represents in schematic form another embodiment of theinvention including a power limiting rectifier; and

Figi.3'represents diagrammatically the optical elements of a film soundrecording system.

InFig: 3, the image of a suitable source of light 10 is focused by the lens 11 on the small slot formed by the ribbons 12 located in the orifice 13 pierced in the pole pieces of the magnet 14. Suitable pole faces concentrate the magnetic field of the magnet 14 to form a uniform magnetic field in which the ribbons 12 are immersed. An electric current fiowing in the ribbons 12 will cause the ribbons 12 to be displaced by the reaction between the magnetic fields due to the current and the magnet 14, and thus will vary the height of the slot formed by the ribbons 12 in accordance with the magnitude of the current. The image of the slot is focused by the lenses l5 and 16 on a photographic medium 17 moving at uniform speed past the point of exposure. An aperture plate 18 holds the film smooth at the point of exposure and prevents diffused light from fogging the record.

In Fig. 1, sound waves, detected by a microphone 19 of known type, are amplified in the amplifiers 20 and 21 and the amplified currents supplied to the primary windings of transformers 22 and 23. A variable resistor or other suitable means may be incorporated in the amplifier 21 to regulate the power supplied to the transformers 22 and 23. The outer ends of the secondary winding of the transformer 22 are connectedto the control elements of the amplifying devices 24 and 25 which for convenience are shown as three element vacuum tubes. The secondary winding of transformer 22 is tapped at a central point and this point is connected through a resistor 26 and biasing battery 2'7 to the filaments of the devices 2a and 25. The output circuits of the devices 24 and 25 are connected in opposed or pushpull relation to the primary winding of the transformer 28. Power is supplied to the output circuits of the devices 24 and 25 from the battery 29 through a reactor 30.

The negative biasing potential applied to the control electrodes of the devices 24; and 25 bythe battery 27 is adjusted to bring the operating point of the devices 24 and 25 in the center of their characteristic curve. So long as the voltage applied to the control electrodes by the transformer 22 is not larger than the biasing potential the devices operate as the well known. push-pull amplifier. However, if the peak voltage applied to the control electrodes exceeds the biasing potential the control electrodes will be given a positive polarity and electrons will be attracted to the control electrodes. The electrons will flow from the control electrodes through the resistor 26 and battery 27 to the filaments of the devices 24 and 25. This flow of electrons will cause a potential to be developed across the resistor 26 which will increase the negative bias applied to the control electrodes. This increase in the biasing potential will reduce the amplification of the devices 24 and 25 so that the power delivered to the transformer 28 cannot materially exceed a value determined by the biasing potential, the value of the resistance and the type of device. Owing to the push-pull connection of the output circuits of the devices 24 and 25,

the distortion which would tend to be produced by the power limiting action is largely neutralized.

The secondary winding of the transformer 28 supplies power to a volume control device which may consist of a variable resistor 31 in shunt to the secondary winding of the transformer 28,

and a variable resistor 32 in series with the primary windingof the transformer 33. A volume control of this type is well known and maintains a constant impedance facing the transformers for all settings. To reduce any inductive pick-up lei 'drawn very close together.

from nearby circuits, the impedance of the secondary winding of transformer 28 and of the primary winding of transformer 33 may be comparatively small, say of the order of 500 ohms.

The operation of the power limiting amplifier comprising the devices 34 and 35 is similar to the operation of the amplifier comprising the devices 24 and 25 and need not be described in de tail. Due to this amplifier, the power supplied by the transformer 23 to the devices 34 and 35 and thence to the transformer 36 cannot materially exceed a predetermined maximum.

The modulated waves from the secondary winding of the transformer 36 are supplied to the ribbons 12 of the light valve in series with a resistor 37. The modulated waves flowing in the ribbons 12 will cause the ribbons 12 to be displaced and to vary the height of the slot formed by the ribbons 12 in accordance with the instantaneous variations in the modulated waves. The variation thus caused in the height of the slot will cause a corresponding variation in the time of exposure of the photographic medium. The resistor 37 may have a resistance of the same order as the motional impedance of the ribbons 12, so that approximately one-half of the power supplied by the transformer 36 is effective in causing the ribbons 12 to move.

A constant current flows from the battery 38 through wire 39, to a tap of the secondary winding of the transformer 36, and there divides, a portion flowing through the ribbons 12 and the remainder through the resistor 3'7. The current returns through the variable resistor to the battery 38. If the resistance of the resistor 37 is approximately equal to the motional impedance of the ribbons 12, in order to maintain a balanced condition for modulated currents the tap on the secondary winding of the transformer 35 should be located so as to have approximately the same number of turns in each part of the winding.

However, if the resistances of the ribbons 12 and resistor 37 are not equal, the constant currents flowing in the two parts of the winding will not generally be equal. As it is preferable that the constant current should divide into equal parts of wire having a higher resistance than the wire used in the other part, or a small resistor may be connected in series with the ribbons. The constant current is adjusted by the resistor 40 so that, in the absence of sound, the ribbons 12 are For convenience, this constant current may be termed the static bias applied to the ribbons 12.

The modulated waves from transformer 33 are applied to a full-wave rectifying device 41. Experiments have shown that the highest eificiency is attained when this device has an impedance of the same order as the impedance of the light valve. As the light valve is inherently a device having a fairly low impedance, in a preferred embodiment of the invention, the rectifying device 41 was a full-wave copper-copper oxide rectifier similar to the device described by Grondahl and Geiger in the Transactions of the American Institute of Electrical Engineers for March, 1927. Other forms of contact rectifier as well as electronic rectifiers have given satisfactory operation in the circuit. The output current of the rectifying device 41 is supplied through a variable resistor 42 and a reactor 43 to the junction of themainder flows through resistor 37 and the other part of the secondary winding of transformer 36. The output current then returns through wire 44 to the secondary winding of transformer 33. The output current from the rectifier 41 flows through the ribbons 12 in the opposite direction to the constant current from the battery 38. The output current from the rectifier 41 will vary with the modulated waves impressed on the transformer 33 and for convenience may be termed a dynamic bias. By suitably proportioning the reactor 43 and the capacitors 45 and 46, the dynamic bias may be made to vary with the envelope of the amplitude variations of the modulated waves and may vary with the mean value or the peak value of the amplitude variation. By adjusting the resistor 42 the rib- Icons 12 may be displaced apart an amount approximately equal to the amplitude of the vibration due to the modulated waves supplied by the transformer 36 and the recording light will thus be substantially completely modulated for all amplitudes of the modulated waves.

A small current will also flow from battery 38, wire 44, secondary winding of transformer 33, rectifier 41, resistor 42, reactor 43, resistor 40 to battery 38. The magnitude of this current may be adjusted by the resistors 40 and 42. The effect of this current is to shift the characteristic of the rectifier 41 so that the operating impedance of the rectifier is less than normal and is reduced to an impedance more nearly equal to the impedance of the ribbons 12. By means of the potentiometer formed by the resistances 31 and 32, the power supplied to the rectifier 41 may be adjusted to the value which is required by the operating conditions when the limiting action of the resistance 26 just begins to take place.

In Fig. 2, sound waves, detected by a microphone 47 of known type, are amplified in the amplifiers 4S and 49, and the amplified currents supplied to the primary windings of transformers 50 and 51. The output currents from the secondary windings of the transformers 50 and 51 may be amplified in suitable amplifiers, such as the amplifier designated by the reference X. As the amplifier associated with transformer 51 may be identical with the amplifier X associated with transformer 50, the description of only one of these amplifiers is necessary.

The output of transformer 50 is supplied to a potentiometer composed of two resistors52 and 53 in series. A second potentiometer 54 is bridged across the resistor 53 and controls the modulated voltage supplied to the control electrode of the vacuum tube 55. The wiper arm of the potentiometer 54 may be moved to increase the input from zero to the maximum voltage developed across the resistor 53. Further movement of the wiper arm 60 will open switch 56 and will move switch arm 57 from contact 58 to 59, and the wiper will again be at the lower end of resistor 54. Still further movement of the wiper arm 60 will increase the input to the maximum voltage developed by the secondary winding of the transformer so. The combined potentiometers thus give a very wide range of control of the voltage applied to the control electrode of the vacuum tube 55. The output of the vacuum tube 55 may be coupled bythe-transformer 61 to a further stage of amplification which may comconnected in series with the battery 38.

from the prise two vacuum tubes 62 and 63 connected in the known push-pull relation.

The output of the amplifier is applied through a transformer 64 to the ribbons 12 of the light valve in series with a resistor 37, and, if desired, a variable resistor.65.. The current flowing in the ribbons 12 will cause the ribbons to move and vary the exposure of the photographic medium as previously described in Figs. 1 and 3.

The output of the transformer 66 is supplied to a control circuit generally similar to the circuit previously described in Fig. 1. Elements in the two circuits having essentially the same functions have been similarly designated and the operation of the circuit will be, in general, the same as the circuit in Fig. 1.

In Fig. 1, ho ever, the amplifiers supplying the control circuit and the recording circuit are so arranged that the power supplied to the circuits will not materially exceed a predetermined maximum. In Fig. 2, regular amplifiers are employed, thus a sudden loud sound will produce a large dynamic bias which may be larger than the static bias and thus tend to force the ribbons 12 apart so that the height of the recording slot would be greater than normal. If the recording slot is permitted to become greater than normal, the time of exposure at maximum modulation will be so long that the range of correct exposure of the photographic emulsion will be exceeded. To prevent this photographic overloading, a rectifier 67, which may be of the contact type such as a copper-copper oxide couple, is The static biasing current flows as before from battery 38 through wire 68 to the secondary winding of transformer 64. Part of the current flows through one part of the secondary winding, resistor 65 and ribbons 12; the other part of the current flows through the other part of the secondary winding and resistor 37. The whole static biasing current then flows through rectifier 6'7 and resistor 40 to battery 38.

The dynamic biasing current will now flow secondary of the transformer 6, through the rectifier 41, resistor i2, reactor 43, resistor 4o, battery 38, wire 69 to the secondary or" the transformer 66. Owing to the presence of the rectifier 67 the dynamic biasing current cannot flow through the ribbons 12, but produces a difierence of potential across the resistor 40 that opposes the flow of the static biasing current. The dynamic biasing current may thus cause the static biasing current flowing in the ribbons 12 to be reduced to zero but cannot cause a reversal of the current flowing in the ribbons 12 even when the input power from the transformer 66 is large.

The resistor 42 assists in securing proper operation of the filter formed by the reactor 43 and capacitors 45 and 45, and reduces the amount of modulated power which, under unbalanced conditions, might tend to flow through the control circuit. I also increases the resistance of the circuit including the rectifier .1 to the static bias current, so that the amount of this current diverted from the ribbons 12 is reduced.

What is claimed is:

1. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light irom said source incident on said medium, a resistor in series with said control means, means for supplying modulated waves from said source to said control means comprising a transformer having a tapped secondary winding, one portion of which is connected to said control means and the other portion to said resistor, a source of unidirectional current connected between said tap of said secondary winding and the junction of said control means and said resistor, and a control circuit comprising a detector energized by waves from said source to vary the unidirectional current from said source in accordance with the amplitude variations of said waves.

2. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, a resistor having a resistance comparable to the static resistance of said control means, means for supplying modulated waves from said source to said control means comprising a transformer having a secondary winding tapped in the center, the outer ends of said winding being connected to said control means and said resistor in series, a source of unidirectional current connected between said center tap and the junction of said control means and said resistor and a control circuit comprising a detector energized bywaves from said source to vary said unidirectional current in accordance with the amplitude variations of said waves.

3. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, a resistor, means for supplying modulated waves from said source to said control means comprising a transformer having a tapped secondary winding, one portion of which is connected to said control means and the other portion to said resistor, a source of unidirectional current connected between said tap of said secondary winding and the junction of said control means and said resistor and a control circuit comprising a detector energized by waves from said source to vary said unidirectional current in accordance with the envelope of the amplitude variations of said waves.

4. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, a resistor having a resistance comparable to the static resistance of said control means, means for supplying modulated waves from said source to said control means comprising a transformer having a secondary winding tapped in the center, the outer ends of said winding being connected to said control means and said resistor in series, a source of unidirectional current connected between said center tap and the junction of said control means and said resistor and a control circuit energized by waves from said source to vary said unidirectional current in accordance with the envelope of the amplitude variations of said waves.

5. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, means for supplying modulated waves from said source to said control means, a source of unidirectional current,a circuit for supplying unied waves substantially no impression is-produced.

in said'medium, and a control circuit energized by waves from said source to vary said unidirectional current, comprising a detector having an impedance substantially equal to the impedance of said control means.

6. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, means for supplying modulated waves from said source to said control means, a source of unidirectional current, a circuit for supplying unidirectional current from said source to said control means whereby in the absence of modulated waves substantially no impression is produced in said medium, and a control circuit actuated by waves from said source to vary said unidirectional current, comprising a detector of the copper-copper oxide type having an impedance substantially equal to the impedance of said control means.

7. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, a source of unidirectional current, a circuit for supplying unidirectional current from said source to said control means whereby in the absence of modulated waves substantially no impression is produced in said medium, a control circuit to vary said unidirectional current comprising a detector having an impedance substantially equal to the impedance of said control means, and circuits for supplying modulated waves from said source to said control means and to said control circuit respectively, each circuit comprising a plurality of space discharge devices connected in push-pull relation, each of said devices having a cathode, an anode and a control electrode, input and output circuits for said devices, means for supplying a constant potential to said control electrodes of such value as to prevent conduction of current from control electrodes to cathodes of said devices when said devices are actuated by modulated waves within an extended range of normal amplitudes, and resistors in the input circuits of said devices for supplying an added potential to said control electrodes in response to and in proportion to an increase in the amplitude of said waves beyond said normal range whereby the gain of said amplifier is reduced.

8. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, a source of unidirectional cmrent, a circuit for supplying unidirectional current from said source to said control means whereby in they absence of modulated waves substantially no impression is produced in said medium, a control circuit to vary said unidirectional current comprising a detector of the copper-copper oxide type having an impedance substantially equal to the impedance of said control means, and circuits for supplying sound modulated waves from said source to said control means and to said control circuit respectively, each circuit comprising a plurality of space discharge devices connected in push-pull relation, each of said devices having a cathode, an anode and a control electrode, input and output circuits for said devices, means for supplying a constant potential to said control electrodes of such value as to prevent the conduction of current from the control electrodes to the cathodes of said devices when said devices are actuated by modulated waves within an extended range of normal amplitudes, and resistors in the input circuits of said devices for supplying an added potential to said control electrodes in response to and in proportion to an increase in the amplitude of said waves beyond said normal range whereby the modulated power transmitted to said control device and said control circuit is prevented from exceeding the value equivalent to the maximum amplitude of said normal range.

9. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, a re-, sister having a resistance substantially equal to the static resistance of said control means, means for supplying modulated waves to said control means comprising a transformer having a centrally tapped secondary winding, the outer ends of said winding being connected to said control means and said resistor in series, a source of unidirectional current connected between said center tap and the junction of said control means and said resistor, a control circuit to vary said unidirectional current comprising a detector of the copper-copper oxide type having an impedance substantially equal to the impedance of said control means, and circuits for supplying modulated waves from said source to said control circuit and the primary winding of said transformer respectively, each circuit comprising a plurality of space discharge devices connected in push-pull 1 relation, each of said devices having a cathode, an anode and a control electrode, input and output circuits for said devices, means for supplying a constant potential to said control electrode of such value as to prevent the conduction of cur- 1 rent from the control electrodes to the cathodes of said devices when said devices are actuated by modulated waves within an extended range of normal amplitudes and resistors in the input circuits of said devices for supplying an added po- 1 tential to said control electrodes in response to and proportional to an increase in the amplitude of said waves beyond said normal range whereby the modulated power transmitted to said control circuit and said transformer is prevented from 1 exceeding the value equivalent to the maximum amplitude of said normal range of amplitudes.

10. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, means for control- 1 ling the light from said source incident on said medium, means for supplying modulated waves from said source to said control means, a source of unidirectional current, a circuit for supplying unidirectional current from said source to said 1 control means whereby a static bias is placed on said control means of such value that in the absence of modulated waves the impression produced in said medium is at the lower end of the region of correct exposure for the emulsion of 1 said medium, a control circuit actuated by waves from said source comprising a detector, an electrical network in the output of said detector whereby the detected current is caused to vary in accordance with the envelope of the amplitude 1 variations of said waves and circuit connections whereby a dynamic bias is applied in said control circuit in such manner as to oppose the static bias, and means comprising a rectifier for preventing the current in said control means re- 1 sulting from the opposition of said static biasing current and said dynamic biasing current from reversing in direction.

11. In combination, a source of modulated waves, a photographic medium moved at con- 1 stant speed, a source of light, means for controlling the light from said source incident on said medium, means for supplying modulated waves from said source to said control means, a source of unidirectional current, a circuit for supplying unidirectional current from said source to said control means whereby a static bias is placed on said control means of such value that in the absence of modulated waves the impression produced in said medium is at the lower end of the region of correct exposure for the emulsion of said medium, a control circuit actuated by waves from said source comprising a detector, an electrical network in the output of said detector whereby the detected current is caused to vary in accordance with the envelope of the amplitude variations of said waves and circuit connections for supplying said detected current in such mannor as to oppose the static bias on said control means, means comprising a rectifier for preventing the current in said control means resulting from the opposition of said static biasing current and said dynamic biasing current from reversing in direction and means for mechanically adjusting said control means whereby the impressions produced in said medium during the time that capable of moving under the influence of electrical currents flowing in said elements, a resistor having a resistance equal to the static resistance of said elements, means for supplying modulated waves from said source to said elements comprising a transformer having a centrally tapped secondary winding, the outer ends of said winding being connected to said elements and said resistor in series, a source of unidirectional current connected between said center tap and the junction of said elements and said resistor, and a control circuit comprising a detector energized by waves from said source to vary the unidirectional current from said source in accordance with the envelope of the amplitude variations of said waves.

13. In combination, a source of modulated waves, a photographic medium movedat constant speed, a source of light, a light valvefor controlling the light from said source incident on said medium comprising a plurality of elements mechanically adjusted to define a slot of normal height and capable of moving under the influence of electrical currents flowing in said elements, means for supplying modulated waves from said source to said elements, a source of unidirectional current, a circuit for supplying unidirectional current from said source to said elements whereby in the absence of modulated waves the height of said slot is reduced until the impression produced in said medium is at the lower end of the region of correct exposure for the emulsion of said medium anda control circuit comprising a detector energized by waves from said source to vary said unidirectional current and having an impedance substantially equal to the impedance of said elements.

14. In combination, a source of modulated waves, a photographic medium moved at'constant speed. a source of light, a light valve for controlling the-light from said source incident on said medium comprising a plurality of elements mechanically adjusted to define a slot-of normal height and capable of movingunder the influence of electrical'currents flowing in said elements,

a detector of the copper-copper oxide type energized by waves from said source to vary said unidirectional current andhaving an impedance substantially equal to the impedance of said elements.

15. In combination, a source of modulated waves, a photographic medium moved at constant speed, a source of light, a light valve for controllingthe light from said source incident on said medium comprising a plurality of elements mechanically adjusted to define a slot of normal height and capable of moving under the influence of electrical surrents flowing in said elements, means for supplying modulated waves from said source to said elements, a source of unidirectionalcurrent, a circuit for supplying unidirectional current from said source to said elements whereby in the absence of modulated waves the height of said slot is reduced to such value that the impression produced in said medium is at the lower end of the region of correct exposure for the emulsion of said medium and a control circuit associated with said source of unidirectional current comprising a detector energized by waves from said source and having an impedance of the same order as the impedance of said elements, and an electrical network in the ouput of said detector whereby said unidirectional current is caused to vary in accordance with the envelope of the amplitude variations of said waves.

16. The combination in claim '7 in'which the means for controlling'the light comprises a light valve having a plurality of elements mechanically adjusted to define a 'slot of normal height and capable of moving under the influence of electrical currents flowing in said elements. 17. The combination in claim 8'inwhich'the means'for controlling the light comprises a'light valve having a plurality of elements mechanically -adjusted to define a slot of normal height and capable of moving under the influence of electrical currents flowing insaid elements.

IBI'The combination'in claim 9 in which the means for controlling the light comprises alight valve'having a plurality of elements mechanically adjusted to define a slot of normal height and capable of moving under the influence of electrical currents flowing in said elements.

19. The combination in claim 10 in which the means for controlling the light comprises alight valve having a plurality of elements mechanically adjusted to define a slot of normal height and capable of moving under the influence of electrical currents flowing in said elements.

20. The combination in claim 11 in which :the means for controlling the light comprises a light valve having a plurality of elements capable of moving under the influence of electrical currents flowing in said elements.

21. In combination, a source of modulated waves, a photographic mediummoved' at'constant speed, a source of light, means for controlling the light from said source incident on said medium, a resistor having a resistance comparable to the static resistance of said control means, means for supplying modulated waves from said source to said control means comprising a transformer having a secondary winding tapped in the center, the outer ends of said winding being connected to said control means and said resistor in serial relation, a source of unidirectional current connected to said center tap and the junction of said control means and said resistor and a control circuit energized by waves from said source to vary said unidirectional current whereby the carrying capacity of said control means is varied proportionally to the amplitude of the waves supplied to said control means.

22. In combination, a sou cc of modulated waves, a photographic medium moved at constant speed, a source of light, means for controlling the light from said source incident on said medium, a resistor having a resistance comparable to the motional impedance of said control means, means for supplying modulated Waves from said source to said control means comprising a transformer having a tapped secondary winding, the outer ends of said winding being connected to said control means and said resistor in serial relation, a source of unidirectional current connected between said tap and the junction of said control means and said resistor, and a control circuit energized by waves from said source to vary said unidirectional current in accordance with the envelope of the amplitude variations of said waves.

23. In combination, a source or" modulated waves, a photographic medium moved at constant speed, a source of light, a light valve having a plurality of elements mechanically adjusted to define a slot of normal height and capable of moving under the influence of electrical currents flowing in said elements, means for supplying modulated Waves from said source to said light valve, means for supplying unidirectional current to said light valve, a control circuit including a detector actuated by Waves from said source to vary said unidirectional current, and means in said control circuit for adjusting the unidirectional current through said detector to a value such that the impedance of said detector is sensibly less than normal.

HAROLD C. SILENT.

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