US1921037A - Sound recording - Google Patents

Description

Aug. 8, 1933. K. F. MORGAN SOUND RECORDING Filed Oct. 1, 1950 FIG. 2

INVENTOR K. E MORGAN A TTORNEV Patented Aug. 8, 1933 UNITED STATES PATENT OFFICE SOUND RECORDING Application October 1, 1930, Serial No. 485,648,

and in Great Britain September 4, 1930 10mi s.

This invention relates to sound recording and particularly to a method of recording by photographic means a wider range of sound volumes than can be recorded by known methods.

5 In any method of recording sound the range of the volumes of sound which can be satisfactorily recorded is limited by the properties of the recording medium used and the handling which the completed record receives. The least volume which may be recorded is the volume which in reproduction can be satisfactorily distinguished above the noise produced by irregularities in the structure of the mediums used in recording and reproducing and by the dirt} l6 scratches, etc. which collect on the record during its useful life.

The object of this invention is a system of sound recording which will produce a reduction of the volume of noise produced from a photographic sound record during the reproduction of the sound from the record. This reduction in the volume of the noise produced permits smaller volumes of sound to be satisfactorilyrecorded and thus increases the range of volumes of sound which may be recorded. Or this invention will provide a larger ratio between the present minimum volume of sound and the volume of noise produced during reproduction.

Experiments have shown that the volume of noise produced during the reproduction of a photographic sound record is approximately proportional to the average transmission of the positive sound record. The average transmission of a photographic sound record is considered to be equal to the transmission of the unmodulated portions of the sound record. The transmission of a photographic medium is defined as the ratio of the light transmitted through the medium divided by the light incident on the medium and 40 for convenience is commonly expressed as a percentage.

A feature of this invention is a method of producing a positive photographic sound record in which the average transmission of the sound 45 record varies with the volume of the sound recorded. Thus for small volumes of sound a dense record is produced and the noise produced during reproduction is a minimum.

In the reproduction of sound from a photographic sound record it has been shown that the volume of the reproduced sound is proportional to the product of the average transmission of the sound record multiplied by the percentage variar tion in this transmission due to the modulation of the transmission in accordance with the sound. In order to prevent distortion or the volume of the sound, when the transmission of the positive sound record is reduced, the percentage modulation of this transmission must be inversely increased. For convenience in operation, the percentage modulation of the transmission is made as nearly 100% as may be practical, as this will give the smallest values of transmission of the positive sound records. Other values of modulation may be used if desired.

A photographic positive is an inverse copy of the photographic negative from which the positive was printed thus if the positive sound record has a small average transmission the negative sound record must have a correspondingly large. transmission. The exposure to which a photographic material is exposed is defined as proportional to the product of the intensity of the light incident on the.material multiplied by the time during which the material is subjected to this light. I If the complete photographic process is assumed to be controlled so that no distortion is produced, then the transmission of the positive sound record will be proportional to 30 the exposure of the negative. Accordingly, to produce a positive sound record having a small average transmission, the exposure of the negative must be correspondingly small. In accord-. ance with this invention the exposure is reduced 35 to produce a negative sound record having a large average transmission.

A further feature of this invention is a method of recording sound by photographic means in which the exposure of the negative sound record varies directly with volume of the sound recorded and the light which performs the recording is virtually completely modulated for all volumes of sound.

In a preferred embodiment of this invention, a light valve of the type disclosed in U. 8. Patent 1,638,555 granted August 9, 1927 to E. C. Wente is employed. In using a light valve of this type for recording sound by the method disclosed in the above patent the intensity of the recording light does not change but the time of exposure is varied as the height of the slot defined by the ribbons is varied by the movement of the ribbons. The average time of exposure will be controlled by the average spacing of the ribbons or, in other words, by the height of the slot formed when the ribbons are not displaced by a current flowing in them.

In accordance with the invention the sound waves are detected as by a microphone and the 110 resulting electric currents amplified in the known way and applied to the ribbons through a large condenser. A portion of the: amplified sound current is tapped off the output of the amplifier and fed to a controlling device, for example, a detector. The output of the detector will be a pulsating unidirectional current Whose magnitude varies in accordance with the volume of the sound. The ribbons of the light valve are connected in series with the output circuit of an amplifying vacuum tube. The grid bias of the vacuum tube is adjusted so that the ouput current of the tube is of such magnitude that the ribbons are nearly touching and the height of the slot defined by the ribbons is small. The

output of the detector is applied to the grid of.

the amplifying vacuum tube in such manner that the output current of the vacuum tube varies inversely with the output of the detector. The height of the slot defined by the ribbons is thus enlarged in direct proportion to the volume of the sound being recorded. At the same time the ribbons are vibrated by the amplified currents from the sound detector and vary the time of exposure of the film in accordance with the modulation produced by the sound. As the amplitude of the vibrations is controlled by the volume of the sound, and as the height of the slot is simultaneously controlled by detected currents varying with the volume of the sound it will be apparent that the slot is always just large enough to allow the vibration. In other words, the light passing through the valve is virtually completely modulated for all volumes of sound.

It is well known that the definition of a photographic sound record is improved when the recording slot is made very small, and, due to the infinite height of the recording and reproducing slots, that there is a loss of definition and power in the reproduction of the higher frequencies of sound. It has been found experimentally that speech and music sounds of low frequency contain greater power or volume than those of high frequency and when recorded the sounds of low frequency will open the ribbons of the light valve to normal spacing producing a record having as good definition as the records produced by previously known methods. The high frequency sound being of low volume, the ribbons of the light valve will be nearly closed, and the definition of the record of the high frequency sound will be better than the records produced heretofore and will partially offset the loss due to the finite height of the recording slot. In the drawing Fig. 1 represents in schematic form an embodiment of the invention;

Fig. 2 represents diagrammatically a film sound recording system.

In Fig. 1 sound waves are detected by a known type of microphone 10 and cause the current from the battery 11 to be modulated in accordance with sound. The sound modulated currents in the primary winding of transformer 12 induce similar currents in the secondary winding of the transformer. The output of transformer 12 is amplified in a known type of amplifier 13. Part of the output of the amplifier 13 is applied through a transformer 14 to the movable elements 15 of a light valve which for convenience is shown as a modification of the light valve disclosed in U. S. Patent 1,638,555 August 9, 1927 to E. C. Wente. The modulated speech currents cause the movable elements of the light valve to vibrate in accordance with the current.

In Fig. 2 the image of the filament of a. suitable lamp 16 is focused by the lens 1'7 on the small slot formed by the movable elements 15 in the orifice 18 pierced in the pole pieces of a permanent magnet 19. Suitable pole faces concentrate the magnetic field of the magnet 19 to form a uniform magnetic field in which the movable elements 15 are immersed. An electric current flowing in the movable elements 15 will cause the elements to be displaced by the reaction of the magnetic fields due to the current and the magnet. The image of the slot is focused by the lenses 20 and 21 on a photographic film 22 moving at uniform speed past the point of exposure. An aperture plate 23 holds the film smooth at the point of exposure and prevents diffused light from fogging the record.

In Fig. 1 a portion of the sound modulated currents from the amplifier 13 is applied through a tapped transformer 24 to the grid of a vacuum tube 25. The bias applied to the grid of the vacuum tube 25 may be varied by means of the potentiometer 26. The sound modulated input to the grid'of the vacuum tube 25 may be varied by means of the taps brought out from the secondary of the transformer 24. The plate current of the vacuum tube may thus be adjusted to any convenient value and the sound modulated input adjusted so that there is obtained the highest coefiicient of detection. Plate current is supplied to the vacuum tube 25 from a battery 27 through the coupling resistances 28 and 29 which may conveniently be in the form of a tapped resistance. The combined resistances 28 and 29 are of suitable value to assist in producing complete detector action in the vacuum tube 25.

The detected currents in the output of the vacuum tube 25 produce a pulsating voltage across the resistance 29 which is applied to the gridfilament circuit of a vacuum tube 30. A steady 115 bias is applied to the grid of the vacuum tube 30 by a battery 32 and the value of this bias may be adjusted by the potentiometer 31 shunted across the battery 32. Plate current is supplied to the vacuum tube 30 from .a battery 33 and 120 flows from plate to filament in the tube, along wire 34, wire 35 and movable elements 15 of the light valve wires 36 and 3'? to the battery. A condenser 38 of large capacity prevents this steady current from flowing in the winding of 25 the transformer 14, yet permits the sound modulated currents to be applied to the movable elements 15 of the light valve. By adjusting the bias on the grid of the vacuum tube 30 by means of the potentiometer 31 the plate current flowing 13 through the movable elements 15 may be adjusted so that when no sound modulated currents are applied the slot in the light valve is small. This adjustment is made easier by adjusting the variable resistance 39 which may be in series with 135 the light valve. The filament heating supply for the vacuum tubes 25 and 30 is conventionally represented by the batteries 40 and 41.

The operation of the circuit shown in Fig. 1 is as follows: When no sound modulated current is 140 flowing the plate current of the vacuum tube 30 will cause the movable elements 15 of the light valve to practically close the slot. The time of exposure of the film is very short and a very light negative film will be produced. When 1455 sound modulated current is applied to the vacuum tube 25 the grid bias is reduced and the plate current increases. The increased plate current flowing in the coupling resistance 29 increases the negative bias applied to the grid of 15 the vacuum tube 30 and causes the plate current to decrease and permit the movable elements 15 of the light valve to open the slot. As the decrease in the plate current is controlled by detected sound modulated currents the slot will be enlarged in proportion to the amplitude of the sound modulated currents. By suitable adjustment of the input, the amount which the slot enlarges may be made equal to the amplitude of vibration of the movable elements so that the light reaching the film is always virtually completely modulated. Complete modulation of the lightallows the production of a negative having the maximum transmission but it will be obvious that the opening of the slot may be adjusted so that the amplitude of vibration is any desired fraction of theopening. The maximum opening will be reached when the plate current of the tube 30 is reduced to zero and will be limited by the construction and mechanical adjustment of the light valve.

A choke coil 42 in the output circuit of the detector 25, in combination with a condenser 43 between the plate and filament of the detector 25 forms a smoothing circuit which aids in cansing the voltage variations developed across the resistance 29 to follow the envelope of the 311T. plitude of the sound modulated currents and not the instantaneous variations. A second condenser 44 connected between the plate and filament of the vacuum tube 30 assists in the smoothing action and aids in causing the ribbons to open promptly when a sudden loud sound is to be recorded.

While the invention has been applied to a specific form of light valve it will be obvious to those skilled in the art that the invention is not limited to the specific type of light valve disclosed.

What is claimed is: l. The method of recording sound which comprises moving a photographic medium at uniform speed through a beam of light of constant invirtually complete modulation of the light passing through said orifice for all volumes of sound recorded.

3. In a system for recording sound on a moving photographic medium, in combination, a source of light of constant intensity, a light valve having movable elements defining a slot controlling the transmission of light from said source to said medium, means for applying electric current varying in accordance with sound to vibrate the movable elements of said light valve, a source of unidirectional current, a circuit for applying unidirectional current from said source to said movable elements to cause said elements to tend to decrease the size of said slot in the direction of motion of the medium, and a controlling device actuated in accordance with the envelope of the amplitude variations of a sound to vary the current from said source.

4. In a system for recording sound on a moving photographic medium, in combination, a source of light of constant intensity, alight valve having movable elements mechanically adjusted to define a slot controlling the transmission of light from said source to said medium, means for applying a sound modulated electric current to vibrate the movable elements of said light valve, a source of unidirectional current, a circuit for applying unidirectional current from said source to said light valve to cause said elements to tend to decrease the size of said slot in the direction of motion of the medium and an impedance element in said circuit to vary the current from said source inversely in accordance with the envelope of the volume variations of a sound.

5. In a system for recording sound on a moving photographic medium, in combination, a source of light, a light valve having movable elements mechanically adjusted to define a slot of normal height in the direction of motion of the medium controlling the transmission of light from said source to said medium, means for applying sound modulated currents to vibrate the movable elements of said light valve, a source of electric current, a circuit for applying current from said source to said light valve to cause said movable elements to tend to reduce the normal height of said slot, an impedance element in said circuit, and controlling means energized by sound modulated currents to vary said impedance element and cause the height of said slot to increase in accordance with the amplitude of vibration of said movable elements.

6. The method of recording sound which comprises moving a photographic medium at uniform speed past an orifice illumined by light of constant intensity, causing the light passing through said orifice to be modulated to a constant percentage for all volumes of sound, and varying the time of exposure of said medium directly in proportion to the volume of sound recorded.

7. In a system for recording sound on a moving photographic medium, in combination, a source of light, a light valve having movable elements mechanically adjusted to define a slot of normal height in the direction of motion of the medium controlling the transmission of light from said source to said medium, means for supplying sound modulating currents to said light valve to vibrate

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