US2193665A - Sound recording system - Google Patents

Description

J. R. BALSLEY 2,193,665

SOUND RECORDING SYSTEM March 12, 1940.

Filed March 6, 1951 INVE/VT'OR J R. BALSLE) A T TORNE) Patented Mar. 12, 1940 R. Balsley', Beverly Hills Calif assignor,

by mesneassignments, to Westinghouse X-Ray Company, Inc/Long Island City, N. Y., a, corporation of Delaware I "ApplioationMarchB, 1931, Serial No. 520,730 111 Great Britain September 4,1930

' 5' Claim'sl (01, 179 -1003 This invention relates to the recording: of sound and. particularly to, the recording of, sound. on auphotographic film. 1 v The object of this invention is a, method of recording sound on aphotographic fllmrffom which a. positive sound, record may-be produced such that the "volume ofnoiseproduc'ed during the reproduction of thesound from the positive sound record is. materially less than thevolum of noise produced from known sound records.

A feature of this invention is arnethodgof re.- cording sound in which the average lightv energy incident on the negativefilm varies directlyjwith the envelope of the volume variations,- cf the 7 sound recorder.

Another feature of this invention is a method of recording sound by photographic means in which the average exposure of the negative sound: record varies directly with the envelope of the volume variations of the sound reco-rdsil.=;

A further feature of this invention is a method of recording sound which results ,in theproduction of. a positive photographic record of the sound in which the average transmissionoi the positive sound recordvaries directlywith the envelope of the volume variations of the, sound recorded. 1

In any known method of recording sound-therange of the volumes of sound which maybe satisfactorily recorded will be limited by'the properties of the recording mediumused and the handling which thecompleted record receives. The least voliune of sound which is of interestin re,- cording'is that volume of sound: which, in reproduction, maybe satisfactorily distinguishedahove the-noise produced by irregularities. in the, structure of the mediums used in recording and reproducing and the dirt, scratches, ,etc., which collect on. the record during its useful life. Becent experiments have shown that the volume; of

noise produced during the reproductionof a film sound record is approximately proportionalto the average transmission, of the positive, sound record. The average transmission-of the-sound record may be considered to be equal to the transmission of the unmoduiated portions of the sound record; The transmission of awphotographic 'medium is defined as the ratio of the light transmitted through the medium diyided by the light incident on the medium and for convenience is commonly expressedas a percentage of the incident light. r w Thus, if the average transmission of the positive-sound record is varied with the volume of thesound, whenqthe volumeyoi; sound issm-al-l,

Case; may beemployed. In known methods of transmission of the sound recordwill also be small and the noise produced during reproduc- -tion willheysmall; A reduction in the volume of noiseproduced from the record will permit smaller volumes of sound to be satisfactorily dis,- tinguished from the noise, thus increasing, the range of volumes of sound which may be, recorded; Or,- if the increased range is not utilized, ,the'invention will provide a larger ratio between the present minimum volume of sound and the volume; of noise produced during the reproduction oi; thesound and will thus produce a more pleasing resultthan known records.

- .As;a photographic positive is an inverse copy of the photographic negative from which the positive was} printed, in order for the positive to have a small transmission, the negative mus have a large; transmission or, as the opacity is by definition the reciprocal of transmission, a small opacity. It is well known, that the opacity of a photographic negative is directly proportional to the exposure when the exposure lies within certain limits, and that the exposureis proportional to the; product of the intensity for the light incident; on the negative multiplied by the time during which the negative is subjected to the. li ht, T us, assuming that the photographic .processes are controlled so that no distortion is produced, the transmission of the positive sound recordwill l e-proportional to the exposure of the negative sound record. Accordingly, to-produce a transmission, theexposure of the negative sound record mustploecorrespondingly small. In accordance withthe invention the intensity of the recording light, is varied in-accordance with the envelope of the volume variations of the sound recQP- edto produce a negative sound record having a large; average transmission or a small average'opacity ,ior small volumes of sound.

- preferred embodiment of this invention, ,a ecordin lampof the typfi disclosed in U. S.

nt1,624,3l,4 patented April 12 1927, to T. W.

recording sound with a lampof this type, a constant current flowing through the lamp is adjusted so that ;in the absence of modulation the light-emittedby the lamp is in the middleof the;cha racteri sticof the lamp; The gain of the vamplifiersupplying sound modulatedcurrents to -waves. are detected as bya. microphone and the the lamp is then adjusted so that the largest volresulting electric currents amplified in the known way and applied to the lamp through a suitable transformer. A portion of the output of the amplifier is supplied through another transformer to the input of a controlling device, such as a thermionic detector. The output circuit of the detector is connected in series with the recording lamp so that the unidirectional current flowing through the lamp also flows through the output circuit of the detector. The impedance of the output circuit of the detector is adjusted so that in the absence of modulation the constant current flowing through the lampv is just suflicient to keep the lamp alight. The sound modulated currents applied to the input of the detector will cause the impedance of the detector to decrease in accordance with the amplitude of the applied currents. This decrease in the impedance of the detector will permit the unidirectional current flowing through the lamp to increase and will thus cause the light energy emitted by the lamp to increase with the volume of the sound. An electrical network having a time constant larger than the period of the lowest frequency to be recorded is inserted in the input circuit of the detector and causes the variation in the impedance of the output circuit of the detector to follow the envelope of the amplitude variations of the sound modulated currents rather than the instantaneous variations in the amplitude.

The drawing is a diagrammatical representation of a typical embodiment of the invention.

The sound waves to be recorded are detected by a microphone l and amplified in an amplifier 2 which may consist of several stages of amplification in cascade. The output of the amplifier 2 is supplied through a transformer 3 to the input circuit of a thermionic amplifier 4. The output of the amplifier 4 is supplied to the primary windings of the transformers 5 and 6. The transformers 5 and 6 may be combined into one three-winding transformer if desired. Batteries 1 and 8 supply a constant voltage to the control electrode and anode of the amplifier 4 in the usual manner. Any suitable means may be used to heat the cathode of the amplifier 4.

The output of transformer 5 is supplied to a resistance 9 bridged across the secondary winding of the transformer 5. A movable contact I0 taps off a controllable portion of the voltage developed across the resistance 9 and applies this portion to the control electrode I l of a thermionic triode l2. An electrical network consisting of a high resistance 13 shunted by a condenser I4 is inserted in the connection between the contact In and the electrode II. A potentiometer I5 shunted across a battery l6 supplies an adjustable negative bias to the control electrode II. The cathode ll of the thermionic triode I 2 is heated by a battery l8. While a thermionic triode has been illustrated, it will be obvious that many other types of vacuum discharge devices may be used and that the invention is not limited to the particular device described.

The anode IQ of the triode I2 is connected through a resistance 20 to one electrode of a recording lamp 2!. The other electrode of the recording lamp 2| is connected through the secondary winding of the transformer 6 to a battery 22 in series with battery 8. A unidirectional current will fiow from ground on the negative terminal of battery 8 through battery 22, second.- ary winding of transformer 6, lamp 2|, resistance 20, anode [9 of triode l2, through triode l2, to cathode I! and thence to ground. The current flowing in this circuit will depend on the characteristics of the lamp 2|, and may be adjusted to any desired value by adjusting the voltage of the battery 22 and the negative bias on the control electrode H.

The light from the recording lamp 2| is directed by a suitable optical system, diagrammatically represented by the lens 23 and slitted plate 24, on a photographic film 25 moved at constant speed by the sprockets 26, 26 in the manner well known in the art.

In an article entitled "Photo-chemical Investigations and a New Method of Determination of the Sensitiveness of Photographic Plates, published in the Journal of the Society of Chemical Industry on May 31, 1890, by Messrs. F. I-Iurter and V. C. Driifield, the characteristic of a photographic emulsion is shown to consist of four parts, a region of under-exposure, a region of correct exposure, a region of over-exposure and a region of reversal. This characteristic of a photographic emulsion has been studied by many investigators since the publication of this article and the four parts of the characteristic are now well known in the art. In the present invention, in the absence of sound, the bias on the control electrode H of the triode I2, and the voltage of the battery 22 are adjusted so that the light emitted by the lamp 2| is at a minimum which preferably should produce an impression at the lower end of the region of correct exposure. This adjustment represents an ideal setting which can only be approximately attained with many of the recording lamps at present in common use. lamps, however, have shown that by proper design, the lamp may be made to produce more light than prior types and that this adjustment may easily be attained with the latest types of lamps.

It is well known that when light is produced by a discharge of electricity through a gas at low pressure, the voltage at which light is produced, that is, the striking or ignition voltage,

is higher than the voltage at which the light disappears, that is, the extinction voltage. In the present invention, the voltage of batteries 8 and 22 in series is preferably adjusted to make the voltage across the lamp 2! slightly greater than the ignition voltage of the lamp so that if a suddenloud sound be detected and the resulting electric current has sufiicient amplitude to put out the lamp, the applied voltage will cause the lamp to again emit light as soon as the applied current has decreased. If the electrodes of the lamp are small and compact, the variation in the light is usually a variation in intensity. However, if the electrodes are fairly large' and of relatively great length the variation in the light will be more nearly a variation in quantity. Also by the addition of suitable gases, such as the vapor of mercury, the energy emitted by the lamp extends beyond the frequencies included in visible light. In this case, the variation in the energy emitted may be measured in terms of the actinic power, that is the power which causes an impression in the photographic emulsion.

Owing to the electrical network formed by the condenser l4 and resistance l3 in the input circuit of the triode I 2, when sound modulated waves are supplied by the transformer 5 to the triode l2, the output impedance of the triode will tend to vary with the envelope of the volume variations of the sound and not with the instan- This action is similar to the taneous variations.

' action. As the current flowing through the lamp 2! must also flow through the triode l2, the change in the impedance of the triode l2 will cause the energy emitted by the lamp to increase and decrease inaccordance with the envelope of the amplitude variations of the waves.

The sound modulated currents supplied to the primary winding of the transformer 6 will cause an induced currentto be developed in the secondary winding of, transformer 65, varying with the instantaneous variations in the sound modulated currents. This induced current is superimposed on the current supplied to the lamp ii and causes the energy emitted by the lamp 2: to vary in accordance with the instantaneous variations in thesound simultaneously with the variation in accordance with the envelope of the volume variations in the sound.

. What is claimed is:

1. In combination, a source of sound modulated waves, a photographic medium moved at constant speed, a source of unidirectional electric current, a lamp, an optical system directing the light from said lamp on said medium, a source of current varied in accordance with the instantaneous variations in said waves, a detector, a series circuit connecting said source of unidirectional current, said lamp, said source of varied current and the output circuit of said detector, and means energized by said waves in the input circuit of. said detector varying the impedance of the output circuit of said detector in accordance with the envelope of the amplitude variations in said waves. i 7

2. In combination, a source of sound modulated waves, a photographic medium moved at constant speed, a lamp, an optical system directing the light from said lamp on said medium, a source of unidirectional electric current, a source of current varied in accordance with the instantaneous variations in, said waves, a detector, a series circuit connecting said source of unidirectional current, said lamp, said source of varied currents and the output circuit of said detector and an electrical network in the input circuit of said detector energized by said waves and having a time constant greater than the period of the lowest frequency of, said waves to be recorded on said medium. v

3. In combination, a source of sound modulated waves, a photographic medium moved at constant speed, a lamp, an optical systeni'directing the light from said lamp on said medium, a source of unidirectional electric current, a source oicui rent varied in accordance with the instantaneous variations in said waves, a detector, a series circult connecting said source of unidirectional current, said lamp, said source of varied current and the output circuit of said detector, means in the input circuit of said detector for adjusting the impedance of the output circuit of said detector so that in the absence of sound modulated waves the lamp is just alight, and other means in the input circuit of. said detector to cause the output impedance of said detector to vary inversely with the envelope of the amplitude variations in said waves.

' 4. In combination, a photographic medium moved at constant speed, a lamp, an optical system directing the light from said lamp on said medium, a source of unidirectional current, a source of current varied in accordance with the instantaneous variations of modulated waves, a detector, a circuit connecting said source of unidirectional current, said lamp, said source of varied current and the output circuit of said detector in serial relation, a biasing voltage in the input circuit of said detector for regulating the impedance of the output circuit of said detector whereby in the absence of, modulated waves said lamp produces a desired minimum actinic eiiect,

network in the input circuit of said detector, comprising a capacitor in parallel relation with a resistor, and a capacitor in parallel relation to the output circuit of said detector cooperating with said input network whereby the output impedance of said detector is varied inversely with the amplitude variations in said waves.

5. A sound-on-film recording apparatus including in combination a film advancing means, means for directing a light beam to the film, a, sound sensitive device, an amplifier, the input of which is connected to the sound sensitive device, a rectifying network connected to the output of said amplifier, a recording lamp, a source of electrical energy connected also to the output of the amplifier and to the output of the network,

and supplementing current source means maintained charged by means of the network whereby the output energy of the amplifier is at all times stored with current of unchanging polarity but in varying amounts at varying potentials.

JAMES R. BALSLEY.

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