US1999951A

US1999951A - Apparatus for recording sound on film

Info

Publication number: US1999951A
Application number: US605993A
Authority: US
Inventors: Stewart C Whitman
Original assignee: Individual
Current assignee: Individual
Priority date: 1932-04-18
Filing date: 1932-04-18
Publication date: 1935-04-30
Anticipated expiration: 1952-04-30

Description

April 30, 1935. s. c. WHITMAN I APPARATUS FOR RECORDING SOUND ON FILM Filed April 18, 1,952

NVENTOR. Z.

CSM/waar e. 917/217 W V||IP Patented Apr. 30, 1935 UNITI-'Efo STATES APPARATUS .Foa RECORDING SOUND WON FILM

stewart. c .fwhitmam Jackson" Heights, N. Y. Application April 18, 1932,' Serial No. 605,993 .7"c1aims. (cuve-100.3)

The invention relatesto Vnew-"and useful improvements' in the art of recordingY sound 'on fiilm and :more Y:particularly to an apparatusiand method 1' for producing i the desired result.

VAn object of Athe invcntion1is toproducean 'apparatus for recording sound 4on nlm VVover :the full range of audiofrequencies.

Afurther'object ofthe invention'is'to produce 'an apparatus to Yrecord sound on Aiilm in s'uch .manner that onlythe desired soundsrresultfupon reproduction.

Another object of the invention is ilto fprovide fan apparatus;` the component 1 partsof which kcan 'be 5 fully supervised and .controlled at `all ftimes 'during operation. f l

Still? another :obj ect7 'of the' invention.; isito-v pro- -videan apparatus Whichmakespossible the-:sav-

ing' ofiilmv in'soundrrecording.

'A lstill "further .fobjectzof :the 'invention i is, `to :produce an: apparatus which :cani beioperated` directly 'fromianalternating' :current 'p ower s`upply.

'With these: and` numerous?otherobjects in view, my invention consists inthe-novel features of construction; combination,v and'4F arrangement of: parts fas will be hereinafterreferredztoz and more g particularly pointed-1out -in .the :specification and 'In' the drawinfgfiorming a Apart voi" this. application: 1 v

The ligurev isca schematic diagramshowing the :arrangement and coupling .among the various component *elements of l my improved apparatus utilized in my system of 1 recordingsound "onf fil-m. VIn-fdescribingtheiinvention, Ishall refer to lthe drawing in -.-which' the ynumeral I designates -a Imicrophone .2 is l:an audio -famplier --for f the microphone signals ;V :3 is `anf instrument .forregulating the amount of amplificationto be obtained by: means vof l 2. .The i numeralwl designates ade- `vice -vwhich supplies, a .continuous -visual indication of the power level measured in decibels; The numeral 5 designatesqa small( .output 1amplierfeeding the loudspeaker-6. f'I'he -signaloperating-5 isvrtapped from 2 and isfreproduced-in .6 Lthereby affording .the f operator a= continuous audible checkonfsound quality and volume. Th volume r'level of monitor 5v sVa-riable.

.-The .numeral 'I designates faccupling .stage through Vwhich the amplified :signal :from s2 .is conducted totheimodulator Vstage .8.k A:'Illenulmeral 9 designates Va1'ractin-frequency vacuum 'tube cscillatonthe output of kwhich is introduced into f the r modulator 8A and is therein .modulated .byftheaudio signal introduced from'i.. Y The numeral :IIS designatesfa radio frequency amplifier. The modulated radiofrequency signal'from 8 is amplified in Ill andfed to the recording 'lamp 'I I. The ynumeral, I2 designates 'a potentiometerfbymeans.of -whichthe output of' I D'maybe tapped to 'operateithe 'earphones I3 thereby affordingthey operator means of checky ing the signal, whenfandjifdesired, arriving at the recording lamp II. "The numeral I4 :designates al device whereby the grid bias ofthe modulator tubei'n may be automatically determined.

and controlled by the strength/of' the audio signal in 2, :as vhereinafter and more particularly set `'forth.-l .-1 V The numeral I 5 designates a rectier'and lter system' forc supplying the 1 necessary potentialsvto r'the-various component parts, thepower being de'- rived from.' f the alternating current source I6. 'The connecting lines indicate coupling -means amongthe-v'arious component parts.

' I shall nowmore particularly'l describe the operation `cfjmy invention. There are two general methods used `at present in :recording soundv on lnlm: 'i the light-valve method and -thefglow l lamp method. In" the' light-valve method, f light '"frcm aconstant-source falls-upon the lm after ybeing modulated bypassing through a mechanical 'light shutter. I'I'his vmechanical shutter isoperated 'electrically vby'fthe audio. signal.` In the :glow lamp method,1a constant y'direct current actu- 'ates -a Ygaseous-conducticn lamp :and Vthe steady light emittedthereby'is modulated by superimposing` the Yaudio signal voltage thereupon.l

'Inherent faults are recognized in Vthese 'systems. The 'frequency range ofthe light-valve system is limited Vdue tothe: inertia ofthe moving mechanism. In the case of thev glow lamp' meth- .o'd'the'steady-D C. exciting current Vproduces vexposure. on ythe .lm with nc signal modulation.

On `the positive .print the unmodulated track transmitslightand ,groundnoises result upon reproduction. The `irregularities in the battery circuit actuating-the lamp. areyreproduced on the :Iilmxrecord and further-aggravate the-ground 'Ihe extinction voltageof a glow discharge lamp is considerably lowerthan-'the ignitionfvoltage. Thus if they audio-modulating' voltage 'rises suiirciently thigh the Vglow lamp 'is extinguishedv and `the modulating voltage must be lowered considerably before .the lamp -re1ights.1 .overloading of this kind occurs frequently in recording and this o llagvin the glowlamp constitutes ay serious dra back. Y Y v It has been proposed toutilize radio-,frequency `Aalternatingcurrent'to activate the glcwpilamp. 55

ka separatev oscillator.

'and output impedances. In ,'tortlon is introduced. 1 l

be impressed across the lamp which would be less.

than Ydirect current of corresponding voltage.

Some improvement in ground noise is thus ob'-" tained.V A

I propose to describe here a method ofglow Vlamp recording and an apparatus which eliminates all the undesirable features. The sound to be recorded is transmitted through a microphone and amplified. The resulting signal is used-to modulate a radioffrequency carrier generated in I have found a frequency of `about 200 K. C. to perform best. Itis im- 'portant that this modulation be accomplished through the medium of a modulator tube. Another important feature in this modulation process is vthe utilization -of Va coupling tube stage between the audio amplifier and the modulator stage for the purpose of Vproperly-matching the input this manner, no dis- By using a separate modulatortube stage an essential improvement can be introduced. n' ,The grid bias on the modulator Atube is determined so that after subsequent amplification, which will be described later, the resultant, unmodulated radio-frequency voltage impressed on the recordy ing lamp is just sufficient to ignitej theflamp. A

imaintained at a fixed value. of the automatic bias changing device just de- 'extinction voltage of the recording lamp. other words,'for the purpose of this discussion,

small portion of the Yaudio modulatingA signal is tapped from theV output of thev audio amplifier.

This signalis amplified and rectified and impressed upon the modulator tube grid bias supply in such fashion that this variable impressed voltage acts in an opposite direction to the grid bias voltage and tends at all times to reduce; the grid bias voltage. This reduction in the `grid bias voltage results in higher radio frequency voltages Vbeing generated in the plateK circuit of the modulator tube. With the gridbias chosen originally, the recording lamp is justbrought to the ignition point. It can be VseenY that even the smallest audio-frequency signal voltageentering the modulator'stage would extinguish the recording lamp if the grid bias applied to the modulator tube were However, by means scribed any 'audio signal must increase the radio frequency carrier. voltage in the modulator tube.

By adjusting the amplification of the tapped audio 'signal properly, the bias is varied so that the carrier voltag'eis just suflicient for complete modulation at all times. By complete modulation Vis meant that degree of modulation which varies the radio frequency carrier voltage almost to the In the extinction voltage of the lamp is considered the zero limit for modulation.A This statement'takes into account the fact that the modulated carrier is later amplified before being impressed upon rthe recording lamp. Y

To summarize, then, we have the followingconditions during operation. The primary advantages of using ra'dio'frequency excitation ofthe recording lamp are obtained.`In addition, the

unmodulated excitation current results in light of minute value, thus producing practically no sound track without modulation. With modulation of the radio frequency current taking place, practically all the light emitted by the recording lamp is a result of the audio signal. On the positive print therefore the unmodulated track is opaque and the modulated track transmits in direct proportion to the recorded sounds. In the reproducing system therefore no light can activate the photocell except that due entirely to the desired recorded sound. Reproduction results which is entirely free from extraneous noises.

From actual experience in the operation of my apparatus, I find the following ratings preferable.

The audio amplifier has an overall gain of approximately decibels. The oscillator used for generating the radio frequency excitation current has a power rating of about '7.5 watts. A 50 watt modulator i! used and feeds into a 50 watt amplifier stage. I haverfound that this high power is necessary in order to properly match all Vthe various components without distortion and still deliver sufficient power to the recording lamp. The amplifier stage following the modulator is necessary in transforming the modulated radio frequency output of the modulator to the proper voltage for transmission to the recording lamp. This Aamplifierrstage also serves as a `coupling stage between-the modulator and the recording lamp wherein the proper impedance match can be'accurately accomplished. Y V- By .arrangingthe apparatus with the audio amplifier, oscillator, *modulaton modulator bias control, and radio frequency amplifier, as separate units, complete control and accurate adjustment of every operation is obtained. The audio amplificationcan be monitored and varied at. will. `The frequency of the oscillator can be raised and lowered. Thecoupling'to the modulator can-*be adjusted? to: produce'any radio frequency' voltage from'zero to `maximum at the recording lamp.

Vabout 1-/50 second. Thus, at no time can any main signal frequency greater than 50 cycles arrive at the modulator grid before the proper adjustment of grid bias has been made. As a. result of this, a wide range of signal intensities can be handledvby the recording lamp with no possibility 'ofextinction due'to'overload and with noy possibility of unnecessary exposure due to unmodulated light. Y Y Y I have successfully recorded frequencies as high 'as fourteen thousand cycles-the practical limit of the amplifying equipment-with the apparatus o disclosed'. These records show a clear cut delineation between adjacent cycles'under the magnifying glass. Using the high power available in the present invention' in connection with aA specially constructed radio-frequency recording lamp, which is described in my 'co-pending application, Serial No. 605,990, filed of an even date herewith, using a rspecially constructed optical system which is described in my copending application, Serial No. 605,992, led of' an even'date herewith, and utilizing the maximum photo- -graphic contrast obtained by my automatic modu- .lator grid bias control, excellent sound records can i be made` -at f- :film` speedsv of 60. feet kper i minute. `Since'this film'speed is. satisfactory for pictorial photography, atremendous savingy in lmstock'is .-zmadepossible. The present sound recording film speed-isusually 90 feet per minute,v

In the presentinvention, allV power. used by: the tubesmaybe supplied from a. rectified and-filtered alternating current line source. ll'Iliis. is a great eaid.. in preservinguniformity Y*since voltages-and currents may' be checked .andfeasily adjusted-at vall times'xwithpno danger of power '.diminutionror failure-such asisxfrequently .encountered with fbatterypowerg.

iWhile I have particularly 'described V:the elements best.- adapted` to perform the; functions: set Sfor.th'it' is. obvious that various changes in form, proportion and in the. minor` detailsV of. construc- Ation maybezresorted'to,.without departing from '.the; spirit vorfsacriicing any of fthe .principles of the invention. f f

`-IIavingfthusdescrbed thel invention, what is claimed is:

1. In an apparatus for recording sound on film, means for providing a high frequency current, a sound pick-up device, an* amplifier therefor, a Vseparate modulated tube stage wherein the high frequency current is modulated by the output of the sound amplifier, a source of fixed grid bias potential for said modulator tube stage, means for tapping a portion of the sound amplifier output current, means for rectifying and amplifying the said tapped current, means controlling said amplification to obtain a balanced relationship of substantial equality between the last mentioned current and the rectied output current of the sound amplier, and means for impressing the balanced tapped current subtractively upon the fixed grid bias potential of the modulated tube.

2. In a sound recording system using a glow lamp wherein substantially no light is emitted from said glow lamp when no sounds are delivered to the input of the system, in combination, a microphone, a speech input amplifier, a source of radio-frequency current, a separate modulator tube, the output of said source of radio-frequency current being delivered to said modulator tube, the output of said speech input amplifier being delivered to said modulator tube for modulating Said radio-frequency current, a recording glow lamp, means for delivering the output of said modulator tube to said glow lamp, means for applying a Xed grid bias potential to the grid of said modulator tube, and rectifying and amplifying means connected between said speech input amplifier and said modulator tube for applying to the grid of said modulator tube a varying unidirectional potential varying in magnitude with the output of said speech input amplifier and in opposition to said xed bias potential, said means for applying a xed grid bias potential to the grid of said modulator tube being primarily adjusted so that when no sound energy is applied,to said microphone and speech input amplier the energy delivered to said glow lamp from said modulator tube is just sufhcient to ignite said glow lamp.

3. In a sound recording system using a glow lamp wherein substantiallyno light vis emitted from said glow lamp when no sounds are delivered to the input of the system, in combination, a microphone, a speech input amplifier, a source of radio-frequency current, a separate modulator tube, an adjustable coupling tube unit connected grid of said: modulator tubel being. primarilyv adjustedfso that-when no soundt 'energy is applied .to:.-sai'drnicrophone andspeech input amplifier, the lenergy delivered to said yglow lamp.' from said ;modulato'rftube is -just suiiioient to vignite said glow lamp. w

4. -In-aa.sound;recording.system using va glow lamp 1 wherein .substantiallym light s .is emitted' from saidvglo'w Vlampiswhen no sounds 4are deliverediA to; the. inputI ofthe system,; injco-mbination, a microphone, a speech input amplieniafsource of. radio-frequency. current, a separate modulator tube, the output of said source of radio-frequency current being delivered to said modulator tube, the output of said speech input amplifier being delivered to said modulator tube for modulating said radio-frequency current, a recording glow lamp, means for delivering the output cf said modulator tube to said glow lamp, means for applying a fixed grid bias potential to the grid of said modulator tube, and rectifying and amplifying means connected between said speech input amplifier and said modulator tube for applying to the Ygrid* of said modulator tube a varying unidirectional potential varying in magnitude with the output of said speech input amplifier v and in opposition to said xed bias potential, said means fory applying a fixed gridbias potential to the grid of said modulator tube being primarily adjusted so that when no sound energy is applied to said microphone and speech input amplifier, the energy delivered to said glow lamp from' said modulator tube is just sufficient to ignite said glow lamp, the level of amplification of said rectifying and amplifying means being so adjusted that the potential delivered to said glow lamp v at no instant goes below the extinction potential.

5. In a sound recording system using a glow lamp wherein substantially nol light is emitted from said glow lamp when no sounds are delivered to the input of the system, in combination, a microphone, a speech input amplifier, a source of radio-frequency current, a separate modulator tube, the output of said source of radio-frequency current being delivered to said modulator tube,

a recording glow lamp, means for delivering the'r output of said modulator tube to said glow lamp, means for applying a fixed grid biaspotential to the grid of said modulator tube, rectifying and amplifying means connected between said speech' input amplifier and said modulator tube for applying tothe grid of said modulator tube a varying unidirectional potential varying in magnitude with the output of said speech input amplifier and in opposition to-said fixed bias potential, and electrical impulse delay means connected between the output of said input speech amplier and the input of said modulator tube forV imparting a slight time delay to the output directly delivered to said modulator tube from said input speech amplifier with reference to the poten- Y means for applying a fixed grid bias potential to the grid of said modulator tube being primarily adjusted so that when no sound energy is applied to said microphone and speech input amplier the energy delivered to said glow lamp from said vmodulator tube is just sufficient to .ignite said glow lamp. Y

V6. InV the recording of'sound on lm employing a recording glow lamp and a separate modulator tube, the methodofvrecording sound with minimum background noise which consistsl in applying to the grid of said modulator tube a xed Agrid bias potential, the level of amplification of "said'portion of the speech input electrical energy being such .that the potential delivered to saidY glow lamp at no instant goes below the extinction potential. y

Vby saidv rectifying and amplifying means, said V!7.'In;the recording'of sound Aon illm employ- 'ing a recording glow lamp and a separate modulator tube, the method of recording sound with minimum background noise which consists in applying tothe grid of said modulator tube a fixed grid bias potential of such magnitude that said glow lampV just ignites when no speechginput electrical energy is deliveredto said modulator tube, amplifying'rectifying and applying to the grid ofsaid modulator' tube a portionrofwthe speech input electrical energy in opposition` to lsaid xed grid bias potential, the levelv of amplication of `said portion of the speech input electrical energy being such that the potential delivered to said glow lamp at no instant goes f below the extinction potential, and imparting a `slight 'time delay to the speech input 'electrical Venergy .directly delivered to the input of said modulator tube *with reference tothe amplied and rectied portion of the speechinput electrical energy applied to the grid of said modulatork tube.-

- 1 fSTEWART C. 'WI-HTMAN.