US2411440A - Light source feed-back system - Google Patents

Description

W. R. LE PAGE LIGHT SOURCE FEEDBACK SYSTEM Filed April 50, 1945 Nov. 19, 1946.

Patented Nov. 19, 1946 LIGHT- SOURCE FEED -BACK- SYSTEM Wilbur labile ,lPage, Pikesville, :Ma., assignonto, fjtadio Corporation of America, ya corporation of Delaware iApplicationiApril'i-bfl, 1943, Serial No. 485,255

"5 ,Claims.

This invention relates to photographic sound .recording and reproducing systems, and, partic u- .larly to ,a system ,-for stabilizing the 1 light, sources .used. in suchsystems or anysystem iniwhich the light output should varydinearly with anapplied .signal, :voltage ,or current which :may consist of oscillations ,of :audio frequencies and unidirectional-components. i

It ,is W811 known :that. in, photographic "sound erecording.systemsdight sources orlampsare-em- .ployed ,toproduce lightibeamswhichmay. be either directly,mod.ulated ,by varying the f energy supplied ,togthelampsiinaaccordance with ,the variationslof soundwavestobe recorded, or whichtmay be modulated by vibrating the beaminaccordance with thesoundwaves. Light. sources cfconstant .intensity are .usedi'for ,recording both variable ,area-and variable density sound tracks, the light being modulated by the vibrations :of agalvan- ;ometer mirror. "Light sourceaisuch as glowlamps, which vary in intensity are also employed for recording variable density records. For reproducing either type of record, *a light source of constant intensity is used to produce the scanning'beam. I

When alight of constantintensity is employed 'for'recording; it is necessary that it be maintained "accurately constantly to prevent undesiredvariati'ons inthe sound levels or modulations and the introduction of distortioninthe recorded or re- 'produced sound. "Unwanted light variations may gizing source, by deterioration 'of the light-producing elements, bythe blackening ofthe lamp envelope 'and other reasons. When the recordingis done'by causing a variation of the light source, "this variation must be linear with the applied signaL-voltage or current, and the linearity must be maintained for frequencies up to the highest frequency of sound to be recorded-While the'average light remains constant or at a controllable value. Thatis, the light'must follow an averagevaluewhich-is determined by the noise reduction'voltage and'also thesoundsignal frequencies andthus linearly respond to voltage or current of-anyffrequencyin theband tob'e-recorded and adirect-currentor voltage. 'Theinventi-on further provides-a feedback system which --requiresveryJlittlelight toproducethe necessary stabilization, modulation,:and control.

"The present inventiongtherefore, is directed to a-systemfor stabilizingand providing linearity of light variation 1 with "an applied alternating and ;direct .;current signal. This is accomplishedjby .detecting a portionxofftheflight;andrregulatingihe (Cl. 250-415) g 2 energy Vsuppliedit'o Ethe :lamp in accordancewvith "the variations inzthe ,deteotedgllight for-stabiliza- .ztiongand in accordancewithsan applied signal for modulationcand ground noise reduction. ziThe 5 general principle ,of suchan opticalieedback stabilizing systemkis knownrin the::art. Incertain .of "these IDIiOI' systems; which provide :both direct current, andalternating-current feedback, the impedance of anqelectronicsrdevice connected in series with the :lamp :and ithe "primary? power :source'is varied inaccordance .with the:light fluc- :tuations. i-The amountx'ofvfeedback in suchra system "depends on theigain .Lof tth'e amplifiergthe eiiiciency of translationof .the :light 'JSEYISltlVSCdG- vice and the amount of ilightgcollected; for acontrol purposes. For a fixed amount-otzamplification and I a fixedramount :of; lightcreceived: by; the 'photocell, :thefeedback can :only ;bex-.increased :by increasin'g the :rcell xload resistance, :or for i a 20 1 fixed-number of #stages ,of; amplificatiomthe 'feedbackmaybe increased by; increasingthe,interstage coupling =resistors. ;However, these methods of increasing ifeedback have the disadvantage :;of causing :phase shift ,and ilossiof gain :at ;,high ifrequencies. i l

I'he zprincipalgobiectaoflthe invention; therefore, ,is'tor-improve light ;source stabilizing, modulating, and controlling systems.

:Another; obj ect.ofxgthezinvention is: torprovide an optical feedback;stabilizing;modulating; and con- ;trolling "system which rhas Iminimum distortion and axhighisensitivityrto the controlling light, making it: unnecessary to appreciably reduce the useful,light,:tmObtaimSuflicient control light.

:A'furt-herobject ofttheiinvention is to-provide :responding tocrapi'dw changes innlight ioutput, was well .asgto slow ,vari-ationsztherein. 1

:Althoughtthe; novelfeatures which are believed tot-be characteristic ,Of'ilhisf invention are pointed ;out with {particularity-1m 1511116? claims "appended rhereta-gfthe manner :jof 1 its rorganization :and the imodeloif iits ,,operation'vvill'cbebetter understood zbyzreferring" to :the following :description, :read in conjunction withrthe iaccompanyinggdrawing, inwhich: r i r {Fig :1 iisra ,rbiock :diagra-m showing :the ":general 1 arrangementofg thezsystenrembodyine the-inven- :tion., V

.Eig. ..-2 is ca schema-tic 5 dia amof thefieedback amolifienof thei-invention-nand 1 Fig.3 is. a schemat-imdetail ,;diagram; of armodimfication of:.the,glastqstagezofuthe system shownmof YEigJQZ. c 55 :Reierringmow to EEiaLl; blockimisranamplifler :an optical feedback system "which is capable ;of

4 sistors 26, 21, and 28. The effective plate loading resistance of tubes l5 and I6 is produced by resistor 26 and resistor 30 in parallel for tube l5 and by resistors 21 and 34 in parallel for tube IS. The constant voltage drops across gas-filled voltage regulator tubes 32 and 35 produce the proper biaseson the tubes l5 and 16, respectively,

. and effectively provide a'direct coupling between conductors l0 upon the input of amplifier A in series with an external voltage which may be considered as co, the total voltage thus appearing across the input to amplifier A being e1. The phase of the voltage ez is such'as'todecrease the energizing lamp current with an increase in light output. 7

The entire system just described can be considered as a feedback amplifier with 100 per cent feedback. Without feedback, the gain G of the I amplifier may be considered as 62/61. It is evident that ez is considered the output voltage, in order to deal with the circuit as an amplifier. Actually, the light is the useful output, but since thelight is proportional to e; the discussion is valid. Thus, in the usual notation in which the feedbackfact'or 5 equals (1), the'well known formula G/(1.GB)=G/(I+G) .for the'gain of a' feedback amplifier applies in this case. If G is large,the gain'is nearly unity, and this is the condition desired. In this analysis the numerical value of the overall gain is based on the assumption that 13:1. tion of operation is thatthe value of G be sufli- Now, the optimum condiciently large so thatthe net gain is very nearly 7 equal to unity.

' of'feedback is limited by the size of the photocell and amplifier load'resistors and any increase therein increases the effects of stray capacity with the associated difiiculties. Of particular concern in this application is the resulting phase shift at the higher frequencies, which may contribute to a tendency to'osoillate. This phase shift disadvantage may be more acute with the use of a glow lamp since the light modulation lags the current modulation by a considerable angle at high audio frequencies' The present invention overcomes the above difficulties by the useof morestages of amplification'in combination with low coupling resistances and a low photocell load resistance. Referring now to Fig.2, four stages l5, l6, l1, and I8, of amplification are shown,; the first three tubes l5, l6, and I1 being comparable to "amplifier A of Fig. 1, and tube l8 being comparable to power unit P of Fig. 1. The three tubes l5, l6, and I! may be amplifier tubes, such as the RCA type 6J7,"while tube IBmay be of the RCA type 6L6. The lamp 5 of Fig. 1 is shown connected in the plate circuit of tube I 8, energy forthe lamp and tube being supplied by a battery 20. The photoelectric cell 6 of Fig. 1 is shown connected back .to a low value load'resistor22 in the input of tube l5.

The external voltage represented by E6 in Fig. 1

24, one of which may be usedfor'introducing a maybe impressed across either resistors 23 or modulating signal voltage and the other for introducinga noise reduction voltage.' A potentiometer 42 is an intensity control for the lamp S'which may'also be obtained, if desired, by a =volta'ge applied across either resistor 23 or resistor 24. A common source of B potential is suppli ed'to the 'plat'sof tubes l5, l6, and l 1 over re- 4i regulates the bias potential on tube 18.

Since a sustaining current of about 5 mils is necessary in the glow tube, thecoupling resistors cannot be very large, and this is particularly desirable in order to provide the low coupling between the amplifying stages. The above -mentioned circuit operated satisfactorily to provide a fiat frequency response characteristic up to 10,000 cycles per'second with negligible distortion. To obtain 20 db. of feedback with resistor-"22 having the small value of 10,000 ohms, only one microampere of photocell current is necessary for each ten milliamperes of lamp current. More photocell current, (i. e. more detected light) will give more feedback. The system may thus supply a considerable amount of feedback with a very feeble photocell current, and therefore has a high sensitivity to detected light. The modulating signal and noise reduction voltages may also be introduced in the control grid circuit offtube [5 instead of across resistors. .23 and 24 as mentioned above.

In Fig. 3, an alternate method of connecting the- 6L6 into the circuit is shown, a method in which the screen grid of the tube is separately utilized. The same low coupling resistances betweentub es is obtained wherein the amount of feedback'fis ,more nearly constant as a function of frequency.

I claim as my invention: a i 1. A feedback amplifier system for stabilizing a light source comprising a photocell, apotential source for said photocell, an input stage of amplification having its input connected tosaid photocell, a light source to be controlled, an .output stage of amplification having its output connected to said light source, coupling means between said amplifying stages, said coupling means having a low value of resistance, and coupling means intermediate said photocell and said input stage of amplification, said last mentioned coupling. means including .a resistor .in; series With said photocell andlsaid potential source, said resistor having a resistance value of the .order of 10,000 ohms, said low value of resistance minimizing phase shift and loss of gain-at high frequencies. a

2. A' feedback-amplifier system for stabilizing a light source comprising a photocell, an input stage of amplification having its input circuit connected to said photocell, a coupling resistor in'said input circuit, a light source to be controlled, an output stage of amplification having its output connected to said light soure -coupling means between said amplifying stages-said coupling means having a low value of resistance, a polarizing source of voltage for said'photocell and the anodes'of the tubes in said amplifying stages, and a degenerative feedback series-circuit from said polarizing source of voltage sistor and back to said polarizing sourcefor coupling said photocell to said input stage of amplification.

3. A feedback amplifier system for stabilizing a light source comprising a photocell, an input stage of amplification having its input circuit connected to said photocell, a coupling resistor in said input circuit, a light source to be controlled, an output stage of amplification having its output connected to said light source, coupling means between said amplifying stages, said coupling means having a low value of resistance, a polarizing source of voltage for said photocell and the anodes of the tubes in said amplifying stages, and a series circuit from said polarizing source of voltage through said photocell through said coupling resistor and back to said polarizing source for coupling said photocell to said input stage of amplification, the value of said coupling resistor being of the order of 10,000 ohms.

4. A feedback amplifier system for stabilizing a light source comprising a light source to be stabilized, a photoelectric cell for detecting a portion or the light from said source, a plural stage amplifier including at least three stages intermediate said photocell and said light source, glow lamp coupling means between said amplifying stages, a high potential source for said photocell and the anodes of the tubes in said amplifier stages, said coupling means having a low value of resistance, and coupling means between said photocell and said first stage of amplification, said last-mentioned coupling means in cluding a resistor in series with said high potential source and said photocell to provide a degenerative ieedback circuit, the variations in voltage across said resistor being amplified by said amplifier stages.

5. A feedback amplifier system for stabilizing a light source comprising a light source to be stabilized, a photoelectric cell for detecting a portion of the light from said source, a plural stage amplifier including at least three stages intermediate said photocell and said light source, glow lamp coupling means between said amplifying stages, a high potential source for said photocell and the anodes of the tubes in said amplifier stages, said coupling means having a low value of resistance, and coupling means between said photocell and said first stage of amplification, said last-mentioned coupling means including a resistor in series with said high potential source WILBUR R. LE PAGE.

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