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Nov. 15, 1933. y ‘B. F. MILLER’ 2,136,723 SOUND RECORDING AND REPRODUCTION . Filed Sept. 25, 1955 V T0 F'ILA 5 Sheets-Sheet 1 ENT HEATE 5 FIG. 1. INVENTOR. Burro/v FM/LLEQ BY M642? ATTORNEY Nov. .15, 1938. ‘ B. F. MILLER ' SOUND RECORDING AND REPRODUCTION Filed Sept. 25, 1955 < < <° ' - 2,136,723 I ‘ ‘ 3 Sheets-Sheet 2 Nov. 15', 1938.‘ B. F. MILLER 2,136,723‘ SOUND RECORDING AND REPRODUCTION Filed Sept. 25, 1955 /m s Sheets-Sheet 5 2,136,723 Patented Nov. 15, 1938 UNITED STATES PATENT OFFICE ‘ 2,136,728 _ g , SOUND RECORDING AND REPRODUCTION Burton F. Miller, Hollywood, Calif., assignor to United Research Corporation, Long Island City, N. Y., a corporation of Delaware . Application September 25, 1935, Serial No. 42,116 . (Cl. 179-4003) 11 Claims. a consequence of this method of recording, the reproduced volume of the lower level sounds is reduced below that of the higher level sounds by an amount correspondingto that at which they were increased above their original levels, thus 5 likewise reducing the ?lm background noise dur ing periods of low level sound reproduction and during silent intervals. It will be shown later thatthe amount of this reduction is greater than that now commonly obtained with usual Well 10 This invention relates to the recording and reproduction of sound and particularly to the recording of sound on light sensitive material to ‘ produce a diapositive record having varying light 5 ‘and dark areas or intensities and the reproduc tion of sound therefrom. In recording, sound on ?lm, the minimum volume level recordable and satisfactorily repro Hduceable, which may be considered the minimum 10 ‘variation in light and shade areas or intensities, is that which is not less than the variations pro known circuits. signal. 25 . ‘ ,When ‘using this anti-ground noise system, duced by the nature of the ?lm, such as graini ness and varying degrees of inherent trans Mparency» or the so-called ?lm background noise 15‘"level. The upper or maximum variations record able are those controlled by the width of the sound track which ‘is of ?xed dimensions or'the greatest variation obtainable between a transparent and opaque section. Both limits represent the go‘ boundaries of sound levels recordable on ?lm, the minimum being perhaps more troublesome, ‘since the cause creating this minimum level is also present in reproduction at the time of' no there is automatically obtained a greater range of volumes within the limits of the ?lm than formerly could be recorded and faithfully re- 15 produced without the use of the invention. .Another object of the invention, is to‘ increase the volume range of reproduction and reduce ground noise from a sound record previously made according to any of ‘the well known methods v20 of noiseless recording, viz shuttering a variable area track,’ orbiasingqa galvanometer mirror or lightv valve strings according to the amplitude of the recording. ' > “ - i ‘ This is accomplished by using the reproduc- 25 ing portion of this system to reproduce standard An object of the invention is to automatically increase the range of volume levels recordable upon the sound track of motion picture ?lm and or non-compressed "anti-ground > noise sound ' faithfully reproduceable therefrom, and thereby records, the total noise reduction being the sum to obtain more effective noiseless recording than . of the noise reduction of the record and of the 30 ‘is obtainable with noiseless recording systems now reproducing system, the latter being determined 30 in common use. variable compression by the maximum volume expansion employed. In this instance, however, the reproduced volume ' This is accomplished according to one feature of the present invention by a method of and means for reducing the minimum volume level of the 35 original sounds which may be recorded and re produced and thus increasing the range of re corded volumes. The method involves the and expansion _ of the volume levels along ‘a predetermined character 40 istic between the minimum and maximum points which the record is capable of handling. That is, for the lower portion of the range of volume in reproduction, a certain volume expansion is pro range is greater than that of the record range, this'belng‘ accomplished by reducing the repro duced volumes of the low level sounds and conse- 35 quently the ?lm background noise. This, of . course, entails expanding the upper level sounds abnormally, which produces dramatic and other presentation eifects of ‘considerable value. When reproducing a non-compressed track, the inci- ‘10 dental electrical .and mechanical noises which oc-‘ curred in‘the recording thereof and those origi 45 ferent over the upper portion of the volume range. The changes in volume expansion may be uni nating‘ in any portion of the reproducing equip ment precedingthe expander are reduced in level by an amount equal to the total‘degree of volume form otherwise. By compressed recording and expanded reproducing of the sound on the ?lm, silent intervals, the reproducing system noises vided, while this volume expansion may be dif expansion during low level reproduction or. at ‘ the record range of levels will be less than the 50 original range of levels but the reproduced range also being reduced when reproducing compressed will be preferably equal to the original range. In recording, the lowest volume sounds are ampli A further object of the invention is to more e?iciently ?ed to such an extent as to be recorded at a level equal to or above that corresponding to the ?lm volumes, when said sound volumes are in the form of electrical currents. This is accomplished by similarly impeding the 55 ‘ 55 background noise level during reproduction. As records. . compress , and/or expand 50 sound 2 2,136,723 opposite half waves of the currents in accord ance with the amplitude thereof. Other objects and advantages of the inven tion will be obvious and the details of the inven tion more fully understood by reference to the following description read in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic drawing of a sound on ?lm recording system involving the invention. 10 Fig. 2. is a schematic circuit diagram of a sound on ?lm reproducing system embodying the invention. Fig. 3 is a graph illustrating the relationship between the input and output’ of each of the systems in Figs. 1 and 2; and, Figs. 4a and 4b are block diagrams showing the transmission level at various stages in the recording and reproducing systems. Referring now to Fig. 1, the light modulation 20 portion of the recording circuit includes a source of light 5 of constant intensity, a condensing lens 6, a light beam forming mask 1, a galvanometer 8, having a movable re?ecting mirror, a light slit mask I0, an objective lens II and a ?lm I2. through those tubes and the resistance 26 is therefore added to minimize the effect of the tube impedances on the time constant. All of the vacuum tubes above mentioned have their ele ments polarized from a power recti?er unit 32 in the usual manner. This recti?er, however, has a variable tapped potentiometer resistance 33, the function of which will be explained herein after. The output of ?lter 30 is connected in series 10 With the grid-cathode circuit of impedance tubes I6 and I1 across the potentiometer terminating resistance 29 of the ?lter 30 and in series with a resistance 36. This connection provides tubes I6 and I"! with a grid bias and therefore a cathode.— 15 to-anode impedance determined by the amplitude of the current output of ?lter 30. The static value of this grid bias is determined by the potentiometer resistance 33, which is given a value in accordance with the characteristics of 20 the tubes used and the function thereof. Filter condensers 34 and 35 are employed across the terminating resistance of the power supply cir cuit. By the use of resistance 36 of the desire-d 25 The operation of this portion of the system is . value, any tendency for the grids to be driven well known in the art and operates in the usual positive by high signal levels will be counter 25 manner; that is, the beam of light is moved longi acted by the voltage drop across this resistance tudinally of the slit in mask Ill, thus varying caused by grid current ?ow. Thus a lower im the quantity of light striking the ?lm. The gal pedance limit for tubes I6 and I1 is thereby es vanometer 8 is connected to the output of an am tablished. Tube I6 is shunted by a series ar 30 pli?er I4, the input of which may be from ter rangement of resistance 38 and a condenser 39, minals 9 connected to the output of tubes I6 while tube I1 is shunted by a similar resistance and I1 which transmit sound currents there 40 and condenser 4|. These elements are sub through serially with the line, or to terminals I3 stantially non-reactive and function to provide 35 connected to conductors I5 which eliminate tubes an upper impedance value by preventing an ef I6 and I1 from’ the circuit. The input of tubes I 6 fectively open circuit through tubes I6 and H. 35 and I1 is connected to the output of an ampli?er The reproducing circuit of Fig. 2 is substan 20,. over terminals I8, the input thereof being connected to a microphone 2 I, or other sound-to~ 40 current translator. The output of ampli?er 20 may also be connected to terminals IQ of con ductors I5. Shunt resistances 22 and 3| func tion to match the impedances of input and out put transformers with their connecting circuits. The output of tubes I6 and I1 is impressed upon. a three stage ampli?er including vacuum tubes 23, 24 and 25, with the usual inter-con necting circuits. The output of this plural stage ampli?er is impressed upon a full wave vacuum 50 tube recti?er 21, 28. The recti?ed current is ?ltered by a low time constant ?lter 30, including resistance 302 and condenser elements 30I and 303 having values such that, together with ter minating resistances 26 and 29, the time con 55 stant thereof is not greater than about .01 sec ond and preferably as'low as .008 sec. The pur pose of this low time constant is as follows: ?rst, to insure that on increase of signal strength the audible ampli?cation will reach its ?nal value in so short an interval of time that no improper ‘variation in signal level occurs, and second, to insure that on decrease of signal strength the background ?lm and system noise will not be unduly ampli?ed afterthe signal has ceased. In order to insure that the rates of growth and decay of voltage across resistance 29 are sub stantially the same, resistance 26 having a value comparable to the ‘minimum impedance of either of tubes 21 and 28, is connected in shunt to the ?lter input circuit between those tubes and the ?lter 30. This will be apparent from the fact that the time constant of the ?lter circuit, for currentpassing through the tubes 2‘! and 28 de pends'in part on the impedance of those tubes. Current cannot pass in the reverse, direction tially identical to that of Fig. 1, except that the input circuit to the ampli?er-recti?er circuit is now not connected to the output of tubes I6 and I‘! but is connected to their input circuit which 40 comprises a photo-electric cell 45 or other light sensitive device upon which light from a constant intensity source 46 is varied in intensity by a sound record on ?lm 41. The output of cell 45 is ampli?ed by an ampli?er 52 to which tubes 23, 24 and 25 are connected while the output of the system terminates in a loud speaker 49 or other sound reproducer connected to the out put of an ampli?er 50. The remainder of the circuit contains all the same elements of the 50 circuit of ‘Fig. 1 but has the connections of the output of ?lter 30 to the grids and cathodes of tubes I6 and I‘! now reversed while a di?erent staticpotential is applied to tubes I6 and I ‘I by potentiometer 33 as will be explained herein— after. The same by-pass switches with conduc tors I5 are also employed in this circuit. Also in this system a unilateral device such as a push pull ampli?er having tubes 42 and 03 is inserted between the output of ampli?er 52 and the input to impedance tubes I6 and I ‘I to prevent unbal ance impedance relations in the circuit of tubes I 6 and I‘! from reacting on the control circuit. The coupling between the tubes I6 and I1 and tubes 42 and43 is shown as a resistance-con denser circuit but these tubes may also be trans former coupled. The impedance control of the above described recording and reproducing circuits, by means of impedance tubes I6 and I1, is the same except that the polarity of the recti?ed current from recti?er 30 is reversed in Fig. 2 from that in Fig. 1. The transmission characteristic of each cir cuit, or the relationship between input and out 75 3 , 2,136,723 put current variations is shown in the curves of Fig. 3 where the recording characteristic is shown extended to the point c,‘which is approximately circuits, it is believed desirable to refer to Figs. to the point D on the reproducing curve, the minus 55 db. since we are still not below the re by the solid line plotted between input record- ' produced noise level limit of minus 35 db. This means that the recordable input range has been ing level and output recording level, and the re producing characteristic shown by the dotted line materially increased over that recordable with . plotted between the record level and‘ reproduced the systems now in use. ' Now in reproduction the point a is transferred level. Before describing the operation of the 4a and 4b, in which the ranges of levels in each . input level now corresponding to an output level 10 between zero and minus 55 db., this minus 55 db. 10 element of the circuit are shown. In Fig. 4a the level being below the ?lm background noise level. input microphone 2| is shown feeding a micro phone ampli?er and mixer 55, the output of which has a level of approximately 40 db. range extending from minus 40 db. to minus 80 db. A 15 second ampli?er 56 increases this range of levels from minus 40 db. to zero still maintaining the same original range. The degree of uniform am pli?cation is arbitrary and is controlled by at tenuation and the usual circuit factors. We then 20 reach compressor 51, which includes the re mainder of the circuit of Fig. 1 between the am pli?ers 20 and [4, this circuit variably decreasing the upper and lower level limits so that the lowest level of minus 40 db. is now minus 45 db. 25. and the upper level is now at minus 20 db., the difference constituting a range of 25 db. as com-. pared with 40 db. The subsequent ampli?er M then uniformly ampli?es this range to levels be tween zero to minus 25 db., at which point it is 30 impressed upon the galvanometer .8 to produce variations in.the light impinging on the ?lm 12 in accordance therewith. This degree of am pli?cation is also dependent on the load. It is thus seen that the original range of minus 40 to 35 minus 80 db. is impressed on the ?lm as a range extending from zero to minus 25 db., this range being within the limits of the ?lm track, the lowest sound level, however, being above the ?lm background level. The reproduction of this record is shown in Fig. 45 wherein the reproducer 60 translates the record range of 25 db., but in view of losses in recording, this range now lies between minus 50 db. and minus 75 db; However, the expander circuit 6| which is that part of Fig. 2 existing ~ between ampli?er 52 and ampli?er 50,. extends this 25 db. range to the original range of 40 db. This means that this background noise may not be reproduced at the loud speakers 49 with suffi cient volume to be audible. In fact I have ob tained with this invention of Figs. 1 and 2 a noise 15 reduction of 20 db. better than the average ob-v tained with standard noise reduction arrange ments well known and in common use. Not only is the ?lm background noise reduced but also the electrical and mechanical noises incident to 20 reproduction are eliminated. In brief, therefore, the ratio between the variations in levels occur ring in the original sound has been reduced for recording and the ratio between the variations in level occurring in the recorded sound is in 25 creased in reproduction. . . The reproduction system of Fig. 2 may also be used to increase the volume range reproduced‘ from a standard noise reduction sound track, this reproduction also following the broken line re 30 producing curve in Fig. 3. Thus, this‘type of track will be reproduced without the present ob jectionable ?lm background noise since the low levels reproduced are below the noise level. This‘ means that ‘the ratio of level variation is in 35 creased to bring the upper levels to zero db. By this variation, special effects such as louder pistol shots, siren noises, etc., will be accentuated. To preventundue modulation of voice repro duction, the ratio of variation between levels may 40 be changed between the upper and lower volume limits as shown by the curves in Fig. 3 thus pro ducing a different ratio of volume variation at particular portions of the level range. This will be further described later. 4.5 The volume compression and expansion above described are accomplished by the circuits of Figs. 1 and 2 and particularly by varying the im by reducing the higher volumes to 52 db. and the lower volumes to 92 db. A subsequent ampli?er ’ pedances of tubes I6 and H in accordance with 52 may bring this range to any desired level such the envelope of the soundcurrents impressed on 50' as plus 10 db. and minus 30 db. for impression the tubes I6 and I1. As mentioned above, these constitute series impedances in the line on loud speakers 49, which translate the currents tubes inter-connecting ampli?ers 52 and 50 in Fig. 2 to sound energy. The range, therefore, im and ampli?ers l4 and ‘20 in Fig. 1. By the use pressed on the loud speakers, extends over 40 of two of such tubes arranged in a parallel series db., the same as the impressed range which orig 55 arrangement, both halves of the sound currents inally extended from minus 40 to minus 80 db. at are equally impeded, the amount of impedance increased level limits dependent upon the output being a function of the current of recti?er tubes volume required for any particular area. 21 and 28, which is a function of the envelope of Reference to graph in Fig. 3 may further ex plain the general operation of the two circuits the sound currents. [While the relationship be 60 in which it will be noted that the relationship tween the input and output current of tubes 21 between the recording curve and reproducing and 28 is linear, the relationship between the in curve appear as re?ections of one another, about an axis which represents a one to one translation 65 or ampli?cation ratio between the input and out put levels of the compressor and expander cir cuits. That is point a represents a minus 35 db. level for both input and output, this point also being the level of the ?lm background noise. At this point, the signal to noise ratio is unity. This ' put and‘output current for impedance tubes l6 and I1 is non-linear and varies in accordance with a power function dependent upon the rela 65 tionship between the impedance of the tubes and their terminating load. To obtain the various degrees of compression and expansion at dif ferent portions of the volume range and to pre vent the impedance of the tubes from becoming 70 70, ,also means that no lower input levels may be re- _ too high, shunt resistances 3B and 40, of ap corded on the record than those between minus proximately 100,000 ohms are employed, these 35 and zero levels, otherwise they are masked values also determining the points at which the by noise during reproduction. However, with 759 the present invention, this input level may be changes in impedance of tubes l6 and I1 is more effective. Thus the steepness, of the recording 75 2,136,723 and reproducing curves between output and in put levels which represents the ratio of com pression and expansion is controllable. . In order to prevent the transmission impedance from drop pin'g to an extremely low value approaching zero, a 100,000 ohm resistance 36is placed in the gird circuit, which will prevent the grids becoming positive, as any tendency to draw current ine creases the drop across the resistance. This 10 means, of course, that the impedance of tubes I6 and I‘! cannot be materially lowered beyond this point. The general range of variation may be determined and controlled by the position of potentiometer 33. In recording, a small bias is 15 placed on the tubes, the recti?ed currents in~ creasing this bias in accordance with their ampli tudes, and thus varying the transmission loss through tubes l6 and H. In the reproducing cir cuit of Fig. 2, a high initial bias is employed, the 20 recti?ed currents decreasing this bias and thus decreasing the transmission loss as the envelope of the sound currents increase. Having thus described the invention, what is claimed as new and desired to secure by Letters 25 Patent, is: _ 1. In a sound on ?lm reproducing system, the combination of a carrier for a sound track hav ing noise reduction, means for translating the modulations of said track into electrical currents corresponding thereto, an ampli?er circuit, and a series-parallel impedance circuit connected in series intermediate said translator and said ampli?er circuit for varying the amplitude trans mission therebetween in a ratio different from that existing upon said carrier, said impedance circuit including the cathode-anode circuits of a pair of thermionic devices having a common of thermionic devices in shunt to said ?xed im-' pedances whose bias is controlled by said recti ?ed currents and whose input and output cir cuits are in series with the cathode and anode of said devices. ' 6. A system of sound recording and reproduc tion comprising a source of current having a certain range of volume levels, a light sensitive medium for recording light variations in ac cordance with sound variations as a single track 10 record, a light modulation system, an electrical circuit inter-connecting said source of electrical currents and said light modulating system, said means including a ?xed impedance and variable impedance in parallel,- said variable impedance 15 being a pair of thermionic vacuum tubes con trolled by the envelope of said electrical currents for compressing the ratio between the original volume levels a predetermined amount, means for translating the single track record so produced 20 into corresponding electrical currents, means for amplifying said currents, and means inter-con necting said translating means and said amplify ing means for expanding the ratio between the volume levels of said record currents, the amount 25 of expansion corresponding to the amount of compression produced during recording. 7. A variable impedance circuit comprising a thermionic device having cathode, grid and anode elements, input and output circuits of said device including said cathode and anode elements, a resistance shunt circuit to said cathode and an ode, and a resistance in series with the grid to cathode circuit of said device, the impedance of said cathode to anode circuit varying in a 35 manner and between limits determined by the values of said shunt resistance circuit and said grid circuit. ' grid to cathode circuit. 2. A sound recording system comprising a car— 8. An electrical circuit comprising a transmis~ rier for a sound record, said record having a sion path, a control circuit for controlling the certain percentage noise reduction, means for , transmission of electrial currents over said path, translating said record into electrical currents a recti?er, a ?lter, said recti?er and ?lter being having volume levels corresponding thereto, an connected intermediate said control circuit and ampli?er for amplifying said volume levels pro portionately thereto, and a series-parallel im pedance circuit intermediate said translator and said ampli?er for varying the ratio between the volume levels of said transmitted electrical cur rents, said variations differing at different por tions within the range of said volume levels, and said impedance circuit including ?xed impedance elements in parallel. 3. A recording system‘ in accordance with claim 2 in which said last mentioned means includes thermionic vacuum tubes in shunt to said re spective ?xed impedance elements, each of said tubes and associated elements transmitting oppo site cycles of said currents with substantially equal variations. 4. In a sound reproducing system, the combi nation of a ?lm carrying an anti-ground noise sound record, means for translating the varia tions of said record into corresponding electrical currents, an ampli?er, a balanced impedance cir cuit inter-connecting said translator and said ampli?er, said circuit including ?xed impedances in parallel, a recti?er inter-connecting said translator and said impedance, and means in cluded in said impedance responsive to the recti ?ed current from said recti?er for variably changing the value of said transmission im pedance, said change in impedance di?ering for said transmission path, a ?lter output terminat ing resistance and a ?lter input terminating re sistance element intermediate said recti?er and said ?lter, said ?lter input terminating resistance element having a value approximating the mini mum impedance value of said recti?er to pro vide said ?lter with substantially equal rates of ' voltage growth and decay across said ?lter out put terminating resistance. 9. An electrical circuit comprising a plurality of ampli?ers, means having impedance and con necting said ampli?ers, a control circuit for vary ing the impedance characteristics of said con necting means, a recti?er intermediate said con trol circuit and said connecting means, a ?lter in the output of said recti?er having a low time constant, said ?lter having an output terminat ing impedance, connections whereby the voltage 60 across said terminating impedance is adapted to be applied to said transmission means, and a re sistance intermediate said recti?er and said ?lter and in shunt thereto for providing the time con stant of said ?lter with substantially equal rates of growth and decay of the voltage across said terminating impedance. 10. A system of sound recording comprising means for generating electrical currents cor responding to sound waves to be recorded, a re cording device for translating electrical currents changes in recti?ed current of different values. into modulated light, and an impedance circuit 5. A' reproducing system in accordance with’ interconnecting said current generating means 75 claim 4 in which said impedance includes a pair and recording device, said impedance circuit in 75 2,136,723 eluding a pair of thermionic devices having re spective cathode, grid and anode elements, the input and output circuits of said devices in cluding said respective cathode and anode ele ments, resistance elements in shunt to each cathode and anode circuit and a resistance in series with a common grid circuit, the impedance of said cathode-anode circuits varying between the limits determined by the values of said shunt v resistance elements and said grid circuit resist ance. 11. A system of sound recording comprising means for generating electrical currents cor - responding to sound to be recorded, a recording 5 device for translating electrical: currents into modulated light, and impedance circuit intercon necting said current generating means and said recording device, said circuit comprising a pair of thermionic devices having a common grid cir cuit and having their cathode-anode circuits con nected in series with each other and with said generating means and said recording device, and an ampli?er-rect?er circuit having its input con nected to the output of said thermionic devices and its output connected to the common grid circuit of said thermionic devices. BURTON F. MILLER.