US2495809A - Signal expansion or compression - Google Patents

Description

Jan. 311, 1959 GRAHAM 2,495,809

SIGNAL EXPANSION 0R COMPRESSION Filed Sept. 6, 1945 5 Sheets-Sheet 1 UV IV 20 I INVENTOR fimfige [C Grafiam BY YI Z/WW ATTORNEY Jan. 31, 1950 v G. K. GRAHAM 2,495,809

SIGNAL EXPANSION 0R COMPRESSION Filed Sept. 6, 1945 3 Sheets-Sheet 2 s; I Y

1 i .1 5 w w AMP aarpur M :l j 7 MAGA/t-T/C 3 i MAGNET/C MAHR/A L M 144 TEE/AL INVENTOR (Zearge K Grafiam ATTORNEY Jan. 31, 1950 G. KJGRAHAM 2,495,899

SIGNAL EXPANSION OR COMPRESSION Filed Sept. 6, 1945 3 Sheets-Sheet 3 Jay/v0 WAVE 7 AMPL/F/ER 007 07 JOUND WAVE 49 I'l'l' ,55 48 1: T

INVENTOR Geazg ff. Grafiam ATTORNEY Patented Jan. 31, 1 950 SIGNAL EXPANSION OR COMPRESSION George K. Graham, Oceanside, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application September 6, 1945, Serial No. 614,736

8 Claims.

This application relates to an improved method of and means for regulating the sensitivity of a transducer such as, for instance, a microphone or phonograph pick-up element. By means of my invention the transducers sensitivity is regulated inversely or directly in accordance with the amplitude of the stimulus to which the transducer is subjected, so that compression or expansion of the currents representing the sound wave is accomplished.

In many radio systems sound waves are converted into electrical currents and transmitted from one point to another by way of conductors such as lines or by radiation. The transmitted material may represent voice or music or television or as a matter of fact intelligence of any type and may be put to various uses. In one particular application of my invention sound waves are converted to electrical currents, amplified, and supplied as output to be used as desired. This use may include modulation of a radiant energy transmitter or distribution to listening points, etc.

In the transmission of program material the volume level range which is transmitted must lie between two extremes, the upper limit established by the maximum handling capacity of the transmission medium and the lower limit by the level of noise present. In the present art it is customary to reduce the level of the higher level passages to a safe value and to increase the level of the lower level passages to overcome the effect of noise.

An object of my invention is to regulate the level of program material within predetermined limits. This object is attained by providing a new method of and means for regulating the sensitivity of the microphone, phonograph pick-up or other transducer supplying the material for transmission.

Some present day compression and expansion amplifiers depend for their operation on the change in grid bias of a variable mu type of tube which is provided by a rectified portion of the output of the compression type amplifier. Using such a method, wide variations in grid bias due to large output level changes tend to shift the operating point of the tube in such a way as to cause serious distortion.

A further object of my invention is a better method of and means for compressing or expanding the range of program material.

This improved regulation or control by compression or expansion is accomplished in accordance with my invention by regulating the sensitivity of the transducer per se rather than of a network and/or amplifier to which the current, resulting from the conversion from sound waves to electrical current, is supplied.

A further object of my invention is improved sensitivity control of a transducer of the nature described above.

An additional object of my invention is improved automatic control of the sensitivity of a transducer of the nature described above.

The above objects and others which appear hereinafter are attained in accordance with my invention by utilizing some of the converted energy after, if desired, amplification to regulate the sensitivity of the microphone, phonograph pick-up, or transducer or other means for converting from sound waves to electrical currents. Where compression is desired, if the program level is low the control is such as to increase the sensitivity of the transducer, while if the program level is high the sensitivity of the transducer is reduced. Where expansion is desired, if the program level is low the control is such as to decrease the sensitivity of the transducer, while if the program level is high the sensitivity of the transducer is increased. In an embodiment the field strength is made non-uniform, with an intensity pattern such that compression or expansion as desired may be obtained.

My invention is applicable to transducers of various types, including microphones and phonograph pick-up devices. In the detailed description which follows, I have disclosed application of my invention to transducers of the electromagnetic type and of the electrostatic type.

In disclosing my invention in detail reference will be made to the attached drawings, wherein- Fig. 1 illustrates schematically an embodiment of my invention wherein the sensitivity of a transducer of the electromagnetic type is controlled. The transducer in the embodiment is a ribbon microphone.

Fig. 2 illustrates schematically a modification of the arrangement of Fig. 1. In Fig. 1 the sensitivity control is carried out by control of the field strength. In Fig. 2 the position of the pickup element in the field is controlled.

Figs. 2a and 2b are modifications of the arrangement of Fig. 1.

Fig. 3 illustrates schematically the application of my invention to a different type electromagnetic microphone. In this embodiment the microphone is of the dynamic type, having a diaphragm operated coil moving in a magnetic field.

Fig. 4 shows my improved compression and expansion means as applied to an electrostatic 3 transducer, while Fig. is a modification of the arrangement of Fig. 4.

In the embodiments of Figs. 1 and 2, I have applied my invention to an electromagnetic microphone wherein sound pressure is converted to electrical energy by means of a moving sound pressure operated conductor which is caused to cut the magneticfield. In such a microphone the E. M. F. induced in the conductor is roughly proportional to the displacement of the conductor and to the strength of the magnetic field. The induced E. M. F. for a given displacement of the conductor is greatest when the moving conductor cuts the greatest number 'of lines of magnetic flux, and is diminished as the conductor assumes a position in which it cuts a smaller number of lines of magnetic flux.

In Fig. l, the conductor or microphone ribbon I2 is located in the field of a magnet 14. The magnet 64 may be of the field winding type or of the permanentmagnettype, the latter type-being shown. Current-inducedin the ribbon misused to excite winding A to control the effectiveness (strength) -of the magnetic field, thereby to control-thetransducer s sensitivity. A compensating winding B, the operation of which will be described-later, is'also-mounted on themagnet. In

the embodiment-of Fig. '2 I control the position of the ribbon -l2 in-the magnetic field to-control the sensitivity of thetransducer.

In Fig. 1, l0 designates a transducer or microphone of the ribbon type. In this microphone a conductor-in the 'form of ,a ribbon l2, acted on by soundwaves, isllocatedbetween the poles of a magnet M, which may bepermanent ormay be of the type having afield producing winding. The ribbon 12, as stated above, is locatedbetween the pole pieces and-connected in series with theprimary wiring of a transformer 16 whereby the F.-inducedin the ribbon l2 during itszmovement by the sound waveszisfed by lines !3 to unit l8. The ribbon conductor l.2 isv also in series with a winding B,-the purpose of which appears hereinafter. The unit l8 .may include an amplifier if desired and .theamplifier output maybe put to any use, such as being :fed to a loudspeaker or transmitter. The magnet .lll also has mounted thereon a second coil .A which when energized varies .the magnetic field,thereby controlling the sensitivity of thetransducer. .Coil A is connected by apolarityreversing switch 26 to the load impedance '22ofa dioderectifier 24. This coupling -may, if desired, include a direct current amplifier 25. The diode24 hasits anode tapped to a potentiometer 30 across theoutput'of the amplifier I8.

The cathode of the diode viscOnnected to the low potential end of the potentiometer resistancei30, so that a portion of Lthepctentiometer is included inseries with the load resisto1x22 and the cathode to anode impedance of the diode 24.

The potential across the load impedance 22'is proportional to the :magnitude of theprogram at the output of the amplifier 18, so that the current passed throughcoil .A is proportional to the output voltage ofthe amplifier l8. ".Under these .con-

ditions, the sound energy .or waves impinging on the microphone'ribbon 12 will cause variations in through the coil causes a. magnetic flux to be set up which opposes the normal flux in the magnet 14. Then any increase in sound pressure will cause a decrease in magnetic flux in which the conductor moves. This decrease in magnetic flux will cause a reduction in the E. M. F. impressed on the input of the amplifier I8. Conversely, a decrease in sound pressure will cause an increase in the magnetic flux. Under these circumstances, the electrical output of the microphone will be proportionately greater for low level sounds than for high level sounds. This will cause the louder passages of sound to be compressed. Reversing the switch 20 will reverse the above and cause expansion of the loud passages. The diode circuit may have suitable electrical networks including, for example, a condenser 23 so that the voltage impressed on the coil A will follow the average level'of energy or be responsive to the peaks of energy, so that by a suitable combination of circuit elements, the required degree of expansion or compression can be achieved. If it is desired that the sensitivity be controlled in accordance with the average level, then the condenser 23 is large so that the network has a time constant which isrelatively large with respect .to the. average variations in volume intensity. If the sensitivity is to be controlledby peaks only, then the time constant is made relativelysmall and condenser 23 Wouldhaveiless capacity.

Coil B is in series with .the moving conductor and is so arranged in position and number of turns that any .E. .M. F. .induced in the moving conductor, due to the variations in the magnetic field under the influenceof the current in coil A, will be opposedby an equal E. .M. F. induced in .coil .B. This will minimizethe effect of any volt- .age which .might be induced in the moving conductordue to a change in magnetic flux. Complete ,cancellation'ofzthe effect of the potential set 11p in.A appearspossible.

.In practice in many .cases, my invention will be applied wherecompression only or expansion only is desired. For example, in broadcast program distribution, compressionwould be used in order to.r estrict the dynamic range to suit the transmission facilities. .If the transducer control of my invention'is to be:used where the noise level is high, then expansion may be used. If used on phonograph pickups expansion may be used to compensate for the intentional.compression used in the recording'art. 'When applied as described above the switch 20 .is omitted and the winding A is then coupled directly to the amplifier 26 in the proper polarity. The apparatus is then other wise as described hereinbefore.

In the embodiment of Fig. 1 the sensitivity of the transducer is controlled by controlling the magnetic field intensity. Means is also provided to eliminate any effect due to the change in the magnetic field intensity. vIn the embodiment of Fig. 2 now to be described, the sensitivity of the transducer is controlled by controlling the efiectiveness of the field onthe element cutting the field. This is done by :moving the static or mean the microphone.

The ribbon element I2 is acted On by sound waves as before and is connected by leads l3 to the input of an amplifier asbefore. It is noted that transformer coupling is not shown in this embodiment, but maybe used I! desired. As a matter of fact, the transformercoupling may be included in the amplifier l 8. The diode and its connections to the amplifier l8 may be as described hereinbefore, so that across diode load 22 a potential proportional to the magnitude of the current at the output of I8 is developed. This potential is supplied by the reversible switch 20 to the winding of the driving means M which may be considered a motor. Ihe motor M has a shaft S connected to the mounting element for the ribbon l2. In the embodiment illustrated, the shaft S is shown as being passed through the magnet in in a manner so that its axis is at right angles to the plane of motion of the ribbon l2as acted on by the sound.

In this arrangement, the E. M, F. induced in the ribbon [2 by cutting the magnetic field is proportional to the extent of motion. The induced E. M. F. is greatest when the conductor moves in the entire magnetic field at right angles to the direction of the magnetic flux and is diminished as the conductor assumes the position in which it moves in a reduced magnetic field. The ribbon i2 is mounted on supports X and Y which extend in to the axis of rotation and are connected to shaft S separated by insulator 36, which also supports and strengthens the ribbon l2. The shafts S may pass through insulators 35" in the magnet 4. The ribbon is rotated in one direction or the other by the motor M depending upon the position of switch 20, to an extent proportional to the magnitude of the potential across impedance 22. The motor M may be a device similar to the moving element in a direct current meter, actuated by the potential drop across 22. As illustrated the motor M comprises a magnetic structure MS and a coil MC free to move in the magnetic field. The coil MC is connected by an insulator 35' and shaft to the shaft S on which the ribbon i2 is mounted for rotation in the field of magnet it. The ribbon 12 may be further supported by the insulator 36. The motor M rotates the ribbon I2 from its position at which the maximum current is induced in ribbon .l 2 to a position at which less current is induced in ribbon I2 when the magnitude (peak or average) of the current at the output of i8 is high and compression or reduction of sensitivity is desired. The ribbon I2 is then at an angle with respect to the magnetic pole pieces and the conductor moves in a smaller magnetic field and the output is reduced. Thus, when the switch 20 is adjusted for compression, the conductor I2 is caused by the spring 37 to assume a position parallel to the magnetic pole pieces in the absence of applied sound waves. When sound wave energy is applied, the amplified output of the microphone is rectified by the diode 24, causes a current to flow through the motor winding MC and rotate the conductor 12 so that its motion causes it to cut a reduced magnetic field, and the'electrical output of the system and the sensitivity of the transducer is reduced.

If the switch 26 is reversed, a source 38 and an adjustable resistance 39 is included in the motor driving circuit wherein the direction of current flow has been reversed. The source 38 and resistance 39 is such as to cause the motor MC to rotate the ribbon [2 on its axis to a position'whereat it is nearly at right angles to the magnetic pole pieces in the absence of output from the amplifier! (i, 9., no sound waves on the ribbon). The source 38 polarity opposes the polarity of thepotential developed across'impedance 22x-when soundn.waves are applied. I However, it will be understood". that the magnitude of this'opposing voltagev is adjustable. a

Now when sound waves-strikethe ribbon l2 the voltage developed'across impedance 22increases. The motor MC, actuated by, the potential across 22, is caused to move the ribbon 12 to a position at which it becomes nearly parallel to the magnetic pole pieces, and the output of the system is expanded. Thus, under expansion condition the ribbon l2, the motor M, and so forth, are set so that the conductor normally assumes a position nearly at right angles to the pole pieces in the absence of sound Waves and little electrical output results. This is accomplished byarranging bias voltage at 38 to position the conductor E2 at an angle approaching with respect to the pole pieces, or by supplying a member 5i attached to MC and arranged to strike amechanical stop 43 so that the motion of the conductor is restricted to a position at right'an'glesio the magnetic pole pieces in case th voltage supplied at 38 is more than required to move MC to said position at which it approaches 90 with respect to the magnetic pole pieces. The sou'rce38' and resistance 39 are means for selecting'si'gnal magnitudes below which there is substantially no output from the system. This will be clear when it is kept in mind that the current to MC results from the difference in the magnitudes of the opposing potentials at 22 and 38 and that the ribbon cannot be made parallel to the magnetic pole pieces until the two potentials are about equalso that the spring 3'! can move" the coil MC and ribbon 12 into said position. Such operation'is useful on transducers used, for example, in a noisy location. Then the sensitivity of the system will be greater for desired high level sounds.

The compensation for the effect of the motion of the ribbon l2 and its mounting means in Fig. 2 is inherent, in that movement of the ribbon by the motor MC as described above does not produce distortion by changing the current output. This is because the mounting X, Y is symmetrical and the current induced by the movement in the two halves of the ribbon mounting are of opposed polarity and balance out in the transducer output. This will be apparent by inspection of the ribbon I2 of Fig. 2. When the same is rotated both halvesmove away from the 90 relation, but in opposite directions.

The flux producing field may be of irregular shape, so that the eifectiveness thereof on the moving member l2 increases or decreases with movement of the member l2 from the no sound wave position. This non-uniform flux field may be arranged to have either a decreasing flux, as shown in Fig. 2a, or an increasing fiux, as shown in Fig. 2b, at points removed from the conductor. Then the potentiometer 30, diode 24, impedance 22, amplifier 26, switch 20 and windings Aand B, all as shown in Fig. 1, are omitted. The magnet in has a cross section as shown in Figs. 2a or 2b. Where the field decreases in intensity, as in Fig.

; 2a, the sensitivity of the transducer is reduced for higher level signals and compression takes place. Where the field increases in intensity, as in Fig. 2b, the sensitivity of the transducer is increased for higher level signals and expansion takes place.

As stated hereinbefore, my invention is applicable to transducers of any type, and a further application thereof is illustrated in Fig. 3 of the drawings. In this figure a transducer or micro- "phone of the cone and voice winding type-is used. In describing this embodiment reference: char,

7. acters and letters corresponding to those usedin the prior figures are used in order tosimplifythe description. The microphone 1 or. transducer has acone C responsive-to the soundwaveso The cone C carries a-fieldvwinding FW mounted between the poles of a magnet 12:". The moving winding FW cutting the flux field has-induced thereina current responding to the, sound Waves, which is fed to the Y amplifier -l 8-as in a the prior embodiment. As in -the prior-embodiment, a-potential'which varies ina mannercorresponding-to the variations in the output 7 of" I 8 appears inthe impedance-22and is appliedthrough switch '26 to windings-A-mounted on-theflux field producing magnet 12'. excited by current from 22 is similar to 'the'operation'of the winding'coil'A in Fig. --1: The current in the winding A 'is in'a direction'to increase the strength of the magnet with increasesin amplitude of" thecurrentoutputof i8 tdacc'omplish expansion: of the said currentsf'to increasethe sensitivity of the microphone; or 'thecurren t in the winding A may'be in the opposite direction to decrease the flux" field'and' the "strength of the magnet, to decrease'the current'at the inputof 18 'when the "current intensity at the "output thereofgoes upand to'thereby compress the signalling the'reof" by decreasing the sensitivity of the microphone, A winding Bin serie's' 'with the Winding FW Selv es the 'sarnef urposje as crewma- 'in'gB'of Fig l'. i

In Fig. I have illustratedfmy improved sensitivity control a's'applied to transducers or micro'pho'n es 'of the electro static type. In Fig. 4, the microphone A comprises two plates El and P2 spaced fronrea ch other to f orm, the plates of; a variable condenser,,across impedances 4'! and 49. h l m ed cl fis is. a r ss h i p oit e amr li t e 5. he t i l. rop cr s res stt m 4. i a fun n, of th .c pas t the can: den f' L P? an 9 h v lta a o s he. se denser plates. [Ihelatter voltage is-made up of the voltage o so r e 4 a d th Potent a rQ across the output oi D. C, amplifierjfi. The a p ifier may c m r se the firstst eqfme amplifier: I}; of the prior figures. .flhe' condenser microphone l3! and 1?; ha s a fixed plate P'Zand a movable plate Pl. Sound waves impingingjon plate PI vary the capacity of the condenser to sorrgspondingly var i ate of. ch ge. nd t voltage on the grid of tube 5! and atthe outputof am ifi a d r s .Pfi im da q 2.2-1 To .dws se th sen t the. vel 's ap e rin across the output of D. C amplifierjli is so conm e to i pedan '4] s. b op oset eliqlt s o S rc 4 Wi th r s t th tl he. v lta e a p rin a rq t e p ats? tth e e se and I32 is reducedj To increasethe sensitivityof t e r 'nsd qetf hepel ity of th poten 1 d. impedan e 22 sr vle i a i ad s to'thevoltage at 43. The values of these voltages are adiustahleat potentiometer Q and at source n. th bo ment F 5;. th j 1a ef i m n on afiqofanjlc ma ne mwe- .n dl ev g a Wind n q ct d qth c u te i s o t e swi chi t, n t i ar an ment. as t Sound. Wav i pin e on. the m rop o plate PI itsposition with respectto P2, is changed The operation of this wvinding A as I ii i . current to the magnet winding, 6| to move the plate P2 toward .or fromthe plate ,Plto increase or decrease the capacitybetween the p1ates.,,lf the current in the winding 6! isin adirectionto move plate PZYaWay from- PI and reduce thecapacity when the signal strength increases, compression of the signal takesplace, so that the transducers sensitivity is reduced. a If the current throughthe winding 6 l is ina direction to movethe .plate P2 closer to the plate Pl to, increase the. capacity when the magnitude-of the voltage appearing at the outputof the amplifier l3 increases, the sensitivity of the microphone isincreased.

--The core 60 forthe winding 6| may be mounted in any manner, and the mountingof the sameis shown schematically in Fig.5. The core 60 is held by aspring S in such a position that the desired relationship betweenplates PI and P2 is maintained.- -Gurrent through the winding 6i compresses oreXpands-the spring S to change the-position-ofthe core :60- and plate P2.

"Whatis claimed-is: I l'wIn signalling apparatus, -a-ribbon-type microphone, having-its ribbon in a magnetic field, for" converting motion representing signals to electrical currents-an" electrical circuit connected to the ribbon,'-and means responsive to currents'in saidcircuit for changing the-static position'of the ribbon in the magnetic field. V 2. Atransducer including a movable element in'a'magnetic field and ma circuit wherein cur rent is produced by'said movementyand apparatus including a 'rectifier responsive to the produce'd currents for producing a control potential. and a' drivingelement-for saidmovable element controlled by said control potential for changing the static'position' of said element in the field to change'the'sensitivity of said'transducer. i

'35 A transducer'including'a movable element in'a magnetic field and in acircuit wherein current i's produced by said movement, and a motor controlled by said'producedcurrent for automatically changing the static position of said elemerit in the field in accordance with the strength of theproduced'current to change the in thafield 'jtochange the sensitivity of said transducer comprising amovable support for said element, and means actuated by' the produced for"'controlling the position of the supy 5.1I n' signalling apparatus, in combination, means for converting sound waves to electrical v r at o s .cam ri iaa e element 6n wh ch t esp n wavesl m l a circuit assodefi d with sa mov b e lem nt he e q age is produced of an amplitude which varies with movement of said element, anamplifier cou le with a c r u t, sa m v an Q t ut..wh n am l fied e ec l va t ons corresponding to the variations in. the sound waves appear, a rectifierin a circuit including an impedance wherein a voltage is, developed proportional to the magnitude of the. signaLin theoutput. of the amplifier, and electrical .or mechanical means excited by-said last.named voltage forregulating the sensitivity of the converting means..,

6..In signalling?apparatualin combination,

means forconverting. soundwavesto electrical variations comprising means for producing a magnetic field, a movable element in saidfield on which the sound waves impinge, a circuit associated with said movable element wherein a voltage is produced of an amplitude which varies with movement of said element, and an amplifier coupled With said circuit, said amplifier having an output wherein amplified electrical variations corresponding to the variations in the sound waves appear, the said magnetic field being irregular so that the intensity thereof changes with movement of said element.

7. In signalling apparatus, in combination, means for converting sound 'waves to electrical variations comprising a movable element on which the sound waves impinge, a circuit associated with said movable element wherein a voltage is produced of an amplitude which varies with movement of said element, an amplifier coupled with said circuit, said amplifier having an output wherein amplified electrical variations corresponding to the variations in the sound waves appear, a diode having an anode and a cathode, means including an impedance connecting the cathode and anode in shunt to a part at least of the output of said amplifier whereby a voltage is developed in the impedance proporl 0 tional to the magnitude of the signal in the output of the amplifier, and electrical or mechanical means excited by said last named voltage for regulating the sensitivity of the converting means.

8. A transducer including a movable element in a magnetic field and in a circuit wherein current is produced by said movement, means for automatically changing the static position of said element in the field in accordance with the strength of the produced currents to change the sensitivity of said transducer, and means for compensating the effect of changes in the static position of said element on the produced current.

GEORGE K. GRAHAM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,519,211 Martin Dec. 16, 1924 1,565,548 Clark Dec. 15, 1925 1,967,125 Miller July 17, 1934 2,113,219 Olson et a1. Apr. 5, 1938 2,309,109 Hathaway Jan. 26, 1943

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