US2449971A - Apparatus for reducing noise in microphone circuits - Google Patents

Description

Sept. 28, 1948. a J. F. ANDREWS 6 2,449,971

APPARATUS FOR REDUCING NOISE IN MICROPHONE CIRCUITS Filed Feb. 26, 1944 INVENTOR JOSEPH FRANK ANDRE ws.

Patented Sept. 28, 1948 uN TEDfSTAT I Appucamnfr mm 2s, 1944,; Serial No. 523.993.. 1' cam ici. 175 1) (Grantee under-t em of Mai-chd, ate, it";

-- amended April 30, 1928; 0,6. 751) This invention relates to'microphone or speech input circuits of communication systems and has particular reference to radio communication systems for aircraft.

In currently designed aircraft communication systems of the class described and particularly where oxygen mask equipment is utilized,.much distortion of speech in the receivers has-beenfound to exist. This distortion is caused pri: marily by too high an amplification of th lower frequency bands of speech frequency components. The components of speech frequency in the lower bands i. e. those below about 350 cycles, approach the natural resonant frequency of the various voice cavities which include the voice passages of the throat and the cavities in the oxygen mask. Because of this, sympathetic vibration of such cavities at their natural resonant frequency is set up and fed into the speech amplifier at too high an amplitude level and thereby produce distortion in the speech when it is received. Furthermore, certain other low frequency noises which are inherent in the operation of the aircraft find their way into the speech amplifier to produce additional distortion. Also present are high frequency noises i. e. those above about 3,000 cycles, and these likewise are also stepped up to too high a level in the speech amplification and interfere with intelligibility of the speech as it is received.

Since the naturalness of one's voice is determined primarily by those components of speech frequency which lie between about 350 and 3,000 cycles, and which may be referred to as the essential voice frequency band, it is evident that much 7 of the aforesaid distortion can be eliminated by providing a system in which frequencies below and above the aforesaid limits are substantially attenuated.

The primary object of this invention is therefore to provide a novel microphone input circuit for communication systems in which substantial attenuation is obtained with respect to frequencies which lie below and above predetermined limits.

A more specific object of the invention is to provide an input circuit of the class described which comprises a series connected capacitor and a parallel connected varistor having a capacitor in series therewith, the function of which is to substantially attenuate frequencies which lie both above and below predetermined limits,

These and other objects will become more apparent from the detailed description to follow and from the accompanying drawing, th single figureof which is a schematic circuit diagram ,of a preferred embodiment-of the-invention. I 1

Referring now to the drawing, the speech in put circuit includes amicrophone III which is connected in circuitwith a battery. H and primarylwindingllof step up transformer l3...

The secondary .wind ing 14 of transformer I3 is connected at one side thereof. tothegrid, l5 of,amplifl er-.-tube l6. The othersideof the; sec,-

. y winding is connected to the cathode, |1 oftube it... U

The anode ll of tube i6 is connected in cir cuit with a source of potential I9 and the primary winding 20 of step-up transformer 2 i. The secondary winding 22 of transformer 2| is connected to other amplification stages if desired and thence fed into the telephone receivers. Such parts of the circuit are conventional and hence have not been illustrated as the invention described and claimed herein is directed to novel improvements in that portion of the speech input circuit which is shown in the drawing.

A capacitor 23 is connected in series in the speech input circuit and is of such capacity as to present a very high impedance to frequencies below about 350 cycles but which decreases toward the normal impedance within the essential voice frequency band which lies above this frequency. Thus the gain of tube I6 is reduced materially with respect to speech frequency components below 350 cycles.

Connected in parallel with the secondar M of transformer I3 is a second capacitor 24 and a varistor 25 in series therewith. Varistor 25 has a negative resistance characteristic. One suitable type which may be used is made from silicon carbide and is described in the October 1940 issue of the Bell Laboratory Record. The resistance characteristic of the varistor is such that its value decreases exponentially as the potential impressed across it is increased. Thus a very small change in the potential impressed across the varistor will result in a considerable change in current through it. The function of varistor 25 and capacitor 24 in series therewith is to present a high shunt impedance to allcomponents of speech frequencies below about 3,000 cycles. However, with respect to frequencies which lie above 3,000 cycles-and which are normall at too high a level of signal potential, the impedance presented by capacitor 24 and varistor 25 drops appreciably and hence increases the shunting effect of capacitor 24 to reduce the gain of tube IS.

A parallel connected resistor 26 may also be 3 utilized and functions to furnish grid bias to tube I5.

While the proper values of capacity and resistance of the various parts shown in the drawing will vary somewhat with the type of microphone, transformers"aud t-amplifier tubesguse'd tin ,thei'-: system, I havefo'und that'very good results =may be obtained when the capacity of capacitor 23 apparatus used with an oxygen mask for attenuating sounds impinging on a, microphone therein, which sounds have a, high magnitude at frequencies above and below the essential speech frequency band, the combination of a circuit for userwithztsuoh apparatus including 12a :first capacitdn-ofifsuchtsizeiasxfto offer :a:?high"- impedance to frequencies of the order of 350 cycles and below,

is .025 mf., that of capacitor 24 .008 mf., that? means having a negative resistance characterisresistance of varistor 25 variable"betweennlfinfln iltiohsaidameans.b i in Series W a Second and 10,000 ohms, and the resistance oniresistori:' ana'citbri'i saidm ans and said se nd capa itor 26 2 megohms. being inishunt across said circuit for efiecting a In conclusion, it should be evidentstnat lowrimncdancec hcncc ase shuntin novel circuit arrangement wkrichgha-sl-been.ide-i effect to signals of high, magnitude at frequencies scribed will function to attenuate" substantialiy*'15rof the-order'of- 3000*cyclcs n above, whereby those voltages impressed across the grid'il5a-iof the eoutput of the circuit has an essentially flat frequency response within the speech frequency quencies which lie below aboutr'350 cycleszand'ifl and-:2 above about 3,000 cycles, with the result "that speech input to the amplifier tube Hi from the ,20 microphone r l Wis? amplifid iand transmitted *td f the various receiv ersin the 'systemrwith very-little distort/lone Th inventiozi descflbd hereirr may bein'a'nn fa cturedrand used by jor for the Goyernmenuon 2 JOSEPH FRANK ANDREWS.

' REFERENGESvCITEDzE file 'ofithi's patent r V a UNITEDSTATESUPATENTSH 'rrietrcnswmg:references:cr of record in the the unitcd 'states crmmencagrefg vemmentcr N'lirhbcr Name Date pu-rpq'seg -witfibut, 'the pa ymen'tf ofz fany royafltigs f J 1 1 D965 5;E 1933 "I m a r w 1,90%,762 Bierworth-; Apr. 16,-1935- Havingithus-" fully describedmy inventionril 2,930,937 n Feb; 1936 wy n -l I 30"? 2,04 -3;-16-1" Foster June '2, 1936* In animproved distortionless communication

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