US2370216A - Noise suppression circuits - Google Patents

Noise suppression circuits Download PDF

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Publication number
US2370216A
US2370216A US451847A US45184742A US2370216A US 2370216 A US2370216 A US 2370216A US 451847 A US451847 A US 451847A US 45184742 A US45184742 A US 45184742A US 2370216 A US2370216 A US 2370216A
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signal
limiter
potential
amplifier
amplitude
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US451847A
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Jr Joseph A Woreester
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General Electric Co
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General Electric Co
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Priority to BE476014D priority patent/BE476014A/xx
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    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03G—CONTROL OF AMPLIFICATION
    • H03G11/00—Limiting amplitude; Limiting rate of change of amplitude
    • H03G11/004—Limiting amplitude; Limiting rate of change of amplitude using discharge tubes
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03G—CONTROL OF AMPLIFICATION
    • H03G3/00—Gain control in amplifiers or frequency changers
    • H03G3/20—Automatic control
    • H03G3/22—Automatic control in amplifiers having discharge tubes
    • H03G3/26—Muting amplifier when no signal is present

Definitions

  • My invention relates to noise suppression circuits, and it is particularly applicable to noise suppression circuits for automatically rendering the output circuits of a frequency modulation receiver inoperative when no signals are being received or when the level of a received signal is below a predetermined selectable value. More specifically, the present invention is an improvement upon the arrangement shown in my coopending application Serial No. 407,543, filed August 20, 1941, and assigned to the same assignee as the instant application. V
  • noise suppression or squelch circuits heretofore proposed have 4been based upon the principle that the sound reproducer should be muted until the signal voltages appear which are of themselves suciently strong Asubstan tially to saturate the limiter.
  • suppression circuit having means for rendering f the circuit completely and substantially instantaneously inoperative upon theoccurrence of a signal voltage of predetermined value.
  • the noise voltages supplied to the limiter under no-signal orweak signal conditions are detected andD utilized to provide bias potentials for reducing the transfer elclency of a portion of the transmission channel following the Preferably one of the amplifying stages following the limiter is completely blocked and the sound Yreproducer thereby eiectively muted. If the noise voltage indication is takenfrom the llmiter output, such an arrangement in itself will provide automatic cuto of the noise suppression eiect as soon as a signal voltage of lust suillcient magnitude to saturate the limiter is received, by reason of the fact that at this point amplitude modulation in the limiter output is prevented and the squelch'voltage auto# matically collapses. It will be understood of course that noise voltagesmay also, if desired, be detected at the limiter input. In another modication this arrangement is used. Where such a system 4is usednolse voltages are still detectable even though .the limiter is saturated,
  • I combine in o'pposing relation with the detected noise bias voltage a biasvoltage of controllable and substantially constant value to obtain a net or differential bias voltage of selectable value whereby by dim inution of the noise bias voltage with increasing carrier voltage the net squelch voltage may be brought to zero at any predetermined selectable signal level.
  • a sensitive trigger action of the squelch circuit is insured by providing means for insuring immediate collapse of the differential bias voltage as soon as it tends to reverse due to predominance of the constant controllable bias. When such a differential voltage is used the noise voltages may be taken either at the l limiter input or output.
  • frequency modulation I mean to include 'any form of modulation produced by varying the frequency or phase of the carrier wave as distinguished from modulation produced by varying the amplitude of such a wave.
  • Fig. 1 is a schematic representation, partly in the form of a conventionalized oneline diagram, of a frequency modulation receiver embodying Amy invention: and Fig. 2 schemati- ⁇ cally represents a modied form of my invention
  • Fig. 1 has Ybeen utilized to illustrate two modifications of my invention by the interposition of 'a selector switch for choosing various circuit connections. 'In order to facilitate comparison o f the two figures and to simplify the accompanying description, like reference numerals have been placed upon corresponding elements of the tlg ⁇ urea. l
  • these voltages may be transmission channel; they may be caused by thermal agitation' eifects, shot effects, and related phenomena which occur in the receiver itself and give' rise to disturbances of the socalled continuous'ior "hash type; or they may be caused by natural atmospheric static, 'ignition interference 'and the like which occur outside the receiver and give rise to disturbances ofthe socalled impulse type.
  • Buch voltages normally vary ulti-utensili! or amplitude at irregular and rapid n
  • I intend to include all amplitude variations in the envelope of high -frequency voltages appearing at specined points of tbe Jtransmission channel, irrespective of the precise character of these voltages.
  • V'I'hus for the purpose of my invention these voltages Imay constitute periodic signal voltages either unmodulated or modulated heterodyne type.
  • Signals received at an antenna III are amplied in the usual radio frequency am pliilers il and combined in a mixer i2 with 1ocally generated waves supplied from an oscillator I3.
  • 'I'he carrier waves of the resulting intermediate frequency are then amplified in intermediate frequency ampliers Ill.
  • frequency modulation receivers of the usual design it is often the practice to pass the intermediate fre# quency waves through two amplitude limiters "n cascade in order to securebetter limiting action.
  • some of the amplitude modulation is re- .moved in a met limiter Il, and practically all of scribed in Patent No. 2,121,103, Seeley. granted June 21', 1938.
  • the demodulated audio signals are reproduced by supplying them first to an audio frequencyv ampliner Il, further amplifying them in-an audio power amplifier il, and then supplying them to a suitable signal translating device such as a loudspeaker 20.
  • the second limiter I0 comprises a pentode amplifier tube Il having an input circuit including 'a coupling capacitor Il and a grid resistor I! connected between its control grid 33 and its cathode M, and an outputcircuit including a tuned circuit Il connected to its anode 38.
  • Anode operating potentials are supplied in conventional 50 manner from a suitable source of power, lsuch as a battery 31, through the usual decoupling resistor 88.
  • the screen grid l! of the pentode tube Il is similarly supplied with operating potentials from the common power supply source l1 through a decoupling resistor and is coupled to the cathode M by means of the usual screen grid by-pass condenser Il.
  • the output circuit of the limiter It is also provided with a lay-pass condenser 42 of conventional form.
  • the limiter is selfbiased by means of the capacitor 3i and the grid resistor 82. It is so adjusted as to draw anode vcurrent only between the limits .at which positive grid current flows and at which anode current cutoi! takes place.
  • , 32 is such that the pentode 30 functions essentially as a grid power detector for amplitude modulation components within the audio frequency band. Therefore it produces audio voltages across an audio frequency load in the anode circuit.
  • a load is provided by the decoupling network comprising the resistor 38 and the capacitor 42. Accordingly, noise voltages at audio frequency appear at the'point 50.
  • These voltages are supplied through a low pass filter comprising a resistor 56 and a. capacitor 51 and through a blocking condenser 55 to a diode rectifier 58.
  • the low pass filter 56, 51 has for its purpose theV removal of intermediate frequency, while the condenser 55 removes any direct current in the limiter output.
  • Any detected audio frequency voltages which are thus applied to the diode detector 58 are rectified and appear across a load resistor 59 as unidirectional voltages having a magnitude proportional to the degree of amplitude modulation present in the 'noise voltages.
  • the unidirectional voltages appearing across the resistor 59 are filtered through a low pass filter comprising a resistor 60 and a capacitor 6
  • the voltages appearing across the load resistcr 59 are of such polarity as to bias the grid.
  • the bias voltages impressed upon the grid .62 are sufficient to reduce the amplification of the audio amplifier 64 to a very low level and preferably to bias it completely to cutoff, thereby reducing the transfer efliciency of or preventing transmission of signals to the sound reproducer comprising the power amplifier I9 and theloudspeaker 20.
  • I also provide a diode rectifier l connected across the terminals of the load resistor 59. Accordingly, when the noise voltages at 'the output of the limiter .I6 are sufficient to bias the audio ampliiler'grid 62 negative -with respect to its associated cathode 63, the plate il of the diode 'l0 is also biased negatively with respect tothe cathode 'l2 of this tube and the diode i0 does not conduct and has no eiect upon the operation.
  • the function of the diode lil will be described hereinafter.
  • the noise voltages impressed upon the limiter grid 33 develop self-bias potentials approaching values required to saturate the pentode tube 3B even under rio-signal conditions.
  • invention includes means for selectablyA controlling the amount of.cutoff bias supplied to the grid 62 of theamplier tube 64.
  • I provide Afor complete collapse of this bias upon any predetermined degree of diminution of the noise voltage appearing across the resistor 59 occasioned by the reception of a weak signal voltage.at.the output of the limiter I6. ⁇ 'Io this end au substantially constant positive or opposing bias of selectable value is applied by means of an adjustable potentiometer l5 between the grid 62 and the cathode 63 of the-amplifier tube 64 from the battery-31 through a resistor 16.
  • the purpose of the resistor ⁇ 16 is to-reduce the positive bias to a suitable value, and the resistor is preferably so selected that the maximum negative noise bias under no-signal conditions exceeds the normal positive bias by an amount just suflicient to bas'the tube 64 to cutoff.
  • a positive bias of seven or eight volts may be applied from the potentiometer 15, so that the net bias of two or three volts negative is just suicient to maintain the amplifier tube 64 inoperative.
  • the modified form of myinvention represented at Fig. l with the selector switch transferred from the .position shown in the vdrawing to the contact Il differs from that described above only in certain'particulars.
  • the source of positive potential from which a positive bias is applied to the grid l! of the ampliner tube through the potentiometer 1B is shown as a point in the limiter output circuit rather than the positive terminal of the battery 31.
  • the potentiometer Il is connected to the plate return of the limiter Il through a conductor Il and a ,low pass nlter comprising a resistor i! and a condenser I3.
  • This modification of the invention has the additional advantage that the positive potential of the plate 30 ofthe limiter Il.
  • the net squelch bias on theA ampli- I der tube 04 approaches pero more rapidly as soon as a weak carrier voltage is received, thereby further reducing the minimum carrier input necessary .to ldisable the squelch circuit.
  • the squelch circuit shall not function in the presence of a. predetermined minimum signal voltage; (c) the squelch circuit shall not be set in operation by the presence of a reasonable amount of amplitude modulation in the signal voltage.
  • these purposes are accomplished by receiving the noise indication from the grid of the first limiter and applying noise voltages thus received to the squelch circuit through a special squelch amplifier in combination with a high pass filter for removing low frequency components from the amplifier output.
  • the circuit there shown is in many respects similar to that illustrated in Fig. 1, and to simplify the description and facilitate a ready understanding of the invention, corresponding elements of the system been assigned like reference numerals.
  • Voltages appearing atthe control grid 33a of the first limiter I5 are utilized to set the squelch circuit in operation.
  • the first limiter comprises a pentode tube 30a and a tuned output circuit 35a,
  • the limiter grid voltage at' 33a is applied to a control lgrid 90 of a squelch amplifier 'triode 9
  • the rere'sistors 93 and 94 are shunted by a capacitor DI, which, in combination with the resistor l2 and certain stray capacities present, serves as a low pass filter for removing voltagesv of intermediate frequency. Thus, only audio frequency voltages are applied to the grid of the squelch amplifier 9i.
  • An audio frequency bypass condenser lla shunts the resistor 93.
  • the anode-cathode circuit of the amplifier 9i is supplied from a suitable source of direct current supply such as the battery31.
  • This output cir# cuit also includes an iron core choke coil l1 which with thevcondenser 55 acts as a high passnlter to remve from the output of the amplifier tube li any low frequency components due to amplitude modulation of the audio frequency out-
  • amplined and detected audio frequency'voltages y are applied to the diode detector 58 and are rectified to develop across the load resistor Il a negative bias potential of a magnitude proportional to the degree of amplitude modulation present in the noise voltages at the limiter grid um
  • the negative noise potential developed across the load resistor il is filtered in the low pass filter Il, Il and supplied through the resistor I! to the ⁇ control grid B2 of the first audio amplifier tube I4 in the same manner as has been hereto fore. explained with reference to Fig. 1.
  • the load resistor Il is filtered in the low pass filter Il, Il and supplied through the resistor I! to the ⁇ control grid B2 of the first audio amplifier tube I4 in the same manner as has been here
  • the magnitude of the negative bias voltage thus applied to the grid 82 is considerably greater'4 than that developed with a circuit arrangement similar to that of 1.
  • the developed negative bias may not be suicient to cut off the squelch amplifier 9
  • the: net negative bias on the grid 62 under .no-signal conditions may be controlled by means of the potentiometer 15, 15a, thereby selectably to' determine the minimum signal strength at which the squelch control opens or becomes ineffective.
  • the circuit of Fig. 2 isl so arranged that an increased negative bias applied to the .control grid 90 of the squelch amplifier 9
  • the voltage dividing resistors 93 and 98 are so selected that an increase in the negative bias applied to the squelch amplifier 9
  • a signal channel adapted to transmit a signal wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said channel being also eiective to transmit noise voltages appearing as audio frequency amplitude modulation of said signal wave, signal reproducing means coupled to said channel, means responsive to said amplitude modulation of said signal wave for at least partially disabling said'sig.
  • a signal amplifier adapted to amplify a carrier wave modu iated in frequency withinV a band of frequencies in'accordance with an impressed signal, said nimmer being also effective w supply in its outf lput substantial noise voltages modulated in amplitude at audio frequency, signal reproducing means coupled to said amplier, means for developing a lunidirectional potential proportional v in value to the degr'eeof amplitude modulation of waves supplied by'said amplifier.
  • means for reducing .said ⁇ potential in accordance with in- -lated in frequency stantially to zero when said waves attain a prel determined level, means for selecting said predetermined level comprising a second unidirectional potential of selectable value opposing said ilrst potential thereby to provide a selectable and variable differential potential, and means controlled by said dierential potential for atleast partially disabling said signal reproducing means thereby to preclude the reproduction of noise voltages so long as said carrier wave is below said predetermined level.
  • a radio receiving apparatus o. signal amplifier adapted to amplify a carrier wave modull in frequency within a band o irequem cies accordance with an impressed signal, said amplifier being also eective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said amplifier, a signal reprodncer coupled to said limiter, moans associated with said limiter for developing a unidirectional potential proportional in value to the degree of amplitude modulation present in waves below a predetermined level supplied to said limiter, ad-
  • instable means for opposing said potential with a second unidirectional potential of selectable value thereby to provide a differential potential, and Imeans controlled by said differential potential for at least partially disabling said signal reproducer, said adjustable means being operable selectably to render said disabling means ineffective in Ithe presence of carrier waves at any level below said predetermined level.
  • a radio receiving apparatus a signal amplifier adapted to amplify a carrier wave modulated in frequency within a band of frequencles in accordance with an impressed signal, said amplier being also eflective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled-to said amplifier, signal reproducing means including an electron discharge de- ⁇ vice having a control electrode coupled to the trode coupled to the output of'said limiterl amplitude modulation detection means for developing a negative bias potential upon said control electrode proportional tol amplitude modulation of waves supplied to said limiter below said limiting level, and means for supplying to said control electrode a positive bias potential of selectable value less than said negative potential so long as the amplitude of said carrier wave remains below a desired value thereby to suply to said control electrode a net negative alas potential sulcient substantially to disable said reproducing means until said carrier wave exceeds said desired value.
  • a radio receiving apparatus a signal ampli'ler adapted to amplify a carrier wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said amplifier being also eective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said amplifier, signal reproducing means including 'an electron discharge device having a control electrode coupled to said limiter, means associated with said limiter for developing a, unidirectioal potential proportional in value to the degree of amplitude modulation present in waves below a predetermined level supplied to said limiter, adjustable means for opposing output of said limiter, means responsive to amplitude modulation of waves below a predetermined level supplied to said limiter to apply to said control electrode a bias potential proportional to said amplitudeinodulation, anddiustable means for applying to said control electrode an opposing bias potential of selectable value thereby to provide a diilerential bias potential of such polarity as to tend to disable saidelectron discharge
  • a plifler adapted to signal am# within a band of frequencies in accordance with an impressed signal, said ampliiler being also effective to supply in its output 'substantial undesired noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled toibaid ampliner, said limiter being effective to remove: amplitude moduf lation at carrier frequencies from thewaveupplied to said limiter so long as a carrier wave exc'eeding a predetermined limiting level in supplied thereto,'a signal-reproducer including an l electron 'discbargedevice having a control elecamplify a carrier wave modu-1 said potential with a second unidirectional potential of selectable value thereby to 'provide a differential potential, means for applying said differential potential to said control electrode to disable said electron discharge device, said adjustable means being operable selectably to render said disabling means ineffective in the presence of carrier waves below said predetermined level, and means for preventing reversal of said differential potential upon
  • a signalempliiler adapted to amplify a carrier wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said ampliiler being also eilective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said amplifier.
  • signal reproducing means including an electron discharge device having 'a control electrode coupled to said limiter, means associated with said limiter for developing a negative bias potential proportional in value to the degree of amplitude modulation present in waves below a predetermined level supplied to 'carricr waves below said predetermined level,
  • a signalantpliner adapted to amplify a carrier wavemodulated in frequencyl within a band of frequencies in accordance with an impreascdlimll. said ampliiier being also elective to supply lin ih out- ,a
  • an amplitude limiter coupled to said amplifier
  • signal reproducing means including an electron discharge device having a control electrode coupled to the output of said limiter, means responsive to amplitude modulation ofNaves below a predetermined level supplied to said' limiter to apply to said control electrode a bias potential proportional to said amplitude modulation, adjustable means for applying to said control electrode an opposing bias potential of selectable rsubstantially.
  • said adjustable means belngarranged to maintain said differential potential suiiicient substantially to disable said electron discharge device in the absence of a carrier wave and selectably to reduce said differential potential to zero in the presence of carrier waves below said predetermined level, and means associated with said electron discharge device for preventing a reversal of said differential bias potential.
  • a signal ampliler adapted to amplify a carrier Wave modulated in frequency within 'a band of frequencies in accordance with an impressed signal, said amelectron discharge device, the magnitude of said differential potential in the absence of a carrier wave being selectable by said adjustable means thereby to provide for reduction of said differential -potential to zero in the presence of a carrier wave' of desired amplitude.
  • a signal amplifier adapted' to amplify a carrier wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said amplier being also effective to supply in its output substantial undesired noise voltages modulated in amplitude at audio frequency, an amplitude limiter coupled to said ampliner, said limiter including an electron discharge device having an output circuit, a signal reproducer including a second electron discharge device having a control electrode coupled to said output circuit,
  • vamplitude Amodulation detection means also plifler Vbeing also' effective to supplyfin its output substantial undesired noise voltages modulated in amplitude at audio frequency
  • an amplitude limiter coupled to said amplifier, said limiter comprising an electron discharge device having an output circuit and being eiective to remove amplitude modulation at carrier frequencies from the wave supplied to said limiter so long as a carrier wave exceeding a predetermined limiting level is supplied thereto, a signal reproducer including a second electron discharge device having a control electrode coupled to the output of said limiter, amplitude modulation detection ⁇ means for developing a negative bias potential upon said control electrode proportional to.
  • amplitude modulation of waves supplied to said limiter below said limiting level means associated with said output circuit of said first electron discharge device for supplying to said control electrode a positive bias potential of selectable value less than said negative potential as long as the amplitude of said carrier wave remains below a desired value, and means responsive to a predominance of said positive bias potential to maintain the net potential upon said control electrode substantially at zero.
  • a ,signal amplifier adapted to amplify a carrier wave modulated in frequency within a. band of frequencies in accordance with an impressed signal, saldamplier being' also effective to supply in its output substantial noise voltages modulated in amplitude at audio-frequencman amplitude limiter coupled to said amplifier, said limiter comprising an electron discharge 'device having an output circuit, signal reproducing means including,- a second electron discharge device having 4a con-- trol electrode coupled to said output circuit', means responsive to amplitude modulation of waves appearing at the output of said limiter for applying to said controlelectrode abias potential of proportional Value, adjustable means for applying to said control electrode an opposing biasof selectable substantially constant value.
  • a signal amplifier adapted 'to amplify a carrier wave modulated in frequency' within a band of frequencies in accordancevwith an impressed signal, said amplifier being also eective to supply -in its output substantial noise voltages modulated in amplitude at audio frequency
  • an amplitude limiter coupled to said ampliner including an electron discharge device having an anode circuit
  • signal reproducing vmeans including a second electron discharge device having a control electrode coupled to said anode circuit
  • amplitude detection means also coupled to said anode circuit for applying to said control electrode a negative bias potential Prwortional to' the degree of amplitude modulation of'waves appearing in said anode circuit
  • adjustable 'means coupled to the anode of said ilrst electron discharge device for applying to said control electrode ana opposing bias potential of selectable value thereby to provide a differential bias upon said control electrode tending to disable said electron discharge device, the magnitude of said differential potential in the absence of a carrier Wave being normally suicient completely to disable
  • a signal amplier adapted to amplify a carrier Wave modulated in frequency Within a band of frequencies in accordance with an impressed signal, said amplifier being'also effective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, signal repron ducing means coupled to the output of said amplier, means including an electron discharge device for amplifying and detecting amplitude modulated voltages appearing in the output of said amplifier, said means including means to reduce the gain of said electron discharge device only in the presence of a carrier wave, means for developing a potential proportional to said detected voltages, means for opposing said potential to provide a differential potential, and means responsive to said differential potential for at least partially disabling said signal reproducing means.
  • a signal amplifier adapted to amplify a carrier wave modulated in frequency.within a band of frequencies in accordance with an impressed signal
  • said amplifier being also effective to supply in its output substantial noise voltages modulated in amplitude at audio frequency, signal reproducing means coupled to the output f said amplifier, means ⁇ including an electron discharge device for amplifying and detecting audio frequency voltages appearing in the output of said amplifier.
  • said means including means to'redce the gain of said electron discharge device in the presence of a carrier wave, said gain remaining substantially constant upon increase of said noise voltages, means fordeveloping a potential 'proportional to said'detected audio frequency voltages,
  • selector means for opposing said potential to provide a differential potential, means responsive to said differential potential for at least partially disabling said signal reproducing means, and means associated with said slgnalreproducing means for preventing a. reversal ofsaid differential potential.
  • a signal amplifier adapted to amplify carrier waves modulated in frequency within a band of frequencies in accordance with an impressed signal, said amr pllner being also effective to supply in its output substantial noise voltages modulated in amplitude.- at audio frequency,v signal reproducing means coupled to said amplifier, means includins asf/0,216
  • noise voltages means for developing a potential proportional to said detected audio frequency voltages, means rendering said last-named means nonrespo-nsive to amplitude modulation of said detected audio frequency voltages, selector means for opposing said potential to provide a differential potential, means responsive to said differential potential for at least partially disabling said signal reproducing means, and means associated with said signal reproducing means for preventing a reversal of said dierential potential.
  • a signal amplifier adapted to amplify a carrier wave modulated in frequency within a band'of frequencies in accordance with an impressed signal, said amplifier being also effective to supply in its output substantial noise voltages modulated in arnplitude at audio frequency
  • signal reproducing means coupled to the output of said amplifier, means including an electron discharge device having a control electrode for amplifying and detecting 'audio frequency waves appearing in the output of said amplifier, the output of said amplifier being coupled to said cmtrol electrode through a circuit so proportioned that the output of said electron discharge deviceremains substantially constant with variations in intensity of said noise voltages, said circuit being arranged to reduce the gain of said electron discharge device in the presence of a carrier wave of predetermined intensity, means for developing a potial proportional to said detected audio frequency Wave, selector means for opposing said potential to provide a differential potential, and means responsive to said differential potential for at least partially disabling said signal reproducing means.
  • a signal amplifier adapted to amplify a carrier wave modulated in frequency within a band of frequencies in accordance with an impressed signal, said amplifier being also effective to supply in its output substantial noise voltages 4modulated in amplitude at audio frequency, signal reproducing means coupled to said amplifier and including a first electron discharge device having a control electrode, means including a second electron discharge device having a.
  • control electrode for detecting and amplifying audio frequency waves appearing in the putput of said amplifier, thev said detected audio frequency wave, selector means for applying to the control electrode of said first electroresponsive device a positive bias potential less than said negative bias potential u so long as the intensity -of said carrier wave an electron discharge device for' amplifying and detecting audio frequency voltages appearing in the output of said amplinenssid means including means for reducing the gain offsaid electron.

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US451847A 1942-07-22 1942-07-22 Noise suppression circuits Expired - Lifetime US2370216A (en)

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Cited By (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2447564A (en) * 1943-01-30 1948-08-24 Zenith Radio Corp Noise reducing circuits
US2459675A (en) * 1941-04-05 1949-01-18 Motorola Inc Interference reducing radio receiver
US2493446A (en) * 1946-01-23 1950-01-03 Rca Corp Radio receiver noise muting circuit
US2513362A (en) * 1945-09-19 1950-07-04 Joseph L Richey Apparatus for improving the reception of telegraph signals
US2533543A (en) * 1947-12-06 1950-12-12 Bell Telephone Labor Inc Off-channel squelch circuit for radio receivers
US2546987A (en) * 1946-12-06 1951-04-03 Standard Telephones Cables Ltd Noise suppression circuit
US2552585A (en) * 1947-01-09 1951-05-15 Nielsen A C Co Apparatus for determining listening habits of radio receiver users
US2569348A (en) * 1946-05-28 1951-09-25 Bendix Aviat Corp Squelch circuit
US2589711A (en) * 1948-05-05 1952-03-18 Bell Telephone Labor Inc Off-channel squelch circuit for radio receivers
US2602885A (en) * 1946-03-30 1952-07-08 Edwin H Armstrong Radio signaling
US2679000A (en) * 1949-01-13 1954-05-18 Harold B Reynolds Noise squelch system for frequency modulation receivers
US2756328A (en) * 1951-06-27 1956-07-24 Hartford Nat Bank & Trust Co Audio-noise responsive squelch circuit
US2914664A (en) * 1956-03-26 1959-11-24 Browning Lab Inc Apparatus for frequency-modulation reception with means for muting noise
DE102019205694A1 (de) * 2019-04-18 2020-10-22 Volkswagen Aktiengesellschaft Geschwindigkeitsabhängige Rauschunterdrückung bei Audiosignalen in einem Fahrzeug

Cited By (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2459675A (en) * 1941-04-05 1949-01-18 Motorola Inc Interference reducing radio receiver
US2447564A (en) * 1943-01-30 1948-08-24 Zenith Radio Corp Noise reducing circuits
US2513362A (en) * 1945-09-19 1950-07-04 Joseph L Richey Apparatus for improving the reception of telegraph signals
US2493446A (en) * 1946-01-23 1950-01-03 Rca Corp Radio receiver noise muting circuit
US2602885A (en) * 1946-03-30 1952-07-08 Edwin H Armstrong Radio signaling
US2569348A (en) * 1946-05-28 1951-09-25 Bendix Aviat Corp Squelch circuit
US2546987A (en) * 1946-12-06 1951-04-03 Standard Telephones Cables Ltd Noise suppression circuit
US2552585A (en) * 1947-01-09 1951-05-15 Nielsen A C Co Apparatus for determining listening habits of radio receiver users
US2533543A (en) * 1947-12-06 1950-12-12 Bell Telephone Labor Inc Off-channel squelch circuit for radio receivers
US2589711A (en) * 1948-05-05 1952-03-18 Bell Telephone Labor Inc Off-channel squelch circuit for radio receivers
US2679000A (en) * 1949-01-13 1954-05-18 Harold B Reynolds Noise squelch system for frequency modulation receivers
US2756328A (en) * 1951-06-27 1956-07-24 Hartford Nat Bank & Trust Co Audio-noise responsive squelch circuit
US2914664A (en) * 1956-03-26 1959-11-24 Browning Lab Inc Apparatus for frequency-modulation reception with means for muting noise
DE102019205694A1 (de) * 2019-04-18 2020-10-22 Volkswagen Aktiengesellschaft Geschwindigkeitsabhängige Rauschunterdrückung bei Audiosignalen in einem Fahrzeug

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BE476014A (enrdf_load_stackoverflow) 1947-10-31

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