US2151778A - Reduction of noise - Google Patents

Reduction of noise Download PDF

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Publication number
US2151778A
US2151778A US71380A US7138036A US2151778A US 2151778 A US2151778 A US 2151778A US 71380 A US71380 A US 71380A US 7138036 A US7138036 A US 7138036A US 2151778 A US2151778 A US 2151778A
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United States
Prior art keywords
signal
channel
voltage
tube
noise
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Expired - Lifetime
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US71380A
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English (en)
Inventor
Vernon D Landon
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RCA Corp
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RCA Corp
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Priority to US71380A priority Critical patent/US2151778A/en
Priority to FR818731D priority patent/FR818731A/fr
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    • 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
    • H—ELECTRICITY
    • H03—ELECTRONIC CIRCUITRY
    • H03G—CONTROL OF AMPLIFICATION
    • H03G11/00—Limiting amplitude; Limiting rate of change of amplitude
    • H03G11/04—Limiting level dependent on strength of signal; Limiting level dependent on strength of carrier on which signal is modulated

Definitions

  • noise impulses may be separated from the signal
  • Fig. 1 is a wiring diagram of a noise reduction impulses without substantial distortion of the circuit involving the use of a single limiter or signal impulses. control tube,
  • Fig. 2 illustrates a similar circuit involving the 10 heretofore been proposed or used.
  • Many of these use of a pair of limiter tubes which operate by devices are not altogether satisfactory for the means of time delay circuits to increase the time reason that they operate to eliminate the signal during which the signal path is short circuited, impulse during the continuance of the noise imthus eliminating the edge portions of the noise pulse thus producing objectionable distortion of impulse passed by the single cutting tube of Fig. 1, 15 the received signal.
  • Fig. 3 illustrates a circuit which differs from ent invention, this difficulty is avoided by the prothat of Fig.
  • a vacuum tube is so ar- Fig. 4'. illustrates a modified noise reduction ranged as effectively to short circuit the amplifier circuit which operates to maintain a substantially 5 when the input voltage exceeds a predetermined constant ratio between the signal strength and 5 value. Under these conditions, signal impulses of the voltage required for operation of the limiter a voltage below this predetermined value are alor cutter, and
  • FIG. 5 is a block diagram which illustrates the sence of relatively high voltage noise impulses application of the noise reduction circuits of 0 but the signal impulse is eliminated during the Figs. 1 to 4 to a radio receiver.
  • the circuit of Fig. 1 includes input terminals impulse. H! from which signal impulses similar to those In an improved modification employing a indicated at H may be supplied to output termiplurality of vacuum tubes, signal delay circuits nals l2 through a circuit including coupling caare utilized to eliminate portions of the noise pacitor l3, resistor l4 and. coupling capacitor [5.
  • the signal is deleted during the vention, the tube I1 is biased beyond cutofi and period of the noise impulses and the deleted sigdraws no plate current in the absence of high nal portion is replaced by a portion of the signal level signals.
  • the path of the signal under these immediately preceding the deleted portion. conditions is through the circuit l3--I4l5 and
  • the present invention is in some respects an the ground return circuit.
  • the signal consists of a carrier modulated by an unvarying audio sine wave I I, for example, and if the noise impulses 26 are repeated at regular intervals, then the phenomena may be observed on an oscillograph.
  • the composite wave I I26 is a reproduction of such an observation.
  • the composite wave I l-2'l is a similar reproduction illustrating the efiect of the limiter tube l7.
  • the apparatus of Fig. 2 is similar to that of Fig. .1 .in several respects, but differs therefrom in that it includes a low pass filterv or delay network 29-3D-3l.32 and an additional limiter tube .33 connected in parallel with the limiter tube I 1. Itv will be noted that the limitertubes 11 and 33 are subjected to out-of-phase grid control potentials derived from different points A and C of the delay network 2930- 3l..32.
  • the tube 33 is caused to pass plate current before the noise impulse has established :an appreciable voltage at its plate.
  • the tube l! passes current later and continues effectively to short circuit the signal path while the impulse is dying away at B thus releasing control an instant later when the impulse dies away at C.
  • the net result is an increase in the time during which the signal path is short circuited'as indicated at 28, thus :reducing or eliminating that portion of thenoise impulse which is passed by the single tube I! of Fig. 1.
  • the two tube circuit of Fig. 2 leaves something to be desired in that it still cuts a hole in the signal as indicated at 28 and in that the resulting noise :is still audible enough to be objectionable.
  • this difiiculty may be avoided by the addition of a circuit including a tube 35 which amplifies the 'voltage passed by the limiter tubes.
  • This amplified voltage is passed through a-step down transformer 36 which gives the same voltage amplitude as that applied to the grid.
  • a variable resistor 3'! may be connected across the secondary windingfor adjusting the signal to the same level as at the grid.
  • Capacitors 38 and 39 may be connected across the primary or secondary or both to give a slight delay to the secondary voltage. This voltage is applied to the cathodes of the limiter tubes.
  • a low frequency signal can not cause the limiter tubes to act'because their grid and cathode voltages are equal and of near the same phase "so that there is no efiective input voltage applied between the grid and cathode.
  • high frequency modulation or a steep wave front such as a noise impulse arrives
  • the sudden application of grid voltage to the limiter tubes causes them to act before the voltage has time to reach the cathode.
  • the voltage level to which the limiter tubes pull the circuit is determined by the momentary potential of their cathodes. This potential corresponds to that of the signal at a time immediately preceding the noise impulse.
  • low frequency modulation is scarcely disturbed at all by :the noise pulse and the noise output is almost completely eliminated, as indicated at 34. This slight irregularity of one cycle of the Wave indicates the point where the noise impulse occurs.
  • the amount of current required to pull the plate potential down to about cathode potential depends on the amplitude of the noise peak appearing at the point B.
  • the potential to which the plates drop is not quite as low as when the required current is lowered.
  • more complete noise elimination may be accomplished if all noise impulses have the same amplitude.
  • all such impulses may be made to have about the same amplitude by employing a limiter in a previous portion of the circuit. For example, the plate voltage on the last intermedate frequency tube may be reduced to a value only two or three times that required to pass the signal.
  • automatic volume control of the type Which allowsstrong signals to come through slightly stronger than weak signals, is employed, then it is of some advantage to use a series plate resistor in the (by-passed) B supply to this tube so that the automatic volume control will cause limitation to work at a higher level when stronger signals are being received.
  • a 6L7 type tube may be used and signals applied to one grid and automatic volume control to the other. Certain advantages may be realized by using 6L7 type tubes throughout the receiver for similar reasons. To reduce strong signals to the same level as weak signals at the limiter tubes, the tube just preceding the limiters may be a GUI type with audio applied to one grid and automatic volume control to the other.
  • the invention may be modified to cause the adjustment of the limiter tube to vary automatically with the signal strength so that the ratio of the signal voltage to the voltage required for limitation is maintained approximately constant.
  • the first audio tube 40 is diode-biased by thedetector and its cathode returns to a point about 30 volts negative to ground through a resistor 4
  • the bias developed by the diode 42-43 shown as enclosed in the same receptacle as the first audio amplifier
  • This variation in cathode voltage is applied to the grid of the limiter tube I! through resistors 44 and 45 thus biasingthis grid in accordance with the signal level.
  • the gain of the audio tube 46 is adjusted so that a 100% modulated signal is of barely sufficient amplitude to activate the limiter tube ll.
  • the cathode of the audio tube 4! is not by-passed to ground because it is impractical to use a large enough, by-pass to avoid undesirable distortion of and prolongation of the noise impulse.
  • the resistor 44 and a capacitor 46 are used to filter out the audio impulses from the bias lead.
  • the resistor 44 may be much larger than the bias resistor M.
  • this circuit improves the automatic volume control circuit performance limiting strong signals to a lower value at the detector. This results from the fact that the diode anode "4243 negative swing is increased by the negative swing of the audio tube 40 so that a given radio frequency signal produces a greater automatic volume control voltage.
  • the automatic volume control lead is indicated at 41.
  • Fig. 5 illustrates diagrammatically the application of-the circuits previously described to a complete radio receiver. This figure will be readily understood from the legends applied to it without detailed explanation of its various features.
  • control potentials are shown as derived from batteries, it will be apparent that such potentials may be derived from a source of rectified voltage or the like and that other modifications may be made without departing from the spirit and scope of the invention.
  • the limiter tubes l1 and 33 may be replaced by a single coplanar grid vacuum tube discharge device wherein the coplanar grids are each connected to a diiferent point of the delay network.
  • means providing a signal amplifying channel comprising a pair of limiter devices provided with cathode-anode circuits connected across said channel, and with grid control circuits energized from different points of said network, means operable to subject said limiter device cathodes to controlling potentials with respect to ground, and means responsive to changes in the amplitude of a signal supplied to said channel for varying said potentials.
  • the method of operating a noise reduction system including an audio frequency signal channel which includes deleting from the signals passing through said channel the minute signal portions containing noise impulses and, while the remainder of the signals less the deleted portions are passing through said channel, replacing said deleted portions by an audio frequency signal portion immediately preceding each deleted portion.
  • an audio frequency signal channel subject to high amplitude interference pulses, means for amplifying audio frequency signals and received interference voltage pulses, means for reducing to zero that portion of each :pulse which exceeds a predetermined value, and means in said channel for substituting therefor in said channel an undeleted portion of the signal wave immediately preceding the interference pulse.
  • means for amplifying audio frequency signals and re ceived interference voltage pulses means for reducing to zero that portion of each pulse which exceeds a predetermined value, means for adjusting said last named means to operate at said predetermined value, and means including delay cir cuits for reducing to zero portions of the pulse immediately preceding and following the portion Whose amplitude exceeds said predetermined value.
  • electron discharge tubes having anode-cathode circuits connected across the signal channel and having control grids, means for maintaining the anodes at substantially cathode potential at times when either grid is actuated, means providing an actuating interference voltage on the grid of one tube at a time preceding the arrival of the impulse on the anode, said means including a time delay circuit, and means for maintaining an actuating voltage on the grid of the other tube following the impulse.
  • electron discharge device having an anode connected to one side of the signal channel and a cathode to the other side of said channel, means for applying signals to said channel and across said device, additional means for maintaining the anode at substantially cathode potential for the duration of voltage pulses exceeding a predetermined amplitude at another point in the signal channel, and means providing a voltage wave on the cathode corresponding to the signal previously passed through said channel.
  • a signal channel two multiple electrode tubes having control grids and having anodes connected to a point on one side of the signal channel and cathodes to the other side of the signal channel, means for maintaining the anodes at the potential of the cathodes at all times when actuating voltages are applied to the control grids of said tubes, means including a delay circuit providing an actuating voltage for the grid of one tube beginning at a time immediately preceding the arrival of an interference high amplitude impulse on the anodes, means including a second delay circuit, providing and maintaining an actuating voltage for the grid of the other of said tubes, and means for causing said last named actuating voltage to cease at a time immediately following the termination of an interference impulse at the point from which the anodes receive the signal voltage.
  • a signal channel two multiple electrode tubes having control grids and having anodes connected to a point on one side of the signal channel and cathodes to the other side of the signal channel, means for maintaining the anodes at the potential of the cathodes at all times when actuating voltages are applied to the control grids of said tubes, means including a delay circuit providing an actuating voltage for the grid of one tube beginning at a time immediately preceding the arrival of an interference high amplitude impulse on the anodes, means including a second delay circuit providing and maintaining an actuating voltage for the grid of the other of said tubes, means for causing said last named actuating voltage to cease at a time immediately following the termination of an interference impulse at the point from which the anodes receive the signal voltage, means for applying signal po tential to the cathodes of said tubes, and means for deriving said last named signal potential from the signal channel at a point beyond the short circuiting connection provided by the tubes.

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US71380A 1936-03-28 1936-03-28 Reduction of noise Expired - Lifetime US2151778A (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
US71380A US2151778A (en) 1936-03-28 1936-03-28 Reduction of noise
FR818731D FR818731A (fr) 1936-03-28 1937-03-05 Perfectionnement à la réduction du bruit dans des radio-récepteurs

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US71380A US2151778A (en) 1936-03-28 1936-03-28 Reduction of noise

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FR (1) FR818731A (fr)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2448027A (en) * 1943-11-23 1948-08-31 Standard Telephones Cables Ltd Static reducing pulse receiver
US2680806A (en) * 1949-12-24 1954-06-08 Du Mont Allen B Lab Inc Noise reducing synchronization circuit
US2710347A (en) * 1952-06-03 1955-06-07 Technicolor Motion Picture Interference reducing circuit
US2803700A (en) * 1952-03-01 1957-08-20 Rca Corp Signal level control of noise cancellation tube conduction threshold

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2448027A (en) * 1943-11-23 1948-08-31 Standard Telephones Cables Ltd Static reducing pulse receiver
US2680806A (en) * 1949-12-24 1954-06-08 Du Mont Allen B Lab Inc Noise reducing synchronization circuit
US2803700A (en) * 1952-03-01 1957-08-20 Rca Corp Signal level control of noise cancellation tube conduction threshold
US2710347A (en) * 1952-06-03 1955-06-07 Technicolor Motion Picture Interference reducing circuit

Also Published As

Publication number Publication date
FR818731A (fr) 1937-10-02

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