US2369952A

US2369952A - Background noise suppressor

Info

Publication number: US2369952A
Application number: US507409A
Authority: US
Inventors: George F Devine
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1943-10-23
Filing date: 1943-10-23
Publication date: 1945-02-20
Anticipated expiration: 1962-02-20

Description

Feb. 20, 1945.

G. F. DEVINE BACKGROUND NOISE SUPPRESSOR Filed Oct 23, 1943 Inventor": George Fipevine. y Z

His .ttorney Patented Feb. 20, 1945 2,369,952 BACKGRGUND NQISE SUPPRESSOR George F. Devine, Easton, Conn assignor to General Electric Company, n'eorporation of New York Application October 23, 1943, Serial No. 507,409

8 Claims. (Cl. 179-400.!)

My invention relates generally to noise sup- Dression circuits for audio frequency channels of thetype in which the appearance of background noise cannot be prevented. It is an object of my invention to provide an improved noise suppression circuit for such channels which operates normally to remove high audio frequencycurrents only from the output of the channel and which is rendered inoperative by the appearance of high audio frequency currents at the input of the channel to permit translation of such currents by the channel.

- In audio frequency translating channels which employ electronic amplifiers, it is customary to employ a noise suppression circuit to reduce noise voltages in the output of -the channel in the absence of signals to be translated. The time of operation of such circuits is a factor which enters into their successful application. This time must be long compared to the lowest frequency voltages which it is desired to transmit. In order to avoid distortion of the audio signal being translated,

such noise suppression circuits usually operate at syllabic speed. It is an object of my invention to provide an improved noise suppression circuit, the suppressor action of which is limited to higher audio frequencies to permit a very short time constant in the control circuit.

My invention relates more particularly to noise suppression circuits for electric phonograph record reproducers and it has for one of its objects to provide an improved circuit for reducing the record background noise in the output of such a reproducer.

The background noise usually encountered in record reproducers is most annoying during quiet passages of recorded music, when higher frequency tones are not present, and at the beginning and end of records played on automatic record changers, but is not noticed when the recording level is high. Accordingly, it is an object of my invention to provide an improved background noise reducing circuit for record reproducers by which the high frequency response of the reproducer is reduced when higher frequency tones are not present on the part of the record being reproduced or are below the level of the background noise andis returned to'normal operation when desirable higher frequency tones, having an intensity greater than that of the backdetermined with a high degree of accuracy. Accordingly, a further object of my invention is to provide an improved background noise reducing circuit for a record reproducer in which a threshold level for disabling of the noise reducing circuit may be set or adjusted above the intensity of the high frequency components of the background noise which it is desired to suppress and below the intensity of high frequency passages on the recording which it is desired to reproduce.

One of the features of my invention is the employrnent in a noise reducing circuit for an audio frequency translating channel of a low-pass filter which includes an electron discharge device functioning as a reactance device having a predetermined frequency characteristic and varying the reactance of said device in accordance with the intensity of the higher frequency components of the audio signals at the input to said channel.

Another object of my invention is to provide an improved noise reducing circuit for audio amplis fiers which includes an electron discharge reactance device having a determinable frequency characteristic. V

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 represents a. portion of an electric phonograph record reproducer embodying the noise suppression circuit of my invention, and Fig. 2 is a modification of the circuit of Fig. 1. In the two figures corresponding reference numerals have been placed on corresponding elements to of the reproducer amplifying circuit and, ultimately, to a. suitable sound reproducing device. Connected across the channel Ill between the terminals ii and I2 is my noise suppressioncircuit nected in shunt to the crystal Iii to control the low frequency response of the pick-up device. Connected between terminal i I and ground is a high-pass filter circuit comprising capacitance l6 and resistance ii. The common point of capacitance IB and resistance I1 is connected to control electrode |3 of the triode section of an electron discharge device I! of the diode-triode type. Anode of the triode section of device I! is supplied with operating potential through a load resistance 2| from any suitable source of potential, such as the battery 22, and is connected to ground through a capacitor 23 and a resistance 24. The cathode of device I3 is connected to ground for unidirectional currents and low audio frequency currents through resistance and for high audio frequency currents through by-D S capacitor 26. The capacitance of capacitor 28 is small so that only high audio frequency currents, i. e. currents having frequencies above approximately 600 cycles per second, flow readily through capacitor 26. Capacitance I8 similarly is a relatively small capacitance so that the potential of electrode It for low frequencies and which is due to the low frequency audio currents flowing through resistance H is quite small. Since low audio frequency currents flow from the cathode of device iii to ground through the high impedance of resistance 25, degeneration occurs and the output current of the triode section of device I! contains but a small component of audio frequencies below about 600 cycles.

Anode 2l of the diode section of device I9 is connected to the common point of capacitor 23 and resistor 24 and to ground through a filter circuit comprising resistor 28 and capacitor 23.

The audio frequency currents in translating channel In flow through a low-pass filter comprising series resistance 30 and electron discharge device 3| functioning as a reactance tube. The anode of device 3| is connected to battery 22 through a load resistance 33 and to the righthand terminal of resistance 30 through blocking capacitor 34 and capacitor 35 inserted in the filter circuit to enhance the reactive effect of device 3|. The device 3| is shown as a pentode having its cathode connected directly to ground and its control electrode 36 connected to the common point of

capacitors

34 and 35 and to ground through resistance 31 and capacitor 29. The screen electrode 33 is supplied with operating potential from battery 22 through a voltage dropping resistor 33 and is connected to the cathode through a capacitance G0. The electrode II is connected directly to the cathode to act as the usual suppressor grid.

In the operation of the low-pass filter comprising series resistor 30 and the electron discharge device 3|, the device 3| functions as a well-known reactance tube having a capacitive reactance which varies with the gain of the tube. In order that the capacitive reactance of the device vary also with the frequency of currents flowing through that device, capacitor 40, connected between screen electrode 38 and the cathode of the device, is made small so that the screen electrode operates practically at ground potential for higher audio frequencies and at potentials considerably above ground for lower audio frequencies. In this manner the device 31 operates as a reactance tube whose reactance varies with the frequency of the currents flowing through the tube. The gain of the device 3| is controlled by the biasing potentials supplied thereto through resistance 31 from the common point of res stance .23 and capacitance 29 in the filter circuit connected between the anode 21 and ground. The value of resistance 31 is made low, in the order of 50,000 ohms, so that this resistance not only functions to prevent reactance device 3| from acting to change noticeably the apparent volume level of the output of channel In, as device 3| is rendered operative or inoperative to by-pass high frequency currents from channel In, but also forms in conjunction with capacitor 33 a tone compensating circuit for the crystal pick-up N. As stated previously, loading resistor l5 reduces the low frequency response of the crystal pick-up and is commonly found necessary in an electrical phonograph to prevent rumble. The compensating action of

tone compensating circuit

34, 31 is limited to the middle frequencies in the output of pick-up H and reduces these middle audio frequencies in a degree commensurate with the amount that the lowest frequencies are reduced by resistor l5 so as to produce a pleasing overall tonal quality.

In the operation of the complete noise suppressor circuit i3 high audio frequency signals are amplified in the triode section of the device I9 to provide a potential to the anode 21 which is rectified in the diode section of the device I9. Low audio frequencies are prevented from bein reproduced in the circuit of anode 20 through the operation of the cathode bias resistor 25 and the high-pass filter circuit i6, IT. The high audio frequency currents are rectified in the diode section of device I! and filtered by resistance 28 and capacitor 29 to provide a negative bias voltage for control electrode 36 which reduces the gain of device 3| and changes its reactive effect to that of a very small capacity. When the reactance of the device 3| is thus reduced, high audio frequencies in the translating channel l0 are permitted to proceed to the terminal l2 without attenuation.

In order to provide a threshold level so that high audio frequency currents, present in the channel I0 and due to surface noise alone in the output of the phonograph record reproducer, cannot reduce the gain of device 3| to permit the high frequency components of this surface noise to pass to the output terminal l2, resistance 25 is made considerably larger than required merely for providing a bias voltage for the trlode section of the device it. In this manner high audio frequency voltages of a predetermined intensity are required to reduce the by-passing effect of device 3 I. It is apparent, of course, that the flow of current in the diode section of device I9 is delayed also by the voltage developed across resistance 25 so that the gain reducing bias for control electrode 38 is developed only after high frequency audio signals of predetermined intensity are supplied to control electrode l8.

When a crystal pick-up of the type shown is employed in an electric phonograph record reproducer, the noise level of the stylus attached to that pick-up device and of the average record can be determined with considerable accuracy. Also, it is well known that the minimum recording level which can be successfully reproduced can be definitely ascertained. Hence, by properly selecting the value of the cathode bias resistance 25, the operating point of the diode section of device l9 may be set or adjusted with sufficient precision that high frequency audio tones are by-passed from the translating channel Ill by the reactance device 3| when the intensity of audio frequency voltages at terminal II is below a definite value. In this manner scratchy high frequency background noise present on the recording is not carried to the output circuits of the phonograph record reproducer during quiet passages in the recording,.when higher frequency tones are not present, and at the beginning and end of the records being played. When, however, the high frequency notes of the record produce voltages at terminal H above a threshold value, biasing potentials are developed by device I! to reduce the gain of reactance tube 3! and permit the passing of these high frequency tones to the output circuit of the reproducer.

Since only high frequency currents are rectified by the diode section of device ID, the charging time of capacitor 28 can be relatively short so that negative biasing potential is supplied rapidly to control electrode 38 when desirable high frequency voltages appear at the terminal H. Thus, the bypassing effect of reactance device 3! is reduced quite rapidly and the switching off and on of high frequency notes in the output circuit of the translating channel occurs at a rate which is substantially imperceptible to the human ear. This control action. while seeming almost instantaneous to the ear, remains relatively slow in relation to the audio frequencies being controlled so that there is no distortion of these audio frequency notes by the control circuit.

' In the modification of my invention shown in Fig. 2, the screen electrode 38 of reactance device 3| is by-passed to ground for higher audio frequencies by the capacitor 40 and the cathode of the device is connected to ground through a resistance 45 across which is connected a by-Dass capacitor 46. The resistance 45 provides normal bias for device 3| as well as degeneration at the frequencies not by-passed by capacitor 46. Both capacitors 40 and it have a capacitance value such that they by-pass only higher audio frequencies. As a result, in this modification of the noise suppressor circuit, the reactance of the reactance tube 3| is increased for higher audio frequencies to be by-passed from channel l and has a very small value for low audio frequencies.

The operation of the circuit of Fig. 2 is essentially the same as that of the circuit of Fig. 1. The main difference between the two circuits lies in the fact that low frequency currents flowing through device 3| are reduced to a very small value by the degenerative voltages developed across cathode biasing resistor l5 and the high impedance of capacitors 40 and 46 to these low audio frequency currents. on the other hand. for higher audio frequency currents the cathode and screen electrodes of the device operate practically at ground potential and the gain of the device is controlled by the potential of control electrode 35.

By way of illustration only, and not in any sense by way of limitation, the following representativc values are given for a circuit constructed in accordance with Fig. 1 of my invention. A 6SQ7 type of electron discharge device was used as the device I! with the two diode-anodes of the device being connected together to comprise the anode 21 and a type 6SG7 tube was used as the reactance tube 3|. The values of the impedance elements of the circuit are as follows:

Capacitor I 6:.001 microfarad Capacitor 26:.1 microfarad Capacitor 29:.05

microfarad Capacitors

35, 40:.01 microfarad Capacitor 34:.00025 microfarad While I have shown a specific embodiment of myinvention, it is apparent that various changes may be made. Thus, instead of using the bias resistance 25 to obtain a threshold level for the two sections of the device IS. a bias voltage may be used. Similarly, when the audio frequency voltages at the terminal II are or high level, or are amplified before passing through the series resistance 30, it is desirable to use. a greater bias for both tubes in the device 3|. However, in all applications of the noise suppressor circuit, it is desirable that the circuit be applied to stages of an amplifier preceding the volume control means in the amplifier. In this way the circuit remains independent of the output level at which the reproducer is operated and rapid, "reliable, and accurate operation of the suppressor circuit is obtained.

While I have shown a particular embodiment of my invention, it will of course be understood 7 that I do not wish to be limited thereto since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to. secure by Letters Patent of the United States, is:

1. In combination, a channelfor translatin audio frequency signals, means coupled to said channel for attenuating the high audio frequency signals therein while permitting translation of lower audio frequency signals, and means responsive to the presence of intense high audio frequency signals in said channel for reducing the attenuation of said signals by said attenuating means. a q

2. In combination, a translating channel for audio frequency signals, means coupled to said channel for substantially removing. audio signals having a frequency greater than a predetermined frequency from the output of said channel while permitting translation of lower audio frequency signals, means responsive to the appearance of audio signals of a frequency greater than said predetermined frequency in the input to said channel to disable said signal removing means, and means to provide a threshold level for the operation of said disabling means.

3. In combination, a background noise reducing circuit for the audio frequency translating circuit of a record reproducercomrprising, a capacitance and a resistance connected in series across said audio circuit, and an electron discharge device connected in shunt to said resistance, said device having a control electrode and said noise reducing circuit being adjusted to. remove undesired high frequency currents from said audio circuit, and means to disable said noise reducing circuit to permit translation of desired high frequency currents comprising, means to rectify high frequency voltages present in said audio circuit, and means for applying said rectified voltages to said electrode in a gain reducing sense.

4, In combination, a surface noise reducing circuit for the audio frequency translating circult of a record reproducer comprising, 8. capacitance and a. resistance connected in series across said audio circuit, and an electron discharge device connected in shunt to said resist once, said device having a control electrode and said noise reducing circuit being adjusted to remove high frequency currents having an intensity less than a predetermined intensity from said audio circuit, and means to disable said noise reducing circuit to permit translation of high frequency currents of an intensity greater than said predetermined intensity comprising, means to rectify high trequency voltages present in said audio circuit, and means for applying said rectified voltages to said electrode in a gain reducing sense.

5. In a filter circuit for an audio frequency translating channel, the combination of, an input terminal, and an output terminal, an input circuit connected between said input terminal and a point of fixed potential, an output circuit connected between said output terminal and said point, a resistance connected between said terminals, an electron discharge device having an anode connected to a point between said resistance and said output terminal, a cathode connected to said point of fixed potential, a screen electrode and a control electrode, said screen electrode being connected to said point or fixed potential through a capacitance having a large reactance at low audio frequencies and a small reactance at high audio frequencies, and means to bias said control electrode in accord with the intensity of high audio frequency voltages across said input circuit.

6. In a filter circuit for an audio frequency translating channel, the combination of, an input terminal, and an output terminal, an input circuit connected between said input terminal and a point of fixed potential, an output circuit connected between said output terminal and said oint, a resistance connected between said terminals, an electron discharge device having an anode connected to a point between said resistance and said output terminal, a cathode, a screen electrode, and a control electrode, said screen electrode being connected to said point of fixed potential through a capacitance having a large reactance at low audio frequencies and a small reactance at high audio frequencies, means to bias said control electrode in accord with the 40 intensity of high audio frequency voltages across said input circuit, and degenerative means for low audio frequency currents connected between said cathode and said point of fixed potential.

'1. In a filter circuit for an audio Irequency translating channel, the combination or, an input terminal, and an output terminal, an input circuit connected between said input terminal and a point 0! fixed potential, an output circuit connected between said output terminal and said point, a resistance connected between said terminals, and a variable reactance connected across said output circuit comprising an electron discharge device having an anode connected to a point between said resistance and said output terminal and a cathode connected to said point of fixed potential, and means to vary the reactance of said device in accordance with the frequency of the current flowing through said device.

8. In a filter circuit for an audio frequency translating channel, the combination of, an input terminal, and an output terminal, an inrput circuit connected between said input terminal and a point oi fixed potential, an output circuit connected between said output terminal and said point, a resistance connected between said terminals, a variable reactance connected across said output circuit'comprising an electron discharge device having an anode connected to a point between said resistance and said output terminal and a cathode connected to said point of fixed potential, means to vary the reactance of said device in accordance with the frequency of the current flowing through said device, and means to vary the gain 01 said device in accordance with the frequency of the currents in said input circuit.

GEORGE F. DEVINE.

DISCLAIMER 2,369,952.-George F. Davina, Easton, Conn. Bacxonormn Nolan surranssoa.

Patent dated Feb. 20, 1945. assignee, General Electric Company.

Disclaimer filed Sept. 20, 1945, by the Hereby enters this disclaimer to claims 1 and 2 of said patent.

[Oflicial Gazette October 30, 1945.]

than said predetermined intensity comprising, means to rectify high trequency voltages present in said audio circuit, and means for applying said rectified voltages to said electrode in a gain reducing sense.

GEORGE F. DEVINE.

Patent dated Feb. 20, 1945. assignee, General Electric Company.

[Oflicial Gazette October 30, 1945.]