US1947822A - Automatic tone control - Google Patents

Description

Feb. 20, 1934.

W. S. BARDEN AUTOMATIC TONE CONTROL Filed April 15, 1931 MEANS .SOIIRLT 0F 5/5/4541 ENE/P6 Y N m n R Q/W N O E mm A IIIL I W Patented Feb. 20, 1934 UNITED STATES PATENT OFFICE 1,947,822 AUTOMATIC TONE CONTROL.

Application April 15, 1931. Serial No. 530,268

8 Claims. .(Cl. 250-20) My present invention relates to radio receivers employing gain control, and more particularly to an improved method of, and means for, maintaining a predetermined background noise level in receivers utilizing automatic gain control arrangements. Subject-matter concerning the relation between attenuation and background noise level disclosed in this application, and common to application Serial No. 545,704 of G. C. Beers, filed June 20, 1931, is not claimed herein, but is claimed in the said Beers application. 7

The use of automatic volume control devices in radio receivers has greatly alleviated the problem of maintaining'a desired signal reproduction level during reception. However, it has been recognized that the employment of such devices simultaneously results in an increase in the background noise level during certain periods of reception. For example; when thereceived field strength is not high relative to the noise level, the inherent action of the gain control will tend to raise the latter. Again, when no signals are beingreceived, background noises are greatly amplified, and appear to detract from the advantages otherwise secured by means of the automatic gain control.

It has been found that theparticuiarly annoying background noises comprise high audio frequencies. This fact can be utilized to advantage in contending with background noises in automatic gain control receivers. I have found that if the high audio frequencies be automatically attenuated when the received field strength is' not high relative to the noise level, that background noises are not apparent to the listener.

Accordingly, it is one ofthe main objects of my present invention to provide a method of, and means for reducing the reproduction of background noises in radio receivers equipped with automatic gain control arrangements whenever theratio of received field strength to background noise level reaches a predetermined value, the method consisting in attenuating'the relatively high audio frequencies in accordance with variation in signal field strength.

Another important object of the present invention is to provide in combination with an amplifier, an arrangement for automatically attenuating the relatively high audio frequencies, little or no attenuation being introduced at signal field strength exceeding. a predetermined value, the arrangement comprising apath adapted to by-pass the high audio frequencies in accordance with signal field strength variation.

Another object of the present invention is to provide in'combination with a radio receiver comprising a radio frequency amplifier and a detector, an automatic volume control circuit adapted to maintain the gain of said radio frequency amplifier at a predetermined level, and additional means comprising a path designed to by-pass high audio frequencies, the path including an electrical element whose impedance is varied automatically in accordance with the functioning of said automatic volume control circuit. 7

Still another object of the present invention is to provide in combination with a radio receiver comprising a radio frequency amplifier and a detector, an arrangement for automatically disoriminating against predetermined audio frequencies in accordance with received signal field strength.

Still other objects of the invention are to improve generally the efiiciency of radio receivers equipped with automatic gain control devices, and to particularly provide a receiver of this type which is not only reliable in operation, but economically assembled.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by. reference to the following description taken. in connection with the drawing in which I have indicated diagrammatically one circuit organization whereby my invention may be carried into effect. a

'Referring to the accompanying drawing there is shown a radio receiver including a device 1 functioning as a source of signal energy. The device 1 may be, for example, an antenna system, and may be coupled, as at M, to the tuned input circuit of a radio frequency amplifier tube 2. The latter is a space discharged device, and while shown as comprising three electrodes may obviously be of the screen grid type if desired. The input circuit of the tube is tuned by a variable condenser 3, while the anode, or output, circuit of the tube includes a source of anode potential B. A resistor 4 in the grounded side of the'cathode produces, in a manner well known to those skilledin the art, bias for the contro electrode of the tube 2. While a single stage of tuned radio frequenc amplification has only been shown, it is to be clearly comprise one, or more, preceding stages of radio understood that the device 1 can readily am 1 rents.

frequency amplification. The output circuit of the tube 2 is coupled, as at M, to the input circuit of a detector stage. The latter comprises a space discharge tube 5 whose input circuit is tuned by means of a variable condenser 6. The conventional capacity 7 and grid leak resistor 8, both disposed in series with the control electrode of the tube 5, provide detection. The anode circuit of the detector tube includes a source of anode potential B.

The anode circuit itself, is coupled, as at M, by an audio frequency transformer to any well known type of utilization means. The latter may comprise one, or more, stages of audio frequency amplification, and a succeeding loud speaker, head phones, or any other well known type of reproducer. As is well known to those skilled in the art, the rotors of the variable condensers 3 and 6, as well as the rotors of any tuning condensers in stages of amplification prior to the tube 2, may be arranged for mechanical unicontrol, as conventionally represented by the dotted lines designated by the numeral 9.

In order to maintain a predetermined radio frequency amplifier gain level there is usually provided an automatic gain, or volume, control arrangement. The latter, in one of its well known forms, comprises a space discharge dBVl-Cu 10 having its input electrodes connected to the input circuit of the detector tube. A source of bias C is connected to the control electrode of the device 10, while a source of anode potential B" is connected to the anode.

A resistor 11 is connected between the grounded side of the cathode of device 10 and the negative terminal of the source B". As is well known to those skilled in the art, the grids of one, or more, of the radio frequency amplifier tubes are connected to the anode circuit of the control tube 10. Thus, the grid of tube 2 is shown connected by a conductor 12 to a point in the anode circuit between the negative terminal of source B" and the resistor 11.

The action of the volume control tube 10 is very wellknown. As the signal field strength decreases below a predetermined value, there is a lesser flow of current through the anode circuit of the device. In other words, the potential drop across the resistor 11 becomes less with the result that the bias on the grid of tube 2 is diminished. The capacity 11 is shunted across the resistor 11 to by-pass radio frequency cur- Therefore, it will be seen that the amplification is increased. The reverse action takes place as the signal field strength increases above a predetermined level.

In other words, the device 10 functions to regulate, or control, the gain of the amplifier tube 2 (and other amplifier tubes if desired) in an automatic manner in accordance with the variation of the signal field strength. The magnitude of the source 0 is such that with no radio frequency input (that is when no station is being received) there is a zero drop of potential across the resistor 11.

Obviously at such times the radio frequency tubes are not amplifying signal energy. Background noises become amplified to a disconcerting extent. In general, it has been found that it is highly desirable to considerably diminish the background noises when the received field strength is not high relative to the noise level. Since, as already pointed out, the particularly annoying noises reside in the upper audio frequency range, this can be accomplished by attenuating the high audio frequencies whenever the signal field strength decreases below a desired value. By arranging the attenuation mechanism to be automatic in its action, and designing the mechanism so that little, or no, attenuation is introduced at signal field strengths greater than a specific value, for example 200 microvolts, effective suppression of background noises is secured.

Specifically, the attenuation mechanism comprises a space discharge tube 13 having its control electrode connected, by a conductor 14, to a point on the conductor 12. That is to say, the bias for the tube 13 is produced by the drop across the resistor 11, as in the case of the tube 2. The anode circuit of tube 13 includes a bypass capacity 15 connected between a high potential point in the anode circuit of tube 13 and a similar point in the anode circuit of detector tube 5. The capacity has a low impedance to the high audio frequencies, and is shunted by a choke coil 16 having a high impedance to high audio frequencies.

It will be appreciated that, from an electrical viewpoint, there is a variable impedance connected in series with capacity 15, the impedance.

being the anode impedance of tube 13. With change in bias on the grid of the latter, the value of the anode impedance will vary. Obviously, this variation is not only dependent on the operation of the automatic volume control tube, but functions in an automatic manner to regulate the by-passing of high audio frequencies through the capacity 15.

The operation of control tube 13 will, then, be such that there is a minimum by-passing through capacity 15 when the drop across the resistor 11 is a maximum thereby increasing the bias on the. grid of tube 13 with the result that the series anode impedance is a maximum. The by-passing of high audio frequencies through capacity 15 is at a maximum when the drop across the resistor 11 is a minimum (that is zero, when no stations are received) thereby decreasing the bias on the grid of tube 13 with the result that the series anode impedance becomes sufficiently great to block the capacity 15.

It will, therefore, be seen that there has been provided a radio receiver arrangement for automatically discriminating against relatively high audio frequencies in accordance with received signal field strength. Furthermore, it should be clear that the arrangement employed herein comprises a variable impedance path cooperating with an automatic gain control device to attenuate high audio frequencies at pre-determined periods in the operation of a radio receiver.

While I have indicated and described one arrangement for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a radio frequency amplifier stage including an electron discharge tube, a detector stage including a space discharge tube having its input electrodes coupled to the output electrodes of the amplifier tube, a gain control circuit including a space discharge tube having its input electrodes coupled to the input circuit loo of the detector tube, means connecting the grid circuit of said amplifier tube. to the output circuit of said gain control tube, an attenuation circuit including a space discharge tube having its grid circuit connected to the output circuit of said gain control tube, a path of low impedance to relatively high audio frequencies between the anode circuit of said attenuation tube and the anode circuit of said detector tube, and a path of high impedance to said audio frequencies in shunt with said first path.

2. In combination, a radio frequency amplifier stage including an electron discharge tube, a detector stage including an electron discharge tube having its input electrodes coupled to the output circuit of the amplifier tube, an automatic gain control circuit for said amplifier stage including an electron discharge tube having its input electrodes connected to said detector stage, means connecting an electrode of said amplifier tube to the output circuit'of said gain control tube, an attenuation circuit including an electron discharge tube having its grid circuit connected to the output circuit of said gain control tube, a condenser of low impedance to relatively high audio frequencies connected between the anode circuit of said attenuation tube and the anode circuit of said detector stage, and an .inductance of high impedance to said audio frequencies in shunt with said condenser.

3. In combination, a radio frequency amplifier adapted to have audio modulated high frequency energy impressed upon its input; a rectifier having its input connected to the output of said amplifier, means responsive to variations in the intensity of the energy impressed upon said amplifier input to vary the transmission efficiency of said amplifier in a direction to maintain constant the intensity level of said modulated high frequency energy supplied to said rectifier, and a capacity path including a control tube means connected directly between the anode of the rectifier and ground, and responsive to said amplifier transmission efiiciency control means, for by-passing to ground the higher audio frequency components of the rectifier output energy whenever the transmission efiiciency of said amplifier increases above a predetermined level.

4. In combination, a detector tube circuit, a source of signal energy to be detected coupled to the detector circuit input, an attenuating circuit in shunt across the detector tube output circuit, said attenuating circuit including in series a space discharge device and an inductance coil of high impedance to high audio frequencies, and means for controlling the conductivity of said device in response to variations in signal energy amplitude derived from the detector tube input circuit.

5. In combination, a detector tube circuit, a

source of signal energy to be detected coupled to the detector circuit input, an attenuating circuit in shunt across the detector tube output circuit, said attenuating circuit including a space discharge device and an inductance coil of high impedance to high audio frequencies, a condenser, across-said coil, which has a low impedance to said high audio frequencies, and means for controlling the conductivity of said device in response tovariations in signal energy amplitude derived from the detector tube input circuit.

6. In combination, a detector tube circuit, a source of signal energy to be detected coupled to the detector circuit input, an attenuating circuit in shunt across the detector tube output circuit, said attenuating circuit including in series a space discharge device and an inductance coil of high impedance to high audio frequencies, and an electronic relay means for controlling the conductivity of said device in response to variations in signal energy amplitude derived from the detector tube input circuit.

7. In a signal energy detector tube network, a transformer coupling means connected between the detector anode and cathode, an audio frequency by-pass path connected in shunt with the anode and cathode of the said tube, said path including a condenser of low impedance to higher audio frequencies and a tube, one side of said condenser being connected to one of the transformer windings, and a by-pass auxiliary tube, connected between the detector input circuit and the control electrode of said by-pass tube, for controlling the conductivity of the latter.

8. In a signal energy detector tube network, an audio frequency Icy-pass path connected in shunt with the anode and cathode of the said tube, said path including a condenser of low impedance to higher audio frequencies and a by-pass tube, and a choke coil in shunt with the condenser, and an auxiliary tube, connected between the detector input circuit and the control electrode of said by-pass tube, for controlling the conductivity of the latter.

WILLIAM S. BARDEN.

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