US2023458A - Muted automatic volume control - Google Patents

Description

Dec. 1 0, 1935. J. YoLLEs MUTED AUTOMATIC VOLUME CONTROL Filed oct. 2e, 1955 2 Sheets-Sheet l AAAAA.

INVENTOR JACOB YOLLES AY/L f /wz/'OL/ ATTORNEY Dec. 1 0, 1935. J. YoLLEs MUTED AUTOMATIC VOLUME CONTROL Filed Oct. 26, 1935 2 Sheets-Sheet 2 F. A m

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Patented Dec. 10, 1935 UNITED STATES PATENT OFFICE MUTED AUTOMATIC VOLUME ooNTaoL Jacob Yolles, Brooklyn, N. Y., assgnor to Radio Corporation of America, a corporation of Dela- Waffe My present invention relates to automatic volume control arrangements of radio receivers, and more particularly to muter circuits for receivers employing automatic volume control networks. Modern broadcast radio receivers utilize automatic volume control arrangements in order to maintain the signal energy input to the demodulator of the receiver subsantially constant regardless of received signal carrier amplitude variations. The employment of automatic volume control, however, gives rise to a serious source of annoyance when the signal carrier amplitude decreases below a predetermined instensity level. For example, when the signal carrier amplitude decreases to practically zero, the gain controlled amplifiers are at maximum sensitivity. This results in an excessive amplification of undesired electric background noise impulses. This amplification manifests itself as exceedingly annoying sounds in the lo-ud speaker.

Various arrangements have heretofore been proposed to avoid the reproduction of such background noise impulses when the received signal carrier amplitude decreases below a predetermined intensity level. In the muters, or background noise suppressors, suggested heretofore at least one circuit in the receiver has been rendered inoperative to render inefficient the transmission of electric impulses to the reproducer. In order to impart a selective characteristic to the background noise suppressor arrangement, it has been shown by M. G. Nicholson, Jr., in application Serial No. 577,235, filed November 25th, 1931 that the input tol the suppressor network should be tuned to the desired signal frequency.

It may be stated that it is one of the main objects of the present invention to provide an intercarrier noise suppressor arrangement for a radio receiver utilizing automatic volume control wherein the input circuit of the suppressor network is resonant to the desired signal frequency, and hence highly selective, the noise suppressor function beingperfo-rmed upon an audio amplifier which is disposed within the same tube envelope which houses the receiver detector.

Another important object of the invention is to minimize intercarrier noise in a receiver employing automatic volume control by means of a selective circuit, the muted. tube being of the duplex diode triode type and embodying diode detector and audio amplification functions.

Another object of the invention is to provide a background noise suppressor system for a receiver utilizing automatic volume control, the

necessary negative cut-off bias for the noise suppression function being secured by means of a' diode rectier which is housed within the same tube envelope as an amplifier for the rectified signal voltage to be employed for the muting ac- 5 tion.

Still other objects of the invention are to improve generally the efficiency of muting systems for receivers, and to particularly provide an interstation noise suppressor for a receiver em- 10 ploying automatic volume control which is not only reliable in operation, but readily assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in par- 15 ticularity in the appended claims. The invention itself, however, both as to 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 20 indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings:

Fig. l diagrammatically shows a receiver em- 25 playing one embodiment of the invention,

Fig, 2 diagrammatically shows a modified form of Vthe invention.

Referring now to the accompanying drawings,

wherein like reference characters in the two g- 30 ures represent similar circuit elements, in Fig. 1 there is shown a radio receiver of a purely conventional type. Since superheterodyne receivers are most generally employed at the present time,

the arrangement shown in Fig. 1 is of such a type. 35

The numeral l designates a tunable radio frequency amplifier stage whose gain is to be automatically controlled. The numeral 2 designates the first detector, local oscillator and intermediate frequency amplifier stages of a super- 40 heterodyne receiver; It is not believed necessary to show the circuit details of the network 2, since those skilled in the artrare well acquainted with such circuit design. It is merely necessary to point out that the tuned circuit 3 has impressed 45 upon it the amplified signal which exists in the plate circuit of tube l. The tuned coupling network 4 is maintained fixedly tuned to the operating intermediate frequency, which may be of the order of 175 kilocycles for example, and the inter- 50 mediate frequency energy is impressed upon a seconddetector for demodulation.

The numeral 5 designates a type of tube known as a duplex diode triode, or a 55 tube. This is a multiple duty tube and performs simultaneously 55 second detection and audio frequency amplification functions. It additionally performs the automatic volume control function. The tube 5 is schematicaly shown since its specific construction has been disclosed and claimed by T. M. Shrader, in application Serial No. 622,140, filed July 12th, 1932, Further, the use of such a 55 type tube for the simultaneous three functions named has been disclosed and claimed by T. M. Shrader in the aforesaid application.

The second detector circuit consists of a half wave diode rectifier network including in series auxiliary anodes 6, tuned circuit 4', resistor 1 shunted by radio frequency by-pass condenser 8, and grounded cathode 9. The direct current component of the voltage developed across resistor I is used for automatic volume control, this function being provided by connecting the lead 9 including filter resistor 9" (marked A. V. C.) between the grid circuits of the controlled tubes and the grid side of resistor I. The audio component of rectified signal voltage is impressed on the grid I8 of the triode audio amplifier section of tube 5 through a path including direct current blocking condenser II, adjustable tap I2 adapted to act as a manual volume control element, and ,resistor I3 connected between lead 9 and ground through condenser I4.

The plate I5 of tube 5 is coupled, as at M, to an audio frequency network which may comprise, one or more amplifier stages and/or a reproducer. Signal energy is also impressed upon a background noise suppressor network through a path which includes the condenser I 6 and a signal selective network II. The noise suppressor arrangement comprises a pair of tubes I8 and I9, the tube I8 functioning as a noise control tube. The variation of current flow in the plate circuit of rectifier tube I8 regulates the flow of current in the plate circuit of tube I9, and the variation in the current flow in the latter plate circuit regulates the transmission efficiency of the triode section of tube 55. The potentials required for the operation of tubes I8 and I9 are derived from a common bleeder which comprises resistor portions R1, R2, R3, R4, R5 and Re. The positive side of portion R6 is grounded, and consequently is at the same ground potential as the cathode 9 of tube 5. The grounded side of portion Re is connected through further resistor portions to the positive terminal of the direct current supply source, and it is to be understood that this supply source may also provide the potentials for the remainder of the receiver tubes. A resistor R7 is connected in shunt across portion R1 between the negative terminal side of R1 and the side thereof connected to the cathode of tube I 8. The highly selective circuit I'I, which is maintained fixedly tuned to the operating intermediate frequency, is connected between the signal grid of tube I8 and a desired point on resistor R1 through a path which includes the resistor 20 and the adjustable tap 2|, the tap side of resistor 20 being connected to ground through a by-pass condenser 22, and a condenser 23 connecting the cathode of tube I8 to the low potential side of the selective circuit II. The cathode of tube I8 is connected to the junction of resistors R1 and R2, while the screen grid of tube I8 is connected to the junction of portions R2 and R3.

The cathode of tube I9 is connected to the junctionof resistors R3 and R4, while the anode of tube I8 is connected to the signal grid of tube I9 through a resistor 24. A grid leak resistor 25 is connected between the anode side of resistor 24 and the cathode of tube I9, and the anode of tube I9 is connected to the signal grid I0 of tube 5 through a path which includes resistor 2S and lead 21. The anode of tube I9 is connected to the junction of resistor portions R5 and Re through a resistor 28, and the screen grid of tube I9 is connected to the junctions of resistor portions R4 and R5.

The lead ZI is designated by the letters N. S., and it is to be clearly understood that this designates the background noise control path, or the intercarrier noise suppressor path. Merely by way of illustration, and not by way of restriction, or limitation, it is pointed out that the potential difference between the negative terminal of portion Ri and the ground side of portion Re may be 211 volts, and the junction points between the resistor portions of the voltage supply bleeder may have the following respective potential values: 37.5 volts; 44 volts; 121 volts; 161 volts; 203 volts. With regard to the regulated stages, and specifically amplifier tube I, there may be employed a potential of about 400 volts on the plate of tube I, and in that case it will be noted that the grounded side of the cathode of tube I will be at a potential of 211 volts. For further specic illustration it is stated that the resistor R7 may have a value of 500,000 ohms, the resistor R2 may have a value of 10,000 ohms; the grid leak resistor 25 and the resistor 28 may 'each have a value of 1A; of a megohm; the resistor 24 may have a value of 1/ of a megohm; and the resistor 26 may have a value of 1 megohm.A Of course, it is quite obvious that any of these values given above may be changed to suit the particular requirements to which a broadcast receiver is to be put.

In actual operation, and with the tuning instrumentality 39 adjusted between station settings, normal transmission by the amplifier section of tube 5 is prevented. This transmission prevention function is secured by virtue of the fact that tube I9, with no signals passing through condenser I5, has sufficient potential drop across resistor 28 to bias the amplifier section of tube 5 to cut ofi. That is to say, the potential drop across resistor 2B adds to the normal negative bias obtained across resistor portion Re, with no signals being collected, and cuts ofi the amplifier section of tube 5, thereby muting the receiver between desired station settings of the tuning device 39. This muting action is desirable, as is well known, because the AVC arrangement is not effective at the no-signal condition of the receiver whereby the high frequency amplifiers are operating at maximum sensitivity, Undesired background noise impulses are greatly amplified when sensitivity is at its maximum, and such undesired impulses are transmitted through the audio amplifier section of tube 5 and finally reproduced.

As soon as the signal energy above a predetermined intensity level is impressed upon the selective circuit I? the plate current fiow of tube I8 increases, with the result that the signal grid of tube I9 has its negative bias increased to a point such that the tube i9 is biased to cut ofi. This means that the potential drop across resistor 28 decreases sufficiently to remove the cut ofi" bias from the signal grid I8 of tube 5. This action effectively restores normal transmission through the audio amplifier section of tube 5. Of course, adjustment of tap 2I regulates the point at which background noise suppression action will become effective.

In Fig. 2 is shown a simplified modification of the arrangement shown in Eig. 1. The audio amplifier section of tube 5 includes a screen grid electrode, and the audio component of the demodulated signal energy is derived from the resistor 'l which is arranged in series between the grounded cathode 9 and the low potential side of tuned circuit 4. Otherwise the circuit involving tube 5 and its immediately associated circuits are exactly the same as shown in Fig. 1. The functons of the tubes I8 and I9 are performed simultaneously in Fig. 2 by the tube 3|. This tube is of the same type as the tube 5 of Fig. 2, the signal rectification function of tube I8 being performed in Fig. 2 by the half wave diode rectifier circuit which includes in series the diode anodes 32, the selective circuit l1, the resistor 33 shunted by the radio frequency by-pass condenser, and the cathode of tube 3|. The direct current component of the voltage developed across resistor 33 is impressed upon the signal grid of the amplifier section of tube 3l through a resistor 34. The bleeder of Fig. 1 is simplified in Fig. 2 to comprise four resistor portions Ra, R4, R5 and Re. Otherwise the resistors employed in Fig. 2 whose numerals correspond to the numerals of the resistors in Fig. 1 function similarly. The background noise suppression path includes the lead 21 and resistor 26, and the potential drop across resistor 28 again regulates the transmission efficiency through the audio amplifier section of tube 5.

In operation, and with no signals being impressed upon the selective circuit Il, there is a maximum flow of current in the plate circuit of tube 3l, and the potential drop across resistor 28 adds to the normal bias potential across resistor Re to bias they signal grid of tube 5 to cut off. Thus, normal transmission of signals through the audio amplifier section of tube 5 is prevented. When signal energy of sufficient amplitude is impressed upon selective circuit I1, there is a flow of rectified signal current through resistor 33, and the signal grid of the amplifier section of tube 3l is biased to a point such that the flow of current through resistor 28 sufficiently decreases to remove the signal grid of tube 5 from its cut off condition. Accordingly, it will be appreciated that in the arrangement shown in Fig. 2 a pair of multiple function tubes are utilized to perform signal demodulation, audio frequency amplication, automatic volume control, and muting functions simultaneously. It is pointed out that the condenser I6 loosely couples the primary circuit of the intermediate frequency coupling 4 to the selective circuit I1.

While I have indicated and described several systems 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 organizations 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:-

l. In a radio receiving circuit, an audio amplifier, means for biasing the amplifier beyond plate current cut off, a tuned diode rectifier, said means. including a muter stage, an automatic volume control system provided with a signal rectifier which is independent of said diode rectifier, means for impressing the direct current component of the diode rectifier current upon said muter stage, the coupling between said muter stage and said audio amplifier being direct and such that when signals are impressed 'upon said diode rectifier removal of said plate current cut off bias is effected, and said muter stage and rectier being both provided by a tube provided with a diode section and an amplifier section, said diode section acting as the rectifier and being arranged to have signals impressed thereon, and said amplifier section acting as the muter stage and having an input electrode connected to a point of direct current potential of the diode rectifier.

2. In combination, at least two multiple function-tubes, each of said tubes including a diode section and an amplifier section, means for impressing desired signal energy upon the diode section of each of said tubes, each diode section including a tuned circuit for resonating it to the desired signal, the control grid of each of the amplifier sections of said tubes being connected to the diode section of their respective tubes, a resistive coupling between the output electrode of the amplifier section of one of said tubes and the control grid of the amplifier section of the other tube, and an automatic volume control connection to the diode section of said last named other tube.

3. In a radio receiver including a multiple function tube provided with at least one diode section and an amplifier section, a signal input circuit connected to the diode section, an automatic volume control network connected to a resistor in the signal input circuit across which is developed a rectified signal voltage, means between said resistor and the control grid of said amplifier section for impressing the audio component of rectified signal energy upon said amplifier section, a rectifier, a noise control electron discharge device having its input electrodes coupled to the said rectifier, a direct current connection between the anode of said noise control device and the control grid of the amplifier section of said first tube, and a highly selective signal input circuit coupling said first named signal input circuit and said rectifier, said highly selective circuit being relatively loosely coupled to said first signal input circuit.

4. In a radio receiver including a multiple function tube provided with at least one diode section and an amplifier section, a signal input circuit connected to the diode section, an automatic volume control network connected to a resistor in the signal input circuit across which is developed a rectified signal voltage, means between said resistor and the control grid of said amplifier section for impressing the audio component of rectified signal energy upon said amplifier section, a rectifier physically independent of said tube, a noise control electron discharge device having its input electrodes coupled to the said rectifier, a direct current connection between the anode of said noise control device and the control grid of the amplifier section of said first tube, and a highly selective signal input circuit coupling said first named signal input circuit and said rectifier.

5. In a receiver as defined in claim 3, the electrodes of said rectifier and noise control device being disposed within a common tube envelope and including a common emission element.

JACOB YOLLES.

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