US2337196A - Signal and noise control system - Google Patents

Description

1943{ R HOLLINGSWORTH 2,337,196

SIGNAL AND NOISE CONTROL SYSTEM Filed March 2'7, 1942 2 Sheets-Sheet l Z6 70 2 0 5; JPEA R FAMPL.

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2 Sheets-Sheet 2 R L. HOLLINGSWORTH SIGNAL AND NOISE-CONTROL SYSTEM Filed March 27, 1942 Dec. 21, 1943.

' INVENTOR 15.5 1 /044 wgsn aam.

\ ATTORNEY Patented Dec. 21, 1943 SIGNAL AND NOISE CONTROL SYSTEM R Lee Hollingsworth, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application March 27, 1942, Serial No. 436,421

8 Claims.

The present invention relates to automatic gain control systems, and more particularly to an improved signal and noise control system suitable for use both in telegraph and telephone systems.

In known types of threshold automatic gain control (AGC) systems, now commonly used for telegraph receiving operation, if the threshold is manually set high when the incoming signals are rather weak and these incoming signals become strong in a short period of time, there is a tendency for the AGC action to split the telegraph characters at the start as they pass through the receiver, if the fastest range of AGC is used, as determined by the values of the time constant circuit. The fastest range is seldom used for relatively strong signals in these known systems, except for facsimile keying speeds, Where the slowest keyed impulses occur at a rate faster than the time delay action of the AGC. The fastest range is used for comparatively weak signals Where very deep rapid fading is encountered, in order to prevent short drop-outs of the keyed signal to th line from the output of the receiver that take place within the time duration of a keyed impulse under these conditions.

The difficulty mentioned above, experienced with known AGC systems, may be better understood by reference to the graphical representations illustrated in Figs. 1a, 1b and 10.

Fig. 10. represents the rectified output of a telegraph receiver, as might be observed in an oscilloscope while receiving weak signals with fast AGC, in known systems. The horizontal dotted line represents the point at which a succeeding tone keyer will be manually set to operate.

Fig. 1b represents the rectified output of the same telegraph receiver in known systems, as might be observed in an oscilloscope for a strong signal using fast AGC. It should be noted that the transient eiTect at the top and beginning of each character dips down below the original point of keyer operation. This transient effect, here represented by Wiggly lines, is caused by the output of the receiver feeding back to the input through a relatively fast time constant to control the overall gain of the receiver.

Fig. 1c illustrates the effect of this undesirable transient of Fig. 1b on the tone keyer output of the receiver. It should be noted, in Fig. 1c, that the tone characters for the dot and the dash are split at those portions corresponding to the points at which the transient of Fig. 1b dips down below the keyer operation point. As mentioned before, this splitting of the characters is highly undesirable because of the distortion that it creates in the signal output of the telegraph receiver.

One of th objects of the present invention is to overcome the foregoing difficulty of split telegraph characters caused by using the fastest range of AGC in telegraph receivers.

Another object of the present invention is to produce a faster acting AGO system to minimize or eliminate completely the efiect of strong peak noise, such as might be caused by ignition, sharp static, or sudden bursts of signal.

Still another object of the invention is to enable the transmission of sloped telegraph characters which can be converted at the receiver to square wave characters for proper recording.

By the use of a telegraph receiving system, in accordance with one embodiment of the invention, employing transmitted character of sloped form, the apparatus at the transmitter can be of simple design and the system as a whole can utilize a reduced frequency spectrum in the transmission medium between the transmitter and the receiver, compared to a system where the transmitted characters are of square wave form.

In accordance with one embodiment of the invention useful for telegraph reception, the control to the AGC circuit increases automatically as the signal increases above a predetermined level, and this control at the same time automatically sets the input voltage to the tone keyer to a higher voltage to prevent splits from showing in the keyed signal at slow keying speeds if very fast AGC is used. In another embodiment of the invention, particularly applicable to peak noise re duction of speech or modulated wave reception, the time constant circuit of the receiver is shunted or by-passed on the charge cycle, thus reducing, in part, strong short duration static impulses. This last embodiment makes no use of a keyer circuit.

A more detailed description of the invention follows in conjunction with the drawings, Where- 1n:

Figs. 1a, 1b and 1c graphically illustrate observed signals in telegraph receivers of the prior art;

Fig. 2-illustrates a telegraph receiver in accordance with one embodiment of the present invention.

Figs. 3a., 3b, 3c and 3d graphically illustrate the transmission of sloped characters and the conversion to square wave tone impulses at the receiver in accordance with one embodiment of the invention and Fig. 4 illustrates a speech or modulated wave receiver in accordance with another embodiment of the invention.

In Figs. 2 and 4, the same parts are represented by the same reference numerals.

Figs. 111, lb, 10 have been described above in connection with a description of the difficulties experienced with known types of telegraph receivers, and for this reason need not be further referred to in greater detail.

Referring to Fig. 2, there is shown a telegraph receiver having a directional multi-dipole antenna l coupled to a radio frequency vacuum tube amplifier 2 the output from which is coupled to multi-stage vacuum tube apparatus 3 representing a mixer or frequency converter and one or more subsequent intermediate frequency amplification stages. Oscillator 22 is the vacuum tube heterodyne oscillator which beats with the signals impressed upon the mixer stage in apparatus 3 for producing intermediate frequency energy. The intermediate frequency output from apparatus 3 passes through an inductance coil L1 which is coupled to a pair of coils L2 and L3. Coils L1, L2 and L3 are individually tuned by means of variable condensers (as shown) to the desired intermediate frequency. Connected to the high potential end of coil L2 is a rectifier tube 4 whose rectified output passes to ground through serially connected resistors 5 and B. The high potential end of coil L3 is connected to rectifier tube 1 whose output also passes to ground primarily through resistance 6. It will thus be seen that when rectifier tube 1 is conductive, resistor 5 will be predominantly by-passed or almost short circuited by the current passing through this rectifier. Resistors ill and I! which are preferably of like value and condenser l2 form a time constant circuit for supplying AGO action over lead AGC to the radio frequency amplifier 2 and to one or more of the intermediate frequency amplifier stages of apparatus 3. Suitably connected to a point on resistor 6 by means of a tap and through a battery it are a tone keyer tube !5 and a threshold tube l4 whose control electrodes are connected electrically in parallel relation. The output or the anode circuit of threshold tube M is connected through polarizing battery It and through a resistor ii to ground, a point on which resistor is connected by means of lead 24 to one terminal of resistor H of the time constant circuit. The output or anode circuit of tone keyer I5 is conn cted to ground through resistor [3. Apparatus 23 is a balanced modulator system comprising, in effect, a relay constituted by a pair of vacuum tubes i9 and 20 whose corresponding input and output electrodes are connected in push-pull relation. A source of audio frequency tone 9 is connected to both halves of the input coil coupled between the grids of the tubes [9 and 29. An audio frequency transformer 2i constitutes the output circuit of these two tubes for supplying keyed tone signals to the line TL, for utilization by a suitable recorder (not shown). The balanced modulator 23 is well known and forms no part of the present invention per se.

When the tone keyer tube I5 is conductive and passes current, the bias on the tubes 59 and 20 caused by the IR. drop through resistor I8 is suflicient to prevent the tubes is and 2%] from passing current, in which condition the tone from source b will not be passed through the balanced modulator to the line TL. However, when the tone keyer tube I5 is non-conductive, there will be no current passing through resistor I 8, as a result of which the negative bias on tubes [9 and 28 is removed to an extent sufficient to render these tubes conductive to allow tone from source 8 to pass through transformer 2! to the line TL for recording purposes.

Threshold tubes I l and are normally conductive in the absence of signals, as a result of which current will flow through resistors l! and [8. From What has been said above, the flow of current through resistor is will prevent current from being passed through the line TL. The flow of current through resistance ill will supply a negative voltage to lead 2% and thence to the time constant circuit by Way of resistor H, as a result of which negative bias is supplied to the AGC lead during the absence of incoming wave signals.

Rectifier tube i is so connected to the circuit through coil L3 that this tube does not pass current until the intensity of the incoming signals is above a predetermined level, whereas rectifier tube d is so connected to the circuit through coil L2 that it passes signals at a much lower level of intensity. To obtain this diiference in operation or" the rectifier tubes i and i at the different signal levels, the coil L2 may be coupled more closely to coil L1 than coil L3, or alternatively, the same result may be obtained by having both coils L2 and L3 coupled similarly to coil L1 and placing a negative bias of predetermined magnitude on the grid of rectifier l by means of battery 8. Obviously, the operation of the two rectifier tubes 4 and i can be obtained by a combination of a difference in coupling of the coils L2 and L3 to coil L1 and by a suitable selection of the bias on the grid of tube '5.

The operation of the system of Fig. 2 will now be given. Let us assume that the rectifier tube is so biased or coupled to the system that it will operate when the intensity of the incoming signal is at a predetermined value above the signal intensity necessary to produce rectification in tube 1. By Way of example, and not by way of limitation, rectifier tube 1 may commence rectifying when the signal intensity is ten times that required to produce rectifying action in tube 4-. Upon receiving a weak signal, not sufficient to cause rectification of tube 1, tube 4 will rectify the signal to produce a flow of current through resistors 5 and 6. The voltage drop across 6 will, through the time constant circuit l0, H and I2, control the gain of apparatus 2 and 3 by way of lead AGC. A portion of the voltage drop through resistor 6 will be applied to the control grid of threshold tube 14 and reduce the conductivity of this tube to thereby reduce the fiow of current through resistor ll. The reduction in current flow through resistor [1 will have a corresponding effect on the AGC lead by virtue of connection 24. During the time of this weak received signal impulse, if the voltage originally selected by the tapping point of lead 24 on resistor I1 is less than the signal voltage supplied across resistance 6 and available at point A, the gain of the receiver will increase during this received signal impulse. On the other hand, if the original bias voltage selected by the position of the tapping point on the resistor El and supplied to the AGC circuit by lead 24 is greater than the signal voltage across resistor 6 and available at A, the gain of the receiver will be decreased during said received signal impulse. If the weak signal which reduces the current in threshold tube I4 is not immediately sufiicient to cut ofi the flow of ourrent through resistor H, the reduction in the current through resistor l7 and the increase of the gain in the receiver by virtue of the AGC action will increase the gain of the signal to an extent which will completely out oit current through tube It. In the telegraph system in which it is contemplated using this receiver, particularly in a diversity receiving arrangement, the presence of a weak signal will be sufficient to immediately cut off the flow of current through tube 14. Since tubes l4 and i5 are preferably controlled in parallel, the action which cuts off the flow of space current in tube 54 will simultaneously cut off current through tone keyer tube l5 and thus permit tone from source 9 to pass out over the line TL. When the signal intensity of the received signal increases sharply beyond the critical level at which tube l commences to rectify, for reasons heretofore mentioned, the fiow of current through tube ll will produce a low impedance path in shunt to resistor 5, thus in effect by-passing resistor 5. Thus, the flow of current through the rectifier E will cause a considerable increase in the current flow through resistor S, as a result of which a considerably large voltage will be available at point A for use in the AGC lead to more positively control the gain of the receiver in such direction as to reduce the gain. Thus, it should be evident that any sharp increase of signal strength, such as may be caused by static crash or a strong signal burst, will immediately reduce the gain of the receiver and increase the negative voltage applied to the control grids of tubes i l and i5, thus preventing splits or mutilations in the characters of the telegraph signals sent out over the line TL.

By using a system of the type shown in Fig. 2, it is possible to reduce the band width required in the frequency spectrum of the medium between the transmitter and the receiver. band width is preferably reduced by the transmission of characters with sloping sides instead of standard or accepted square wave character transmission, as at present in general practice. This may be more readily understood by referring to the graphs of Figs. 3a to 3d, inclusive. In Fig. 30. there are shown signal characters comprising a dot and dash of the generally accepted form of transmitted signals, in accordance with known systems. According to known practice, a central oi'hce will transmit signal characters of square wave form which because of the line faci ities become distorted and of sloping character before the signals arrive at the transmitting or radiating point. It has been customary, heretofore, at this transmitting or radiating point to convert the sloping characters to square wave characters before transmission over the ether. By means of the present invention, we can directly transmit the sloping characters over the ether without the need for converting them to square wave characters at the transmitting point. Since the transmission of square wave form at the transmitter requires a wider frequency band than the transmission of sloping characters, I am able to reduce the band width of the frequency spectrum between the transmitter and the receiver required for a certain speed of transmission. This is done by transmitting over the ether signal characters having sloping sides, in the manner shown in Fig. 3b, and by converting these characters of sloping form, when received, to square wave form as originally transmitted from the central office.

Fig. 3c graphically shows how this is done in the telegraph receiving system of Fig 2. The gain of the receiver is adjusted by means of a tapping point on resistor H to a relatively low value. As the signal begins to pass through the receiver by way of rectifier 4, resistance 5 and 8 to ground, threshold tube it begins its process of being cut oil. During the process of this tube cutting off, the gain of the receiver (due to a reduction in the flow of current through resistance ll) will greatly increase to a point where rectifier 1 suddenly comes into action, increasing the current through resistance 6 and thereby further reducing the gain of the receiver but at the same time considerably increasing the negative bias applied to the control grids of tubes i4 and I 5. In effect, rectifier tube 1 provides a snap action which decreases the gain of the receiver suddenly in order to cut off sharply the how of current to tone keyer E5 to thereby produce square wave tone output characters from the balanced modulator 23. By suitable selection of the values of anode battery l6 for tube I l and the anode battery 25 for tube l5 and the grid bias battery lit for tube M, the diiferential point of operation between tubes M and can be chosen to make the nonconductive point of operation of tube l5 coincide with the operation of the rectifier tube "i. The point of operation of the tone keyer i5 is represented by point B on curve 3C. From what has gone before, it will be apparent that the fiow of current through tube 14 is not immediately cut off and that current flows through tube i l from point C on graph 30 to point B, in a continuously decreasing amount, and that when point B is reached, both tubes Hi and I5 cease conducting altogether. The tone signal characters in line TL are represented by graph the durations of which correspond to the fiat top portions of curve 36.

The automatic volume control circuit of the present invention operates as explained in my two patents, Numbers 2,104,324 and 2,243,423. In the absence of carrier wave energy, threshold tube I4 is conductive, supplying a negative automatic gain control voltage supply that is passed through resistance ii to control the gain of the receiver. With carrier current present causing rectifier tube t to become conductive, furnishing an automatic gain control supply voltage across resistance 6, threshold tube M- is wholly or partially cut oif, in turn wholly or partially eliminating the negative voltage drop across resistance ll. Therefore, there is a presence of negative voltage supply at all times available to the automatic gain control circuit to properly control the receiver in such a manner that the gain is never allowed to rise to a point where noise would become excessive in the output circuits. Furthermore, if the automatic gain control voltage manually tapped from across resistance ll is exceeded during incoming wave signals by the voltage drop across resistance 6, the gain of the receiver will be further reduced. If the signal voltage developed across resistance 5 decreases as it normally would du ing a condition of fadthe gain of the receiver will rise to a value corresponding to or within the noise level signal values permitting same. If the voltage developed across resistance 6 happens to be exactly equal to the voltage developed across resistance it, the gain of the receiver does not change; the automatic gain control supply is simply fed automatically through resistances l8 and H during signal or no signal conditions.

It will be appreciated that there is an equalization efi'ect produced by the continual interchanging of the automatic gain control supply from resistance 5 and from across resistance H, as a result of voltage variation across resistance 6, which partially cancels the modulation components feeding into the automatic gain control circuit. For instance, let us assume that a carrier is being received without modulation. There is little or no bias at all supplied from resistance ll due to reduced conductivity or complete cutoff of threshold tube E4. The automatic gain control bias is then fed through resistance llJ. However, when modulation peaks rise to a height to be of consequence, by feeding through resistance it into condenser l2 and which tend to demodulate or cause distortion to the incoming Waves through the automatic gain control circuit, threshold tube l4 becomes conductive to supply a negative voltage through resistance H to replace the electrons in the automatic gain control circuit taken away momentarily from across resistance 6 during modulation peaks.

Fig. 4 illustrates a receiver of the present invention which is primarily for use in receiving speech or modulated Wave signals. The elements of Fig. 4 which bear the same reference numerals as those of Fig. 2 are substantially the same and operate in similar fashion. Since the receiver of Fig. 4 is intended only for telephonic or modulated wave signals, it lacks the tone keyer of Fig. 2 and includes the audio translating circuit here represented by audio frequency transformer 25, the primary winding of which is connected across the resistor 5. It should be noted that the control grid of tube it of Fig. 4 obtains part of its bias through a cathode resistor 2'! which is preferably employed in this particular receiver rather than the use of the bias battery I3 shown in Fig. 2. It should also be noted that the lower terminal of coil L3 is coupled to the time constant circuit through a resistor 28 which is relatively low compared to the value of resistor H. The time constant circuit also has a resistor Hi which in this case is also of a low value compared to the resistor H,

As for the operation of Fig. 4, normally this receiver operates as a standard modulated wave receiver with the additional noise reduction characteristics provided by the use of rectifier F tube '7 and its associated elements. Let us assume that the receiver is designed to pass With equal gain the signals between one and fifty decibels, and that rectifier tube 1 becomes conductive when the signal intensity approaches the upper limit of the modulated Wave, let us say at around forty-eight decibels. The reception of a strong static crash or strong signal above forty-eight decibels would thus cause rectifier l to become conductive and cause an increased flow of current through resistor E. An increased current will fiow through low resistor 28 directly to the AGC lead, causing the gain of the receiver to instantaneously decrease sharply for the duration of the stat c crash. Because of the relatively low value of resistor l8 compared to resistor l l, a general increase in the flow of current in resistor 6 will occur, by virtue of which the modulated wave appearing in transformer 26 will be slightly increased during the duration of the static crash. Explaining further, the current through elements consisting of rectifier i, inductance L3, resistance 28, resistance l0 and resistance 6 to ground potential, encounters less resistance than through resistance 5. Therefore, since resistance 5 is suitably placed in parallel With a resistance of less value, the total plate load circuit of rectifier t is lessened, causing the rectified current to increase to ground through resistance 6. This slight increase in the intensity of the modulated wave during the excessively strong incoming signal will prevent a drop out or complete cut oif of the signal appearing in the translator coupled to transformer 25, during the interval of the excessively strong signal. Putting it in other Words, the system of Fig. 2 prevents a hole from being knocked out of the incoming modulated wave, as available for utilization, during the static crash, and this hole knock out is prevented by by-passing the current from rectifier tube 1 through low resistors 28 and IE! to thereby increase the current through resistor 6. By suitable choice of the resistors 28 and 10', we can obtain equalization between the voltage added to the AGC circuit and the voltage added across resistor 6 to thereby give the desired effect of substantial reduction or of apparent reduction in the modulation output.

In the operation of Fig. l, if it is desired to Wholly or partially remove a strong static impulse, the anode circuit of rectifier tube 7 may be connected through coil L3 directly to ground. Upon becoming conductive, due to an excessive static impulse or excessive modulation levels, the output of rectifier l during this period of time becomes a very low impedance circuit, thus bypassing the excessive noise energy or excessive modulation energy to ground around the audio output system; but on its return to its cathode, it meets and opposes the flow of current through resistors 5 and 6 produced by rectifier 4, thus creating opposing voltages which wholly or partially equalize themselves, thus producing an effect of partial compression of the noise or modulation components. Another Way of explaining this cancellation effect is that the rectified current passing through resistors 5 and 6 is suddenly connected through the low resistance path of rectifier 1 to ground, as a result of increased signal or noise excitation in inductance L3 from inductance L4. During extreme signal peaks the electrons collected at the top end of resistance 5 suddenly pass through rectifier 1 creating a partial deficiency of electrons for the moment at the top of resistance 5. Thus, as the cathode return current of rectifier l replaces this deficiency of electrons, an effect equal to partial cancellation exists, reducing the noise or excessive modulation components appreciably through resistance 6.

The term ground used in the specification and claims denotes any point or surface of fixed alternating current potential or of zero radio frequency potential.

What is claimed is:

1. In a signal receiving system, a signal transmission tube, a rectifier tube including a resistive connection in its space current path across which is developed a direct current voltage varying in magnitude with signal intensity variation, an automatic gain control system comprising a time constant circuit including a condenser-resistor combination connected between a gain control electrode of the transmission tube and. a point on the resistive connection which becomes more negative with signal intensity increase, and another rectifier tube in shunt to a portion of said resistive connection located between said time constant circuit and said first rectifier tube,

both of said rectifier tubes being individually coupled to the output of said transmission tube and so constructed and arranged as to become conductive at different values of incoming signal voltage, said second rectifier tube controlling the rate of charge of the condenser in said time constant circuit and the rapidity of response of said gain control system in response only to large increases in signal voltage, to thereby reduce the effective resistance of said portion and thereby decrease the effective time constant for strong signals.

2. In a receiving system, an amplifier adapted to amplify the incoming waves, means for rectifying the products of said amplifier, a time constant circuit including a resistance and a con denser in circuit with said means for impressing a potential obtained from said rectified waves upon said amplifier to control its gain, a direct current impedance located between said time constant circuit and said means through which the rectified currents pass to thereby limit the rate of charge of said time constant circuit, and an electron discharge device in shunt to said direct current impedance and coupled to the output of said amplifier, said device being only responsive to an increase in the intensity of the incoming waves above a predetermined value for providing a path of low impedance across said direct current impedance for strong signals, said predetermined value being appreciably above the value at which said means begins rectifying the products of said amplifier, whereby said condenser is more rapidly charged to increase the speed of control over said amplifier.

3. In a signal receiving system, a signal transmission tube, a rectifier tube including a resistive connection in its space current path across which is developed a direct current voltage varying in magnitude with signal intensity variation, -:a

automatic gain control connection between a gain control electrode of the transmission tube and a point on the resistive connection which becomes more negative with signal intensity increase, said automatic gain control connection including a, time constant circuit having a series resistor and a condenser connected between a point on said resistor and a surface of fixed alternating current potential, and another rectifier tube in shunt to a portion of said resistive connection, both of said rectifier tub-es being individually coupled to the output of said transmission tube and so constructed and arranged as to become conductive at difierent values of incoming signal voltage, said second rectifier tube being responsive only to sudden large increases in signal voltage, an electron discharge device having its input electrode coupled to a point on said resistive connection outside of said shunt portion and its output electrode connected through a resistor to said surface of fixed alternating current potential, and a connection from a point on said last resistor to a point on the series resistor of said time constant circuit, and means for biasing the input electrode of said device such that said device is conductlve in the absence of incoming signals and in this condition supplies a negative voltage to said gain control connection.

4. In a radio telegraph receiving system, a signal transmission tube having a parallel tuned output circuit, a second parallel tuned circuit coupled to said first tuned circuit, a rectifier tube having an electrode coupled to the high potential terminal of said second tuned circuit and another electrode coupled to the low potential terminal of the second tuned circuit through a pair of series resistors, a third parallel tuned circuit coupled to said first tuned circuit, said third tuned circuit being in shunt to one of said pair of series resistors over a path including another rectifier tube, said three parallel tuned circuits being tuned to the same frequency, an automatic gain control circuit including a time constant element between a gain control electrode of the transmission and a point on the other resistor of said pair, said second and third tuned circuits being so coupled to said first tuned circuit that said rectifier tube is responsive to a lower intensity of incoming signal than said other rectifier tube, a t e iieyer tube having its input electrode connec d to a point on the other resistor oi said pai and its output electrode coupled a relay system, the bias on the input electrode of said tone keyer being such that the keyer is conductive in the absence oi signals and is nonconductive in the presence oi signals, to thereby control the operation of said relay accordingly.

In a radio telegraph receiving system, a signal transmission tube having a parallel tuned output circuit, a second parallel tuned circuit coupled to saidf'rst tuned circuit, a rectifier tube having an electrode coupled to the high potential terminal oi said second tuned circuit and another electrode coupled to the low potential terminal or the second tuned circuit through a pair of eries resistors, a third parallel tuned circuit coupled to said first tuned circuit, said third tuned circuit being in shunt to one or said pair of series resistors over a path including another rectifier tube, said three parallel tuned circuits being tuned to the same i'requency, an automatic gain control circuit including a time constant element between a gain control electrode of the transmisslon tuoe'and a point on the other resistor of said pair, said second and third tuned circuits being so coupled to said first tuned circuit that said first rectifier tube responsive to a lower intensity or incoming signal than said other rectifier tube, a tone keyer tube having its input electrode connected to a point on the other resistor of said pair and its output electrode coupled to a relay system, a threshold tube having its input electrode in electrically parallel relation to the input electrode of said tone lseyer and its output electrode connected through a resistor to ground, and a connection from a point on said last resistor to said time constant element, whereby the fiow of current through said last resistor supplies a negative voltage to said automatic gain control circuit, the bias on the input electrodes of said threshold and tone lieyer tubes being such that they are conductive in the absence of signals and become non-conduct1ve during the reception of signals.

In a modulated wave receiving system, a signal transmission tube having a parallel tuned output circuit, a second parallel tuned circuit coupled to said first tuned circuit, a rectifier tube having an electrode coupled to the high potential terminal of said second tuned circuit and another electrode coupled to the low potential terminal 01 the second tune-d circuit through a pair of series resistors, a third parallel tuned circuit coupled to said first tuned circuit, said third tuned circuit being in shunt to one of said pair of series resistors over a path including another rectifier tube, said three parallel tuned circuits being tuned to the same frequency, an automatic gain control circuit including a time constant element between a gain control electrode of the transmission tube and a point on the other resistor of said pair, said second and third tuned circuits being so coupled to said first tuned circuit that said first rectifier tube is responsive to a lower intensity of incoming signal than said other rectifier tube, a vacuum tube having its input electrode connected to a point intermediate the ends of the other resistor of said pair of series resistors and its output electrode connected to ground through a resistor, a connection from a point on said last resistor to said time constant element, the bias on the input electrode of said last vacuum tube being such that the tube is conductive in the absence of signals during which time it supplies a negative bias to said automatic gain control circuit, and an audio frequency transformer having its primary winding connected across the other resistor of said pair of series resistors and its secondary winding coupled to a utilization circuit.

'7. In a multi-stage system, an intermediate stage vacuum tube wave amplifier, a transformer having an input winding and two output windings, connections between the output of said amplifier and said input winding, a first rectifier tube connected to one of said output windings, an output resistance for said first rectifier with a connection to a succeeding input transformer, a

second rectifier connected across the other output transformer winding and adjusted to become conductive after the first rectifier becomes conductive, both of said rectifiers being connected in electrically parallel relation to said output resistance, whereby the application of a strong Wave to said amplifier increases the IR drop across said resistance, thus producing an expanding characteristic for said strong amplified wave.

8. In a multi-stage system, an intermediate stage vacuum tube wave amplifier, a transformer having an input winding and two output windings, connections between the output of said amplifier and said input winding, a first rectifier tube connected to one of said output windings, an output resistance for said first rectifier with a connection to a succeeding input transformer, a second rectifier connected across the other output transformer winding and adjusted to become conductive after the first rectifier becomes conductive, both of said rectifiers being connected reversely to said output resistance so that the opposing voltages partially cancel the stronger waves, whereby a limiting or compression action is produced as the waves pass through said Wave amplifier.

R. LEE I-IOLLINGSWORTH.

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