US2282131A

US2282131A - Electrical signaling system

Info

Publication number: US2282131A
Application number: US366121A
Authority: US
Inventors: Hadfield Bertram Morton
Original assignee: Associated Electric Laboratories Inc
Current assignee: Associated Electric Laboratories Inc
Priority date: 1940-01-12
Filing date: 1940-11-18
Publication date: 1942-05-05
Anticipated expiration: 1959-05-05

Description

M. HADFIYELD 2,282,131

ELECTRICAL SIGNALING SYSTEM May 5, 1942.-

3 Sheets-Sheet 1 Filed Nov. 18, 1940 I ATTORNEY y 1 B. M. HADFIELD ELECTRICAL SIGNALING SYSTEM- Filed Nov 18, 1940 3 Shee ts-Sheet 2 mvuron BERTRAM MORTON= HADF'IE'LD ATTORNEY y 5, 1942- B. M. HADFIEL'D 2,282,131

ELECTRICAL SIGNALING SYSTEM Filed Nov 18, 1940' s Sheets-Sheet :5

mvs'nmk BERm/m MORTON HADFIELD ATTORNEY Patented May 5, 1942 232122.131 ELECTRICAL SIGNALING SYSTEM Bertram Morton Hadfield, Harrow Weald, England, assignor to Associated Electric Laboratories Inc., Chicago, Ill a corporation of Delaware Application November 18, 1940, Serial N 0. 366,121 In Great Britain January 12, 1940 20 Claims.

The present invention relates to electrical signaling systems and more particularly systems of the type in which alternating current signals are transmitted over lines which may also carry composite non-signaling currents including currents of the signaling frequency or frequencies. The object of the present invention is to provide circuit arrangements whereby such alternating current signals may be converted into corresponding direct current signals with a minimum of distortion and mutual interference, whilst currents of alien frequencies are rendered ineffective. The invention is particularly applicable to the reception of voice frequency signals over telephone or telegraph lines whose nature prevents the distortionless transmission of direct current signals of distinctive types. In such cases the alternating current signals may range from those representing impulse trains such as are used in automatic telephone and telegraph systems to relatively long signals for supervisory purposes comprising one or more frequencies transmitted successively or simultaneously. In addition in the case of telephone lines it is generally required in order that complete supervision may be effective that the reception apparatus shall not respond to alien signals such as speech and automatic tones as used in direct current systems.

As is well known the reception apparatus usually consists of an arrangement of thermionic valves, in the preliminary stages of which is incorporated a device which renders the alternating current output substantially independent of received level variations. In subsequent stages the signal paths are defined by filter networks according to the signal frequencies used so that operation of relays individual to certain frequencies can be ensured. Prevention of response of these relays to alien signals which may include the signal frequencies is generally attained by a further path whose guarding effect is applied to all signal paths. The extent to which the guard path is rendered effective at frequencies above the highest signal frequency is dependent on the harmonics introduced by the transmission system and received level compensating stage. Provided the design of the latter is such as to introduce no appreciable signal harmonics below the fifth and only small percentages of higher order harmonics, it is well known that the guard path need only be tuned so as to reject the signal frequencies. The guarding effect so obtained is generally rendered the more effective to alien signals which may include the signal frequencies by aggregating the responses and applying the effect to all the signal paths so as to render them ineffective. The aggregation is generally accomplished by rectification and utilisation of the direct current response as a negative bias on the control grids of the signal valves.

The filter networks used to define the signal paths are generally of the simple acceptor resonant circuit type that is to say a given path is made most responsive to a particular signal frequency. In order to secure efiective operation of a given path to only one frequency, plus a band-width to allow for received frequency variations, the resonant circuit used must have a gain at resonance of at least 10:1 over the input where for instance discrimination between the two commonly used frequencies of 600 and 750 cycles per second is desired. In general the gain or Q of the acceptor resonant circuit must not be less than 20:1. An upper limit to the value of Q is set by the consideration of approximation to the signal envelope Wave shape. This approximation must, ideally, be very close if the inherent distortion of the apparatus is to be small. Although distortionless operation of the signal circuit can be secured by adjustment of the operating and release levels in relation to the distorted envelope, itis desirable that the inherent distortion shall be low so that changes in the operation and release characteristics of the apparatus shall cause the least change of output.

It will be seen therefore that to secure a minimum of inherent distortion together with a degree of discrimination such that for instance no single frequency can operate more than one signal path involves conflicting requirements in relation to the Q value of the acceptor type resonant circuit. As indicated before, a Q of not less than 10 has been found necessary.

When it is desired to receive more than one frequency at a time in the form of a compound signal, a further consideration conflicting with the use of low Q circuits is obtained. For instance, if the compound signal consists of the two

frequencies

600 and 750 cycles per second, then the response of either of the two acceptor circuits to the unwanted frequency is appreciable (some 30% for a Q of 10). This results in the amplitudes of the signals as selected by the resonant circuits being modulated at the difference frequency to an extent of :30%. Consequently utilisation of this signal is limited by the frequency and depth of modulation, since unless marginal operation is resorted to, a component at the difference frequency may cause the responding apparatus such as a relay to chatter. Elimination of this component is difficult without distorting the signal waveform envelope.

According to one feature of the present invention it is therefore proposed to use rejector resonant circuits in each signal path of number equal to the number of unwanted signal frequencies. By resonant rejector circuit is meant a circuit arrangement whose response is a minimum at a certain frequency. Each signal path therefore will respond to any frequency, including the wanted one, except the remaining unwanted signal frequencies. In order to prevent effective operation at frequencies other than the wanted one plus a suitable bandwidth, a guard circuit is employed which has rejector circuits at all the signal frequencies.

By these means the envelope of the signal frequency pulses as applied to each signal relay stage remains substantially unaltered, since only those frequency components corresponding to the rejector circuit frequencies in each signal path are removed from the input signal pulse. This state of affairs is made to apply only over the bandwidth of the wanted signal by arranging that the guard path response over this bandwidth is completely suppressed. This response will of course be at a minimum due to the rejector circuit of corresponding frequency in the guard circuit, and it is therefore possible to suppress not only the static response of the guard circuit, but also the surges produced at the beginning and end of a signal pulse to an effective degree.

The effective suppression of both static and dy-- namic responses of the guard circuit over the signal frequency bandwidths is regarded as an other feature of the invention, since within limits it enables the ratio of guard to signal effects outside the operating bandwidths to be altered without affecting the inherent distortion of the required signals. This ratio will of course to a very large extent determine the efficacy of the guard circuit in preventing response of the signal stages to alien frequencies which ma contain some signal frequencies.

It has been shown that the above arrangement enables the inherent distortion to be rendered negligible, whilst retaining adequate separation between signal frequencies by the combined action of the rejector circuits at the unwanted frequencies in each of the signal paths and the rejector circuits at each signal frequency in the guard path. This arrangement is also efiective in securing non-marginal operation of the signal paths when two or more signal frequencies are simultaneously applied. As shown heretofore, this can only be secured by reducing the percentage modulation at the difference frequency or frequencies in each signal path. It has been found that provided the rejector circuits in the signal paths are of high Q, i. e., present a maximum rejection effect at their resonant frequencies, the maximum interference from an unwanted signal frequency of 600 cycles per second rises to some at the extreme limits of a bandwidth of :20 cycles on the I cycles circuit. For smaller divergencies from the nominal frequency, the interference decreases to a minimum at the resonant frequency. Over such a range of bandwidth frequencies the wanted signal response is, of course, substantially unaltered, and therefore the modulation effect is considerably less than in the well-known acceptor signal circuit arrangement. In the latter case the modulation effect remains substantially constant over such a bandwidth if acceptor circuits having a Q value of about 10 are used, since the variation of response over the bandwidth is small. The modulation effect can, of course, be reduced over smaller variations by raising the Q value of an acceptor circuit, but the inherent distortion of the signal envelope will then increase.

It will be realised that with a guard circuit of the type incorporating rejector circuits resonant at each signal frequency, the output, although diminished over the signal frequency bandwidth, will be approximately 100% modulated at the difference frequency when two or more signal frequencies of equal amplitudes are simultaneously applied. While it might be possible to suppress this diminished and modulated output over the signal bandwidths, such modulation will give rise to chatter of the signal relays at frequencies just outside the bandwidths. This action would be deleterious in securing sharply defined operational bandwidths and will also tend to give such chattering operation to alien frequencies such as speech.

As another feature of the invention it is there fore proposed to smooth the guard circuit direct current output to an efficiency of at least at all frequencies higher than thelowest difference frequency of any combination of signal frequencies, by means of circuit elements designed to give an attenuation/frequency characteristic of some 20 db. over the above range of frequencies, whilst giving at least zero attenuation to frequencies below some one-third of the lowest difference frequency.

The invention will be better understood from the following description of several methods of carrying it into effect which should be taken in conjunction with the accompanying drawings. These show it applied to a signaling arrangement suitable for use in a telephone system employing for signaling purposes two voice frequencies of 600 and 750 cycles per second.

Figure 1 shows an arrangement making use of a source of very high internal impedance giving in effect a constant current arrangement.

Figure 2 shows a similar arrangement using a low impedance source, that is to say a constant voltage arrangement Figure 3 shows an alternative method of smoothing in the application of grid bias to the signal valves.

Figure 4 shows a modified arrangement using direct current operation of the signal valves, while Figure 5 indicates how the guard circuit may be used to control a variable permeability coil.

As explained above owing to the use of a re jector type guard circuit, the preliminary stage or stages in the apparatus, while delivering a substantially constant output over the desired range of input levels, must not produce any appreciable harmonics below the fifth. This applies more particularly for the band of input frequencies including the signal frequencies since it is only necessary that no additional guarding action due to harmonics of the signal frequencies shall be obtained. The means whereby the output of such preliminary stages is rendered reasonably pure therefore depends on the total range of the signal frequencies. In the case assumed of signaling by the two frequencies of 600 and 750 cycles per second, an elementary band pass filter network extending from say 5.00 to 980 cycles would be sufficient for the purpose. Alternatively any wellknown distortionless limiter may be used.

Given a substantially constant and undistorted alternating output whose signal envelope is similar to that of the signal input, this may then be used to energise either the guard circuit alone or both the guard and signal circuits. In the former case selection of the wanted signals takes place at a later stage, which may or may not be desirable according as to Whether immediate conversion into a direct current signal is desired. The guard circuit however must be energised and produce its guarding action at this stage, since its action must be independent of any particular signal path but common to all.

It will be realised that the guard and signal paths have the common feature that rejector circuits are used to prevent action at the unwanted frequency. In the assumed case of signals of 600 and 750 cycles per second therefore, the guard circuit will include two rejector circuits resonant at or about these frequencies connected in parallel with one another whilst the 750 cycle signal path will include a 600 cycle rejector circuit and Vice-versa. Two methods are available for the connection excitation of rejector circuits in any given path:

(a) By using a source of very high internal impedance (i. e., constant current) together with an external circuit consisting of series-connected impedance (i. e., constant voltage), together with i an external circuit consisting of parallel-connected resonant circuits, each constituting a rejector circuit, in series with each other and with a resistance. The voltage and/or current in the latter may be used for giving the desired control.

By a series-connected circuit is meant same. The difference in response about resonance is due to the mutual impedance interaction of the two circuits at frequencies between 600 and 750, whereby the rate of change of response is made greater.

In order to utilise the aggregate effect of the frequencies energising the guard circuit, the alternating output across the terminating resistance is converted into direct current by means of any well-known rectifier system, preferably the full wave rectifierbridge shown comprising the rectifiers M. It should be noted that since the terminatingresistance is placed in parallel with the series rejector. circuits, a transformer may be used having differing and appropriately designed windings to supply the rejector circuits andterminating resistance. It will be noted also that the terminating resistance forms the direct current load on the rectifiers.

By a parallel-connected circuit is meant the shunt connection of an inductance and capacity. This arrangement is shown in Fig. 2.

The arrangement of Fig, 1 is generally preferable, since a pentode valve, more particularly when used with negative feedback of the current type, adequately fulfils the requirements of a very high resistance source and also gives a greater power output than a triode valve.

In Fig. 1 the signal responding apparatus is connected at the points a, b to the telephone line or other circuit over which the alternating current signals are received. These signals are passed through the level compensating stage indicated by the equipment LCS. The output from LCS is fed to a series circuit consisting of a resistance S from which the signal paths are supplied and the primary of transformer T9 from which the guard circuit is supplied. One secondary winding of this transformer supplies two resonant circuits GI and G2 of high Q value (say a Q of 40) which are connected in parallel with one another while another secondary winding supplies the terminating resistance Ry. It is preferred to tune the circuits G! and G2 to nominal values differing from 600 and 750 cycles by, say, 10 cycles so that the resonant frequencies are 610 and 740 cycles, in order that the responses at the bandwidth limits of 120 cycles about each signal frequency shall be substantially the It will be appreciated that the wavefront of th guard circuit direct current output should preferably be as rectilinear as possible, in order adequately to forestall incipient operation of the signal relays to alien frequencies. At the same time the direct current output must be reasonably smoothed as mentioned above down to the lowest difference frequency which in the case of signal frequencies of 600 and 750- cycles per second will be 150 cycles.

In the preferred method as illustrated, the required effect of the guard circuit is produced by using the guard circuit output as a direct current bias on the signal valves, and smoothing is effected by the action of the signal valves.

It will be seen that the signal stage makes use of the two pentode valves V, V in pushpull, and guard circuit output is applied to the common grid-cathode path. As is well known, with such an arrangement any voltage applied to the common grid/cathode path is amplified by the two valves in parallel, and only produces a modulation effect on the pushpull alternating output by altering the mean mutual conductance. If therefor the gain/frequency response curve of the two valves considered as parallel connected is made the same as the desired attenuation/frequency characteristic of the smoothing circuit, application of the unsmoothed guard circuit rectified voltage to act as a negative bias on both valves simultaneously will result in the grid/cathode voltage being smoothed. This type of gain/frequency characteristic may be obtained by inserting in thecommon anode or cathode paths a network consisting of a capacity and series resistance all shunted by an inductance, and applying the voltage developed across the resistance element as negative feedback to the common grid path. that is in series with the unsmoothed guard circuit voltage.

This arrangement has the advantage that the values of the circuit parameters (L, C and R) are dependent only on the valves and the degree of feedback required and therefore place no restrictions on the design and output voltage of the guard circuit. A further advantage is that owing to the invariable square law characteristics of most thermionic valves, the smoothing action of the above arrangement is most effective with small outputs from the guard circuit since the mutual conductance increases with small grid biases. It will be seen that the voltage across the resistance S is applied to the primary windings of the transformer Ts and the alternating output from the secondaries to the grids of the valves V, V arranged in push-pull. The unsmoothed guard voltage output on Rg is applied as additional grid bias by way of the rectifier M3 and the resistance P and in series with the E. M. F. e2. This E. M. F. effects substantially complete suppression of the guard circuit response over the signal bandwidths by ensuring that the guard circuit is ineffective as long as the voltage developed therein does not exceed e2 which will be the case since the frequencies concerned have already been substantially eliminated by the rejector circuits GI and G2. The rectifier prevents the steady voltage 62 from disturbing the normal circuit, potentials when the guard circuit voltage does not reach this value. The normal grid bias for thevalves is provided by the voltage 61.

The smoothing of the guard voltage on the lines described above is effected by the network of inductance L shunted by condenser C and resistance R in series which is inserted in the feed back path from the centre of the primary of the output transformer T0. The primary of the feedback transformer Tfg is connected across resistance R and the secondary is included in the grid-cathode circuit so as to exert the appropriate smoothing effect.

With regard to rendering the signal bandwidth. responses of the guard circuit ineffective, this is further ensured by the following arrangement. It is well known that if negative feedback of the current type be applied to a thermionic valve stage, the mutual conductance is stabilised to a degree dependent on the amount of such feedback and hence changes of valve bias over a wide range do not appreciably alter the output. towards the zero anode current value, the gain of the valve stage will remain substantially constant and then rapidly drop to zero, instead of gradually tending thereto when no feedback is used. This effect is used in the present case by applying negative current feedback to the push-pull paths of the signal valve stages. The split primary of the transformer Tfs is inserted between the halves of the primary of the output transformer To and applies push-pull negative current feed back to the valves V, V. The transformer Tfs is arranged to have a suitable step-up ratio, and the secondary is loaded by the variable resistance Rf and connected with each half winding in series with the half secondary windings of the input transformer. The value of the resistance R controls the degree of current feedback.

The alternating output from the valves V, V is applied by way of the secondary winding of the transformer To to a series circuit consisting of series resonant rejector circuits, each shunted by an appropriate resistance, as there are signal frequencies. Eachsignal circuit will of course include as many series resonant rejector circuits in parallel as there are unwanted signal frequencies. In the specific case under consideration each signal circuit will comprise one resonant rejector circuit at the unwanted frequency in parallel with its terminating resistance. The signal output on the latter may then be rectified and applied to further valve However, as the bias is increased relay stages, or preferably direct to the relays.

that they act on the rejector circuits, at any rate in the steady state condition, as if they were non-inductive resistances of value equal to the relay resistances, provided they are shunted by an appropriate capacity.

This arrangement is the one illustrated and it will be seen that for the 750 cycle signal path the responding relay RI shunted by the condenser Cl is fed from the bridge of rectifiers MI. Ihe bridge is shunted by the resonant rejector circuit J I which is tuned to 600 cycles, the only unwanted frequency concerned. Similarly the responding relay R2 for the 600 cycle path is shunted by the condenser C2 and fed from the rectifiers M2, the bridge being shunted by the rejector circuit J2 tuned to 750 cycles.

The arrangement of Fig. 2 is similar to that of Fig. 1 except that it is suitable for a level compensating stage of low impedance, that is constant voltage type. Though the general layout is the same this involves some alteration in the nature and arrangement of the components. In this case the guard path and the signal paths are fed in parallel from different windings of the input transformer and the rejector circuits in the guard path are of the parallel type connected in series with each other and with the terminating resistance. The signal feed back is of the voltage type and is provided by tertiary windings on the output transformer. The signal relays are supplied in parallel from the secondary windings of the output transformer and each is connected in series with a parallel type rejector circuit tuned to the unwanted frequency.

It is not necessary that the use of a high impedance source should involve the use of the full circuit of Fig, 1 or a low impedance source the full circuit of Fig. 2. In fact the parts on opposite sides of the chain dotted lines in Figs. 1 and 2 may be considered as interchangeable between the two figures. That is to say the lefthand portion of Fig. 1 may be combined with the right-hand portion of Fig. 2 and vice versa. By a low impedance source is meant one which has considerably lower impedance than the load impedance over the working frequency range and similarly for a high impedance source. practice it may be desired to use sources which may be termed partial in that their impedance is of the same order as that of the load. The rejector circuit principle may still be applied to such cases since the only effect of having a partial source will be to alter the frequency responses of the resonant circuits in a manner easily capable of calculation.

Fig. 3 represents an alternative arrangement for smoothing the guard circuit voltage directly without reference to the signal valves. This consists in adding series inductance to the terminating resistance and shunting the whole by a capacity. As the generator of Fig. 1 is of the constant current type, this system is designed for a Q value of approximately unity and the resonant frequency of f=%1r/i% will give a frequency response such that at 34 the alternating component in the output is not more than. 10% of any frequency below 1. Accordingly the design value of j may be taken as 50 cycles and the direct current output on the terminating resistance will then have a wave front of some 6 milliseconds. It should be explained that the arrangement depends for its action on the impedance/frequency characteristic of the current in the resistance R expressed as a fraction of the current I supplied to the circuit. The sharpness of this characteristic is in practice dependent on I and the best result is obtained when I is constant, hence it is preferable to use a high impedance source.

This circuit may be used on any of the terminating resistances or impedances, i. e., guard or signal, but depending as it does on the value of R, the voltage or current output required, the

power output of the source and the lowest fre-' quency to be smoothed, it may involve unwieldy inductances or capacities. In the case of the guard circuit the lowest frequency in question is 150 cycles and this is the limiting factor, but in the case of the signal frequencies where it may be assumed that the 150-cyc1e component is attenuated by the use of rejector circuits, the lowest frequency is the signal frequency and the circuit then becomes practicable.

It may be mentioned that where the inverse principle is applied to the smoothing of the guard circuit by producing a complementary gain frequency characteristic, the difficulties just mentioned do not arise to the same extent, since the design frequency can be made the same as the lowest frequency to be smoothed because the attenuation of the feed back path must be zero at this frequency and all higher frequencies.

It has already been pointed out that whilst energisation of the signal circuits may be 'delayed in the progression of the signals through the apparatus until the relays are reached, the guard circuit must be energised and enabled to perform its guarding action before or at the same part of the circuit as the signal selection. Fig. 4 shows an arrangement using rejector circuits in which both signal and guard circuits are energised and converted into direct current voltages before application to the signal relay valves. The arrangement shown is that adopted for a source of high impedance but there is no difliculty in modifying the circuits so as to be suitable for a low impedance or partial source. In the circuit shown in Fig. 4 the rectified signal and guard voltages are unsmoothed and use is made of the smoothing action previously described in the feed back path of the signal valves. In this case the static grid bias 6! on the valves V, V will need to be increased since a direct current change of anode current is now required to operate the relays RI and R2. It is. possible, however, to use the alternative method of individual smoothing on each terminating resistance in accordance with Fig. 3 and this will not then impose the wave front of some 6 milliseconds on the signal envelopes. It will be seen that as in Fig. l the guard circuit is fed from the transformer Tg having two secondary windings one shunted by the two series rejector circuits GI and G2 which are connected in parallel and are tuned to 610 and 740 cycles respectively. The other secondary winding feeds the terminating resistance Rg supplying the guard voltage. The input circuit also includes in series the primary windings of two further transformers Tsl and T52 corresponding respectively to the two signal frequencies. The secondaries of these transformers are shunted by series rejector circuits corresponding in each case to the unwanted frequency and supply the terminating resistances Rsl and Rs2 forming the loads on the groups of rectifiers MI and M2. The output from the resistances is applied to the valves V, V which then operate as direct current amplifiers. This arrangement may be satisfactory in some circumstances but in general is subject to two possible disadvantages. In the first place since the guard circuit output is applied to the signal valves in parallel, its feed back smoothing circuit will determine the gain/frequency characteristic of the individual signal valves. This will mean in general that no further feed back smoothing circuits are needed but unfortunately it limits the wave front of the signal current, givin inherent distort-ion. In the second place since the signal operating potentials oppose those from the guard circuit, the square law characteristics of the valves will adversely affect the smoothing of the signalsassuming that, as is usuahthe signal potentials act as positive grid bias.

Fig. 5 indicates a possible method of using the current developed in the guard circuit to control a variable permeability coil as an alternative to arranging for the guard circuit to alter the grid bias voltage. 1 In this case also special arrangements are made for securing complete suppression of the guard circuit response over the signal bandwidths. The circuit element in question comprises three coils I, 2 and 3 wound on a core of material such as mu-metal or permalloy. The coil 3 is excited by the guard current which is arranged to be opposite in effect to a local current through coil 2 froma battery 13 by way of a resistance T. Over the signal bandwidths the diminished guard current i made insufiicient to neutralise the effect of the local current and hence the impedance of the 0011 i is low. This coil is included in the negative feed back path of the signal valves and therefore in these circumstances their action is unimpaired. Outside the signal bandwidths however, the increased response of the guard circuitneutralises the effect of the local current and the impedance of coil l is increased and thus reduces the gain of the signal valves. In order to obtain the maximum impedance of the coil, the resistance T must be high and may conveniently be inductive.

In a modified form, coil 2 may be inserted in the anode'lead of a high impedance valve so that thesteady anode current is used as the local current. The coil I is then connected in series with a grid lead so as to give negativ current feedback.

It should be explained that at thebeginning and end of a signal pulse a surge will be generated on the guard circuit of maximum amplitude much greater than the static response and of duration dependent on the tim constant of the circuit. This surge would be detrimental to the signal envelop-e as applied to the signal valves. Its partial elimination can be secured by increasing the suppression bias described abov but the methods of smoothing the guard circuit voltage are found in practice to remove this surge. This is due to the imposition of a relatively slow wave front on the guard voltage by the extension of thesmoothing range to include cycles.

I claim: i

1. In a signaling system wherein a simple wave predominantly of a certain audio frequency is impressed upon a certain line at times, wherein another simple wave predominantly of a diiferent audio frequency is impressed upon said line at other times, and wherein acomplex wave including said two audio frequencies and other audio frequencies is impressed upon said line at still other times, two responding devices connected to said line, a rejector resonant circuit connected to one of said devices for preventing that device from responding to said first simple wave, a rejector resonant circuit connected to the other one of said devices for preventing said other device from responding to said other simple wave, and a guard circuit responsive to said other audio frequencies effective to prevent either of said devices from responding to said complex wave.

2. In a signaling system wherein a simple wave predominantly of a certain audio frequency is impressed upon a certain line at times, wherein another simple wave predominantly of a different audio frequency is impressed upon said line at other times, and wherein a complex wave including said two audio frequencies and other audio frequencies is impressed upon said line at still other times, two responding devices connected to said line, a rejector resonant circuit connected to one of said devices for preventing that device from responding to said first simple wave, a rejector resonant circuit connected to the other one of said devices for preventing said other device from responding to said other simple wave, and a rejector resonant circuit responsive to said other audio frequencies effective to prevent either of said devices from responding to said complex Wave.

3. In a signal receiver for discriminating between an essentially simple wave predominating in a predetermined audio frequency and a more complex wave made up of a plurality of audio frequencies including said predetermined frequency, a thermionic valve, a guard circuit, means for applying each of said waves, whenever received, to the grid of said valve and to the input of said guard circuit, a resonant circuit tuned substantially to saidpredetermined audio frequency and associated with said guard circuit to render said guard circuit non-responsive to said predetermined audio frequency without pre venting its response to others of said audio frequencies, means controlled by said guard circuit for varying the negative bias on said valve in accordance with the response of said guard circuit to said other audio frequencies, a negative feed back circuit controlled by said guard circuit whenever the response of Said guard circuit to said other frequencies is appreciable to also vary said bias in accordance with the output of said valve, said feed back circuit being ineffectiveif said response of said guard circuit is small, and a device in the output of said valve responsive to said predetermined frequency,

4. A signal receiver as claimed in claim 3, wherein said second circuit is tuned to an audio frequency slightly different than said predetermined frequency, thereby to cause said receiver to discriminate between said complex wave and a simple wave whether the simple wave predominates in said predetermined frequency or in some frequency differing slightly from said pre determined frequency.

5. A signal receiver as claimed in claim 3, wherein the last-named means includes a saturated inductance having one winding traversed by current from said guard circuit, thereby to vary the impedance of said inductance, and having another winding in said negative feed back T circuit.

6-. A signal receiver as claimed in claim 3. wherein the last-named means includes an inrent potential connected to one winding to saturate said inductance, wherein another winding of said inductance is traversed by current from said guard circuit, thereby to vary the impedance of said inductance, and wherein still another winding of said inductance is connected in said negative feed back circuit.

7. In a signal receiver for discriminating between an essentially simple wave predominating in a predetermined audio frequency and a more complex wave made up of a plurality of audio frequencies including said predetermined frequency, an input via which said waves are led into the receiver, a thermionic valve having its grid linked to said input to receive said waves, a rectifier also linked to said input, a resonant circuit associated with said rectifier andv tuned substantially to said predetermined audio frequency to prevent said rectifier from receiving said predetermined audio frequency from said input, said rectifier effective to receive others of said audio frequencies from said input and to rectify said other frequencies, means controlled by the output of said rectifier for increasing the negative bias on said valve in proportion to increases in said output, a negative feed back circuit for also varying said bias in accordance with the output of said valve, and a device in the output of said valve responsive to said predetermined audio frequency, v

8. A signal receiver as claimed in claim 7, wherein said rectifier comprises a full wave rectifier bridge having its opposite corners linked to said input, wherein a resistance and inductance are connected in series across the other opposite corners of said bridge, wherein a condenser is also connected across said other opposite corners of said bridge, and wherein the last-named means is controlled by the voltage across said resistance.

9. In a signal receiver for discriminating between an essentially simple wave predominating in a predetermined audio frequency and a more complex wave made up of a plurality of audio frequencies including said predetermined frequency, an input via which said waves are fed into the receiver, a thermionic valve having its grid linked to said input to receive said waves, a rectifier also linked to said input, a resonant circuit associated with said rectifier and tuned substantially to said predetermined audio frequency to prevent said rectifier from receiving said predetermined audio frequency from said input, said rectifier effective to receive others of said audio frequencies from said input and to rectify said other frequencies, a circuit connecting the output of said rectifier to the grid of said valve to bias same, said last circuit including a source of constant voltage connected in opposition to the voltage developed in the output of said rectifier, thereby to render the voltage in said output ineffective whenever it is smaller in value than said constant voltage, the voltage of said output effective, whenever it exceeds said constant voltage, to increase the negative bias on said valve in proportion to the amount by which it exceeds said constant voltage, a negative feed back circuit for also varying said bias in accordance with the output of said valve, and a device in the output of said valve responsive to said predetermined audio frequency.

10. In a signaling system as claimed in claim 9, a second rectifier in said circuit connecting the output of said first rectifier to said grid, said second rectifier poled to permit current to flow over said circuit from said first rectifier but to prevent current to flow thereover from said source of constant voltage.

11. In a signal receiver for discriminating be tween an essentially simple wave predominating in a predetermined audio frequency and a more complex wave made up of a plurality of audio frequencies including said predetermined frequency, an input for said waves, a rectifier linked to said input, a resonant circuit associated with said rectifier and tuned substantially to said predetermined audio frequency to prevent said rectifier from receiving said predetermined audio frequency, said rectifier effective to receive and rectify others of said audio frequencies, a thermionic valve, a biassing circuit for said valve including a source of constant direct current potential and a resistor in series, means connecting the output of said rectifier in bridge with the portion of said biassing circuit including said source and said resistor in series so that the direct current potential produced by said rectifier in said portion of said circuit opposes the potential of said source, means controlled by the plate potential of said valve for introducing a corresponding negative feed back potential into another portion of said biassing circuit, means for superimposing the waves in said input upon said biassing circuit, and a device in said plate circuit responsive to said predetermined audio frequency.

12. In a signal receiver for discriminating between an essentially simple wave predominating in a predetermined audio frequency and a more complex wave made up of a plurality of audio frequencies including said predetermined frequency, a thermionic valve, a transformer, means for applying each of said waves, whenever received, to the grid of said valve and also to the primar winding of said transformer, a series resonant circuit tuned substantially to said predetermined frequency and connected across another winding of said transformer, means controlled by the output from the secondary winding of said transformer for varying the negative bias on said valve in accordance with said output, a negative feed back circuit controlled by said output, whenever same is appreciable, to also vary said bias in accordance with the output of said valve, said feed back circuit being ineffective if said output from the secondary of said transformer is small, and a device in the output of said valve responsive to said predetermined frequency.

3. In a receiver for audio frequency waves, a pair of thermionic valves connected in push-pull, a guard circuit, means for applying said waves to the grids of said valves and to the input of said guard circuit, a rejector resonant circuit associated with said guard circuit and tuned to a predetermined audio frequency for rendering said guard circuit non-responsive to a limited band of audio frequencies including said predetermined frequency without preventing its response to other audio frequencies outside said hand, means common to said valves controlled by said guard circuit for varying the negative bias on said valves in accordance with the response of said guard circuit to said other audio frequencies, a negative feed back circuit effective whenever the response of said guard circuit to said other frequencies is appreciable for also varying said bias in accordance with the output of said valves, and a device operated by the output of said valves whenever the received wave is comprised essentially of a single frequency lying within the limits of said audio frequency band.

14. In a receiver for audio frequency waves, a pair of thermionic valves connected in pushpull, a guard circuit, means for applying said waves to the grids of said valves and to the input of said guard circuit, two resonant circuits associated with said guard circuit and each tuned to a difierent audio frequency, said resonant circuits effective to make said guard circuit nonresponsive to two limited bands of audio frequencies, each including one of said different frequencies, without preventing its response to other audio frequencies outside said bands, means common to said valves controlled by said guard circuit for varying the negative bias on said valves in accordance with the response of said guard circuit to said other audio frequencies, a negative feed back circuit effective whenever the response of said guard circuit to said other frequencies is appreciable for also varying said bias in accordance with the output of said valves, two devices in the output of said valves, means for rendering one of said devices responsive to substantially all audio frequencies except those in one of said bands, and means for rendering the other of said devices responsive to substantially all audio frequencies except those in the other of said bands.

15(In a receiver for audio frequency waves, a pair of thermionic valves connected in push-pull and having a common grid bias circuit, a guard circuit, means for receiving said waves and impressing them upon the grids of said valves and upon the input of said guard circuit, a rejector resonant circuit associated with said guard circuit and tuned to a predetermined audio frequency for rendering said guard circuit nonresponsive to a limited band of audio frequencies including said predetermined frequency without preventing its response to other audio frequencies outside said band, a rectifier in said guard cir cuit effective to receive and rectify said other frequencies, means for applying said other frequencies, after rectification, to said common cir cuit to negatively bias the grids of said valves, means for also applying a portion of the output of said valves to said common circuit to negatively bias the grids of said valves, and a device operated by the output of said valves whenever the received wave is comprised essentially of a single frequency lying within the limits of said audio frequency band.

16. In a receiver for audio frequency waves, a pair of thermionic valves connected in push-pull and having a common grid bias circuit, means for receiving said audio frequency waves and impressing them upon the grids of said valves, other means for receiving said waves, said last means effective to filter from the received waves a predetermined limited band of audio frequencies and to rectify the remainder of said waves, means for applying said remainder, after rectification, to said common circuit to negatively bias the grids of said valves, means for also applying a portion of the output of said valves to said common circuit to negatively bias the grids of said valves, and a device operated by the output of said valves whenever the received wave is comprised essentially of a single frequency lying Within the limits of said predetermined audio frequency band.

17. In a receiver for audio frequency waves, a pair of thermionic valves connected in push-pull and having a common grid bias circuit, a source of constant direct current voltage in said circuit, means for receiving said audio frequency waves and impressing them upon the grids of said valves, other means for receiving said waves, said last means effective to filter from the received waves a predetermined limited band of audio frequencies and to rectify the remainder of said waves, means for applying said remainder, after rectification, to said common circuit in opposition to the voltage of said source, said rectified remainder of said waves effective, whenever its voltage exceeds that of said source, to negatively bias the grids of said valves, means for also applying a portion of the output of said valves to said common circuit to negatively bias the grids of said valves, and a device operated by the output of said valves-whenever the received wave is comprised essentially of a single irequency lying within the limits of said predetermined audio frequency band. i

18. In a receiver for audio frequency waves, a pair of thermionic valves connected in push-pull, said valves having a common grid bias circuit and a common external anode/cathode circuit, means for receiving said audio frequency waves and impressing them upon the grids of said valves, other means for receiving said waves, said last means effective to filter from the received waves a predetermined limited band of audio frequencies and to rectify the remainder of said waves, means for applying said remainder, after rectification, to said common grid bias circuit to negatively bias said valves, means linking said common anode/cathode circuit to said common grid bias circuit to also negatively bias said valves in accordance with the output of said valves, said last means including a network for controlling the gain/frequency characteristic of said valves, and a device operated by the output of said valves whenever the received wave is comprised essentially of a single frequency lying within the limits of said predetermined audio frequency band.

19. In a receiver for audio frequency waves, a pair of thermionic valves connected in push-pull,

said valves having a common grid bias circuit and a common external anode/cathode circuit, means for receiving said audio frequency waves and impressing them upon the grids of said valves, other means for receiving said Waves, said last means efiective to filter from the received waves a predetermined limited band of audio frequencies and to rectify the remainder of said waves, means for applying said remainder, after rectification, to said common grid bias circuit to negatively bias said valves, an inductance connected in series in said anode/cathode circuit, a circuit including a resistor and a capacitor in series bridged across said inductance, a transformer having its primary winding bidged across said resistor and having its secondary winding connected in said common grid bias circuit to also negatively bias said valves, under control of the output of said valves, in accordance with the attenuation/frequency char-' acteristic of said inductance bridged by said resistor and capacitor, and a device operated by the output of said valves whenever the received wave is comprised essentially of a single frequency lying within the limits of said predetermined audio frequency band.

20. In a receiver for audio frequency Waves, a pair of thermionic valves having a common grid bias circuit, means including a tuned circuit for filtering from the received waves a predetermined limited band of audio frequencies and for applying the remainder t0 the grid of one of said valves, means including another tuned circuit for filtering from the received Waves a different predetermined band of audio frequencies and for applying the remainder to the grid of the other of said valves, means including still other tuned circuits for filtering from the received waves both of said bands of audio frequencies and for rectifying the remainder, means for applying said rectified remainder of the waves to said common circuit to negatively bias the grids of said valves,- and a device in the plate circuit of each of said valves responsive to a flow of current in that plate circuit.

BERTRAM MORTON I-IADFIELD.