US1911850A

US1911850A - Signaling system

Info

Publication number: US1911850A
Application number: US417203A
Authority: US
Inventors: Sandeman Edward Kenneth; Carter Robert Owen
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1929-01-09
Filing date: 1929-12-28
Publication date: 1933-05-30
Anticipated expiration: 1950-05-30
Also published as: FR689304A; GB327709A

Description

y 1933- E. K. SANDEMAN El AL 1,911,850

SIGNALING SYSTEM 2 Sheets-Sheet 1 Filed Dec. 28, 1929 5 K. SANDEMAN R. 0. CARTER A UPNEY May '30, 1933- E. K. SANDEMAN ET AL 1,911,850

SIGNALING SYSTEM Filed Dec. 28. 1929 2 Sheets-Sheet 2 EKSANDEMAN //Vl EN7'OPS. Ra CARTER A OP/VE Y Patented May 30, 1933 UNITEQ STATES PATENT OFFICE EDWARD KENNETH SANDEMAN, OF LONDON, AND ROBERT OWEN CARTER, OF

ALDWYCH, LONDON, ENGLAND, ASSIGNORS T0 WESTERN ELECTRIC COMPANY, IN-

CORPORATED, OF NEW YORK, N. Y., .A CORPORATION OF NEW YORK SIGNALING SYSTEM Application filed December 28, 1929, Serial No. 417,203, and in Great Britain January 9, 1929.

This invention relates to wave transmission systems and more particularly to transmission of the energy of given waves or impulses by means of waves of lower frequency than those whose energy is to be transmitted. In certain cases it is necessary or advantageous to reduce the width of the band of frequencies (such as the speech frequency band) before transmission in order to bring the frequency range to be transmitted within the efficient transmission limits of the transmitting medium used. It is necessary in some cases to shift in the frequency spectrum the frequency of the band to be transmitted. This may be done before or after frequency band reduction.

In numerous transmission systems it is de sirable to reduce the width and/or the position in the frequency spectrum of the band of frequencies sent over the transmission medium; for instance if the transmission medium is a submarine cable (loaded or not) it is well known that the attenuation of such H cables increases so rapidly with frequency that it is practically impossible to transmit telephone conversation over long distance submarine cables. Likewise in radio trans mission systems it is also necessary to occupy as small a frequency band as possible for each transmission channel, in order to prevent interference between the various channels or stations in simultaneous use.

Several systems have been suggested for frequency band reduction. For instance, in British Patent No. 226,338 a signaling system is disclosed in which the band of frequencies of the signals sent over the transmission medium is reduced at the sending station by means of a plurality of modulators and restored at the receiving station to their original range. Another arrangement having the same object is disclosed in British Patent No. 259,328.

The present invention has for its object the provision of an improved wave transmission system of the type referred to above which will provide a high grade of performance.

A feature of the present invention. consists in a signaling system wherein the number of different frequencies, as well as the frequency band width necessary for transmitting waves or signals such as spoken words or the like, is reduced by periodically and with a suitable speed picking up portions of the frequency spectrum, of the wave to be transmitted, by scanning means movable with time relatively to said spectrum. The said scanning means may be if necessary provided with frequency changing means such as modulators (with constant or varying modulating frequencies) for bringing the frequencies of the components of the waves picked up by the scanning means Within a smaller frequency band width than that of the original waves to be transmitted; and the said reduced frequency band may then be shifted in position to occupy in the frequency spectrum a range within the efiicient limits of the transmitting medium. The said selected portions of the waves are transmitted in turn with time selectivity over one channel, to the receiving station where the transmitted portions of the waves are restored to their original frequency range, and if necessary the said portions are reproduced a number of times to compensate for the portions of the frequency range of the waves which are not transmitted.

According to another feature of the invention the frequency band of the waves to be transmitted (which will be for convenience termed hereinafter the speech band) is moved, for instance step-by-step or cyclically in front of a pick up device or scanning means; whereas according to a further feature of the invention it is the pick up device which is moved relatively to the speech band.

A further feature of the present invention consists in a speech band reduction system in which the energy spectrum of the signal or signals to be transmitted is scanned periodically (continuously or not) by a band pass filter having fixed cut off frequencies. This is accomplished as a result of the conversion of said signal or signals into a modulation product (e. g. a side band) of a periodically varying carrier frequency or frequencies in such a manner that the essential frequencies of the signal or signals to be transmitted during a period. of the varying carrier frequency or frequencies are brought within the pass range of said filter.

Another feature of the invention resides in a system of the type referred to in which the reduced frequency band is shifted in position in the frequency spectrum, for instance by modulating said reduced frequency band with one or several suitable modulating freq uencies so as to bring said band into the desired position in the frequency spectrum.

According to a further feature of the inventi on the signals to be transmitted are impressed upon a plurality of modulators rendered operative in turn and supplied with continuously (e. g. cyclically) varying car rier frequencies which vary according to such a law that during a cvcle of variation of the carrier frequencies at least portions of the modulation products (e. g. a side band) fall within a range of frequencies of width smaller than the range of frequencies of the signals to be transmitted, the signals or portions thereof thus compressed in their frequency range being transmitted to the receiving stations where the signals are restored to their original frequency range.

According to another feature of the invention, the signals to be transmitted or portions thereof are impressed in turn and at a suit able rate at the sending station upon a plurality of modulators, each of which is sup plied with a constant frequency of such a value that the signals or portions thereof in pressed upon said modulators are brought within a frequency band width smaller than that of the original signals; and the said signals or portions thereof reduced in their frequency band width are transmitted in turn to the receiving station where they are im pressed synchronously upon a plurality of oscillators supplied with suitable frequency for restoring the signals to their original frequency range.

According to a still further feature of the invention the speech band may be scanned by means of a filter of variable cut off frequencies.

According to a further feature of the invention, the synchronizing devices at the sending and receiving stations may be dispensed with and replaced by several transmission channels which are equal in number or less than the number of sub-bands in which the speech frequency band is divided. For instance, if the speech frequency band is divided in say three portions or sub-bands which are transmitted over three separate channels, the said three separate channels may be conveniently provided by the phantom and side circuits of a four wire circuit such as a quad.

According to the invention a transmission system is provided which is adapted to transmit vibratory energy such as a signal or sigmils and in which the frequency spectrum of the signal or signals to be transmitted is scanned periodically by means of a pick up device which is adapted to traverse etfectuah ly a substantial portion of the frequency spectrum of the signal to be transmitted and to pick up therein portions of the signals, said portions being or not shift-ed (e. g. by moduiation. and selection of a side band) in the frequency spectrum and then transmitted to the receiving station where said portions are restored to form an intelligible signal.

Below several arrangements embodying the invention will be described with reference to the accompanying drawings which illustrate the nature of the invention.

In Fig. 1 there is shown schematically an important feature underlying the invention.

Fi

2 and 3 are diagrams showing the frequency spectrum of the signals to be transmitted and their position at different periods in the cycle of operation of the system.

Fig. a shows the general arrangement of apparatus at the transmitting end.

F 5 shows the arrangement of app-.i ratns at the receivingstation.

Fig. 6 is a modification of the arrangement of F' 4 and 5, using oscillators of constant =3; and a pair of distributors, one at q aiding station and one at the receiving station.

Fig. 7 illustrates in a diagrammatic form the operation of the arrangement of Fig. 6.

Fig. 8 represents an arrangement similar to the one shown in Fig. 6 and using a plurality of pairs of distributors.

A signal such as a telephone conversation may be considered for the present purpose as a quantity having two dimensions, one with respect to time and the other to freqiuncy. hiorc accurately a third dimension should be added this being the amplitude characteristic of the signal. In some cases it may be necessary to take into account the relative phase of the various components of the signals. Therefore if a specific signal does not exceed the frequency B and the time 7', ail the elements of said signal Will be represented by points within the

rectangle

1, 2, 3, 4. of Fig. 1A.

According to the invention the frequency scanning devices select at the transmitting station. certain portions of the rectangle 1, :2- 3, 4. Those selected portions may have any convenient shape. In the first embodiment of the nvention hereinafter described, portions of the signals are selected for transmissien as indicated by the hatched portions of the rcctau l-iof 1B. According to another on :ndinnnt described below portions are sel cted for transmission as shown by the hatched portions of Fig. 1D.

Fig;- ED shows the shape of the selected portr of the signals in the case in which the frequency band B is divided by two. It

should be noted however that in the embodiment shown in Fig. 6 the frequency band is divided by 3 whereas in Fig. 8 it is divided lay-,4. :Figs. 1C and; 1E show other manners of selecting portions of the

rectangle

1, 2, 3,

In the. diagramofFig; 113- it is assumed that the picleup device scans the speech range ith a uniform velocity- 'However, this is not essential to the invention and the rate of travel ofthe pickup device may vary in ClLI- ferent parts of the frequency range according to any desired law which may be found to improve the quality of the signals transmitted. For instance in some cases it may be. advantageous to move the pick-up means with a velocityincreasing towards the upper portions" of the frequency range B.

This result may be obtained by adapting the frequency of the scanningdevice oscillator broscillatorsof variable frequency, such for example as those in the system of Figs. 4 and 5 described hereinafter, to vary according to a predetermined law. For instance when said oscillators are provided with rotating condensers the shape of the vanes of said condensers may be determined so that the frequenc'ythat is to say'the speed of scanning variesv according to the desired law. When oscillators of constant frequency are used as in Figs. 6 and 8 described hereinafter, it is possible in the case of Fig. 8'to rotate the arms R and R of the distributors at difierent speed to to provide a different number of contacts on different pairs of rotating arms.

It should'be noted that blank of signals with respect to time may be provided as shown in- Fig. 1E.

It has been assumed above that it is the scanning device which is moved with time along the frequency band B but it is also possible to move the frequencyba'nd B with time along the scanning means for instance by carrying the frequency band B upon one-or several carrier frequencies varying according to a suitable law in function of time.

In order to obtain. effectually scanning mean movable along the frequency band 13,.

filters of variable cut off frequencies (not shown) may be used.

It will be seen' that the-two above arrangements may be used-together, the essential being to obtain relative displacement of the scanning means with regard to the speech. band. i

Referring now to Figs. 2 and 3 one manner of carrying into practice the-invention will be described.

On the axes Off and. Ofi" of Fig. 2 are represented the pass ranges of transmitting and of receivingfilters in their relative'position in the frequency spectrum. 1 Inthisfigure F 0 represents the frequency band width of the signal to be transmitted and F3 the band of. frequencies which may betransmitted over the transmission medium. F 1 shows the position of the band of frequencies to be transmitted after the first modulation whilst F2 represents the pass range of the transmitting and receiving filters in their relative positions in the frequency scale.

According to an embodiment of the invention, when it is desired to transmit signals extending over a band of frequencies from f to f f f =B) over a transmission path capable of transmitting with tolerable attenuation, distortion, etc. only a band of frequeneies from f to, f (f f =b) the following system may be used,'with apparatus'such for example as indicated in Figs. 4 and 5 described hereinafter.

By modulation and selection of the lower side band (for example in modulator m and filter F l in Fig. 4). the original band of frequencies A to f is first raised to a range f to f such that f is greater, and preferably considerably greater than (f f The purpose of this is to facilitate elimination of the upper side band products of second modulation which immediately follows (for example in modulators m in Fig. 4). In order that no trouble shall be experienced from the upper side band product of modulation the frequency of the first carrier must be removed from f by a frequency greater than (f -f and either a band pass filter having its pass range from f to 2 must be emloyed or else the frequencies of the variable frequency oscillators must be chosen so that the upper side band of the primary modulator lies outside the pass band of the scanning filter described below.

The frequencies f to f will be referred to as M the first modulation band. From the nature of modulation it follows that The first modulation band M is supplied (for example from modulator m in Fig. 4) to g modulators (for example modulators m in Fig. 4) performing the second, or variable frequency modulation. In addition to M these 9 modulators are each supplied With a carrier frequency from one of an 'ual number 9" "of secondary oscillators (0 O and 0 in Fig. 4). The frequency. of each oscillator is varied between the same frequency limits i and f by means for instance of a separate condenser: of varying value (not shown), consisting of an air condenser with one set of vanes on a common rotating shaft making 11, revolutions per second. Both fixed and moving vanes are in the form of sectors of circles concentric with the rotating shaft. The sectorial angle 0 of the moving vanes and the fixed vanes with which they inter-act is the same in every case, but where'the fixed vanes on a projected View alongthe axis of the shaft are situated in the same sector, the moving vanes of each oscillator are consecutively displaced by an angle B B+b"' Arrangements may be made so that the out put from each modulator is only transmitted during the time that each condenser is increasing in capacity (or alternatively during the time that each condenser is diminishing in capacity). This may be achieved by the use of commutators on the rotating shaft.

The frequency range f f of each secondary oscillator is adjusted so that f f 7 f1 f4 fs The lower second modulation band M therefore varies from the position X Fig. 3 (wherein theaxis 0} represents frequencies and the axis 0A the amplitudes) in which the frequency range is Upper frequency=f f Lower frequency=f -f to the position Y Fig. 2 in which the frequency range is Upper frequency=f f Lower frequency= f f Between f f and f f there is a gap of width fofe (lb 1 f4fa from Equation (2).

The lower modulation bands M from each second modulator are all supplied to one and the same band pass filter (scanning filter),

for example filter F in Fig. 4, whose pass range extends from f f5 1' 0 fs fef from Equation (3).

f8 f7 f4 fa It is therefore evident that if f1' (f4 f the upper side band products of modulation will always be outside the pass range of the scanning filter. In the event that primary modulation may be dispensed with.

Considering the band M from any one modulator it is evident that the scanning filter effectively scans the band n times per second, by virtue of the fact that the band M traverses the pass range of the scanning filter every second. The speech range is therefore scanned m times per second with q modulators. The purpose of employing a plurality of modulators is to ensure that the pass range of the scanning filter scans the speech spectrum continuously, since by this means it is arranged that as any second modulation band M leaves the filter it is followed contiguiusly by another second modulation The following will help to make this clear. Let the series of second modulation bands M be designated as M M etc. corresponding to oscillators O 0 etc. controlled by condensers Ca, Cb, etc. When condenser Ca=0 and is just about to start increasing, band M is at X, Fig. 3. When band M has arrived at Z, Fig. 3 condenser C has rotated through an angle since the plates of the condensers must be so shaped that equal changes of angle give equal changes of frequency (standard type straight-line frequency condensers). At this moment a second condenser C must start to increase and therefore lags behind condenser 0 by an angle In general there Will be a total of 9 such rotating condensers each lagging behind (or leading on) the next by 45 degrees. It is naturally desirable that the number of such condensers shall be finite and small. This may be achieved by arranging that the phase of the (0+1) th. condenser is 360 behind the first condenser 0,. Since the condensers are formed by the capacities between moving and fixed plates in the shape of sectors of circles concentric with the axis of the shaft carrying the moving plates, the smallest integral value of g is given by the equation B B+b where 0 is not greater than 180.

Example 1 b /;B. Therefore,

Putting 9 2, 0=270 which is not allowable.

Putting q=3, 6= 180. 120.

Ewample 2 1 o 6 B. .g %0 360 Putting 9 3, 0 160", 4 =120.

If g is made equal to unity and 6=180 then incomplete scanning occurs, since each second modulation band leaves the filter pass range completely before another enters it.

Such a system may give tolerably good results.

The output from the band pass filter (F in Fig. 4) which lies in the range f to f is modulated (for example in modulator m in Fig. 4) with a steady frequency of the value of f -f (for example from oscillator 0 in Fig. 4) and so reduces the band frequencies to the range f to f. (which is the past range of filter F3 in Fig. 4), for transmission to the receiving end of the system (over a line or medium L in Figs. and 5) At the receiving end the iifrcqnencics received are beaten (for example in demodulator D in Fig. 5) with a steady frequency of the value f f (from oscillator G in Fig.

and then traverse a band pass filter having the range f to f The output from the hand is supplied with time selectivity to q demodulators, (for example demodulators D in F ig. 5). each demodulator being supplied with an oscillator (G G and G in Fig. 5) whose frequency at any instant is the same as that of the corresponding modulating oscillator at the transmitting end.

Each modulator is connected in circuit only during' the same time that its corre sponding modulator (0 O and 0 respectively, in Fig. 4) is operative at the transmitting end. For the purposes of the above it is assumed that times at the receiving end are the same as those at the sending end when displaced by an amount corresponding to the mean delay of the circuit.

The combined output from the demodulators (D in Fig. 5) lies in the range f to f and it is necessary to employ a third oscillator and demodulator (G and D in Fig. 5) to reduce this to the range 7"; to f (which is the past range of filter F0 in Fig. 5).

Synchronism between the events at the transmitting and receiving ends may be accomplished by any known means such as the transmission of 50 cycle current along the transmission path.

The resulting product at the receiving end is effectively that which would he produced by causing the pass range of a variable band filter of constant band width to traverse the original band Zn times per second. In this case each part of the range would be trans" mitted during only of any time considered. It is therefore proposed to substitute the missing parts, by re peating these parts which are reproduced any required number of times. If

then repetition must be made once; if

two repetitions must be made and so on. Repetition may be secured by supplying the third demodulation products (from filter F O in 5) to a delay network (N in Fig. 5) and combining (for example in amplifiers A and A in Fig. the voltages taken from different points along its length, for transmission to the receiving circuit, indicated at R in Fig. 5.

then it is only necessary to combine the voltages from the input and output of a network having a delay of 2q'n seconds.

then it is necessary to combine the input voltage with voltages delayed by 1 35?, and

3 an seconds.

B 1; 4 then the delays are 1 2 m e and 3 m and so on.

The general arrangement of apparatus at the transmitting end is shown in Fig. 4, as indicated above.

O and m are respectively the primary oscillator and modulator the speech band to be compressed being supplied at S.

F1 is a band filter passing the range f to 7'2-

O

0 and 0 the secondary oscillators and m are the secondary modulators.

F2 is a band filter passing the range from f to O and 772 are respectively the tertiary oscillators and modulators.

F3 is a band pass filter passing the range from i to f The compressed band is supplied to the line at L.

The general arrangement at the receiving end is shown in Fi 5, as indicated above.

G and D are respectively the oscillator and modulator converting the band from f to f to the range f, to f and F2 is a band filter passing this range.

G G and G are the variable frequency oscillators corresponding to O O and and D are the demodulators cor responding to m G is synchronized with 0 G with O and G with 0 so that each to each the frequencies are the same at instants separated in time by the time delay of the transmission path. The synchronizing means are not shown.

F1 is a band filter passing the range from f to 72.

G and D are respectively an oscillator and modulator converting the band from f, to f to the range f to f F0 is a band filter having its pass range from f, to f N is a delay network for producing repetitions.

A A and A are one-way devices, which may conveniently be amplifiers of high impedance input with their outputs parallel.

The restored band is delivered at R.

Three oscillators have been found a convenient number but it should be emphasized that the invention is not limited to this particular number of oscillators. It will be seen from the above that an important feature of the above system is the use of multiple modulation which in the particular instance described is triple modulation and demodulation. In order to show another way of carrying out the invention, in the following there will be described a system in which the frequency band of the signals is moved step by step in front of a plurality of scanning devices provided with frequency changing means and said signals or portions thereof are transmitted to the receiving station either in turn over one channel with synchronizing means at the sending and receiving stations or over a plurality of channels without synchronizing means at the terminal stations.

The velocity of scanning must be large enough so as to preserve the intelligibility of the signals. For instance when it is desired to transmitspeech it has been ascertained that each individual frequency of a composite wave representing a vowel or consonant lasts for a considerable time greater than one hundredth of a second with practically constant amplitude.

It should be noted that in systems of the type herein set forth it may be necessary to provide means for compensating for phase distortion or phase compensation. In some cases it may be necessary to reduce transient effect in the systems.

Considering more closely the question of phase distortion, let N be the number of times the speech band is scanned per second, then =q where g total number of IIlOflllhltOrs n number of revolutions per second of the varlable condenser spindle.

Hence the time for one scanning is Let B=band width of original speech then the velocity of scanning is BN cycles per second. If new any frequency is displaced in time from its original position by S seconds, on demodulation the frequency will be displaced by F cycles per second where (6) F=B-N-S cycles per second for example considering a practical case and taking cycles as the maximum permissible displacement of the received speech 1 20 and then If we suppose a speech band width B equal to 2000 cycles and 3 modulators (9 3) we get 1 8 soon (8) The value given by (8) represents:

(a) The maximum permissible deviation is time delay between any two frequencies in the range considered and also,

(6) The maximum permissible total time delay at any one frequency.

Two methods of adjustment may be used for correcting for an excessive time delay:

(1) By the use of two sets of oscillators vanes moving in correct relative phase. For illustrating the above point suppose that the time delay for which it is required to adjust is S microseconds, then the leading edge of one set of receiving condenser vanes must lag behind the leading edge of one set of transmitting condenser vanes by a time equal to S microseconds that is it must lag by Sn 360 degrees.

If the condenser vanes of the receiving apparatus lag behind the transmitting ones by any integral multiple of 120 (taking q=3 as in the above example) it simply means the two condensers are occupying similar positions and that the same capacity will be inserted in the oscillators circuit at both ends altl'iough a different set of "anes may be in position. From the considerations above it will be seen that multiplying the speed by 2 will have the same effect as increasing the lag of the receiving vanes by 120 and consequently if the circuit has been correctly adjusted for any given scanning speed then this speed may be multiplied by any integer Without upsetting the adjustment.

(2) By the addition of a uniform time delay to the value of S. It has been shown above that if both receiving and transmitting vanes can occupy the same position in space, that is no adjustment is necessary. Consequently, if a uniform time delay is added to the value of S to make the total delay equal to complete adjustment for the particular frequency under consideration will be obtained. Such a uniform time delay may be obtained by the use of lattice networks.

The deviation in time delay may be phase compensated for instance by means of lattice networks to a degree of accuracy determined by the scanning means used.

In the systems hereinafter described in connection with Figs. 6, 7 and 8/, it will be assumed that it is desired to divide the frequency band to be transmitted by three, but it will be understood that the frequency range may be divided to any desired extent.

A schematic of this arrangement is shown in Fig. 6. The signals, for instance speech, extending over the range 0-2700 cycles, are impressed on the input I of the system and passed to a distributor comprising a brush R1 rotating at say 100 revolutions per secend which makes contact with each of segments 8,, S S for one third of each revolution.

It will be seen that the speech energy is applied to band filters BF BF BF in turn the said filters, filtering respectively the ranges 200-900, 900-1600, 1600-2700 cycles.

Thus during the first 300th of a second energy in the range 200-900 is applied to device AH which may be an amplifier; during the second 300th energy in the range 9-00-1600 is applied to modulating or frequency changing means AH and during the third 300th energy in the range 1600-2700 is applied to modulating or frequency changing means AH Modulating means AH and AH may for instance include amplifiers, each coupled with a heterodyne oscillator. The frequency of the oscillator included in the modulating means AH is 700 cycles, so that the resultant beat frequencies produced in AH lie in the range 200-900. The oscillator in the modulating means AH generates 1400 cycles, which produces beat frequencies between 200 and 900 cycles also. AH requires no oscillator, since its energy already lies in the range 200-900. The outputs of AH AH and AH are then transmitted down the transmission channel TC.

On Fig. 6 AN and VN are networks designed to equalize the attenuation and Velocity of transmission of the transmitting channel for the range 200-900 cycles, A is an amplifier.

At the receiving end, the line is connected through an amplifier A to a brush R synchronized with R The three segments associated with R are connected to an amplifier AH and demodulators AI-I and AH,. Since R and R are synchronized, all energy received from AH is passed into AH all energy from AH goes to AH and so on. Demodulators AH and AH include amplifiers, each coupled to a local oscillator which restores the speech energy to its correct frequency range. AH, does not require an oscillator, since its speech energy is already in the correct range.

The outputs of AH,, AIL, AH are now exact copies of the inputs to AH AH AH which will not produce intelligible speech. For instance, during one three hundredth of a second, energy in the range 200-900 is received from AH but during the next two hundredths no energy in this range is received.

In order to compensate for the missing portions of the speech band, for example the portion 200-900 cycles, this portion is reproduced during the next two intervals during the time brush R passes from contacts S S to contact S thereby greatly improving the intelligibility of the speech, since as explained above, each frequency lasts for a long time with practically constant amplitude.

This may be achieved by passing the output of AH through a delay network DN, having a total delay of about one hundredths of a second. A coil C is coupled to the first coil of the network, and a second coil C to a coil in the network having a delay of one three hundredth of a second behind the first coil, and a third coil C coupled to a coil in the network with a delay of two three hundredths behind the first coil. These three coils, C C C are placed in series, and connected to the input of an amplifier, A of very high input impedance, so that the current in C C C is negligibly small.

During the first 300th of a second C, picks up frequencies received by the network from A during the second 300th, C, no longer does so, but the Wave train has then reached G which picks up during this interval, during the third interval C picks up in a similar Way. At the beginning of the fourth interval, a fresh wave train is received from AH and picked up by C The voltage impressed on A is therefore continuous, and owing to the long persistence of each frequency in speech, practically indistinguishable from the original speech wave.

Frequencies in the bands 900-1600, and 1600-2700 received from AH and AH, are repeated in a similar way by the delay network.

The diagram in Fig. 7 shows the wave trains received from AH AI-I AH by the delay network. In the figure the axis 0 t represents time and the axis 0 A the amplitude. The time T is the period of the operation and the waves WS S WS are the waves sent over the contacts S S2, S of the distributor.

In order to avoid transient effects in the filters BF BF etc. it is desirable that the input to these filters should be connected to an impedance equal to the characteristic impedance of the filter itself, and that it should be closed through this impedance regardless of the position of R This is brought about by inserting a small amplifier (not shown) before each filter.

It is essential that the time taken for each speech band to travel from R to the line should be substantially the same. As the delays introduced by the filters may vary with frequency, it may be necessary to equalize the velocity of transmission by phase compensating networks introduced between each filter and its associated amplifier.

In the following description, the term contact time is used to indicate the time during which the brush R is on one contact.

The contact time must be short compared to the shortest transient it is desired to transmit; it must also be large compared to the time of one complete cycle at the lowest frequency to be transmitted if distortion is to be avoided. Distortion at the change over at the end of each contact time will always occur unless the transmitted frequency is an exact multiple of the interruption frequency. But provided the former is much greater than the latter, this will only produce a superimposed hum of interruption frequency, which can easily be removed for instance by a filter. There will also be harmonics of the interruption frequency which will, however, be small and the lower harmonics which are below the speech band can be cut out by the same filter which removes the fundamental. Those which occur within the speech band are generally too small to cause trouble.

From the above, it appears that in an arrangement for dividing the speech band by n, the contact time must not be greater than seconds. This is based on the assumption that the shortest transient lasts for more than 1/ 100th second. In addition, the contact time must be longer than the time of about four or five cycles at the lowest frequency it is desired to transmit.

These two conditions are irreconcilable, if the speech band is 300-2300 cycles and n is 3 or 4. To overcome this difficulty, a modification of the above arrangement can be used. This is shown in Fig. 8. The speech range is first divided into two bands, 3004300, 13o0 2300, by two filters BF, and each band is applied to a rotating contact R or R with four contact segments. The output from each of these is treated in the same way as before, and the two outputs are transmitted on two adjacent frequency channels, say 200-450, and 450-700 cycles per second.

The speed of R is about 25 revolutions per second, so that its contact time is l/ 100th second. Thus even at 800 cycles, three complete cycles will occur in one contact time. R rotates at 100 revolutions per second, its contact time being 1/400th second. It'thus includes more than three cycles for a speech frequency of 1.300 cycles per second.

Viththe above arrangement, it may be desir-able to keep the two channels separate until after the delay network stage. It will be necessary to provide a delay networkfor each channel, but the design of these networks will be simpler because the frequency band dealt with by each is narrower.

The band which passes through the delay network may be still further reduced by inserting the network before the received energy is separated into the different speech bands. In this case only one network dealing with the transmitted frequency range 300-700 cycles will be required.

The brushes at the sending end may be driven by synchronous motors (not shown), and the AC supply driving them is transmitted down the line. At the receiving end it is amplified and used to drive the rotating brushes R and R Exact synchronism is thus obtained. lVhere two brushes are used at each end, two synchronizing frequencies must be transmitted. These will be at a very low frequency sufficiently removed from the speech band to prevent interference between the two.

In the figures a certain disposition of the various pieces of apparatus is shown which convenient in certain cases: it will be understood, however, that the sequence of the various operations may be modified in any manner which may be found convenient. For instance the distributor or distributors may be inserted after the frequency selecting and/or frequency changing devices. Likewise the delay network or networks may be inserted at the receiving stations in any convenient position with regard to the other terminal apparatus. Other modifications will be obvious to those skilled in the art. Although mechanical distributors of the rotary type are shown in the above embodiments, We may use instead any other electrical or mechanical switching means well known in the art, such as arrangements of thermionic valves, etc.

What is claimed is:

1. The method of transmitting signals with reduced width of the frequency band transmitted, which comprises periodically scanning a substantial portion less than the wnole of the frequency spectrum of the signal to be transmitted, and transmitting only the frequency components derived from said portion.

2. The method of transmitting signals which comprises periodically scanning a substantial portion of the frequency spectrum of the signal to be transmitted and picking up a fraction only of the signal components in said portion at a time, and altering the frequency of at least a portion of the components picked up.

3. A signal transmitting system comprising scanning means for periodically traversing effectually a substantial portion of the frequency spectrum of the signal to be transmitted, to pick up portions of said signal, shifting the frequency components so picked up to occupy a narrower total band and transmitting said band of components.

4. A signal transmitting system comprising scanning means for periodically traversing effectually a substantial portion of the frequency spectrum of a signal to be trans mitted and picking up portions of said signal, and means for shifting at least some of said portions in the frequency spectrum.

5. A signal transmitting system comprising scanning means for periodically traversing effectually a substantial portion of the frequency spectrum of the signal to be transmitted and picking up portions of the signal, means for transmitting the latter portions to a distance, and means for reprodueing a number of times at least some of the transmitted portions to compensate for the portions of the signals which are not transmitted.

6. A signal wave transmission system comprising scanning means movable periodically with time relatively to the frequency spectrum of the signal to be transmitted for picking up such portions of the frequency spectrum of the signal to be transmitted that the number of different frequencies as well as frequency band width necessary for transmitting the signal is reduced, a transmission channel, means for transmitting said portions over said channel in turn with time selectivity, and signal receiving means for producing from the transmitted waves a signal representative of the signal to be transmitfed.

7. A signal transmitting system comprising a band pass filter with fixed cut-off frequency, a source of carrier frequency waves,

means for modulating the signal to be transmitted with said carrier frequency waves, means for transmitting the modulated Waves to said filter, and means for causing such periodic variation of the frequency of said carrier Waves that the essential frequencies of the signal to be transmitted during a band of the varying carrier frequency are brought Within the pass range of said filter and the lrequency spectrum of the signal is scanned.

8. A system for transmitting signals comprising a plurality of modulators, means for supplying said signals to said modulators, means for supplying to said modulators carrier waves so varying that during a cycle of the variation portions of the modulation products fall within a range of frequencies of width smaller than the range of frequencies occupied by the corresponding portions of the signals to be transmitted, and means for transmitting said portions of the modulation product to a distance.

9. A system for transmitting signals comprising a plurality of modulators, means for supplying said signals to said modulators, means supplying to said modulators carrier waves so varying that during a cycle of the variation portions of the modulation prod ucts fall within a range of frequencies of width smaller than the range of frequencies occupied by the corresponding portions of the signals to be transmitted, and means for shifting said portions of the modulation products in the frequency spectrum.

10. A signal transmission system comprising a transmitting station, a receiving station, a plurality of modulators at said receiving station, means for impressing the signals to be transmitted on said modulators in turn and at a suitable rate, means for supplying to said modulators, respectively, modulating waves having constant frequencies of such values that the signals impressed on the modulators are brought within a frequency band width smaller than that of the original signals, means for transmitting the signals thus reduced in their frequency band width to said receiving station, a plurality of modulators at said receiving station, means for synchronously impressing the signals transmitted to' said receiving station on the latter modulators, and means for supplying the latter modulators with waves of suitable frequencies for restoring the transmitted signals to their original frequency range.

11. A signal transmission system as set forth in claim 10. the means at the trans mittiug and receiving stations for impressing the signals on the modulators comprising a pair of distributors, one at each station, adapted to rotate in synchronism, and the means at the transmitting station for impressing the gnals on the modulators comprising also a plurality of band pass filters selecting portions of the signals to be transmitted.

12. A system as set forth in claim 10, comprising a delay network at said receiving station for repeating a number of times the signals arriving at the receiving station, to compensate for the portions of the signals which are not transmitted. 4

13. A system as set forth in claim 10, the means at the transmitting and receiving stations for impressing the signals on the modulators comprising a plurality of distributors at each stat-ionadapted to rotate in synchronism, a number of pairs of the distributors being used for transmitting the higher frequencies of the signals to be transmitted and the remaining pairs of distributors being used for transmitting the lower frequencies of the signals.

14. A signal transmitting system comprising scanning means for periodically and at varying velocity traversing effectually a substantial portion of the frequency spectrum of the signal to be transmitted, to pick up portions of said signal, shifting the frequency components so picked up to occupy a narrower total band and transmitting said band of components.

15. The method of conserving the frequency band width employed in transmitting speech which comprises selecting components representing (liiferont portions of the frequency spectrum of speech from different time elements of the speech, shifting the frequency of certain of the components so selected and transmitting them to the exclusion of other components.

16. The method which comprises selecting signal components representing different portions of the frequency spectrum of a signal from different time elements of the signal to the exclusion of other components and relatively changing the frequency of said selected components to cause the signal com ponents to occupy a narrower total band of frequencies While preserving their original time relation.

In witness whereof, We hereunto subscribe our names on the 24th day of October, 1929, and the 15th day of November, 1929, respectively.

EDWVARD KENNETH SANDEMAN. ROBERT OWEN CARTER.