US2390869A - Telephone transmission system and apparatus therefor - Google Patents

Description

Dec. 1l, 1945. c. w. CARTER, JR 2,390,869

TELEPHONE TRANSMISSION SYSTEM AND APPARATUS THEREFOR Filed OCT.. 2, 1942 Patented Dec. 11, 1945 TELEPHONE TRANSMISSION SYSTEM AND APPARATUS :THEREFOR Charles W. Carter, Jr., New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 2, 1942, Serial No. 460,520

(cl. lis-44) 6 Claims.

The invention relates to communication systems and particularly to telephone transmission systems and apparatus therefor.

In telephone transmission it is often desirable to provide additional telephone circuits between certain points quickly and economically. For example, under emergency conditions, such additional circuits might be required for providing telephone circuitsv over an alternative route between two points because of cable failure on the regular route caused by damage due to bombing, sabotage, floods, tornadoes or other storms, etc. Also, it might be necessary to provide such additional circuits in response to a sudden demand for service at a military camp, factory or construction project at a location not sufllciently supplied with telephone circuits, or to relieve overloads on congested long haul routes until regular equipment may be installed.

In such situations, economy and the quickness in supplying the Service may be more important than quality considerations. As the number of such circuits which will be required, their locations, and the kind of system to which application is to be made, may be unpredictable in advance,.this uncertainty emphasizes the need for simplicity and flexibility of application of the additional circuits. Simplicity is desirable in order that the circuits may be installed quickly by any available men of comparatively little skill in telephone practice under conditions of stress. Flexibility of application requires that the new circuits be adapted for application to systems of different types, for example, to voice frequency and carrier frequency telephone systems, and that the apparatus required for modification of the existing systems to give the additional service be in a, form permitting easy shipment from place rto place. Ordinarily, such points would be in toll telephone offices easily accessible to railroads but under some circumstances it might be necessary to install the new telephone equipment at remote, somewhat inaccessibue points, for example, in manholes, requiring that the size and weight of the telephone equipment be limited so that it may be brought to the point of application by a truck or other smaller vehicle and installed without the use of special facilities.

An object of the invention is to increase the number of telepho-ne circuits available between two points quickly and economically, without increasing line facilities and without excessive interference between the circuits or greatly reduced transmission volumes.

This object is attained in accordancewith the invention by the provision of a small, portable apparatus unit containing a small amount of transmission apparatus, at each terminal of an existing circuit transmitting a sufficiently wide band of signal frequencies, to split up the latter into two or more' circuits each transmitting a narrow intelligible band within the original signal band. The apparatus units are adapted for insertion in the circuit terminals either by patching or cross connection.

In on embodiment applicable to a four-wire toll telephone system, filters are employed in the apparatus unit at each terminal for dividing the normal speech signal band of a single channel into two channels each comprising the lower half of the speech band (frequencies below 1850 cycles). The reduced Width speech band in channel A is transmitted over the system unchanged in frequency whereas the reduced width speech band in channel B is translated by modulation from its normal location in the frequency spectrum to the upper half of the speech frequency range before being transmitted, and at the other station is demodulated to return it to its normal location.

The Various features and objects of the invention will be understood from the following detailed description when read in conjunction with the accompanying drawing in which:

Figs. 1 and 2 show schematically the invention applied to a four-wire toll telephone circuit on a four-wire basis and a two-wire basis, respectively.

At the west station of Fig. 1, the two-wire circuits L1 and L2 of the four-wire tenninatins circuit TC@` are coupled by the hybrid coil HA and associated balancing network NA in conjugate relation with each other and in energy transmitting relation with the two-wire subscribers telephone line LA, and the two-wire circuits La and L4 of a second four-wire terminating circuit TCB are coupled by the hybrid coil HB and associated balancing network Nn in conjugate relation with each other and in energy transmitting relation with the two-wire subscribers telephone line LB.

Similarly, at the east station the two-Wire circuits L1 and L2 of the four-Wire terminating circuit TCA' are coupled by the hybrid coil HA and associated balancing network NA in conjugate relation with each other and in energy transmitting relation with the east subscribers telephone line LA', and the lines La and L4' of a second four-wire terminating circuit TCB are coupled by the hybrid coil HB' and associated bal-- ancing network NB' in conjugate relation with each other and in energy transmitting relation with a second subscribers telephone line LB'.

The apparatus unit AW at the west station and the apparatus unit AE at `the east `station are usedv to connect the two four-wire terminating circuits at thei two stations to opposite ends of the westto-east one-way transmission pathEA and the east-to-west one-wayV transmission path WA of the intermediate four-Wire toll telephone circuit TL to provide two telephone channelsA and B in eachdirection between the stations,

The apparatus unit AW at the west station comprises two transmitting branches TA and TB having their inputs respectively adapted for connection to the circuit L1 of the four-wireterminating circuit TCA and to the circuit L3, of the fourwireterminating circuit TCB, and their outputs connected in parallel to'the'i'nput of a low-pass line lter I the output Yof which is adaptedfor connection to the west end'of the path EA; and two receiving branches RA and RB the outputs; of which are respectively adapted for connection to the circuit L2 of the four-wire terminating circuit TCA, and to the circuit L4 of the four-wire terminating circuit TCaand the inputs of which are connected in parallel to the 'output of the lowpass line filter 2 the input of which is adapted for connection to the west Vend of the path WA.V

'I'he transmitting branch TA includes aloss pad '3 in its' input and a low-pass filter 4 in its output. YThe transmitting branch TB includes in order reading from west to east the loss pad 5, the low-pass filter 6, the modulator 1 and the high-pass filter 8..; The receiving branch RA includes `the low-pass filter {ifin its input and the one-way amplifier I in its output. The receiv- K ing branch RB includes in order reading from east to West the high-pass filter`Y I I, the demodulator I2, -the low-pass lter I3 vand the one-way ,am plilcr I4. Carrier waves are suppliedv to the m`odulator 'I- of the transmitting branch TB and to the demodulator I2 of the receivingbranch RB from the oscillator I 5. The oscillator` I andthe ampliers I Il and I4 are suppliedwith energizing current from the common power supply I6.

' Similarly, the apparatus unit AEat the east station comprises two Ytransmittir1g branches YTA and TB having theirinputsrespectively adapted for connection to the two-.wire circuitV L1' in the four-wire terminating circuit TCA' associated with subscribers line LA and the two-wire circuit L4 of the four-wire terminating circuit TCBQassociated withr the subscribers line LB', and their outputsl connected in parallel to the input of the low-pass` line filter I', the output of-V which is adapted for connection to the east end of the. east-to-west path WA; and'two receiving branches RA' and RB having their outputs respectively adapted for connection to the two-wire circuit L2' of the four-wire terminating circuit rTCA' associated with subscribers line LA' and to the twowire circuit L' of the four-wire terminating circuit TCB' associated With'the subscribers linev LB', and their inputs connected in parallel to theoutput ofv a low-pass line filter 2',v ther input o f. which is adapted for connection to the east end` of the eastfto-west path EA. I VVVjThe transmitting branch TA vincludes in its input the loss pad 3' andan its output the 10W- pass i'llter 4', and the transmitting branch TB rincludes in order reading from east to west the loss pad 5', the low-pass lter 6', the modulator put, and the receiving vbranch RB includes in order readingv from west to east the high-pass lter II', thedemodulator I2', the low-pass filter I3 and the one-way amplifier Ill'.V Carrier waves are supplied to the modulator 1 of the transmitting branch TB' and to the demodulator I2' of the receiving branch'R' by a common oscillator I5'. Energizing currents aresupplied to the oscillator I5 and to the amplifiers I0' and I4 by the common power supply I6'. Y The low-pass channel filters 4, 6, 9 and I3 in the apparatus unit AW at the west station are adapted to pass all frequencies below 1850 cycles in the voice frequency band, and the high-pass channel filters 8 and II in that unit are adapted to pass all frequenciesin the voice frequency band above 1850cycles. The low-pass line filters I and 2 .in the apparatus unit AW are adapted to transV mit all'frequencies below 3600 cycles.

Similarly, the low-pass channel filters 4', r6', 9 and I3 in the apparatus unit AEA at thel east station are adapted to transmit'the frequencies in the voice frequency range below 1850 cycles, and the high-pass channel filters 8 and II'fin that unit are adapted to pass frequencies in the voice frequency range about 1850 cycles. Also, the lowpass line filters I' and 2 in the apparatus unit AE at the east'station are adaptedto pass all `frequencies'fbelow 3600 cycles.r i n `The loss pads 3 fand 5 in the transmitting branches of the apparatus unit at'the west station and theV corresponding loss pads 3' and 5' in the transmitting branches of the apparatus unit AE at the east station, which may be of the adjustable plug type, are providedfor'the purposeYV of maintaining the losses in the transmitting branchesat each station equaland for'prov'iding level lowering, as needed, to reduce'modulation crosstalk between channels. The amplifiers and oscillators in the apparatus'units'at the twol sta# tions are preferably of the vacuum tube type. The

power supply I6, I6', ywhich may be designed to Vto the modulator and demodulator at each station. f

Assuming that `the apparatus unitsV AW' and AEfat the two stations Yare connected through tothe associated four-Wire terminating circuits and the incoming and outgoing klines'EA and WAV of I the Vcircuit TL, the system of Fig. l operates asV follows:

Speech waves received at the west terminal station over the subscribers line LA will be impressed by the associated hybrid transformer HA on the two-wire circuit L1 of the terminating circuit TCA from whicnit will be transmitted into the transmitting branch TA of the apparatus unit AW. The received Vvoice waves will be transmitted in the latter branch through the loss pad 3 and then through the low-pass filter 4 which will limit the transmitted speech frequencies to the band below 1850 cycles. The latter. band will pass through the 360o-cycle low-pass line filter l to the West-to-east path EA of the four-wire circuit TL, over which it will pass to the east station.

At the east station the received voice band will pass through the line filter 2 and then through the low-pass filter 9 in the receiving branch RA', which will select the frequencies below 1850 cycles passed by the lineiilter. These frequencies will then be transmitted to the amplifier l which will amplify them sufficiently to compensate for the loss introduced by the preceding transmission equipment in channel A for the west-to-east direction. The amplified voice frequencies will then be impressed by the hybrid coil HA on the line LA' over which they will be transmitted to and heard by the east subscriber associated with that channel.

The voice frequency band of the message wave received at the west station over the subscribers line LB will be impressed by the hybrid coil HB on the circuit L3 from which it will be transmitted into the transmitting branch TB of the apparatus unit AW. In the transmitting branch TB the voice frequency band will be transmitted through the loss pad 5 and then will pass to the low-pass filter 6 which will restrict the transmitted speech frequencies to the band below 1850 cycles. The restricted frequency band will be modulated in the modulator 'l with the carrier wave of frequency 3700 cycles received from oscillator l 5. Of the modulation products in the output of modulator 1, the frequencies-above 1850 i cycles will be selected by the high-pass filter 8 and the selected frequencies transmitted to the 3600 cycle cut-off low-pass line filter i which will suppress the upper side-band of the impressed frequencies and will transmit the lower side-band occupying the frequency range from 1850 to 3600 cycles, to the west-to-east line EA.

At the east station, the received 1850 to 3600- cycle band will pass through the 3600-cycle lowpass line filter 2 and will be selected by the 1850- cycle cut-off high-pass channel filter Il in the receiving branch Re'. I'he 1850-3600 cycle band in the output of the high-pass filter il will be combined in the demodulator l2 with the carrier wave of 3700 cycles from the oscillator I5'. The lower side-band of the resulting modulation products will be selected bythe 1850-cycle cut-off low-pass filter i3 and will be amplified by the amplifier I4 to compensate for the loss produced in the waves by the preceding transmission equipment in channel B. The amplified band occupying the lower half of the Voice frequency range below 1850 cycles will then be passed over the twowire circuit L3' through the hybrid transformer HB to the subscribers line L B over which it will be transmitted to the east subscriber associated with channel B. v

The operation of the system for the speech waves in the opposite direction from the subscribers line LA at the east station to the subscribers line LA at the west station and from the subscribers line LB' at the east station to the subscribers line LB at the west station, is identical with that which has been described for transmission in the west-to-east direction for the similar apparatus for the latter direction.

Fig. 2 shows the application of the circuits of the invention for obtaining two low quality telephone channels in each direction over a fourwire telephone circuit on a two-wire basis. 'I'he apparatus unit AW at the west station includes a combined transmitting-receiving branch TRA and a combined transmitting-receiving branch TRB adapted respectively for connection on one side through telephone lines (not shown) to the transmitting and receiving telephone apparatus of telephone subscribers for the A and B channels, respectively, and on the other side in parallel to one side of the S600-cycle cut-off low-pass line filter il. The other side of line filter Il is adapted for connection to the line lterminals of the hybrid coil Hw at the west end of the fourwire toll telephone circuit TC including the oppositely directed one-way repeating paths RE and RW.

The branch TRA of apparatus unit AW includes a loss pad iB and the low-pass lter i9 having an upper cut-off at 1850 cycles, and the branch TRB includes a low-pass filter having an upper cut-off at 1850 cycles, a high-pass filter 22 having a lower cut-off at 1850 cycles, and an f intermediate modulator-demodulator 2l supplied with a carrier wave of 3700 cycles from the oscillator 23 energized from the power supply 24. Similarly, the apparatus unit AE at the east station comprises a transmitting receiving branch TRA' and a transmitting receiving branch TRB', respectively adapted for connection on one side through subscribers telephone lines (not shown) to the transmitting and receiving telephone apparatus of the subscribers associated with channels A and B, respectively, and on the other side in parallel to one side of the 3600- cycle cut-off, low-pass line filter l1', the other side of which is adapted for connection to the line terminals of the hybrid transformer Ha at the east terminal of the four-wire toll telephone circuit TC. The branches TRA and TRB include the same transmission apparatus as the corresponding branches at the west station as indicated by the use of similar characters for identifying the individual pieces of apparatus at the two stations, except that at the east station the characters are followed by a prime mark. Because each branch of the apparatus unit AW', AE' at the stations must transmit in both directions, no one-way amplifiers can be used in those branches. Therefore, in addition to the usual one-way amplifiers (not shown) inthe oppositely-directeci one-way paths RE and RW of the intermediate toll telephone circuit, a one-way amplifier A1, A2 is inser-ted in each of the latter paths to make up the losses of the transmission apparatus in the added apparatus units for the two directions of transmission. lf the volume loss is not serious a two-wire line can be used instead of the four-wire circuit. The operation of the system of Fig. 2 for one direction of transmission will now be describedl The speech frequency band of the talking currents of a west subscriber having his transmitting and receiving apparatus connected directly or through a switchboard to the transmission branch TRA of the west apparatus'unit AW' will first pass through the loss pad I8 provided to adjust the loss of the branch TRA to make it approximately equal to that of the branch TRB which includes more transmission apparatus, and to provide 4the level lowering required to reduce the modulation crosstalk which occurs in the wide band intermediate four-wire circuit TC. The waves then pass through the low-pass filter I9 which limits the transmitted vfrequencies to the band below 1850 cycles, which will pass through 360o-cycle cut-off low-pass line filter I1 and will be impressed by hybrid transformer Hw on the West-'to-east repeating path RE ofthe four-wire `toll circuit TC. As previously pointed out, the additional amplifier A1 in path RE will make up the additional loss introduced in the path* of the transmitted waves bythe transmission apparatus in AW'. f f l j The amplified limited frequency speech band inthe output of RE will pass through the hybrid Vtransformer HE to the 3600-cycle cut-ofi' low pass line filter Il of apparatus unit AE and willbe transmitted through that filter. @The limited frequency Vband `speech waves passed by the line filterllV will be selected by the 1850-cycle cut-off low-pass filter f I9 Vin branch TRA and willpass through the loss pad I8 utilized for a purpose similar to that of the loss pad I8 in the corresponding branch atY the west station, to a circuit (not shown) leading to the receiving apparatus of the east subscriber associated With'that channel. l

The frequency band of the speech waves of a second westtelephone subscriber is transmitted tothe transmission branch TRB of the apparatus unit AWf at the west station and will be limited to the frequencies below 1850 cycles by the lowpass filter 20 in that branch. The limited voice frequency band will be combined in the modulator-demodulator 2l with thecarrier wave of 3700 cycles supplied by the associated carrier oscillator 23.

Of the products of modulation the frequencies above 1850 cycles will be selected by high-pass filter 22v and the selected frequencies will be transmitted kto the 3600-cycle' low-pass rfilter I1 which will suppress the upper side-bandof the impressedv frequencies vand will` transmit the lower side-band occupying the frequency range from 1850 to 3600 cycles. This lower side-band will pass to the hybrid transformer YHw which will impress it on the repeating path RE of the four-wire toll circuit, in which it will receive the required'additional amount of amplification in amplifier A1 and then Willpass through hybrid transformer Hn'and the line fllter l1' of the east apparatus unit AE. 'I'he voice band'1850 to 3600 cycles in the output vof filter l1 Vwill then be selected by the high-pass filter 22' in transmission branch TRB of that unit and will be combined in modulator-'demodulater 2 I with the 3700-cycle carrier wave supplied from-oscillator 23,'. Of the products of demodulation the 1850- cycle cut-off, low-pass filter 20 will suppress'the upper side-band and transmit the lower sideband which will then be transmitted over the associated circuit (not shown)v to the receiving apparatusv of the east subscriber associated with the transmission branch 'I'RB' and' will be heard by him. y e

The operation of the system of Fig. 2 for the two vchannels in the direction from east'to west is identical with that forrthe west-to-east direction as described above. As noted above, in Fig. 2 the four-Wire'circuit may be a two-wire circuit if wide enoughand if the loss of volume is not serious.

Each apparatus unit of a'system such as shown in Fig. 1, which has been built and operated satisfactorily, had outside dimensions'of 19 inches by`12 inchesr by 9 inches, and weighed less than 40 pounds.

Improved operationl of the systems of Figs. l and 2 maybe attained without increasing the cost, size or weight appreciably, by the addition of a voltage limiting device, such as a peak chopper, ofany of theA well-'known types, in the transmitting branch of each apparatus unit, for

the purpose of further minimizing crosstalk be- 4tween channels. Y

Although the invention has been described only as applied to four-wire voice frequency circuits it is apparentV that it is applicable Valsolto any four-wire carrier `telephone circuit transmitting the frequencies below 3600 cycles with realsonable fidelity. Although it was primarily developed for splitting up the frequency-range of ,single existingl telephone channels into two channels of approximately 'half the frequency range of the single channel, itis apparent that its principles may be applied to obtain three or more narrow band channels inreach direction within the frequency range of the system with which it is used, if that range is'sufliciently wide 1 to allow the frequency width of the several channels to be made such as to provide thedesired amount of intelligibility.Y

f It is tobe understood that the values which have been given for the frequency ranges of the filters and the frequency of the carrier waves applied to the modulators and demodulators in the systems illustrated and described, are by-wayv and shifted in frequency range as in thecase of the 3700-cycle carrier. In both cases', the

crosstalk will be unintelligiblebutfthe decreaseV in intelligibility provided by the inversion plusV frequency shift favors ythe use of lthe S700-cycle carrier. f

Various other modifications of the circuits. ilf lustratedY and 'described which are within thev y spirit and scope ofthe invention will be apparent to persons skilled in the art.

What is-.claimed is:` Y 1. An apparatus unit for attachment to a telephone line or speech channel for securing two speech channels of lowered quality for emergency. use, said unit having a rst pair of tel;-

minals for connection to said linefor channel and having a second'pair and a third pair oi.' terminals for respective connection to a diiierent one of two telephone instruments Vor. lines, said second pair of terminals having connection within the unit through a low-pass filter to said first pair of terminals, said third pair of terminals having connection within the unit through a low-pass filter, a frequency shifting means and a high-pass filter in tandem, to said first pair of terminals, each of said low-pass filters having a pass-band suflicient to embrace the lower speechV frequencies essential for 'intelligibility said frequency shifting means'serving to shift the frequency of the Waves from the pass range of one of said two tandem connected filters between said first and third pairs of terminals to the pass range of the othertandem jconnected filter.

2. Anapparatus unit according to claim 1, in whichy the circuit connection betweensaid first pair and saidsecond pair of terminals is a twothird pair of terminals is a two-way transmission circuit, said frequencyishifting meanscomprising a bilateral modulator and a source of continuous frequency carrier waves therefor.

3. An apparatus unit for connection to the transmitting channel of a four-wire telephone circuit for securing two speech transmission paths of substantially half the frequency width of the speech band which said channelis adapted to transmit, said unit having a first pair of terminals for connection to said channel and a second pair and a third pair of terminals for respective connection to a different one of two incoming speech circuits, said second pair of terminals having connection Within the unit through a lowpass filter to said first pair of terminals, said third pair of terminals having connection within the unit through a low-pass filter, a modulator and a high-pass filter in the order named to said first pair of terminals, each of said low-pass filters having a pass range substantially equal to half the speech band width which said channel is adapted to transmit, said high-pass filter having a lower cut-off frequency just above the upper cut-off frequency of said low-pass filters, and means to supply said modulator with waves having a frequency of such value as to raise the frequencies of the incoming waves, in effectively passing through the modulator, from the passband of the last-mentioned low-pass filter into the pass-band of said high-pass filter.

4. An apparatus unit for connection to the ree ceiving channel of a four-wire telephone circuit for securing two speech transmission paths of substantially half the frequency width of the speech band which said channel is adapted to transmit, said unit having a first pair of terminals for connection to said channel and a second pair and a third pair of terminals for respective connection to a different one of two outgoing speech circuits, said second pair of terminals having connection within the unit through a lowpass filter to said first pair of terminals, said third pair of terminals having connection within the unit through a low-pass filter, a demodulator and a high-pass filter in the order named to said first pair of terminals, each of the low-pass filters having a pass range approximately equal to half the speech band width which said receiving channel is adapted to transmit, said high-pass filter having a lower cut-off frequency just above the uppery cut-off frequency of said low-pass filters, and means for supplying said demodulator with waves of such frequency as to lower the frequency of applied speech waves received from said one pair of terminals through said high-pass filter, in effectively passing through said demodulator, from the pass-band of said high-pass filter into the pass-band of said low-pass filter.

5. An apparatus unit for attachment to each terminal of a four-wire telephone circuit to provide two telephone channels in each direction, said unit having two pairs of terminals for connection respectively to a different side of said four-wire circuit, and having four other pairs of terminals two of which are for respective connection to a different receiving telephone instrument or line and the other two for respective connection to a different transmitting telephone instrument or line, one of the first two and one of the other two of said four pairs of terminals having connections within the unit through an individual filter passing only the lower frequencies essential for intelligibility in the speech frequency band, to a different one of the first-mentioned two pairs of terminals, the other of said first two and the other of said other two of said four pairs of terminals having individual connection with- Ain the unit through a filter passing only said lower speech frequencies, a filter passing only the higher speech frequencies in the speech band, and an intermediate frequency shifting means in tandem, to a different one of said first-mentioned two pairs of terminals, the order of connection of the tandem filters being reversed in the latter two connections, the intermediate frequency shifting means in one of said latter connections serving to shift the frequency of waves received through the preceding filter of lower speech frequency pass range to the pass range of the succeeding filter of higher speech frequency pass range, and the intermediate frequency shifting means in the other of said latter connections serving to shift the frequency of waves received through the preceding filter of higher speech frequency pass range to the pass range of the succeeding filter of lower speech frequency pass range.

6. In combination with a two-way telephone system including stations connected by a fourwire telephone circuit having oppositely directed one-way transmission paths for transmitting telephone signals in opposite directions between said terminals each adapted to transmit a wide band of speech frequencies, means to secure two speech channels of lowered quality in each direction over said system, comprising an apparatus unit at each terminal comprising two transmitting circuit branches and two receiving circuit branches, the two transmitting branches being adapted for connection in parallel to the outgoing one-Way path of said four-wire circuit, the two receiving branches being adapted for connection in parallel to the incoming one path of said fourwire circuit, one of said transmitting branches and one of said receiving branches being adapted to transmit substantially the same half of said wide band of frequencies, the other of said transmitting branches and the other of said receiving branches respectively including in their inputs one filtering means to select from the applied Waves a narrow band of frequencies Within said wide band, of substantially the same width as the frequency band transmitted by said one transmitting branch and said one receiving branch, modulating means to shift the selected band to a different position in the frequency spectrum within said wide band and other filtering means in their outputs for respectively selecting the shifted frequency band, the width of the narrowfrequency band transmitted by said one transmitting and said one receiving branch and that selected by said one filtering means in the other transmitting and receiving branches being such as to embrace sufficient essential speech frequencies for intelligibility.

CHARLES W. CARTER, JR.

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