US2379055A - Signal wave transmission system - Google Patents

Description

June 26, 1945. l., G. ABRAHAM SIGNAL.l wAvE TRANSMISSION SYSTEM Filed may 12, 194s ALZQRNEV Patented `Func 26, i945 UNirao STATES PATENT oFFIcE 2,379,055 SIGNAL WAVE TRANSMISSION SYSTEM Leonard G. Abraham, MadisonN. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., `a corporation of New York "biiutoii stay V12, 1943, Serial 'No1l` 486,688" C'lims. (C1. rvs-44) This invention relates to signal wavetran n'n's'- sion systems and particularly. to systems for transmitting a Wide band of signal' 'frequericies.

An object ofthe invention is toA transmitefliciently and economically a signalwave compri..- ing a Wide band of frequencies. f y

A related object is totransmit eiiic'iently a" signal Wave comprising a Widef bandof frequencies, utilizing available transmission lines inherently capablel of transmitting fefliciently orilygan'arrowerfrequency band. t f i' Another object is to cheapen the 'costpf' termi'- nallilte'ring equipment in ajwidej frequencylband carrier wave signaling systeriwithoutiapprecif ably reducing the quality of transmission'i These objects are attained in'accordane with the invention by a particular method fo dividing,r the signal Wave of wide frequency, ,1113, r

a complex carrier wave, into two or morparts tions each containing the whole band of frequenf cies; a narrower frequency subband is selected from one energy portion of 'the'wave'al parto'f thev energy of the selected subband `is used to balance out corresponding frequencies the other energy portion of the-Wave; the' remaining frequencies in the latter energy portion of the signal Wave, and the selected frequency subband Yin the rst energy portion are transmitted loverseparate available transmission circuits?" of' limited transmission frequency range 'to a receiving point, utilizing frequency shifting operations, if necessary, to bring them Within the frequency limits of the available circuits before transmission and to restore them to their riginal frequencies after transmission; and at the receiving point the two `fredencysubbands are combined to reproduce the original signal tvavet In another embodiment, a similar processfis employed at the transmitting end ofi@ Single Sid@- band carrier signaling system for efficiently separating the side-band component ,toibe transmitted from the other side-band Ycornpnnent .and the carrier, utilizing relatively cheapflltersfw/ YThe various objects rvand features -of the invention Will be better understoodV from the following detailed descriptionwhen read vconjunction With the accompanying draining in which are shown in blockl schematic form, different modifications of the invention applied toI program transmission systems. Fig. ,1,Li1lustratesthel application ofpthe method of the4 invention to, the transmission c f,a A program with,freguencies ranging from, sayfpfto 15,009, cycles, between tv/o, widely separated points` A v,and B, 2Where the ,only transmission media available between those points."arela4` pluralitypf toll transmissioncircuits vor lines` ,',IL1f, TLzfe'ach 'capable of transmitting efliciently only a. portion, say 30 toj `8,509 cycles, of that' frequency ranger@ v, Atgstation A, i nthe system of Eig. Ltheprogram Wave comprisingfrequencies ranging 'from about 30,eycle s tg 15,000` cycles receivedeve the line from a `source (not -shown), u fhichmay be a microphone or a .telephene .line, lis fed Ain multiple into'two branch" transmission circuits TG1 and TCz which may include one-Way amp ifying. vrnearls A.1, .A2 for providing ai'. desired amount offamplication in theequ'al energy portions of the. wie@ lfrequenv band. .ware transf mitted over the respective branch circuits.4 In

the' upper branch circuit'TCiisa bandfpassiilter Fi which is `adapted tofpass*a'relatively'narrcvv frequency. Subbanda te y13.',000 Cycles .Qfgthe impressed wave; (this filter u1d bea1owpas,s lte ,hai/ing, a {Gut-Oatfoo Cycles.) and .tb then c utfoff in aY reasonable frequency interval'to very highf loss at, say 8,500 cycles, A portion V of the energy, f, the Selected" 30509 Y,84100 yls' Sub'an'd is transmitted out over the available 4(30 to,V 59() cyclegf-toll line vT L1 tothe receiving 'station B Where ,it is transmitted over the lreceiving circuit RC1` through the one-way amplifying 'deviceAr to the circuit or line L2 leading to signal repro'- ducingapparatus. Another energy portionof the' selected :30 to 8,000 cycle frequency "subband 4in the output of lter F1 at station A passes into the @fle-Way amplifying, device ,All

,Injthe othr'brenh C twit' TCwPStatQl A, thev entire 30 to l15,00() cycle bandV of theimpressed program wave is transmitted through vthe transmission loss device C1, Which "may be f a inputpf the by-pass circuit BCi including the` phase corrector if tests indicate that phase c rrection is necessary. The 'jamplified'O t`o 8 ,pl l0 cycle wave output of the by'fpass 'circuit BCi is afpvparts are combined at the gram frequencies -between 8,000 and 8,500 cycles and the complete frequency band between 8,500 and 15,000 cycles.

The resulting wave comprising frequencies ranging from 8,000 to 15,000 cycles, in the lower transmission branch TG2 then passes to the modulator M1 in which they are combined with a carrier wave of 7.9 kilocycles supplied from the carrier source S1, to produce signal side-bands of frequencies ranging from 0.1 kilocycle to '7 .1 kilol cycles and from 15.9 kilocycles to 22.8 kilocycles, respectively. A cheap band-pass filter F2 in the output of the modulator M1, adapted for passing frequencies up to 7.1 kilocycles and cutting off at 7.9 kilocycles and higher frequencies, selects j filter cutting off above 8 kilocycles with very high loss at 16 kilocycles and above, which would be cheaper than the band-pass filter. Also with the latter modification, the frequencies just above 8,000 cycles in the original program wave would still be near 8,000 cycles on the line Th2 and would be delayed in transmission over that line about the same amount as the nearby frequencies in the other line TL1. This might aid in reproducing the original wave when the two frequency receiving end of the system.

At the receiving station B, the received' frequency subband, '7,100 to 100 cycles, is combined in the modulator M2 with the carrier wave of 7.9 kilocycles from the associated source S2, to step it up to the original frequency position, 8,000 to 15,000 cycles, which it had at the input of the modulator M1 in the transmitting station A. This frequency band, 8,000 to 15,000 cycles, is selected by the cheap band-pass filter F3 in the output of the modulator M2. The selected subband is then transmitted over the receiving circuit branch RC2 through the one-way amplifier A5 to the circuit L2 in which by proper poling it may be made to combine with the amplified to 8,000 cycle subband received from the receiving circuit branch RC1, to reproduce the original 30 to 15,000 cycle band program wave l The phase corrector C1 in the transmitting branch TCz at the station A, should preferably be so designed that, with a turn-over of the frequencies, the band from 30 to about 8,000 cycles in its output will be almost exactly out of phase with the corresponding frequency output of the by-pass circuit BC1 when they are combined at the transmitting station A. vIf this is not accomplished, there will be frequencies below 8 kilocycles at the input of the first modulator M1, and another filter will be required there for cutting off those frequencies. For the alternative scheme referred to above, employing a 16-kilocycle carrier wave for the modulator'M1 and a low pass filter cutting off above 8 kilocycles for the filter F2 in the output of the modulator, this might not be necessary. In either case, a very cheap filter could be used if complete cancellation could be obtained, say for a few hundred or a thousand cycles below 8 kilocycles. In the region from 8 to 8.5 kilocycles, phase correction for the filter F1 in the upper transmitting branch TG1 would be of advantage, but is perhaps not essential. For example, suppose that at 8.2 kilocycles, the wave in the output of the by-pass circuit BC1 and the wave in the output of the phase corrector C1 in the lower branch circuit TG2 would be exactly in phase, instead of out of phase, at the point of combination. With a loss for the filter F1 in the upper branch circuit TG1 of, say, 10 decibels, the level of the currents in the lower branch circuit TC2 should be reduced by about 3.3 decibels by the combination, but with degrees phase error would be increased by 2.4 decibels. With no line distortion, however, there would be no final error at the receiving end of the line so that the only effect would be to slightly change the power on the line. Of course, delays which are great enough to cause echo effects could not be tolerated.

It is apparent, of course, that a system based on the 'same principles described above in connection with Fig. 1, could be devised to transmit a band which is wider than 30 to 15,000 cycles, or to transmit a frequency band by dividing it into more than two parts. For example, let us sup pose it is desired to transmit a 30 to 22,000 cycle band of signal frequencies. VThe upper branch circuit TCi with the 30 to 8,000 cycle band-pass filter F1, and the by-pass circuit BC1, could be used as shown, and the wave in the lower trans mitting circuit TC2 resulting from the combination of the 30 to 22,000 cycle band in the output of the phase corrector C1y with the wave output of the by-pass circuit BC, would have frequencies ranging from 8,000 to 22,000 cycles. This wave then could be split into two frequency parts by transmitting it in multiple into two auxiliary band circuits, similar to TC1 and TCz, one with an 8,000 to 15,000 cycle band-pass (or 15 kilocycle low-pass) filter. A portion of the energy of the frequency subband in the output of that filter could be used in the same way as described for the arrangement of Fig. 1 to cancel out correspending frequencies in the wave in the other auxiliary branch. After shifting all three frequency subbands thus obtained to some desired place in the frequency spectrum determined by the frequency limits of three'transmission lines available and transmitting them so shifted over those lines to the receiving station, they can be there shifted back to their original frequency values and be combined in reverse order to obtain the original signal frequency band of 30 to 22,000 cycles. Another possibilbity would be to produce an 8 kilocycle program circuit from two 4 kilocycle message circuits by similar means.

It is also apparent that different levels and predistortion could be used for the two or more branches on lthe transmission line, since the energy content will be quite different. Compandors could be used on some or all of the bands separately. Some rearrangements of the filter 'cutoffs andthe selection of different carrier frequencies might be desirable in order to have the two branches transmit more nearly the same frequency bands.

The method which has been described above in connection with Fig. 1, is superior to that of using two filters for selecting the frequencies to be transmitted over the two lines, one transmitting 0 to 8 kilocycles and the other 8 to 15 kilocycles, -because of the lower cost of the second (lower) branch than an 8 to 15 kilocycle filter, and because the requirements on the correspondence around 8 kilocycles of the two filters in the latter scheme to give perfect transmission of the total frequency band, are extremely difficult and perhaps impossible to obtain. No such requirement is necessary with the scheme illustrated in Fig. 1 or the modifications thereof described above.

A second useful application of the method of the invention is illustrated in Fig. 2 which shows schematically a high frequency carrier wave system utilized for transmitting a program wave of frequencies ranging from 30 to 15,000 cycles, from station A to station B.

As indicated, at station A in Fig. 2 the program band of 30 to 15,000 cycles is combined in a modulator M3 with a carrier wave of 80 kilocycles from an associated source S3, to produce modulation products of frequencies ranging from 65 to 95 kilocycles. lt is desired to transmit only the upper side-band, 80 to 95 kilocycles, of the modulation products. It is technically difficult to make a band-pass filter passing 80 to 95 kilocycles with sufficiently sharp cut-off to eliminate the carrier and lower side-band. On the other hand, an 80 to 88 kilocycle band-pass filter and an 88 kilocycle high-pass filter or an 88 to 95 kilocycle band-pass filter may be produced easily, particularly of a few hundred or a thousand cycles for cutting off below 88 kilocycles are unimportant.

In the arrangement of Fig. 2, the energy of the modulation products of frequencies ranging from 65 to 95 kilocycles in the output of the modulator Ma is divided between the upper transmission branch TCa and the lower transmission branch TC4. The wave in the upper branch TCS is transmitted through a band-pass filter F4 adapted to pass the frequency subband, 80 to 88 kilocycles, and to cut off with great loss at, say 88.5 kilocycles. The other energy portion of the complex carrier wave in the output of the modulator M3 passes through a phase corrector C2 in the lower circuit branch TC4, having a function similar to that of the phase corrector C1 in the system of Fig. 1. The 65 to 95 kilocycle wave `in the output of the phase corrector C2 is combined with the amplied 80 to 88 kilocycle wave in the output of the by-pass circuit BCz including the one-way amplifier Ae, the input of which is fed from the upper transmission branch TG3 in the output of the band-pass filter F4, in such phase and magnitude as to eliminate from the 65 to 95 kilocycle wave transmitted over the branch TC4 the frequencies below 88 kilocycles. The remaining frequencies 88 to 95 kilocycles of the wave in the lower branch TC4 are transmitted through the high-pass filter F5 having a cut-off at 88 kilocycles, which eliminates the lower sideband, 80 to 88 kilocycles, and the carrier component of 80 kilocycles. The remaining frequencies 88 to 95 kilocycles in the output of the filter F5 pass through the one-way amplifier Aa to the outgoing line La in which they are combined with the other energy portion of the 80 to 88 kilocycle wave selected by the band-pass lter F4 in the upper branch TG3 and transmitted over the output of that branch through the one-way amplifier A1 to the line L3. Thus, the wave transmitted over the line La to station B comprises a single side-band with frequencies ranging from 80 to 95 kilocycles, approximately, the carrier component and the other side-band having been suppressed in the manner just described.

It may be pointed out that when the 88 kilocycle high-pass lter is used for the filter F5 in the lower branch T04, an equivalent effect could be obtained by using only an kilocycle highpass filter for the filter F4 in the upper branch TG3 connected to the output of modulator M3. The use of an 88 to 95 kilocycle band-pass filter in place of the high-pass filter F5 is a preferred arrangement.

Since the filtering and balancing circuits above described are all located at the input terminal of the line La in the system of Fig. 2, phase or amplitude distortion in that line does not affect the output, and phase corrector C2 may not be necessary except to avoid echo effects. At the receiving station B in the, system of Fig. 2, the received side-band, 80y to 95 kllocycles, is combined with an 80 kilocycle carrier wave from the source S4 in the modulator M4 to provide in the output of that modulator, side-bands of frequencies ranging from 0 to 15 kilocycles and from to 165 kilocycles, and a simple low-pass filter F6 in the output of the modulator, cutting off somewhat above 15 kilocycles could be used to suppress the carrier and upper side-band to reproduce the 30 to 15,000 cycle program wave applied to the input of the system.

In the case of Fig, 2, also, more than two divisions of the band in the output of the modulator M3 could be used, either for a wider band or to permit cheaper filters than the 8 kilocycle filters.

It is obvious that the methods described in connection with Figs. l and 2 offer certain practical advantages. For example, the methods would have 'considerable economical advantage in the case where most program circuits are to have an 8 kilocycle'band and the 15 kilocycle band is only rarely required.

Various modifications of the circuits illustrated and described which are within the spirit and scope of the invention will occur to persons skilled in the art.

,What is claimed is:

1. The method of transmission which consists lin selecting from one energy portion of a wave comprising a wide band of frequencies, a relatively narrow frequency subband, utilizing one energy part of the selected subband to effectively balance out the corresponding frequencies in another energy portion of said wave, selecting a second frequency subband from the remaining frequencies in said other energy portion of said wave, transmitting said second frequency subband and another energy part of the first selected subband over separate paths to avreceiving point and combining the received subbands thereat to produce a combination wave containing all of their frequencies.

2. In a signal transmission system comprising a plurality of signal transmission lines of limited frequency transmission range extending between geographically separated stations, the method of transmitting efficiently between said stations a signal wave comprising a wide range of frequencies, which consists in dividing said wave at one of said stations into two energy portions each comprising said wide range of frequencies, selecting from one of said energy portions a frequency subband within the frequency range of one of said lines, transmitting one energy part of the selected subband over said one line to the other station, utilizing another energy part of said selected subband at said one station to effectively balance out corresponding frequencies in the other energy portion of said wave, shifting the frequencies remaining in said other energy portion of said wave to a position in the frequency spectrum within the frequency transmission range of anothery of said lines and transmitting the frequency band in its shifted position thereover to said other station, shifting the frequency band received over said other line back to its original position in the frequency spectrum, and combining at said other station the latter frequency band so shifted with the frequency subband received over said one line in such manner as to reproduce the original signal wave.

3. The method of separating one side-band component from the other side-band component and the carrier component in a complex carrier wave, which consists in dividing said complex i, wave into two energy portions each containing all its component frequencies, selecting from one of said energy portions a frequency subband comprising a portion of the frequencies in said one side-band component, combining one energy part of the selected frequency subband with the other energy portion of said complex wave in such phase and magnitude as to effectively balance out the corresponding frequencies in said other energy portion, selecting from the remaining frequencies in said other energy portion of said wave a second subband of frequencies corresponding to those in the other portion of said one side-band component, and combining said second selected frequency subband with the other energy part o f the first selected frequency subband to produce f said one signal side-band component` 4. The method of transmitting a signal wave of a wide band of signal frequencies which consists in modulating said wide frequency band signal Wave at a transmitting point with a high frequency carrier wave to produce a complex carrier wave comprising upper and lower side-band components and a carrier component, dividing said complex carrier wave into two energy portions each containing all of its frequencies, selecting from one of saidv energy portions a frequency subband comprising a portion of the frequencies within one of said side-band components, comfbining a part of the energy of the selected subband with the other energy portion of said complex wave in such phase and magnitude as to effectively balance out corresponding frequencies in said other energy portion of said Wave, selecting from the remaining frequencies in said other energy portion of said wave a second frequency subband comprising the other portion of the frequencies in said one side-band component, combining said second selected frequency subband with the other energy part of the first selected frequency subband so as to produce said one sidehand component, transmitting said one side-band component to a receiving point, modulating the received side-band component thereat with a carrier wave of the same frequency as that of the high frequency carrier wave utilized in the modulating step at the transmitting point, to produce modulation products including as one sideband component the original wide band of signal frequencies and selecting that side-band component.

5. The method of transmission which consists in dividing a wave of a wide band of frequencies into a plurality of energy portions, selecting a relatively narrow frequency subband from one of said energy portions of said wave, utilizing one energy part of the selected frequency subband to effectively balance out the corresponding frequencies in another energy portion of said Wave, effectively transmitting at least a portion of the remaining frequencies in said other energy portion of said wave and the other energy part of said selected frequency subband over respectively different circuits to a given point and there combining them in a common circuit to produce a combination wave containing all their frequencies.

LEONARD G.ABRAHAM.

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