US1821997A

US1821997A - Compression of frequency range

Info

Publication number: US1821997A
Application number: US377305A
Authority: US
Inventors: William T Wintringham
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1929-07-10
Filing date: 1929-07-10
Publication date: 1931-09-08
Anticipated expiration: 1948-09-08

Description

Sept. 8, 1931, w. T. W'INTRINGHAM 1,821,997

COMPRESSION OF FREQUENCY RANGE Filed July 1o. 1929 ATTORN EY Patented Sept. 8, 193i rar ser estes-"1i WILLEM" T. VJINTR-ING-HAM, OSL."Y BR'OOKEYN, NEW YORK, ASSGNOB TU" ATEERICAN" TELEPHONE &` TELEGRAPI-I COIMPANY; A. CORPORATION" OENEVI YORK COMPRESSION OEl FREQUENCY RANGE Applicationzeieai any 10, 192s.A

This invention relates to* a method and meansfor compressing the frequency range required for signaling.` its purpose is to make possible an-.increasein the-number ofA 5 channels-of communication over any transmission system.. Another pur-pose is toob tain a; greater; measure of secrecy in communication than isnowcommonly present.

It is Well recognizedV that 4if a plurality of messages areY to be transmitted simultaneously over'any transmission medium, the frequency bands ofthe. several zmessages-shouldV not overlap. In order, then, to obtain as large a'number of channels as poss-ibleover 15. givenitransmission medium, it is' desirable that the frequency band Width for each signaling channel should befas-narrovv as possible,.in ordenthatl times T shallbe as small as feasiblef Where F represents the Widthfof thesignaling band. and T the time required -.to ltransmit the message.

In this invention l propose to accomplish the desired ends by transmitting a given portion ofthemessage frequency b'andffor.`

a portion 4of the time onlyand Yarrangingthe system so that the rportionof theband Which is being-transmitted shall moveV fromv oneV end oflqthe signaling bandfto the other periodically, thusffgiving opportunity for` allv frequencyA bands inthe Adesired range to-berepresented, but forl alportion offthetime only. rlfhis lmake its possible to reduce the Width offthe band during transmission and still transmit the message at the normal rate.-

More specifically, I practice my invention by modulating a periodically Varying carrier frequency With the message to be transmitted. Oney side band of the modulation products, Whichv band Will' have the same 40 Width as the originalmessagaristhen reserved'and a portion ofthis side band only is transmitted. In'vievvv ofthe varying nature of thecarrier frequency, it isf possible togive each portion of the original signal 15 a trepresentation.inthe narrow portion ofthe side band Which'is finally transmitted.l

At the receiving. station, thisnarrow side bandis demodulated by a locallyl generated carrier frequency. which varies'in the same i0 manner and in phasewith t'hatat the transmitting station. Among the demodulation products there Willappearansorfiginallmesfv sage With'whichthe Varying carrier atsvthe. transmitting station Was f modulated.

rlhe invention Will be` better understood by'reference to the accompanying. drawing inWhich Figures l, 2 and 3 showfrequency spectra, and Fig.. 4. showsY a signaling system embodying my invention.. Referringmore particularly to- Fig.y l, the. abseissee. 60 represent the frequency and, for. purposes of illustration, I. have assumed amessage such as a. speechl signal' Whosey frequencyvr band extends from` Oltof 3,000 cycles... Sup-1 pose now, this message isi used. for modulating acarr-ier frequency-22. Of the'two'side bands,. one is suppressed, and; in Fig. 1,I L have shown the upper band. entendingfrom. pto p +3 ,000 as beingmeserved. By-,suitabl'e means, such as band'ilters, any-desiredfpor;A 7 0 tion of thisupper lside band may be trans'. mitted, and fin-Fig. lA I-ha-ve shown this band. as. being of.` Width. 1,000 andzbeing..v inthemiddle of the carrier side band;A If the can-- rierffrequency Were. tovremain fixedithensa portion only ofthe originalmessage Would be transmitted;. the upper. third and. ther lower? third havingjnqrepresentation. 1f, lioWever,-. the carrier frequency should. be lowered to p1v by anamountfequal to. onethird the Width ofthe 3,000-eycle'ban'dg5then ity is evident that through the; same. band filterthe upperl third only.- of. the: original signaling frequency bandi Would.` be. Vtranse' initted.V Onthe other? hand, ift'he carrier 55 frequency Were increased top" by anamou-nt equal to one-third the Width` of: the l signaling band, then the lower third'offthefor'iginal signaling. band Would be represented;y in the Wave passing through the band filter. Still further, if. the carrierfrequency Were to vary continuously from -pltopfand back again,.then at one-time orA anothereach' and every port-ion ofthe original si'gnal'band would be represented inthe: output of the 95 band filter.A f

My invention,then, consists lin .partf'in providing means for varyingfthefcarrier. frequencyl by a: suitable amount,.so1that` each portion ofthe originalmessageshall. receive representation. This variation may take place in any desired manner, that is, continuously or step by step. In the continuous change, again, it is evident that the variations may follow a large variety of current relations. That is, the frequency may vary at a uniform rate from p1 to p and back again, or may vary between these limits sinusoidally, or in any other desired manner, certain ones being preferable to certain others. In actual practice, I would prefer to have a uniform rate of frequency change in passing from p1 to 20 and then the same uniform rate in passing from p to 1. Y

Iii general, such modulation on a varying carrier frequency will introduce certain modulation products whose frequencies may lie in the range of the message frequency.V It will be desirable to avoid this, and Fig. 2 represents a modification of the invention for this purpose. In that case again, definite values for frequencies have been used for illustrative purposes, but it is to be understood that the invention is not restricted to the particular frequencies indicated. Re ferring to that ligure, the message signal whose frequencies vary from 0 to 3,000 is first modulated on a fixed carrier frequency such as 7 ,000 cycles. One of the side bands from this modulation is reserved, in this case the lower side band extending from 4,000 cycles to 7,000 cycles. The second carrier frequency is shown as having a mid-value for p of 9,500 and varies as a whole from the value for p1 of 8,500 to a value for ya of 10,500. This varying carrier is now modulated with the band extending from 4,000 to 7,000, and the upper side band only is retained, this band extending from 13,500 to 16,500 when p=9,500. Here, also, for illustrative purposes, the middle third of this upper band is transmitted, and it will be seen, as in the case of Fig. 1, that at one time or another all portions of the original signal band are represented.

The invention may be carried out with apparatus shown schematically in Fig. 4, in which A represents the transmitting station and B the receiving station. In this figure, the apparatus indicated schematically corresponds to the system described in connection with Fig. 2. The input band of frequencies extending from 0 to 3,000 cycles is transmitted through a low-pass filter to eliminate any extraneous frequencies. t is then modulated on a carrier frequency of 7,000 cycles, and the lower Iside band from 4,000 to 7 ,000 cycles is selected from the output of themodulator by band pass filter. As has been previously mentioned, the purpose of this modulation is to eliminate undesired modulationv components at later stages. From the output of this band filter, the band is put into a modulator the carrier frequency of which varies from 8,500 to 10,500 cycles. The rate at which the carrier frequency is varied is subject to a wide latitude of choice but preferably should be of a reasonably high rate in order that no portion of the original frequency band shall be omitted for a long period of time. A very suitable condition would be to have the frequency of the oscillator go through its cyclic change at a rate of 1,000 cycles per second. In the output of this modulator there is a band pass filter passing the band 14,500 to 15,500 cycles, that is, a band whose width is 1,000 cycles, which, it will be seen, lies l in the middle portion of the upper side band resulting from the second modulation. The output of this band pass filter is then impressed upon the transmission medium by means of any appropriate circuits.

Comparing the results thus far obtained, it will be seen that when the second. carrier frequency is at its lowest value of 8,500, the band transmitted corresponds to the frequencies from 6,000 to 7,000 in the intermediate modulation side band, which in turn corresponds to the frequencies from 0 to 1,000 in the original message band. Similarly, the portion of the band which is transmitted for any other value of carrier frequency can be calculated. For convenience, the following table shows the results at a number of different values:

Since in this case the carrier is varied between the limits 8,500 and 10,500 at the rate of 1,000 cycles per second, in the period of one two-thousandths of a second each frequency in the original input band would have been transmitted through the 14,500 to 15,500 cycle band filter for a fraction of the time, and thus every part of the original frequency band will have been represented. The band of frequencies at the output of the last band filter mentioned may be either transmitted over a circuit in this position in the frequency spectrum or may be shifted by modulation into any other band of 1,000 cycle width that may be desirable.

At the receiving end, the l000-cycle band should preferably be remodulated to the position 14,500 to 15,000 cycles if it has been shifted and then impressed on another modulator havinga varying carrier frequency extending between the same limits of 8,500 and 10,500, at the same rate of 1,000 cycles per second and supplied in the proper aannam time phase. The time phase is important, since the remodulated carrier must be at exactly the correct frequency at eachV instant to remodulate the output band into a normal speech band. At the output of this modulator, the side band 4,000 to 7,000 cycles is selected by a band pass filter shown in the receiving circuit of station B, and in turn is modulated on a carrier of 7,000 cycles to shift the band to its original position from 0 to 3,000 cycles and being preferably transmitted through a low pass filter to again eliminate any extraneous frequencies.

Vhile the circuit of Fig. 4 has been described in connection with denite values of frequencies, it is to be understood that the invention is not limited to these frequencies and that the width of the band to be finally transmitted may be modified over a wide range. This can be made clear by a reference to Fig. 3 in which, for simplicity, the intermediate frequency modulation has been omitted. In this Fig. 3, the original signal band is shown as extending over a width Q starting with Zero. This is modulated against a varying carrier frequency whose mid-value is p. Selecting the upper side band, it is seen that this extends from p to p-I-g, and in the middle of this band there is selected for transmission a narrow band of width r. If, now, the carrier frequency is reduced to the value the upper portion of the original band is represented, whereas when the carrier frequency is shifted to the lower portion of the original band is represented, and for intermediate values of the carrier frequency, intermediate portions of the original band are represented.

l/hile .in this description I have assumed that the upper side band is retained after modulation by the varying frequency, it should be pointed out .that the lower side band could have been retained and upon similar clemodulation at the receiving station the message would reappear in correct form. Also, while in Fig. 2 the original message is shown as being shifted to the band 4,000 to 7 ,000, which is below the varying frequency, it could have been shifted to a band above the varying frequency, whereupon by suitable demodulation and shifting of the band the original message would re appear in its correct spectral position. Many other variations may also be introduced as to the side bands which will be retained or as to the particular apparatus used for carrying out the invention, all leading tni substantially:v the; same?. results, and. it: is to] be.` understood;F thatifthey; are; includedv in: thisuinventibn'.. f

Wilate. is claimed; is .s i

1;.. rEhe. method' ofi signalingwhich. con-.- sists.- in transmitting-ai narrow: bande. off frequencies. of definite widthv which represents successively. different portions; of the originalA message band and. suppressing allother fre.-4 quencies.

2i. TheA method. ofi. signaling, which-v con` sists in.` transmitting: a bande of frequencies narrowerthanitliatL of.r therzmessageathe nar-"- rouviT band;representingfsuocessinelyf different portions of.: andi covering.- the band'. width of the original; message to; be transmitted;

3.-. ln a: signaling; system, the method. of signaling; with af reduced1 frequency; band width, whiclrconsis-sinamodiilatinggtlie -messageto aicarrier: wave; of periodically varying frequency and transmittingfaf.portion;of' the modulated' wave.` band of'a.width less than the band lwidth of the` originalmessage to--be` transmitted' and. suppressinggtherex-- mainder..

il.' Inf signaling system, the; method of signaling-'with a. reduced frequency band width, which .consists `iinmodulatingjthe mes-.- sage. to a carrier. wave; of2 periodically varyingY frequency, transmitting. a-A portion only ofthe modulatedwave-band f of' a width :less: than thebandr widthbf'; the originallmessage to. be transmitted, suppressingV the."1 re.- mainder', anddemodulating this wave'at the receiving station. withi a; similarly periodi'- cally varyinggcarrierfrequency.

5.- lny a: signaling system, the. method of Y reducingthe signalfrequency b'andfof; width gfto one -of' anarrower; width .7', which.. com

sists: 1n. modulating; the message to: a car,-

rier waveiof mid-Tfrequency'pzand varying the carrier.A frequency oven aA rangefrom Pt T to WT and: suppressing all frequencies :exceptf those 1n .tlief band of' width r.

6.-.In asigna-ling system, the methodl of transmitting-'a message offrequency band width g byoneofanarro-werwidtho, which consists in modulating the messagev to a carrierv wave of mid-frequency p, varying the carrier. frequency over a range from U Q T,. 19+ 2 2.

to pmeans for modulating the carrier with the signal, means for transmitting a portion of the modulated frequency band narrower than the message band width, a receiving station, a generator of carrier oscillations of a similarly Varying frequency, and means for demodulating the received signal on the said oscillations to substantially reproduce the original message.

8. ln a system for transmitting a message of frequency band width g, means for reducing the band width to a band of Width fr during transmission, said means comprising an oscillation generator for carrier current of frequency Varying periodically by the amount g-r, means for modulating the varying carrier with the message, means for selecting from the modulated side band a band of Width r, a receiving station and means thereat for reversing the process to yield the message of band width Q.

9. The method of reducing the frequency band width required for transmitting a message of a definite band width, which consists in transmitting at one time a portion only of the message frequency band and varying this portion cyclicallythroughout the range of the message frequency band.

10. The method of reducing the frequency band width required for transmitting a message of a definite band width, which consists in shifting the message band cyclicaliy across a definite frequency range and suppressing all but a hand narrower than the shifted message band Width.

ll. The method of signaling on a reduced frequency hand width, which consists in modulating the signal to a varying carrier frequency, transmitting a portion of one side band thereof of narrower frequency than the message band width, and performing the reverse process at the receiving station to yield a-message of original hand width.

l2. A signaling system for signaling with reduced band width a message to be transmitted ofY a certain frequency band width, means for suppressing all but a narrow band of width less than that of the message to be transmitted, and means for causing the whole band to shift periodically over a definite frequency range to permit representation in said narow band of each part of he original message frequency hand to he transmitted.

In testimony whereof, have signed my name to this specilication June, 1929.

VILLAM T. VINTRINGHAM.