US1931038A - Telegraph system - Google Patents

Description

Oct. 17, 1933. A T, STARR 1,931,038

TELEGRAPH SYSTEM Filed Dec. 27, 1930 2 Sheets-Sheet 1 f i l l 0 t, t2 *t Byj A TTORNEV Oct. 17, 1933. A. T. STARR 1,931,038

TELEGRAPH SYSTEM /NVE/VTOR A. 7.' STA RR A T TORNEY Patented Oct.,17, 1933 i 7 i Y.

UNITED STATES PA'iE-NToFFicE/f TELEGRAPH SYSTEM Arthur Tino Starr, Aldwych, London, England,` assignor to Western Electric Company, Incorporated, New York, N. Y., a corporation of New York Y Application December 27, 1930,' SerialV No.` 150356075, and in Great -Britain January 24,

i claims. (ci. 11s-s6) This invention relates to the'electrical trans- (Note--The speed in Bauds isequal to twice the mission of signals in code. dot frequency.) Y It applies to systems in which each signal eie- A Symbol required' inv the specification is de ment complies with` one of -two known voltage or noted by Nt and ,iS defined aS OllOWS i Assume a f 5 current conditions. In D. C. telegraph systems single signal element is transmitted, then the total 55 this corresponds to systems inwhich two voltage time elapsing from the commencement of the or current values, for example, positive and negareeeived Signal C0 the Point Where the Current tive, or positive and Zero, etc., are used to trans- Value beCOineS Substantially negligible iS Nt time mit the intelligence. In carrier telegraph sysnnitS v l0 tems, (that is, where the signals are used to modu- The Signal frequencies may be defined aS lhOSe H0 ist@ asteady carrier frequencw the twoy condi-` which would be produced by the transmitters@- tions are` in general, for example, full carrier and ing On a' direct Cuilenl SOnleel Y no carrier. i Y l p K A feature of the inventionis a telegraph system It has previously been suggested to transmit wherein the band of frequencies transmitted has 'T5' telegraph messages by a system analogous to `that a widthI numerically less than the dot frequency, Q51

used for carrier wave telephony and known as but includes thedot frequency. 1 single side band transmission, but the theory of According t0 a further feature 0f the invention the subject required that, for complete intellia telegraph system is providedzwhereinit is only gence, ther transmitted band should extendfrom necessary to transmita band 0f Signal frequen- D `to s periods per second, where s is the dot frecies of Width Y 'A n 70 quency. From this it follows that after module# tion the frequency nearest the carrier in each .f side band coincides with the carrier frequency. p Y Coilseqllently aitetr'mfthaperpendicular Cutoff* to s where s is the dot frequency and Ntis as which is not'practicaole, would be necessary in defined abo-Va rfetlosepaite ,a side bmd from a' Garner AS One of the objects of the invention is to provide' n? llpolgsoll prtlce l bcgmestnecessary a carrier telegraph system inv which theuintelli p .y Vey Xl'enm'f'an e a om? pnasmg gence-bearing band of frequencies can be sepa,- arrangeinentsto cancel out frequencies of the unwanted side bands which would cause interrated from the carrier andiother frequens by .30V ference. Y' f 4. means of filters which permit of comparatively i Itis an object of this invention to overcome slmpleprfctlcal consum/mon' these dsadvantage5- p According to a further featureof theinvenf The various terms used throughout this specition a' Carrier telegraph System 1s provlded m mation Wm be defined as follows; which the carrier wave and Vone side band4 are 85V i' the intelligence- A unit of intelligence (that is, a letter or sym- Completely suppl-ssed and .f boi) is made up' of a number (Say N) of Signal bearing band of width less thans are transmitted 1' L i f v only. Y elements occupying a total time T seconds.

f According toa st1ll further feature of the inisnrlelr. f Y 40 g a ement 1S a gwen Oltage O1 Cunent vention a telegraph system is provided wherein 90 persisting for a definite time called a time unit.

In the present invention,V alltime units are of equal duration. As the unit of intelligence contains N signalyelements occupying antotal time T` seconds, a time unit is lequal toT/N seconds.

f The 4clot frequency is defined ,as arfrequency is transmitted along with the intelligence-bearing-band nearest thevcarrier, a portion kof unmodulated carrier. Y v. q y

The nature of the invention and other features thereof will Abe apparent from the following def sc ription taken in conjunction `with the accom` quaitor-f v p e 1N Y i Y Y y .panyingdrawings of whichFigs. l, 2, 3 and 4 2-TPPS- y v' .are descriptive of the theory underlying thel 1 l Y invention.

(that is, onetime unit), then (l) becomes 509:1.; (qT/ZN) cos (3)l Equation (3) shows that we have here a con tinuous frequency spectrum of amplitude The shape of S,(q) as a function of the frequency is vshown in Fig. 2.

' Let us now examine the value of S(q) at various instants of time,lremernbering that N/ qT=s S(q)'=0 at q :ZmrN/T except when m=`0,

corresponding tof=m'N/T=2ms.

S(q) (is a maximum just before (2m-|-1)s, provided m='0.

` Now supposel we have a signal as shown in Fig.'

Then by superposition of signals given by (3) we have where The first factors in both expressions of (7) are identical form with S(q) given in (4) and is the shape-factor for the D. C. wave. The second factors in the expressions are the discrimination factors and carry the intelligence conveyed by the signals; the shape-factor may be anything so long as it is knownand does not vanish at any frequency at which we wish to know the discrimination factors. The discrimination factors may be taken as l l Remembering that the speed of signaling is given by f s=N/2T it is seen that C(q) and S(q) are repeated in frequency bands of width s bounded by odd and even multiples of s. It is therefore unnecessary to transmit a wider band than this. But on the other'hand,v theoretically it is not necessary AAto transmit such a wide band, because since Equation (6) above contains an infinity of frequencies in any finite band, however narrow, it would be sufficient to transmit any band, as narrow as we wish, andr any such band will give us thewhole of the intelligence. There are two drawbacks tothis; iirstythe difficulty of extracting from a wave of such narrow frequency width the intelligenceV which it contains, vand second, the interference of preceding and succeeding signals with the signal to be observed. The effect of preceding and succeeding signals is to lump the spectrum in the vneighborhood 0f certain frequencies; the worst possible interference occurring when the precedv ing and succeeding signals are the same as the mid signal, in which case the lumping is complete at the frequencies given bythe frequencies of the representative Fourier series. else the amplitude falls to zero.

In this case it is necessary to take a finite frequency bandof a certain minimumwidthy at a proper part of the. frequency spectrum.k

The preceding theory as has been shown, deals with the infinite frequencyspectrum, (that is the transients) produced by a single pulse as shown in Fig. l.v It is difficult-toA deal with an infinity of equations, and it is therefore preferable to work Ves iso

Everywhere Y witha steady-state signal,

peated indefinitely.

It will now bel shown how` to transform the problem froxna question of transient currents to 5 one of steady-state signals.

It has been assumed vthat after Nr time units from the commencement of the .received signal the current is substantially negligible. Then if We Wish to consider the Signal element occurring in any time unit, we can assume that al1 signals transmitted Ni or more time units before it has no interfering effect on it. Hencev these early signals may be replaced by whatever we-like; so

we choose them to be the'same as those signals in the Nt time units immediately before the considered signal indefinitely repeated. Thus we need only to consider a. signal of Nc elements, indefinitely repeated.

If the signal similar to that shown in Fig. 3

but, with Ne time units, is repeated indefinitely i. e. one which is re-V from t= w tot=+ thefunction-E (t) can beA expressed in a Fourier series, whose `coeficients can be found by the ordinary method.

Hence also .55 need be transmitted to carry thevsignal without ambiguity or redundancy. For Vthis .weushall consider the case of a sequence ,signal when N (-:Nf at the moment) is odd, e.'g. 5. It is reasonable to take N as a small number, and Nf-is prob- 'Y ably also small, and of the same order. lBy co4 incidence Nt is the same as the number of units in the Baudot code, butthe reasoning is'perfectly general f From (11) andr(12) above we see that and occurs at frequencies given byfin) :2m/Nt. Therefore for Ni=5, fm) takes theivalues '2s/5, Lils/ 5, 6s/5, 8s/5, 2s, v12s/5, etc. It is thus seen that the frequencies s, 3s, 5s do not occur, What' ever the shape-factor ,would have been. Also, as

the shape-factor` for asquarewave vanishes atthe frequencies 2s, 4s, 6s, these frequencies,

do not exist. f

C q, C1, C2, Si, Sz. Co is unique inthat it never vappears again, for it could appear only when n andsolve .forthe alfs;

`.apsin (21r/5)'-a5 sin (21r/5). =S4` v v S2=a1sin(21r/5)a2 sin (1r/5)+'a4 sin (W/S)vv a5 sninr(2vr/5v)-TS3 5V n v 1 .Now as the shape-factor vanishes at the fre-- quencies 2s, 4s, 6s,` etc., that is, at n=5, 10, 175, etc., we cannot determine Za from any frequency but zerobecause 2a occurs only at Cno-Csfi-Cim-,Cia etc., and C5, C10, etc. occur at frequencies 2s, `4s',' etc. at which the shape factor vanishes: also We see that the only other independent "combinations of the as are Si, S2, C1 and Cz which ca n be found at various different frequencies, as shown by ,Equationfl just given. Hence unless wetransmit zero frequency'we havenot sufficient equations to determine the five as; ]Thi-S'is the jordi.;`

Vnary D'. C. telegraph system, butin an ordinary practical two-current system the values of the 'as are known to be either one-of the two current values. Ineffect, this is intelligence whichrneeds no frequency spectrum to convey it,` and We Will therefore re-exaznine `our equations in the light ofthisfact. The only independent functions of the as are 11,:5,` 10, 15, and then the shape-factor vanishes.` It contains the sum ofthe as;A C1, Cz, S1, S2 reoccur, the law of recurrence being given in Equation (14) above, and possess the property that they remain unaltered; if the as are each increased bythe same constant. .f f We can take the four equations given 'by C1,` C2, Si and S2, except that ai iscalled ail,y a'zletc. andadd an arbitrary equation 135 The values of a1, a2, :a3L etc. are clearly ail-i-l/5Co, a21+1/5Cp,' etc. for (1) these will satisfy the Equations C1, C2, S1,` S2,'since the als` in them can be` increased byaconstant amount without altering their values, and (2) Vsince we took'2a1=0: S1, S2 and the arbitrary 1 EquationEai-:Uweget theryalues of the vasex- 150 So by taking'CnlCa VlCl diminishing divergence from s.

cept for a constant. 'We could take vEa1'=K and we should then obtain the values less the constant (1/5) (C0-K). If, for example, the as were +1, +1, +l, -l, and +1, then'Ci, C2, S1 and S2 and 2a1=`0 would give values ofas equal to 4/5, 4/5, 4/5, -6/5, -6/5.

In a two-current system all that it is necessary to known is the difference between successive -as as we know that they are either alike orunlike from the given conditions. In the numerical example just given it is seen that the difference between the successiveas of the absolute signal, i. e. +1, +1, +1, 1, -1, are o, o, r2, o, which are clearly the same as the differences between successive values of the as of the signal, 4/5, 4/ 5, 4/5, -6/5, -6/5. Therefore, assuming that a receiving instrument can be designed to operate on the'change of value of the as it is clear that it is only necessary to transmit the two rfrequencies which carry C1 C2, Si, S2. Such areceiving instrument may be, for' example, a siphon recorder or an oscillograph. In this example taken, frequencies 2s`/5 and L1s/5 or 6s/5 Vand 8.9/5 or suffice cas shown inl Fig. 4. These correspond to valuesof n=1 and 2, 3 and 4,6 and 7 (5r-l-l) and (5`r+2), (5r+3) and (5r+4) respectively;

It appears, therefore, that it would be `suiicient to transmit a band such as (gs to gs) or (gs to s),

where r is any integer.

It must be noted that it is not sufficient to transmit any two contiguous frequencies, for the contiguous frequencies 43/ 5 and 6s/ 5 corresponding to n=2 and n'=3, have the same discrimination factor associated with them and hence do not provide sufficient independent functions of the as to vdetermine the as, except for the additive constant, of course. Y Y

Further consideration shows, however, that a peculiar property possessed by the frequency of the bandKnearer to the odd multiple of s modifies the band widthrequired, in general, in order to convey the complete intelligence contained in any succession of signals. v

This property will be explained in the ing:

`If a signal based on an lN unit code is repeated indefinitely the steady state method developed above mayVV be applied and it is found that the frequency band required depends naturally on N but also on whether N is odd cr even. If N is odd the frequency s does not occur, as has been shown forlN=Nt=k5, and if N is even thefrequency s does occur. By work analogous to that done for the case of N=5 it can be shown for example, that if N=4 the band needed is (s/2 to s); for N=5, the band is (2s/5 to ls/5); for N=6, the

followlband is (s/3 to s); N=7 the band is (2s/7 to 6.9/7) etc. Obviously if N becomes large (infinite) the band width tends to become (0v to s) irrespective of N being odd or even.

The point of importance is, that when 'considering the lowest intelligencey bearing band'the lower frequency of the band required continually 'diminishes as N increases, while the Yupper frequency oscillates to Aand froms with successively increasing integral values of N, but with Similar reasoning applies to all othery bands.

Thus the important proposition maybe stated for the lowest band that theAI upper frequencyequired for the frequency band necessary is s and the lower frequency required is dependent fentirely on the time, measured in time units, occupied by the total transients from asingle signal element plus the signal element, i. e. Nt;

In the case of a laboratory experiment, it was found thatthe total pre and post transients associated with' one signal elementgplus the signal element, were contained for all practicalpurv poses within a time equal `toV five time units.V Hence the lower limit vof the frequency band considered was 25/5. I ,v

If in another Vsystem this timey was,l say 6 signal elements, then the lower limit'would `be (2/6)s(='s/3). The required band istherefore seentobe l -Nir Y to s. l .Y 95

Fig. 5 shows a D, C. system in block form. The

D. C. transmitter DCTi works ata dot frequency of s1, and uses a band widthselecteclby the band pass filter BPFl, of

to s1. DCTs works at the same dot frequency but uses a band width of 2s y l Nt t0 381. V .v

All the remaining odd channels use the dot frequency s1 and band widths respectively 231 apart.

.D. C. transmitter DCIz and all the even channels operate at a different dot frequency s2 110 higher than s1 to prevent coincidence of the edge of the required frequency bands, e. g.y if s1=s2=vs channels 1 and 2 would coincide at the frequency s leading to confusion of the message.

Eventually, even withrthis difference between H5 si and s2, as more channels are taken into use, the successive odd and even' channels will get closertogether and eventually over-lap. It `was found that with practical filters a 6 channel system required the ratio. of s1 to s2 to be approxi- 1 mately 23/27 in order to prevent too close spacing of the channels.

Y Channel 1 from DCT1 to R1 is more or less in the nature of a D. C. telegraph channel, butdiifers A in that frequencies'belowv j need not be transmitted. The other channels, although sent `by a D. C. transmitter, are more like carrier telegraph channels. There is a consequent saving, for only one modulator is needed to send these channels, whereas inA ordinary carrier telegraphy'a modulator is needed at the sending of the channels.,Y v Y135K However, frequency-translating devices, i. e.Y de-modulators D2 and D3 etc., arey needed in channels 2, 3 etc., at the receiving end as shown. The correcting networks CN1, CNz, CNS applyfboth attenuation and phase shift adjustments q in known manner, to give the desired received wave. Inthe above reasoning it has-been assumed vthat all thvatis vrequired'is the diiference between frequency fc.

transmission which does not need line time'for transmission. v f' It can be shown that when an A. C. source is switched on to and olf from the line' in the same Way as a D. C. source, then if the A. C. source is of a sufficiently high frequency, the shape-factor of the'resulting wave, assumed indefinitely repeated or non-repeated, is the same as the shapefactor for the direct current case shifted tothe right in the frequency spectrum by an amount equal to the frequency of the A. C. source, and therefore the analysis of the D. C. case .applies here. If the carrier frequency is not high comi pared with the dot frequency, the shape-factor is not the same as for the D. C. case, but the dis` crimination-factors will be the same, and the theorem as regards the band width necessary to transmit complete intelligence will follow in the same way. Hence as the lowest frequency re.- quired is displaced from the carrier frequency .by a denite amount, it is now possible to select the single side band frequency spectrum, this band width being Ythe same as for the D. C. case, i. e.

to s but with a location in some part lof the fre`` quency spectrum depending on the carrier frequency.

Fig. 6 shows how this may be carried out. Instead of passing channels 1, 2, 3, etc., directly out on to thejline, they can be passed in to a modulator of acertain carrier frequency fc andv translated up in the frequency spectrum by a It is now comparatively easy to transmit a single side band for each channel, because the intelligence bearing bands are separated from the carrier frequency, which can thus be easily suppressed. In Fig. 6 it will be assumed that it lis required to transmit the upper side band. The D. C. transmitter DCT supplies signals to the band pass lter BPFa `which selects aband of the corresponding width from 2 3 N l l I t to s, and passes this band on to thev modulator M supplied with a carrier frequency fc. The band and transmits it to the line L. At the receiver the incoming frequency band passes througha similar band pass filter BPFc and the passed band is de-modulated (i. e. translated down in the frequency spectrum) in the de-inodulator DW supplied with a frequency je. The .output from the demodulator passes through a band pass lter BPFd which selects the desiredbandof l Nt H to s, which after passing through the correcting network CN which supplies the necessary atten'- uation and phase adjustment, is applied to the n d quency, and which', in its normal'positionlintheV Fig. 6 is. to make the signal modulate the carrier. directly receiver R.

A better method than the scheme of and thereupon filter out the required side band and`pass this band to the line wthoutfthe intervention of a modulator. The advantages of this scheme are as follows: One filter is saved per sending channel. The harmonics of the D. C.

wave are not formed, as they would be in the rto be `ur`1derstood`v that other modes ofV` applying thej principles set forth above"will-`be irrimedimodulator method; Also, -vattenuatior'i andv phase shift'du'e to the modulator are avoided; I .1 .5.

An alternative method lof transmission consists in sending a separate message in each of the side bands of a single carrier with a portion of unmodulated carrier transmitted in additionV vto the side bands. Y

Referring now to-`Fig. '7, O` is an oscillator which provides thenece'ssary carrier frequency. Thisis fed into two transmitters T1 andfTZ and also into the line Ldirect through the band pass filter BPF. rBPll' lets throughthe flower side band and 'BPFZ the upper'side band;A '#Phase Shifters` PS1 and PS2 vserve the purpose'` of 'cor-f recting thefchanges '(at' different frequencies)` due tothe'band pass filters. Thus `it'willbe seen that a `portion of unmodulated carrierand"'two separate side bands carrying diferentimesfsages arefed into the line'.`l yAt the receivingend each of the band-pass filters vBlVFS"andlBPFlV lets throughone side band with a part ofthe u n' modulated carrier'. Each 'side band is adjusted for` attenuation and phase shift A.due to 4the-:line and band pass filters BPF3 and BPF4', this being provided for in equalizers E3 and E4 and 100 phase Shifters PS3 and PS4. Each signal is now demodulated by the appropriat'edtector y=D3'fo r D4 and the resulting currents operatey there ceiving mechanism R13-and Rito lrecordthe 'sig nals.` The receiver, as in 4'the case ofthe pre- 105 ceding examples, may be any suitable telegraph receiver, for example,` aVsiphon recorder, teleprinter recording oscillograph -or any sensitive recorder. V' Y f In order to minimize interference between the channels` but at thesame time to receive as large a 'carrier as possiblejfor obtaining as strong a signal for demodulation purposes as Apossible, the" sendingfilters vBPFl' and BPF2 should havefat-i tenuation characteristics as 'shown' in` lFig. 8` 115 since the form of the ycharacteristics of 'BPF1-" and BPFZ are such'v as tominimize interference between the channels and the forms of the char#` actersitics of VBPF3 and BPF4" are such as to separate theA bands `andV let through amoderatee ly large'portionfof the carrier-'to each channel.`

Although I have described certain Amethods of carrying the present invention into effect,;itfis` ately apparent to those skilled inthe art; and 'I Y do not, therefore, limit myself to the precise ern` bodiments described above. Y

'In the claims, the word multiplef is used in the generic sense to include single wellfas 1302m plural multiples'. e.l g., the' expression "multiple of the dot frequency may include the dot fre? quency itself. 1 What is claimed' is :"f

1'. A telegraph 'system comprising means for V13.5,

causing each signal element to complywith' one of two known electrical conditions that are used to transmit the intelligence,` andA frequency selec-` tive'means for selecting from the signaling waves," for transmission, frequencies of af band which-140'? has width numericallysmaller than the dot 'fre-' cal conditions that are used to transmit the in.

telligence,` and frequencyselective means for se- 1503' n 6 .lecting from 'the signaling Waves ,-for transmission, frequencies of a band which hasband width and which in its normal Vposition in the frequency spectrum, has an odd integral multiple v of s asone of its boundaries, where s is the dot frequency and Nt is a number as defined specification. v

v3. A telegraph system as claimed in claim 2, comprising frequency-translating means for in the transferring to a given position in the frequency spectrum va frequencyband as specied inclaim 2. 4. A .telegraph system as claimed in claim 1, with receiving means for extracting the intellif gencefrom the'lirnited band of signal frequencies-transmitted. Y

5. A direct current telegraph system comprising message channels and frequency selective means in said channels for selecting for trans- .mission in the respective channels respective frequency bands of width l s N, n spaced throughout the frequency spectrum so that successive bands include successive odd integral multiples of s, Where s is the dot frequency j and N: is a number as defined in the specification.

claimed in claim 5, comprising means for producing a different dot frequency for every channel. i

7. A telegraph system comprising a group of message channels and a group of direct current telegraph transmitters therefor of one dot frequency, asecond group of message channels and a second group of direct current telegraph transmitters therefor having a higher dot frequency, frequency Iselective means in said first channel group selecting from the signalwaves in its channels, for transmission in the respective channels vofy that group, frequency bands each of width numerically smaller than the dot frequency for that channel group and each with an odd. integral multiple of that dot frequency as a limiting frequency, and frequency selective means vin -said second channelgroup for vselecting .from the sig-- nal waves in its channels, for transmission in the respective channels of that group, frequency bands alternating in order in the frequency scale:

with said first mentionedfrequencybands and eachof Widthnumerically smaller thanthe dot frequency forgthe second channel group and each With an odd integral multiple of that dot` frequency as a limiting frequency.

8. A telegraph system comprising a transmis- `Asion line, a plurality of direct current telegraph transmitters, band-pass filters respectivelyconnecting said transmitters to said line, each filter passing a frequency band lying in a different portionv of the frequency spectrum and having a `band width Vnumerically smaller than the dot frequency of its transmitter, and signal receiving means connected to said line and comprising signal receiving channels, filters corresponding v respectively to said first mentioned filters conjnected in said receiving channels respectively,V and frequency vtranslating means connected in 6. AV direct currentY telegraph system las.

each of said receiving channels other than the lowest frequency channel. y f

9. A carrier wave telegraph transmitting system comprising a carrier wave transmission circuit, a source of carrier telegraph signaling waves, Aand'frequency selective means c onnecting said source to said circuitfor selecting from said signaling Waves, for transmission to said circuit, `rfrequencies of a band which-has width numerically less than the dot frequency and vwhich has as one of its limiting frequencies a frequency algebraically differing from thecarrier frequencyby an odd integral multiple of the dot frequency.

l0. A carrier wave telegraph system as claimed in claim 9, in'which the widthV of the frequency band is Y S'Nt-vz:

12. A system comprising means for producing carrier telegraph signal waves, a carrier telegraph transmission circuit, frequency selective means for selecting from said signaling waves,l

for transmission to said circuit, frequencies of a band which haswidth numerically smaller than the dot frequency and whichhas as one of its limiting frequencies a frequency algebraically differing from the carrier frequency by an odd integral multiple of the dot frequency, and means for transmitting waves of the carrier frequency to said circuit.. 'v Y 13. The method which comprises transmitting direct current telegraph signals as a band of fre-..

quencies that has width numerically smaller than the dot frequency and that has an odd integral multiple of the dot frequency as one of the boundaries of the signal frequencies.,

, 14. Ihe `method which comprises transmitting carrier wave telegraph signals as a hand of frethe dot frequency and that has an odd integral multiple of .the dot frequency as one of the boundaries of the vsignal frequencies..

15. The methodoftransmitting Asignals in code which comprises transmitting each of the signal elements in compliance with either of two electrical conditions either of which they are known to fulfill, and transmitting each o f the signal lelements as wave components embraced in the same frequency band having width numerically smaller than the dot frequency and having an odd vinte'-Y gral multiple of the dot frequency as one limiting frequency.

ARTHUR TINO STARR.

quencies thathas width numerically smaller than 3130l

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