US2102472A - Multichannel carrier transmission system - Google Patents

Description

De.14,1937.y H. 'MAYER 2,102,472

MULTICHANNEL ICARRIER TRAANSMISSION SYSTEM Dec. 14, 193,7. H MAYER 2,102,472

MULTICHANNEL. CARRIER TRANsMissIo'N SYSTEM Filed Deo. l2, 1953 2 Sheets-Sheet 2 Petenfed Dee. 14,. 1937 MULTICHANNEL CARRIER TRANSMISSIO SYSTEM Hans Mayer, Berlin-Charlottenburg, Germany,

assigner to Siemens & Halske, Aktiengesellschaft, Siemensstadt,

near Berlin; Germany, a

corporation of Germany Application December 12, 1933, serial Ne. ronces In Germany December 22, 1932 9 Claims.

ferent channels of a multiplex system, require that the frequency bands used for the transmission of information and called hereafter useful bands, do not succeed each other' directly, but that they be separatedfrom one another by unemployed frequency ranges.-

Accordingly to the invention, the useful frequencybands are so distributed within the available'frequency range as to enable such of the products of modulation (harmonics, beat frequenciesbdue to non-linearly working switching umts (e. g. valves) asare most disturbing to fall within the Aafore-mentioned unemployed frequency ranges.` 'I'he total pass-band is hereby most efficiently utilized, since the control'limits can in certain parts of the useful band be made independent of linearity conditions.

According to 'the invention, this may be attained by a subdivision of the available pass-band into equal band components (operating bands) whose border frequencies `are entire multiples of their width and by the allocation, in each of these operating bands, of a single useful band whose width is smaller than that of the operating band.

According to the operating bands are obtained by means of a 55 Fig. 2 shows diagrammaticallyfa system of freanother feature of the invention,`

common intermediate frequency, in an intermediate modulation stage producing frequency (Cl. 179-45) I quencies in which the useful bands are arranged asymmetrieally with respect to the operating bands.4 Y

Fig. 3 schematically shows a wiring diagram according to the invention, Fig. 4 is a diagram showing both transmitting 'and receiving apparatus and Fig. 5 is a partial diagram illustrating how certain results may be Vobtained in accordance with my invention.

The diagram Fig. 1 shows that the available frequency range has been subdivided, according to the invention, into eight equal component bands (operating bands) each having a width of 3000 cycles per second, the border frequencies thereof (0, 3, 6, 9 etc. kilocycle's) being entire multiples of their Width. Within each of the operating'bands'there is a useful band (a h) whose vwidth is less than that of the operating band. vAccording to the arrangement illustrated i by Fig. 1, I utilize, as Vuseful frequencies, the frequencies between 250 and 2750 cycles per second, that is to say, those of a band of 2500 cyclesV second harmonic corresponding to the useful band frequencies a, b, c, and d respectively.

The frequency distribution shown in Fig. 1

embodiesthe following characteristics of my invention:

(a) Beat frequencies (including harmonics) resulting from the interaction oftwo frequencies lying exactly in the middle of two adjacent operating bands coincide with the junctions of the adjacent operating'bands (border frequencies) and can therefore not interfere with the useful bands. s

(b) The same is true of a range in the neighbourhood of the mid-portion of the operating band the width of which range is equal to the ldistance separating the highest or lowest useful frequency from the neighbouring border frequency. l 4

(c) Beat frequencies resulting from the interaction of border frequencies coincide againwith the border frequencies.

(d) The second harmonics of-a useful band fall within the range of only two useful bands.

vAccording to the invention, these properties can practically be utilized as follows:

In the useful bands (transmission of speech,

multi-channel carrier telegraphy, picture transmission, etc.), it is often necessary to transmit individual frequencies or frequency bands which these latter to fall within the mid-portionsA of the operating bands, which results automatically in I -a. coincidence of their beat frequencies with the border frequencies and thus prevents them from producing disturbances in the useful bands. In the system shown in Figgl, for instance, I should prefer to use a signalling frequency or ringing 'frequency of 1500 cycles per second in each of the useful bands.

It is furthermore known that in certain useful l bands. e. g. speech, even if there exists no predominant frequency, part of the band has a higher mean amplitude than has the rest. With speech such an amplitude peak is fou'nd to lie in the neighbourhood of 1000 cycles per second. The invention can also be applied to such a useful band, by locating the frequency corresponding to the peak amplitude, in the mid-portion of the operating band. This offers the special advantage that the points of greatest low frequency amplitude need not be considered in the determination of the control or cut-off limits of the systemand that the transmission system can bel most advantageously utilized.

In Fig. 2, a dierent distribution of frequencies is shown as an example. The width of the operating band has been chosen at 4000 cycles per second and the useful bands have been arranged so as to enable in each case such frequencies as correspond to 1000 cycles per second in the speech band, to fall within the mid-portion of the respective operating band (the midpoints of the several Operating bands being at 2000, 6000, 10,000 cycles per second, etc.). `As contrasted with the system shown in Fig.- 1, the useful band is situated asymmetrically with respect to the operating band. The width of each useful band has again been assumed as of 2500 cycles.

The application of the system embodying my invention, offers advantages even if` the amplitudes are Luiiformly distributed over the entire useful band, since the beat frequencies of a definite range are then also made unobjectionable.

The method by which, according to the invention, the useful bands can be shifted to the desired position within the available pass-band is shown.

in Fig. 3, as an example.

By means of low-pass filters Sp1-Spa, the con- With an additional frequencyh-fa respectively. rIn the` example illustrated, the frequencies f2, la

f4 are equal to the frequencies f5, fe, and f7 respectively, so that the same carrier frequency may be used for each two useful bands. One band of this pair is an upper side. band of this carrier frequency,` and the other useful bar id of between the two modulation stages have the same frequencies (from 3250 to 5750 cycles). The bands received at the' output of the band-pass filters B21-B23 have the values indicated in Fig. 3. The filter outputs are combined in the manner shown in Fig. 3 to form two groups, in order to prevent the interconnection of such filters as allow adjacent frequencies to pass. Each group is connected to a special amplifier V1 or V2. Through another amplifier V3 and a high-pass filter K, the transmission band corresponding to the conversations in the circuits I-8 is connected to the line L. A channel N including the ampliv fier Vn and the low-pass filter Spn is designed'to transmit a single low-frequency band and is also connected to the line L. In much the same manner, provision can be made for another channel not shown in the drawings, which is derived from the intermediate modulation stage, so that one obtains a pass-band of about 30,000 cycles per second including, altogether, 10 component bands of 3000 cycles per second each.

, In Fig. 5 I have indicated diagrammatically one Way of producing the result stated above, namely, that the frequency corresponding to the peak amplitude will lie in the mid-'portion' ofthe operating band. In Fig. 5 this result is obtained by a shiftingcf the useful frequency band within the operating band. Specifically the shift amounts to 1000 cycles, corresponding to the showing of Fig. 2 as compared with that of Fig. 1. Fig. 5 shows only the three upper. channels 6, 1, 8 of Fig. 3 with their low-pass filters Spe, Sp1, Sm and 4the modulators Mis. M11, Mia. It will be understood that the elements to the right of said modulators will be the same as illustrated in Fig. 3. In order to obtain the shift of frequency, I have shown an additional modulation stage, consisting of the modulators Me, M'z, Ms in conjunction with the band-pass filters Be, Bv, Ba together with an additional carrier frequency generator fo producinga frequency of 1000 cycles.

' At' the receiving end, thev original frequencies are regained by an analogous process. From the incoming frequency mixture, a frequency band somewhat wider than the operating band under consideration, is picked out by means of normal filters. yThe addition of the corresponding carrier frequency produces among others a side-band corresponding to the frequency band obtained at the transmitting end by means of intermediate modulation; the border ranges of this side band, however, contain portions of adjacent frequencies, i. e. neighbouringconversations. By means of a filter having a steep characteristic I pick out the prescribed range ofthe intermediate frequency band, thereby reliably separating the conversations from one another. A second demodu- A lation at 3000v cycles per second, corresponding to vthe intermediate frequency fz at the transmitting end, then enables the original speech band to be P obtained in the usual manner. It will easily bey seen that at the receiving end as well, a single demodulation would impose very severe demands on the lters which might be diflicult to fulfill.

The system under consideration can, of course,

. aioac'ra i v also be used for two-way trame, that is for. du-l plex operation. The channels I, 2 an'd 3 can, for instance, be used, according to Fig. 3, for trailic in one direction and the channels 6, I and 8 for that in the opposite direction. The idleness or suppression of the channels 4 and 5 is of advantage, as the interval thereby obtained between the two groups is sufficient reliably to separate the directions of speech byv means of iilters.

It may be noted that, incertain cases, it may be recommendable vto effect modulation simul-V taneously at the transmitting end, lso as to enable the information to be cyphered (by inversion or the like) for purposes oi7 secrecy.

It will ibe noted that, as shown in Fig. 3, the

circuit has a plurality of transmission branches' (eight of them in this particular case). To each of these branches, indicated at I to 8 respectively, I supply intermediate carrier currents through the'modulators M11 to Mis, said intermediate c arrier currents having a definite frequency .fz (in ond modulators Mai-to Mza, branch carrier currents the frequency of which is different from the frequency of saiddntermediate carrier curv rents but is an entire ,multiple of said definite frequency fz, the frequencies of the additional branch carrier currents being indicated at fi to fa.

In Fig.4 I have represented a station arrangement including both transmitting and receiving apparatus. I, y2 and 3 indicate the terminals of three transmitting circuit branches or channels,

v while 6, 1 and 8 indicate the terminals of threey receiving branches or channels; 'that is, the two groups of three channels each serve for traiiie`r in opposite directions, one group for each direction. Thereference characters in Fig. 4 are largely the same as in Fig. 3, and the arrangement is substantially the same. In view of this, detail hdescription of some of kthe elements of Fig. 4 is deemed superfluous, particularly as leg-l f endshave been applied to the various elements.

It will be noted that the elements Mie, M11 and Mis of the receivingi'group are not modulators,

but demodulators, and the same remark will apply to the elements Mza, M21, and Mza. Fig. 4 shows a divided or forked circuit arrangement,4 corresponding to the separation of the transmitting group ofbranches or channels from the receiving group; thus, V1, V2, V3 indicate amplifiers for the transmitting direction, and W4, Vs, Vav

indicate amplifiers for -the receiving direction. Fig. 4 also shows the same generator as supplying the intermediate frequency currents (fs) both tothe modulators of the transmitting portion of the apparatus and to the demodulators of the receiving portion. f

Iclaim:`` 1 l I l. A multiple transmission system comprisinga circuit having a plurality of. 'transmission branches, means for supplying to each of said branches, carrier currents of the same intermediate frequency, means for supplying to each of 'said branches, branch` carrier currents the frequencies of which are different from said intermediate frequency but lare entire multiples of said intermediate frequency, and means' for passing through said branches, transmission currents in frequency bands each of which is narrower than ples of said intermediate frequency.

2. A multiple transmission system comprising a circuit, means for supplying thereto carrier currents` covering a range of gfrequencies subdithe interval between two successive entire multi vided into a plurality of channels of equal width, the border frequencies of each of these channels being entiremultiples of the said channel width, means for supplying to each of the channels of` said circuit, transmission currents in useful fre` quency bands each of which is narrower than the respective channel, modulating means for placing the useful frequency bands within the respective channels, and additional modulating means' for determining the position of each of said bands quency bands each pf which is narrower than the respective channel, this case of 3000 cycles) In addition thereto, I `supply to the several branches, through the secintermediate modulating means having 4a common .carrier lfrequency for placing-the useful frequency bands within the respective channels, and additional modulating means for determining the position of each,of said bands relatively to the vborders of the respective channel.

4. A multiple transmission system comprising:

a circuit, means for supplying thereto carrier currents covering a range of frequencies subdivided into a plurality of channels of equal width,the

borderijfrequencies of each of these channels be-` lating means subsequent to the rst modulating means o'f 'cache group using a carrier frequency which is an entire multiple of the frequency used by the first of such successive modulating means. v

SQA multiple transmission system comprising a circuit, meansfor supplying thereto carrier currents covering a range of frequencies subdivided into a plurality of channels of equal width, the border frequencies of each of these channels'4 being entire multiples of the said channel width, means for supplying to each ofthe channels of said circuit, transmission currents in useful frequency bands each of .which is narrower than the respective channel, and a plurality of successive modulating means, for obtaining the desired position of each useful frequency band within the respective channel. each of said useful frequency bands being a side band of the corresponding car.-. rier frequency. I i

6; A multiplatransmission system comprising a circuit, means for supplying thereto carrier frequency which is anfentire multiple of the frevcurrents covering a range of frequencies sub- Ving `entire multiples of said channel width, and

means for supplying to each of said channels, through a plurality of successive modulating means, transmission currents in useful frequency bands each of which is narrower than the respective channel, said-useful frequency bands being separated fromeacn other by intervals, and a receivingapparatus including means for supplying carrier currents and demodulating means corresponding in number and character to the modulating means of the transmitting apparatus, and

also including normal filters in front of the first demodulating means and additional filters; having a considerably greater sharpness of tuning than said first-mentioned filters, located in front of the last demodulating'neans. y

8. A multiple transmission system comprising anca-iva a circuit, means forsupplying thereto carrier currents covering a range of frequencies subdivided into a plurality of channels of equal width, the border frequencies of each of these channels being entire multiples of the said channel width, 5

means for supplying to each of the channels of said circuit, transmission currents in useful frequency bands-each of which is narrower-than the respective channel, intermediate modulating means having a common intermediate carrier l0 frequency, andadditional modulating means having carrier frequencies which are' higher har- 4monies of said intermediate frequency,each of the useful frequency bands being a side-band of the carrier frequency of the corresponding chanl5 nel.

9. A method of transmission which comprises supplying to each of the branches of a subdivided circuit, carrier currents of the same intermediate frequency, and in addition thereto, branch carrier 20 currents the frequencies of which are different from said intermediate frequency but are entire multiples of said intermediate frequency, and

transmission currents in useful frequency bands each of which is narrower than the intervalbe 25:

tween two successive entire multiples of said, intermediate frequency.

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