US2009438A - Carrier wave transmission system - Google Patents

Description

Jfll 30, 1935.

CHANNELS NOS. 7O 8 'CHANNELS NOS. T0 I2 WNNELS "05.18 7080 /vos.:/ was CMNNELS CHANNELS Mos/3970142 Filed July 31, 1931 2 Sheets-Sheet l '11 I26 KC.

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y ATTOPNEK 2 Sheets-Sheet 2 H. w. DUDLEY CARRIER WAVE TRANSMISSION SYSTEM Filed July 31, 1931 July 30, 1935.

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/N [/5 N TOP A T TORNEV FREQUENCY H. W DUDLEY BYJ V .T V M s M M T A; m E IN m a B c M a m. L 4 4 A 5. w 6 H L m P N 0. I1 6 F A2 h M mm-3 $20G J a J m m F r v s M M N W m A \Al/l m w (N N G 2 E s I M F M m FREQUENCY Patented July 30, 1935 UNITED STATES PATENT OFFICE Bell Telephone Laboratories, Y., a corporation New York, N.

Incorporated, of New York Application July 31, 1931, Serial No. 554,206

6 Claim.

This invention relates to multiplex transmission systems-and more particularly to multiplex systems employing carrier waves.

It is an object of this invention to increase the number of carrier wave channels that may be accommodated in a given range of frequencies, and more particularly to decrease the frequency interval between adjacent channels.

Another object of the invention is to extend manyfold the frequency range of carrier wave systems without increasing the complexity of the channel-separating circuits thereof.

A more specific object of the invention is to translate a plurality of signal channels by a single process of modulation to closely adjacent positions in a carrier frequency spectrum extending upwards of a hundred thousand cycles per second and to translate these carrier frequency channels subsequently by a single process of demodulation to their original frequency positions.

Other objects and the various features of the invention will appear hereinafter in the description of a specific embodiment of the invention.

In a multiplex transmission system employing carrier waves, several factors unite to impose a limit on the number of carrier wave channels that can practically be superposed on a single pair of conductors. One is the increasing attenuation with rise in frequency that is met in the transmission line. With the lines commonly in use, a frequency of the order of forty to one hundred thousand cycles per second has been found the highest it is desirable to employ, the increasing difllculty in preventing cross-talk between adjacent lines militating against extension of the frequency range. By suppressing one of the two side bands resulting from the modulation process, the width of the signal band to be transmitted has been reduced. It is also common practice to suppress the carrier wave during the modulation process and to supply it in the de modulation process from a local source. There is also to be considered the frequency spacing of the carrier waves, or more pertinently, the frequency interval between the signal bands in adjacent carrier wave channels.

In determining the spacing between bands, account must be taken of the fact that the selectivity afforded by filters for selecting a particular band of frequencies to the exclusion of others is proportional rather than absolute so that they are effectively less selective at high frequencies than at low. A dissipational filter having a given ratio of reactance to resistance, such as shown for example in G. A. Campbell Patents 1,227,113

and 1,227,114, dated May 22, 1917, which might require a spacing of 1,000 cycles per second between bands at a frequency of 30,000 cycles per second, would require a proportionately greater spacingat 60,000 cycles per second, viz. 2,000 5 cycles per second. If it were thus made necessary to increase the spacing between bands as the carrier frequencies became higher,'it is readily seen that a condition would quickly be reached where the interval between bands would exceed 10 the width of the band and the available frequency range would be utilized very inefliciently. It

is possible and has been the practice to place a number of filter sections in tandem to improve the selectivity, but long before the selectivity be- 15 comes great enough to separate channels spaced perhaps 1,500 cycles apart at frequencies of the order of a hundred thousand cycles per second, the signal distortion caused by the filters as a result of their discrimination against the extreme 2 frequencies of the signaling band, becomes intolerable. In a long transmission system it may be desirable for switching purposes to reduce the signals impressed on the carrier waves to their normal frequencies at a number of points, each 25 time reapplying the signals to carrier waves. Since any cutting of the side band by the filter is multiplied at each successive point, a high grade system requires a very nearly flat frequency-attenuation characteristic in the pass band of 30 each filter. A deviation from linearity of not more than 1 decibel over a 2,500 cycle band is contemplated in applicant's system.

Heretofore, the range of frequencies actually employed in carrier telephone systems has been so 5 restricted that the filters required but small waste space between channels. In attempting to translate a plurality of low frequency channels to closely adjacent positions at much higher points in the frequency spectrum, however, and subsequently 40 to restore the translated channels to their original frequencies, the lack of selectivity of the filters becomes a serious limitation. It has been proposed to obviate this difficulty by employing successive processes of modulation and successive 45 processes of demodulation. In the first stage, the low frequency channels are divided into several groups. The channels in each group are then applied to respective carrier waves of relatively low 50 frequency, perhaps of the order of thirty thousand cycles per second, which differ from each other by little more than the width of the signal bands to be created. At these frequencies the illters are very effective in separating the bands, 55

so that the side bands to be eliminated can be so effectively suppressed that they do not interfere with the transmitted'side bands of adjacent channels. In the second stage of modulation, the several groups of carrier wave channels are translated to respective positions in the wide range of frequencies to be applied to the transmission line. The groups are not as closely spaced together as are the channels, however, since at the higher frequencies the filters require a greater frequency spacing between the waves which they are to separate. The resulting band of channels, therefore, has in it many intervals that cannot be utilized for signaling purposes. The inverse process is used at the receiving terminal of the system.

While the frequency translating systems used heretofore have been incapable of utilizing completely even a restricted frequency range, their limitations become especially important when an attempt is made to take advantage of the wide range of frequencies which can be efficiently transmitted over a pair of coaxial conductors. With reasonable spacing of repeaters a useful frequency range of a million cycles or more is practicable with this type of transmission line. Even with half this frequency as an upper limit, more than a hundred carrier telephone channels are available, provided that it be feasible to space the channels uniformly close together.

In accordance with applicants invention a carrier wave system is provided wherein, even at fre-- quencies many times greater than now commonly employed on lines, the respective bands of signal waves may be separated at any frequency level throughout a wide frequency spectrum of transmitted waves. A feature of applicants system resides in the use of particular selective circuits. The latter are of a band passing type incorporating piezo-electric crystals, of quartz, for example. The selectivity of the preferred forms of these filters is such that in a system where the highest frequency is above half a megacycle per second and the width of the respective signal bands to be transmitted is 2,500 cycles per second, a spacing of 3,500 cycles per second or less can be maintained between carrier waves. The unused space between channels is 1,000 cycles, i. e., of the order of only one or two tenths of one per cent of the highest frequency transmitted. To separate a number of incoming bands into respective channels at a terminal station it is necessary only to connect these channels in parallel through a plurality of these filters. To divert any particular channel from the main transmission line for transmission over a branch line, similarly, only the selective devices are essential. This avoids the "process that would be required in systems\proposed heretofore of translating a group of channels to a lower frequency position, separating the desired channel from the others of its group and then restoring both, separately, to their original high frequencies.

- The nature of applicants invention will appear more fully in the following description of a system embodying it in specific form. While the signaling sources are indicated as telephone and television apparatus, it will be obvious that waves frgm other signaling sources can be impressed on the carrier waves. Furthermore, the carrier wave system of applicants invention may be incorporated in a system involving double frequency translation so that the advantages inherent in both systems may be combined to still further extend the frequency range over which the selective circuits can effectively be employed. In the drawings,

Fig. 1 shows schematically one terminal of a combined telephone and television carrier wave transmission system in accordance with applicant's invention;

Fig. 2 shows a preferred form of the piezo-electrio selective circuits;

Fig. 3 represents a piezo-electri'c crystal;

Fig. 4 shows the equivalent "electrical circuit thereof;

Fig. 5 shows a preferred form of repeater; and

Figs. 5A and 5B show graphically the successive steps of equalization and amplification occurring in said repeater.

Referring now to Fig. 1 there is shown a terminal circuit for effecting two-way frequency translation of signals between a plurality of relatively low frequency signaling circuits and a pair of transmission lines adapted to transmit carrier waves extending over a wide range of frequencies. Each of the low frequency channels, which are represented here as telephone and television signaling circuits, is associated with individual modulating and demodulating apparatus and through these, with the high frequency transmission line. A separate conductor pair is shown for each direction of transmission, although with a single pair of conductors different frequency ranges could be used for this purpose, in a manner well known in the art. The telephone lines are represented, and their respective associated modulating and demodulating apparatus, are divided into a plurality of groups in order to simplify the problem of emciently connecting them to the transmission line, as will be more fully explained hereinafter. Signals transmitted between the several telephone lines and the carrier frequency line are otherwise subjected to very similar treatment.

Telephone signals from line Z1 enter hybrid coil H, pass through the output winding of the latter to a transmitting channel which includes a low pass filter LPF. The latter, which may be of the type disclosed in the G. A. Campbell patents, supra, is designed to suppress all frequencies above the signal band it is desired to transmit. A 2500 cycle band extending from perhaps-250 to 2750 cycles per second is satisfactory. To modulator M, which may be of the balanced type disclosed in J. R. Carson Patent 1,343,306, issued June 15, 1920, is applied this 2500 cycle band of speech signals together with a carrier wave supplied by high frequency generator G1, so that a speech modulated carrier wave results.

The carrier wave applied to the modulator M of the first channel has a frequency of 21 kilocycles per second. The carrier wave applied to the modulator M in the adjacent channel is 3.5 kilocycles higher. In succeeding channels similarly the carrier waves are increased in 3.5 kilocycle steps, the last channel, the 138th, having a carrier frequency of 500.5 kilocycles per second. In the process of modulation, the carrier wave is suppressed by virtue of the balanced arrangement of the modulator circuit. One side band, preferably the upper one, also is suppressed, as by means of a succeeding band pass filter EBF1. At 60 kilocycles per second an electrical band filter of the type disclosed by Campbell supra is satisfactory, and, as indicated, such filters are used in channels I to 8 where the highest carrier fre- 'l1, 1:, etc., of which one hundred and thirty-eight quency is 56 kilocycles per second. In channels u 9 to I! of thefirst group and in all channels of higher frequency, band passing filters CBF incorporating piezo-electric crystals as will be described hereinafter are employed. Of the two signal side bands produced, the lower one in the first channel extends from 20.75 kilocycles per second down to 18.25 and the upper one from 21.25 kilocycles per second to 23.75. Since the side band applied to the transmission line from the adjacent channel ranges from 24.25 kilocycles per second down to 21.75, the upper side band of channel I must be well suppressed if it is not to cause interference, and so with the upper side bands of the other channels.

The output terminals of the filters EBF and CBF in channels to i2 are connected to a common collecting uis CB5, which is preferably formed from a coaxial conductor having a characteristic impedance of about ohms, and which in turn is connected to transformer Ta. The impedance ratio of transformer Ta is selected so that the mean impedance of the several filters in their pass bands is matched with the impedance into which the secondary of transformer Ta works. In general, the impedances of the filters in their pass, bands as seen from the transformer decreases with rise in frequency. The mean impedance of the first twelve, comprising group A, may be of the order of 600 ohms. Outside the pass bands the filter impedances rapidly become so high as to give practically no bridging effect.

The next eighteen channels, l3 to 30, comprising group B, are similarly connected to a common collecting bus CBb. The mean output impedance of the several filters CBF in this group may be ohms; the impedance matching transformer Tb is designed accordingly. An approximately geometrically increasing number of channels is included in the groups succeeding group B, there being 36 in C and 72 in D. The number of channels to be included in each group is determined by the maximum allowable percentage deviation of the impedance of any filter from the mean impedance for which the transformer is designed, and therefore, by the maximum allowable percentage deviation of the frequency of any given channel of the group from the mean frequency of the group. The higher the frequency the greater the number of channels that may be included in each group. The secondary windings of the several group transformers Ta, Tb etc. are connected to the group collecting bus GCB, which is connected through transformer Tr to the transmitting amplifier TA and to the outgoing line LE. Amplifier TA preferably comprises a suitable number of tandem screen grid stages leading up to a final stage or stages of capacity-neutralized push-pull tubes. Across the input of this amplifier is shunted a resistance; 8000 ohms was found to be a satisfactory value in one case. Transformer T1; was so proportioned that a fixed impedance of 80 ohms was presented to the group transformers. "The transmission line LE preferably comprises a central conductor and a hollow return conductor maintained in coaxial relation by means of insulating washers or beads spaced at intervals along the central conductor. A suitable conductor of this type is described in greater detail in U. S'Patent 1,781,124, issued November 11, 1930 to H. R. Nein. The high degree of freedom from cross-talk of this type of conductor permits the assemblage of a plurality of them within a common cable sheath.

The receiving circuits are arranged in a man- -ner similar to the transmitting circuits, as shown in Fig. 1. through the receiving amplifier RA and transformer Tr to the group distributing bus GBD. The receiving channels are grouped in accordance with the frequencies of the carrier waves employed exactly in the same manner as the transmitting channels. Transformers T'a, T'b, etc., serve to match the mean impedance of the respective groups of filters with the impedance presented acros their respective primary windings. Signals passing through these transformers are applied to the respective channel distributing buses DB5, DBb, etc.

From the distributing buses each of the receiving band-passing filters EBF'1, EBF'z, CBF'io. etc. selects its particular band of modulated signal waves. The bands received may be 2500 cycles wide and spaced with 1000 cycles between their adjacent edges, as are those transmitted. Each filter may be identical with the filter in the transmitting channel using the same carrier frequency; those in receiving channels I' to 8' may therefore be of the electrical filter type and those in channels 9 to I38, of the crystal type. The succeeding demodulators DM may be of the balanced type disclosed in the Carson patent, supra. Preferably both modulator and demodulator are neutralized, as for example in the manner shown in Ballentine Patent 1,560,332, November 3, 1925. The carrier wave which must be introduced to effect demodulation may be applied from the same source as used in conjunction with the associated local modulator. The telephone signals resulting from the demodulation pass through a low pass filter LPF to the input terminal of hybrid coil H, whence they are applied to the telephone lines Z1, In etc. I

In Fig. 2 is shown schematically a preferred form of the crystal band-passing filters utilized in accordance with applicants invention. The filter per se, is the invention of W. P. Mason and together with the theory underlying its operation and design is fully disclosed in his application for patent bearing Serial No. 489,268, filed October 17, 1930. In the diagram, L1, L2, L3 and L4 are inductances of equal values connected in series with the four terminal leads of the filter. The condensers C2, C2, connected between inductances L1 and Lz'and between L2 and L4, respectively, are of equal capacity, as are the condensers C3, C3. The condensers C3 are shunted around the respective identical quartz crystal elements X1, X1. The diagonally connected crystals X2, X2, are likewise identical.

The proportioning of the various elements of this lattice type filter to obtain the desired transmission characteristics may be determined by calculation. When doing so the crystals may be considered as equivalent to the electrical circuit of Fig. 4. This circuit comprises a parallel branch network connected between terminals l3 and I4, one branch consisting of an inducance La in series with a'capacitance Ca and the other branch comprising a. simple capacitance Cb. The magnitudes of these equivalent elements are determined by the dimensions of the crystal as represented in Fig. 3. The length l of the crystal is taken parallel to the mechanical axis MM, the widthw parallel to the optical axis 00 and the thickness 15 parallel to the electrical axis EE. Electrodes H and I2 are applied to the, large faces of the crystal, that is, to the surfaces perpendicular to the electrical axis, preferably by the electrical deposition of a layer of silver or other metal to secure an intimate Signals arriving over line LW pass I contact over the whole surface. For a quartz crystal where the dimensions are in centimeters.

In electrical filters, the ratio Q of the reactance of the coils to the resistance thereof is a measure of the effectiveness with which the illters can transmit a selected band of waves to the exclusion of others. In filters used heretofore values of Q of the order of one hundred or two hundred were obtained, the latter figure being considered quite high. In the case of the quartz crystal filter, however, values of Q up to several thousand can be obtained. Such high Qs are obtainable in fact as to bring in another factor, viz., delay distortion, as the limiting one in the spacing of the channels. In any filter, the attenuation at the edge of the pass banddepends on the amount of resistance therein and on the number of reflections to which signals traversing it are subjected. The higher the value of Q, the greater the number of reflections, and accordingly the greater becomes the-phase difference between waves of different frequencies. Applicant has found, however, that despite the degree of selectivity required in his system and the number of filters that it may be necessary to connect in tandem in the longest circuits, the distortion caused by this phase delay is not prohibitive.

Above the frequency range required for the carrier telephone system a carrier television systenimay be added. Four transmitting and four receiving television channels or more may be provided, two of which are represented in Fig. 1. Each band of television signals may have a range of 100 kilocycles per second. A spacing of 110 kilocycles between carrier waves would be sufficient when band selective circuits using piezoelectric crystals are employed. With the television carrier wave of lowest frequency fixed at 610.5 kilocycles per second an interval of somewhat more than 10 kilocycles is left between the carrier television and the carrier telephone channels.

Signals from television transmitter 'I'Vi are passed through the low pass filter LPF to eliminate extraneous waves that may be present above the 100 kilocycle band it is desired to transmit. In the balanced modulator M the television signals are impressed on a carrier wave of 610.5 kilocycles. The crystal filter BF! suppresses the upper side band and higher products of modulation to prevent interference with the trans mitted side band of the adjacent television channel. The modulated waves from the four channels are applied to a collector bus CB, as in the carrier telephone circuit, for connection with line LE through transformer Te. The inverse process whereby carrier television signals arriving over line LW are reduced to their normal fretelevision signals.

telephone signals by noise currents in the transmission system. While the level of. the carrier telephone signals may have to be maintained at all times at least 65 decibels above the level of noise, the carrier. television signals may be attenuated to as low as 30 decibels above the noise level. This fact is utilized in the design of the repeater circuit shown in Fig. 5. Figs. A and 53 will aid in an understanding of the nature and function of the several elements of the repeater. These latter diagrams sh the energy level of telephone and television signals at successive points in the system. As applied to the transmission line, either from a terminal station or from a repeater station, all signals are at the level S1. Because of the unequal attenuation to which waves of different frequencies are subjected by the transmission line, the level of the signals arriving at the input transformer T1 of a succeeding repeater may be as represented by the solid line Si. The carrier telephone signals of highest frequency is have been attenuated down to the minimum permissible level M'r determined by the noise level N, which is 65 decibels lower. The width of the television band is such that waves of the highest frequency f3 are likewise attenuated to a minimum level Mv, which is only 30 decibels above the noise level N. To equalize the signals, 1. e., to bring them all to the same level, each might be attenuated to the level of the lowest one, viz., to Mv. This is not desirable since the telephone signals would then be only 30 decibels from the noise level and serious interference would result. Again, all signals might be amplified 35 decibels, which would bring those of lowest amplitude, i. e., those of frequency f3, to the minimum permissible telephone level My and then each signal attenuated in an equalizer to a common level MT. The amplification however would raise the signals of low frequency to an unnecessarily high level and thereby demand greater power carrying capacity of the equalizer.

In applicant's preferred form of repeater the telephone signals alone are first reduced to the level MT in an equalizer EQl to which the signals arriving over the transmission line are passed by the input transformer Ti. The attenuationfrequency characteristic of the equalizer, which is represented by the shaded area of Fig. 5A, is such that the telephone signals are reduced to the level M'r without substantially afiecting the The heavy dotted line S2 indicates the signal level at this stage. Following equalizer EQ1 is an amplifier A1. The maximum level applied to the amplifier, it will be noted, is Mr, which is considerably lower than would be the case had amplification preceded equalization. In the-amplifier, the energy level of all signals is raised uniformly to the level represented by the dotted line S3, so that the television waves of lowest energy level are brought to at least the level M'r. All waves are now reduced by the succeeding equalizer EQz to a level S4 which,'as shown in Fig. 5B is at or somewhat above the minimum telephone signal level MT. The characteristic of this second equalizer is represented by the shaded area of Fig. 5B. The succeeding voltage amplifier A: and power amplifier PA raise the signals uniformly to their original level S1 for application through output transformer To to the line.

In a system such as the present one, wherein the channels are uniformly distributed in the frequency spectrum, it is possible to obtain such a frequency allocation that the most objectionable of the modulation products created in the amplifiers fail in the frequency interval between channels. With these products thus located, appreciably more modulation is permissible than would otherwise be the case. Frequency allocation of this sort is effective chiefly because it is a relatively narrow band of frequencies within the speech band that contributes most to interchannel cross-talk. The distance of the center of this narrow, high energy band in a given carrier channel from the lowest frequency of the carrier band transmitted may be represented by d, the mean absolute frequency of this narrow band as it appears, for example, in the nth carrier frequency channel by dn, the width of the speech band by b, and the frequency interval between channels by c. The disturbing modulation products of chief concern are of the second order. Foremost among these are the summation and difference frequencies resulting from the intermodulation of the principal disturbing frequencies (1 dk, (in, etc. of the several channels J, k, n, etc. If the principal disturbing frequencies were at the center of the speech band, i. e., if d were equal to the summation frequencies and the difference frequencies resulting from their intermodulation could both be made to fall exactly in the center of the interchannel dead space, where they would have least effect on the desired signals. It would only be required that the frequency allocation of the several channels be such that the lowest frequency of any channel be expressible The frequency actually contributing the most to interchannel modulation is not the mid-frequency of the speech side band but one corresponding to a speech frequency of the orderof 1000 cycles per second. The summation frequencies and the difference frequencies resulting from the modulation of this most disturbing frequency as it occurs in the several carrier bands cannot both be made to fall at the center of the frequency interval between channels. If either these worst summation frequencies or these worst difference frequencies are made to appear at the centers of the interchannel spaces, the other will fall within the signal bands if the latter are closely spaced. A compromise can be reached, however, by making these two groups of frequencies fall equi-distantly from the center of the interchannel space, where neither will lie in the signal bands provided the dead space is at least 1000 cycles wide. The essential feature of this allocation is that the lowest frequency of any given channel be expressible as While approximately 1000 cycles is the most important single frequency as regards modulation effects and the frequency allocation can be determined from the foregoing equations with it as a basis, greater accuracy is obtainable by considering the fact that frequencies above and below it also contribute disturbing modulation products. It is desirable that the bands of summation frequencies and the bands of difference frequencies contributed by these frequencies occupy the same frequency range between channels and not be offset one from the other. At one extreme is the case where c is equal to 2b; the summation frequencies and difference frequencies just fill the interchannel space if the channels are allocated on the basis that the most I important frequency is at the center of the signal band, i. e., that At the other extreme is the case where c is zero and 1000 cycles is properly considered the most important frequency for the purposes of determining the frequency allocation. For intermediate values of c a corresponding intermediate value of the most important frequency, lying between 1000 cycles and the mid-frequency ,fm of the speech band transmitted, may be assumed. Thus, in a system where a speech band of from 250 to 2750 cycles per second is used and aparticular value 01 of 1000 is selected for c, then the frequency to be considered the most important one, and therefore to be used in evaluating d, is determined from the general expression i 100O+ (f 1000).

Substituting, we have 1000+ (1500 1000) 1100 cycles per second Then, for an upper side band system, the corrected value of d to be used in Equations (2) and (3) is 1100-250:850 cycles per second. For a lower side band system 11 is 2750-1100=1650 cycles per second. The lower side band system shown in Fig. 1 is based on a harmonic relation of the carrier frequencies. By reducing all carrier frequencies by 650 cycles (as by a heterodyning process) the requirements of Equations (2) and (3) can be met. The carrier frequencies would then be 20.35 kc., 23.85 kc., 27.35 kc., etc. A corresponding upper side band system would have carrier frequencies of 21.65 kc., 25.15 kc., 28.65 kc., etc.

The cross-talk between two adjacent carrier systems can be reduced by staggering the frequency bands of one with respect to those of the other. Where the lower side band is used in one system and the upper side band in the adjacent system and the optimum frequency allocation set forth above is observed, a certain reduction in cross-talk is therefore obtained.

While applicants invention has been described as embodied in a specific carrier wave signaling system, it is apparent that it may find application in various otherwave transmission systems within the scope and spirit of the appended claims.

What is claimed is:

1. In a wave transmission system, a multiplicity of circuits each adapted to transmit a band of speech signaling waves, means in each of said circuits to translate said bands to adjacent positions in a frequency spectrum extending to a maximum of several hundred thousand cycles per second, the frequency separation of said translated bands being of the order of one per cent of the highest frequency thereof, piezoelectric band passing filters in said circuits to effect said separation, the frequency-attenuation characteristics of said filters being fiat within their respective pass-bands to within one decibel, and a common transmission element for conductme said translated waves.

2. In a carrier telephone communication system, a high frequency transmission circuit, means to apply thereto a multiplicity of bands of carrier wave signals uniformly spaced and extending in frequency to a maximum of the order of half a megacycle per second, the separation between said bands being not greater than the width of said bands, a plurality of low frequency conducting circuits, means to separate said carrler wave signal bands comprising frequency selective devices including piezo-electric and inductance elements, said elements of said devices being proportioned to transmit a selected band of frequencies to the substantial exclusion of frequencies outside of said band, the frequency-attenuation characteristics of said filters being fiat within their respective pass-bands to within one decibel, and means to demodulate said separated waves for application to said low frequency circuits; a

3. A multiplex carrier wave transmission system comprising a multiplicity of telephone signaling circuits, modulating means associated with each of said circuits to translate the speech signals therein to individual signaling bands, said transmitted, the frequency-attenuation characteris'tics of said filters being fiat within their respective pass-bands to within one decibel.

4. In a multiplex carrier wave transmission system, a high frequency transmission circuit conveying a multiplicity of carrier telephone signal waves occupying respective, closely adjacent frequency bands, said bands being uniformly spaced in a wide frequency range lying below half a megacycle per second, a plurality of signal wave receiving circuits, a frequency-translating device between said transmission circuit and each of said receiving circuits for reducing the frequency of a respective signal wave, the frequencies of the original signal waves being so high and their separation so small that wave filters employing only inductances and capacitances would be incapable of selectively passing one of said bands without an appreciable amount of interference from adjacent bands, and a piezo-electric crystal filter connected between each of said frequency-translating devices and said transmission circuti for passing a respective signal wave band and excluding adjacent bands, the frequency-attenuation characteristics of said filters being fiat within their respective pass-bands to within one decibel, and the input circuits of said filters being conducted in bilateral energy transfer relation with each other.

5. In a multiplex carrier transmission system comprising a plurality of sources of speech signal waves to be transmitted, a signaling circuit, frequency-translating means for raising said signal waves to respective adjacent bands closely and uniformly spaced throughout a frequency spectrum extending to a maximum of several hundred thousand cycles per second, means for applying said translated waves to said signaling circuit, the separation between said bands being less than one per cent of the maximum frequency transmitted, and meansfollowing each of said translating means for separating said bands, said separating means comprising for frequencies of the order of fifty kilocycles per second and less electrical filters, and for higher frequencies filters employing piezo-electric crystals.

6. A multiplex carrier wave telephone system comprising a transmission line, at one terminal thereof a multiplicity of voice frequency telephone circuits, means for translating signals in said telephone circuits to respective uniformly spaced positions in a frequency spectrum extending from a maximum frequency of several hundred thousand cycles per second downwards to a fractional value of said maximum frequency, respective means for applying said translated nals to said line for transmission thereover, each of said last mentioned means including a crystal filter the frequency characteristic of which is fiat throughout its pass-band to within one decibel and the pass-bands of said filters in adjacent frequency channels being separated by not more than one per cent of said maximum frequency, at the other terminal of said line means for translating the carrier signals received from said line to voice frequency signals, a second transmission line, and means for translating said last mentioned voice frequency signals to respective frequency posit ons for transmission over said second line, each f said translating means at said other terminal including crystal filters having the same characteristics as those at said first terminal.

HOMER W. DUDLEY.

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