US2226728A - Transmission device - Google Patents

Description

1940- M. A. LALANDE ETAL I 2,226,723

' TRANSMISSION DEVICE 4 Sheets-Sheet 2 Filed April 1,1939

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4 Sheets-Sheet 4 M. A. LALANDE EFAL TRANSMISSION DEVICE Filed April 1, 1959 Dec. 31, 1940.

1 W I MN Patented Dec. 31, 1940 FATE T QFFC TRAN SMI S SION DEVICE Marc Andr Lalande and Paul Francois Marie Gloess, Paris, France, assignors to International Standard Electric Corporation, New York, N. Y.

Application April 1, 1939, Serial No. 265,476 In France April 23, 1938 11 Claims.

The present invention relates to electric signaltransmission systems, particularly to systems intended for transmitting a very wide frequency band.

In transmission systems transmitting wide bands of frequencies such as systems intended to transmit television signals, difficulties are experienced in transmitting the lower frequencies, since certain transmission apparatus only transmits well the signals above a particular frequency.

The invention more specifically relates to systems in which the frequency band to be transmitted is divided into two bands A and B. The band A contains the lower frequencies which are normally badly transmitted by the transmission apparatus and the band B contains the higher frequencies. modification. The band A is modulated at a particular carrier frequency of such value that the band A is transmitted consequently as a frequency band higher than the band of frequencies B which itself is transmitted without modification.

At the receiving end the products of modulation of the band A are separated from the band B and demodulated. The original frequency band A is thus obtained and is then re-mixed with the band B which is transmitted directly and without modification, the original frequency band 30 A+B, thereby being reconstructed.

An electric signal transmission system, of the kind referred to, according to the present invention is characterised in this, that the higher band of frequencies is obtained by balancing out the lower band of frequencies by current of the same magnitude but of opposite phase in a first path, the lower band of frequencies being transmitted in a second path to a modulator of higher frequency than the signal of highest frequency, the output of both paths being applied together to the transmission channel.

This and other features of the invention will be clearer from the following description taken in conjunction with the accompanying drawings, of a television transmitting system, embodying the invention, but it will be understood that the scope of the invention is not limited to this embodiment which is capable of numerous variations apparent to those skilled in the art.

In the drawings:

Fig. 1 represents the frequency range to be transmitted;

Fig. 2 represents the division'of the frequency band to be transmitted into two bands A and B.

The band B is transmitted without Fig. 3 shows the band B directly transmitted and the products of the modulation of the lower band A;

Figs. 4, 5 and 6 show the division into two bands in accordance with the invention;

Fig. 7 shows a basic diagram of the embodiment of the invention for the division of the two bands transmitted;

Fig. 8 is a diagram employed in the description;

Fig. 9 shows an artificial line employed in the system;

Figs. 10, 11, 12, 13 and 14 are diagrams employed in order to explain the operation of the artificial line of Fig. 9;

Fig. 15 a particular embodiment of this artificial line;

Fig. 16 shows curves relating tothe above artificial lines;

Fig. 17 shows an embodiment of the invention for reception.

In the following description it is assumed for explanatory purposes, that a frequency band of three megacycles as employed in television is transmitted. This frequency band is shown in Fig. 1.

Certain devices employed transmit easily only frequencies higher, for example, than kilocycles, and the theoretical division of the original frequency band of three megacycles of Fig. 1 is provided at kilocyclesas shown in Fig. 2 for reasons which will appear later on.

The band A comprising the frequencies from 0 to 120 kilocycles is modulated after separation at a carrier frequency of 3.5 megacycles so that no modulation product falls in the band B from 120 kilocycles to 3 megacycles which is transmitted without modification.

The two sidebands of modulation obtained by modulating the band A with 3.5 megacycles are represented by A1 and A2 in Fig. 3.

This theoretical case having abrupt division of the frequency band to be transmitted into two frequency chosen, which in the particular case is 120 kilocycles, there is an intermediate range of about 60-200 kilocycles in which the attenuation of the band A increases progressively, while that 5 of the band B also decreases progressively.

In order to eifect this separation without introducing harmful distortion a low pass filter is provided in connection with the circuit shown schematically in Fig. 7. The signals constituting 10 the complete band to be transmitted arrive at V. The current is then divided into two paths, one transmitting the band A and the other the band B. The path serving for the transmission of the lower band A contains a low-pass filter I which 15 practically introduces neither phase distortion nor attenuation up to a frequency of 60 kilocycles.

Such filters which practically introduce no appreciable phase distortion or attenuation dis- 20 tortion up to 0.5 of the cut-off frequency are wellknown in the art. In particular referencemay be made to a low-pass filter composed, for example, of two so-called constant k or proto type cells.

The output of the filter I is applied simultaneously to a modulator 2 and to a valve 3 the plate circuit of which is connected in parallel to the plate circuit of a valve 4 connected in the other path of the circuit and transmitting the higher frequency band B.

The signals from V which pass to the said other path pass through a valve 5, a retardation line 6 which introduces a phase lag equal to that of the filter I, and then to the valve 4.

By reason of the presence of the valve 5 constituting a phase inverting circuit the currents arriving at the valve 4 are in phase opposition with those which have passed through the filter I ,and arrive at the valve 3. By an adjustment of the gain of the valves 3, 4 and 5 it is thus possible to obtain that for all the frequencies which pass through the low-pass filter I without phase distortion or attenuation distortion, the currents 45 in the common plate circuit of valves 3 and 4 are -of equal amplitude and of opposite phase, and cancel each other. There is thus no transmission for frequencies below 60 kilocycles which pass without phase or attenuation distortion through so the filter I For frequencies above 200 kilocycles the filter I entirely cuts off the transmission to the valve 3 and, consequently, the signals passing through the valve 5, the retardation line 6 and the valve 4 55 are normally transmitted without any alteration.

The passage of the intermediate frequencies between the frequencies transmitted without attenuation by the filter I (up to 60 kilocycles) and the frequencies completely eliminated (greater 60 than 200 kilocycles) is explained by Figs. '7 and 8.

Referring to Fig. 8, the vector OV represents the incoming signal V.

The portion of the current passing through the valve 5, the retardation line 6 and the valve 4 65 arrives on the line circuit connected to the plate circuit of this valve 4, and may be represented by the vector OB.

That portion of the current passing through the low-pass filter I is attenuated and retarded,

70 and is divided into two in the output of the filter I. A portion represented by the vector 0A passes directly through the modulator 2. Another portion represented by OA' of the same amplitude but having a phase difference of 180 with re- 75 spect to 0A; due to the'presence of the valve 3,

arrives at the common output circuit. With the portion directly transmitted represented by the vector OB, this vector 0A gives a resulting vector 00 which is transmitted, and the amplitude of which depends upon the transmission through the filter I. The vector OA decreases in proportion as the attenuation of the low-pass filter I increases from 60 to 200 kilocycles. On the other hand for frequencies lower than 60 kilocycles, for

which the retardation in the filter II, and in the retarding line 6 are the same, and for which the filter I transmits without attenuation the vectors 0A and OB are equal and opposite, owing to the fact that the two paths contain the one an even number of valves, and the other, an odd number of valves. There is no effective transmission for these frequencies from these two paths. It is only through the path containing the modulator 2 that these'frequencies are transmitted.

The retardation line 6 (Fig. 7) should introduce practically without attenuation a delay in transmission equal to that introduced by the lowpass filter I. It is possible to employ for this purpose a retardation line of the type shown in Fig. 9. Such lines can also be employed with advantage when it is desired to obtain a constant retardation, that is to say, a variation of phase of the current transmitted in proportion to the frequency.

The retardation lines employed hitherto comprise inductances in series and condensers in shunt, and constitute a low-pass filter shown in Fig. 11' of the cut-off frequency:

and of impedance f Z0: 62'

continually withthe frequency. The difference of transmission time reaches 2.5% of the mean time of transmission for a frequency equal to half the cut off frequency. For high frequencies it has been recommended to employ filters derived from the proto-type filter shown in Fig. 11. The values of the elements of these so-called filters are given in Fig. 11 as a function of a co-efficient m. For a co-eificient m such that is in the vicinity of 1 more uniform times of transmission are obtained. A particularly favourable value of m is 1.275.

In such filters it is found that the inductance to be placed in series with the condenser-of the shunt arm must be negative.- This negative inductance may be obtainedby a well-known theory by means of a mutual inductance between the two series self-inductances as indicated in Fig. 12 the equivalent circuit of which is given in Fig. 13. The connections of the self-inductances must be such that the latter are additive in series. Under these conditions the equivalent self-inductance in the series branch is negative and equal to the co-efiicient of coupling M. I

This method already permits the amount of space taken up to be reduced by reducing the number of elements.

Fig. 9 shows an artificial line in which a coupling is established between all the adjacent se- 6 ries inductances contrary to the arrangements hitherto used.

It is thus possible to arrive at a particularly advantageous construction in which a magnetic or non-magnetic core is uniformly loaded with turns,

10 for example, evenly distributed with regularly spaced tappings, to which the shunt condensers are connected. With a non-magnetic support the ratio of the length of an element (between two tappings) to the diameter of the coil determines the coupling between two successive elements, and consequently thevalue of the co-efiicient m mentioned above which comes into the calculation.

It is possible generally, and this is confirmed by experience, to ignore the coupling effects between the non-adjacent elements. For the value of m, mentioned above, of 1.275 it is found that this ratio of length of an element to the diameter of the coil must be equal to 1.7 which corresponds to acoupling co-efficient of 0.119 between the elements.

The impedance and the cut-off frequency are determined by the value of the condensers employed, the diameter of the wire employed, the

diameter of the support. The termination of such a line may be effected only in mid-series. In this case the coupling between the last section and the final half-section must be equal to 2 times the coupling between consecutive elements, and is thus 0.168 in the example described. It is then necessary to employ a different diameter of wire for the final half-section. It is also possible to terminate the line in mid-series followed by a half-section of a proto-type filter. In

this case it is practically possible to constitute this combined section by the same Winding as the following sections.

This method has the drawback of introducing a section having a slight distortion of the transmission time. It is, moreover, possible to compensate the slight distortion thus produced by utilising sections having values of m slightly different from the value of 1.275, for example, slightly higher.

Fig. 1 t shows an embodiment of a line terminating at one end in mid-series, and at the other end in mid-shunt. The values of various elements as a function of the cut-off frequency ,fc are found as follows:

Since the series self-inductance of the prototype filter whence the co-efiicient of coupling 7622 between 0 two consecutive sections L2, is:

' 5 The left-hand termination shown in Fig. 14 is an example of a termination by. a mid-series cell the self-inductance L1 of which is equal to The right-hand termination shown in Fig. 14 is an example of a termination in mid-shunt of a cell of proto-typefilter. To the adjacent midseries termination a half cell of proto-type filter has been added in such a way that the self- 15 inductance of the termination is equal to and the shunt capacity of termination is equal to 29 Fig. 16 gives the values of Two for various values of m, 1- being the time of transmission, we the cut off pulsation. Y

The construction of artificial lines in accordance with the above characteristic is obviously capable of variations. In particular, it is not necessary forthe windings to be adjacent; there may be a space or interval between the coils, but the coils must be regularly spaced. There may be a modification of the optimum ratio of the length of a coil plus an interval, to the diameter of the 011. 35

In order to reduce the amount of space occu pied it is also possible to replace the condensers by a regularly distributed capacity by effecting, for example, the winding on a partially conducting support 2! as shown in Fig. 15. In this case the turns of the winding may be'spa-ced apart (Fig. 15A) or carried out in the form of groups of turns (Fig.15B). Reverting to Fig. 7, there is also shown auxiliary members, such as the oscillator 1 associated with the modulator Z, and 545 whose frequency of 3.5 megacycles modulates the band A (0 200 kilocycles). The two side-bands are filtered by the pass band filter 8 and by the mixing valve 9 passed onto the repeater l0 common to all the branches and thence on the cable or line I I.

A filter 12 may be associated with the retardation line 6 in order to eliminate frequencies greater than 3 megacycles, and which might fall within the range of frequencies of the band A modulated at 3.5 megacycles and which extends from 3.3 to 3.7 megacycles approximately.

Fig. 1'7 represents a simplified diagram of an embodiment of a receiver system suitable for receiving the divided band of frequencies.

After a general repeater or amplifier I3 the currents received are applied to two valves M and I5 the adjustable amplification of which permits the levels to be adjusted in the two paths of the circuit. 4

In the lower path (corresponding to the reception of the band A) is a pass band filter l8 identical with the pass band filter 8 of the transmitting end which passes the two bands due to the modulation of the band A by the frequency of 3.5 megacycles. A stage amplifier I! if neces sary permits a current of sufficient intensity to be applied to a double diode l8. After demodulation the current passes through a low-pass filter IS the cut-off frequency of which is higher 117.5

cies from 3.3 to 3.7 megacycles and corresponding to the band A modulated to 3.5 megacycles.

A band'elimination filter instead of the lowpass filter has the advantage of having for the same lower cut-off frequency a'lower phase distortion than that of the low-pass filter.

A retardation line 22 serves to equalise the periods of transmission in the two branches 'taking into consideration the retardation produced by the filters l6, l9 and 2|, it may be of the type of the line 6 (Fig. 7) already described in detail with regard to the transmitting end. The valve 23 serves as an output valve, and is jointly connected with the valve 20 in the lower .branch transmitting the band A with a common output valve 24.

By the same means which has been explained with regard to the transmission circuit, the original frequency band, from 0 to 3 megacycles is thus reconstructed by the addition of the two bands A and B from the two circuit paths.

If necessary, in order to re-establish the phase relations desired an auxiliary valve 25 may be inserted in one branch of the circuit in order to change the phase by 180". v

It is clear that although the invention has been described in relation to the specific example it is in no way limited thereto, and is capable of numerous variations without departing from its scope. Y

- What is claimed is: i 1. An electric signal transmission system for the transmission of signals extending over a wide frequency range, comprising two paths in parallel, the first of said paths containing means for selecting the lower band of frequencies,

means for feeding to the second path a portion of the lower frequency currents from said first path equal in amplitude and opposite in phase to the corresponding currents in said second path, means in said first path for modulatingthe other portion of the lower frequency band by a frequency in the higher region of the frequency spectrum beyond'the highest signal fre-. quency, and means for applying the outputs from the two paths to a transmission channel;

2. An electric signal transmission system for the transmission of signals extending over a wide range of frequencies comprising two paths in parallel, the first of said paths including-a low pass filter to pass frequencies falling withinthe lower band, means for feeding a portion of the output from said filter to the second path equal in magnitudebut opposite in phase to the current-s of correspondingfrequencies-in said secondpath, means for modulating the other portion of the output from said filter by a higher frequency beyond the highest signal frequency and means for applying the outputs from the two paths to a transmission channel.

'70 3. An electric signal transmissionsystem for the transmission of signals extending over a wide,

an auxiliary path between the output of the selecting means in the first path and the output of the second path, means in either the auxiliary path or the second path for producing a phase shift of 180 in the currents of the frequencies in the lower band in the one path with respect to the other, means in the first path for modulating the lower band of frequencies at a frequency higher than the highest signal frequency, and means for applying the outputs from the first and second paths to a transmission channel.

4. An electric signal transmission system for the transmission of signals extending over a wide range of frequencies, comprising two paths in parallel, the first of said paths including means for selecting the lower band of frequencies, the second path including means for passing the Whole range of frequencies and means for retarding the lower band of frequencies by the same amount as the said selecting means, an auxiliary path from'the output of said selecting means to the output of said second path, amplification means in said auxiliary path and amplification means in said second path adjusted so that the lower frequency band currents in the outputs of the auxiliary and second paths are equal and means in one of said paths for adjusting the phases of the currents therein in opposition to the currents in the other path, means in the first path for modulating the lower band of frequencies at a frequency higher than the highest signal frequency, and means for applying the outputs from the first and second paths to a transmission channel.

5. An electric signal transmission system according to claim 4, in which the number of thermionic valve amplification stages in the second path and in the said auxiliary path differ by an odd number. Y

6. An electric signal transmission system for the transmission of signals extending over a wide frequency range, comprising two paths in parallel, the first of said paths including a low pass filter, means for modulating the band of frequencies passed .bysaid filter at a frequency higher than the highest signal frequency transmitted, and apass-band filter to the modulation products, the second of said paths passing all frequencies and including a retardation line for producing the same retardation as the low pass filter, an auxiliary path from the output of said low pass filter to the output of said second path and including means for producing a phase shift with respect to the currents in the said second path of 180 and for rendering the magnitude of the currents in the said auxiliary path equal to the magnitude of the currents in the second path, and means for applying the outputs from the first and second paths to a transmission channel.

7. An electric signal transmission system ac cording to claim 6, in which the retardation line consists of a series of cells of T formation, each cell comprising series induetances in the series arms and a negative inductance and capacity in the shunt arm.

8. An electric signal transmission system according to claim 6, in which the retardation line consists of a'series of cells of T formation, each cell comprising series inductances in the series arms, and a capacity in the shunt arm, the said series inductances being such that the mutual inductance therebetween forms a negative inductance in series with the condenser in the shunt arm.

9. An electric signal transmission system according to claim 6 in which the retardation line consists of a series of cells of T formation, each cell comprising series inductances in the series arms, and a negative inductance and capacity in the shunt arm, said negative inductance being formed by the mutual inductance between the two series inductances, which comprise a single coil wound on a core and provided with taps connected to the shunt condensers, the ratio of the diameter to the length of the winding between two successive taps being so chosen that the mutual inductance between two adjacent coil elements has a suitable value, the inductance of each section and the capacity of the shunt condensers being such that the iterative impedance of the line has the desired value, and the cut-off frequency is about 1.5 times the highest frequency to be transmitted.

10. An electric signal transmission system according to claim 6 in which the retardation line consists of a series of cells of T formation, each cell comprising series inductances in the series arms, and a negative inductance formed by the mutual inductance between adjacent ones of said series inductances, and a condenser in the shunt arm, the said series inductances comprising regularly spaced lumped windings on a common core.

11. An electric signal transmission system according to claim 6 in which the retardation line consists of a series of cells of T formation, each cell comprising series inductances in the series arms and a negative inductance formed by the mutual inductance between adjacent ones of said series inductances and a condenser in the shunt arm, said condenser being comprised by the capacity between at least part of the windings of said inductances and a metal electrode.

MARC ANDRE LALANDE. PAUL FRANQOIS MARIE GLOESS.

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