US2031528A - Wire broadcasting system - Google Patents

Description

Feb. 18, 1936.

P. P. ECKERSLEY ET AL 31 9 WIRE BROADCASTING- SYSTEM Filed July 26, 1954 Fig. 2.

28 7 PETER Pzuog ron EcKERsLEY 6a RUPERT EVAN HowARD QARPENTER BY g @ ATTORNEYS Patented Feb. 18, 1936 PATENT orrics WIRE BROADCASTING SYSTEM Peter Pendleton Eckersley, Chelsea, London, and

Rupert Evan Howard Carpenter, London, England Application July 26, 1934, Serial No. 737,120 In Great Britain August 1, 1933 9 Claims.

This invention relates to systems for distributing music, speech and entertainment or other signals, in general from a central station to a number of receivers and including systems of the kind now usually known as re-diifusion systems. The invention is particularly concerned with multi-programme systems by means of which two or more programmes are made available by impressing modulated high frequency currents upon a suitable conducting network used for distributtion purposes and now often known as wirebroadcasting systems.

It is possible to distribute several programmes simultaneously in this way by employing 'a number of different carrier frequencies for carrying the different programmes. It is desirable because attenuation of the signals increases with frequency, to make the difference in frequency between the different carrier frequencies employed as small as is possible consistent with the attainment of good quality of reproduction.

The technique of high frequency diffusion over wires differs from that of radio broadcasting, because the output terminals of each transmitting apparatus must be directly physically connected, while in broadcasting the transmitting aerials are usually neither directly connected together nor closely coupled. When the output terminals of two re-diifusion transmitters are connected in parallel, the modulated high frequency voltages of one transmitter will be conveyed to the anodes of the output tubes of the other transmitter and vice versa. If the direct current to alternating current efiiciency of the output tubes is reasonably high, it will follow that the signals from one transmitter will modulate the carrier wave of the other transmitter since some rectification will take place in the output tubes when their anode potentials are raised and lowered 'by a certain amount. One of the purposes of the present invention is to make it possible to avoid this inter-modulation phenomenon in high frequency wire-broadcasting systems. 7

Such wire-broadcasting systems, however, have other peculiar problems of their own. Thus, it is foundthatthe impedance of the distributing network isvery low compared with the impedance of the output tubes of the transmitters. This means, in effect, that a step-down transformer has to be introduced between the tube anodes and the load. Moreover, as already indicated, it is desirable in order to minimize the attenuation, to use carrier frequencies which at their lowest may be nearly audible. at a frequency, say of 10 kilocyclesper second,

If these are modulated the highest frequency of modulation may be as much as 50 per cent. of the carrier frequency. It follows from this that the high frequency power transformer used must have a sensibly fiat frequency characteristic over a wide band of frequencies and the use of ordinary single frequency tuning arrangements, such as are commonly employed in wireless apparatus, would render it impossible to transmit the required sidebands with substantially the same intensity. On the other hand, the secondary windings of the output transformers of all the transmitters have to be connected to the same load, and unless precautions are taken this would result in the severe inter-modulation mentioned above.

Yet again, it is necessary to take steps to ensure that all, or nearly all, of the power output from one transmitter shall pass into the load and shall not be wasted in other circuits. If, however, for best matching results the impedance of the distributing network is equal to the impedanceof the aperiodic power transformers, then each transmitter will have a large part of its power wasted in every other transmitter.

There is yet another problem tobe solved which is concerned with the generation of harmonics, which, if transmitted, may either beat together in the receivers or produce direct interference on other channels. Harmonics may be generated particularly in the modulator and because of the necessity of using amplifiers giving a substantially uniform performance over a wide band of frequencies, the harmonics may be strongly amplified unless precautions are taken to suppress them. Furthermore, if the amplifying tubes and the output tubes are arranged to have a reasonably high eificiency, they may themselves generate harmonics. Such generation of harmonics may, however, be minimized by operating the tubes in phase opposition.

Thus, the provision of a multi-programme high frequency system working on low carrier frequencies presents at least three distinct problems, comprising the prevention of inter-modulation, secondly the prevention of absorption of power from any transmitter by the output circuits of the other transmitter or transmitters, and thirdly the prevention of the transmission of harmonics.

It is possible to solve all three of these problems, and to that end according to the present invention, a multi-programme high frequency wire broadcasting system includes an amplifier designed toamplify a selected band of frequencies substantially uniformly, a step-down'transformer or other network designed to cause the impedance of the output stage of the amplifier to match the impedance of a distributing network, and a band-pass filter interposed between the amplifier and the distributing network. The band-pass filter may include a pair of series tuned circuits connected in series and having values such that the point of comiection between them is a nodal point of potential, and a low-pass filter may be interposed between the output stage of the amplifier and the distributing network, the low-pass filter being designed to prevent the transmission to the distributing network of harmonic frequencies generated in the amplifier. Practical considerations make it most convenient for the band-pass filtering device to be introduced between the secondary winding of the output transformer and the distributing network, but it is possible to introduce it between the output tube of the transmitter and the output transformer. The band-pass filtering device in question may include a pair of series tuned circuits having components of such magnitudes that the point of junction between them is at a minimum potential to ground, for a chosen frequency, generally the carrier frequency, and then a leg of the filter may be connected between the junction point and ground without disturbing the distribution of energy at the carrier wave frequency. In addition, rejectors may be interposed in the circuits between the amplifiers and the distributing network. In order to prevent absorption of current from other transmitters, these rejectors.

must be connected on the distributing network side of any circuits or circuit element capable of shunting such currents away from the network. The provision of the rejector circuits causes the frequency of any one transmitter to find considerable difficulty in reaching the anode of the output tube of another transmitter connected to the network.

Yet again, in order to avoid interference between two transmitters having their outputs directly connected together and feeding a distributing network by a single conductor of appreciable impedance to the frequencies in question, the output from each separate transmitter may be taken to the network through a separate cable, preferably a sheathed cable, the cores of which form the conductors from the output of the transmitter to the different conductors of the network, while the outer conductor of the sheathed cable forms the grounding point from the output of the transmitter.

In order that the invention may be more clearly understood and readily carried into effect, the output end of a high frequency re-diifusion system designed in accordance with the invention will now be described by way of example in connection with the accompanying drawing, in which:

Figure 1 is a diagram showing the connections between the output ends of three separate transmitters and a mains network, and

Figure 2 shows an alternative form of rejector circuit which may be used.

Three transmitters each generate a selected carrier frequency and have output stages A, B, C in the form of paraphase-connected amplifiers of the type described in patent application Serial No. 455,326, filed May 24th, 1930, of R. E. H. Carpenter. These output stages include tubes I and 2, the grid 3 of tube I being coupled with a preceding modulated amplifying stage through a band-pass filter not shown in the figure, and the grid of the tube 2 receiving an input in opposed phase from a tapping in the resistance connected directly between the anodes of the two tubes 2, 3. The output from the tubes I and 2 is passed through a multi-section low-pass filter 4 including series-connected inductances and shuntconnected condensers, the purpose of the filter being to prevent the transmission to the coupling transformer 5 of harmonics generated in the preceding stages of the respective transmitters. Harmonics of one of the carrier frequencies if so transmitted, might conceivably be sufliciently strong to cause interference with the signals from another transmitter having a fundamental carrier frequency equal to a given harmonic. Again, such harmonics may be picked up by radio receivers near the transmitters, or again by radio receivers using the lighting or power network D as an antenna system.

The transformer 5 has a step-down ratio and is designed to match the high inpedance of the tubes I and 2 to the comparatively low impedance of the mains network D. The material use for the core 6 of the transformer 5 must be such that it will not have a detrimental effect upon the transmission of high frequency currents.

In order to ensure the maximum transmission of power into the mains network D, it is necessary to reduce as far as possible, the: effect of any reactive impedance which may be due, for example, to leakage inductance of the secondary winding of the transformer 5 and the reactance of the mains network D. The reactance of the mains network is usually inductive, and while a condenser connected in series would cancel both the inductive reactances due to the transformer and mains network, it would be fully effective only at one frequency. The secondary winding of the transformer 5 is therefore shown connected to a band-pass filter including series inductances I and 8 and series condensers 9 and Ill tuned to the carrier frequency, a condenser I I being connected in shunt to ground from the connection common to the condensers 9 and I5. It will be seen that as the secondary winding of the transformer 5 is in series with the inductance I and the condenser 9, the leakage inductance due to the secondary winding forms part of the series circuit of the band-pass filter. Again, the inductive reactance due to the mains network is in series with the inductance 8 and condenser I0, and by thus making these undesirable reactances part of the bandpass filter circuit, they are effectively disposed of.

The differing impedances at the sending and receiving ends of the band-pass filter may be accounted for by selecting condensers 9 and III of unequal values in order to ensure that the point of connection between them is as nearly at a node in potential as possible. The band-pass filter, when so adjusted, serves to transmit currents of frequencies equal to that of the carrier wave of the particular transmitter with one or both sets of side bands added. However, while the bandpass filter does not transmit to any appreciable degree outside the range of carrier frequency with one or both sets of side band frequencies, it does, in fact, tend to behave differently at different frequencies within that range. The response curve representing output plotted against frequency exhibits a cusp representing a reduction in response at a frequency which occurs mid-way in the band of frequencies transmitted. At such a frequency, the sending and impedance of the filter rises. The transformer 5, in conjunction with the preceding tube circuits, may be therefore designed to give bad regulation, that is to say, high internal impedance, and then the voltage of the output transformer 5 will tend to rise at the frequencies at which the cusp mentioned above occurs, so that the cusp in the curve will be less pronounced. In order to emphasize the bad regulation, the transformation ratio of the transformer 5 may be chosen to be somewhat less than that which gives optimum matching of impedance. This introduces added asymmetry in the bandpass filter, and it is then further necessary that the condensers 9 and II) should be unequal in value.

The band-pass filter assists in preventing the transmission of harmonics and also assists in preventing inter-modulation between the respective transmitters A, B and C, because as it is tuned to pass frequencies between the respective carrier wave frequencies and the highest side-band frequencies, it presents a high impedance to frequencies outside this band, and therefore highly attenuates such frequencies. By multiplying the separate sections of the filter, it is possible still further to attenuate the frequencies it is desired shall not be transmitted, but attenuation of the carrier frequency then becomes appreciable, due to resistance.

If, in order to ensure efliciency, the image impedance of the band-pass filter of the transmitter A is equal to the terminating or load impedance, the impedance of the output end of the filter is so small at different frequencies as to involve absorption of power from the other transmitters B and C feeding contiguous frequencies into the mains network D.

It will be seen that all the transmitters A, B and C are connected to a common mains network D, the currents from a given transmitter having alternative paths either into the mains network D or through the band-pass filters of the other transmitters. If power from one transmitter does pass into the circuit of another transmitter, not only does wastage of power occur, but the anode voltage of the tubes of one transmitter may be varied in accordance with the combined modulation of the other transmitters connected to the network D. inter-modulation may cause interference in respect of each subscribers receiving and reproducing equipment, and in order to prevent both intermodulation and wastage of power, rejector circuits l2-l3, I4l5, and I6--l'l are inserted in the feeders I8 from the transmitters to the network D. The rejector circuits have a high impedance at the frequencies it is desired to reject. Thus, the filters l2 and I3 are designed to pass intothe network D the carrier and side-band frequencies from transmitter A, but the filter I 2 will present a high impedance to the carrier and side-band frequencies from transmitter B, while the filter l3 will presenta high impedance to the carrier and side-band frequencies from transmitter C. Similarly, the filters l4 and I5 will pass into the network D the carrier and side-band frequencies from transmitter B, while presenting a high impedance respectively to the carrier and side-band frequencies from the transmitters A and C. Again, the filters l6 and I! will pass into the network D the carrier and sideband frequencies from transmitter C while presenting high impedances respectively to the carrier and side-band frequencies from transmitters A and B. It will be understood that as the rejector circuits look like inductances or capacities to the frequencies they are designed topass, the series components 1, 8, 9, and IU of' the band-pass behave aperiodically.

filter must be selected of such magnitudes as to counter-balance the left-over reactance of the 'rejector circuits.

As already indicated, the feeders Hi from the three transmitters A, B, and C to the common network D are preferably separated for each transmitter and only connected at the network load. In other words, it is preferable that there should not be any impedance common to all the transmitters except the impedance of the network D itself, which is inevitable from the nature of' the problem. It does not matter if the separate leads are self-inductive, because their re actance is counter-balanced by the series condensers in the band-pass circuits over the band of frequencies passed, but they should be free from mutual inductance. The feeders l8 are low resistance cables each having cores 19 which form the conducting leads to the network, while the outer sheath 20 of the cable forms'the ground point from the output of the transmitter. The separate conductors 2|, 22, 23, and 24 of the mains network D are capacity-coupled by con- 7 densers 25, 26, 21, and 28 to the transmitters. It

will be seen that the condensers 25, 26, 21, and 28 supplement the series condenser l and in some circumstances, depending upon the capacity in that leg of the band-pass filter, the condenser [0 may be omitted. The feeder cables l8 may take the form of single core cables and in this case the condensers 25, 26, 21, and 28 are connected between the feeder cables and the mains network, at the opposite end of the cable from that shown in the drawing.

It will be understood that various modifications may be made within the scope of the invention. Thus, for example, although the band-pass filter has been shown connected in the secondary circuit of the output transformer of each transmitter, it may be connected in the primary circuit of that transformer. Again, it may be included partly in the secondary circuit and partly in the primary circuit. Actually, instead of a transformer, any other form of impedance transforming network may be employed. Again, instead of the particular form of filter shown, an arrangement may be used including a series condenser, the capacity of which, together with the inductive reactance due to leakage inductance of the transformer and the conducting leads, forms a series tuned circuit which behaves at the resonant frequency as a pure resistance sothat maximum power passes to the mains network D. The same effect can be obtained by adding additional inductance and reducing the size of the condenser to correspond. This arrangement, however, forms a tuned circuit which does not By replacing the condenser by two condensers in series having the same effective capacity and of such relative magnitudes that there is a minimum potential to ground at the junction of the two condensers, it is possible to connect between this junction and ground, a condenser or an inductance or both a condenser and an inductance either in parallel or in series without disturbing the distribution of energy at the carrier wave frequency. In this way, an effective nodal point can be obtained by providing equal condensers and two unequal added inductances between them.

In Figure 2, a preferred form of rejector circuit is shown. 1 This rejector circuit is in the form of a band-pass filter designed to ensure that carrier and all side-band frequencies are rejected.

Rejectors in the form of single tuned circuits as shown at 12 [1 in Figure I, prevent absorption of power from one transmitter by another, and also prevent inter-modulation by the carrier and relatively low side-band frequencies, but it is, of course, desirable that the rejector should be effective over a band of frequencies, and the form of rejector shown in Figure 2 is therefore preferred to be used in the place of the singletuned rejector circuits l2 ll. Rejector cir cuits of the form shown in Figure 2 present a high impedance to a band of frequencies and their reactance at the frequencies it is desired to pass can be absorbed in the band-pass filters of the particular transmitters with which they are associated. It will be understood that instead of the particular form of rejector circuit shown in Figure 2 any other form of band-pass filter having similar image impedance characteristics may be used.

We claim:-

1. A multi-channel high frequency wire-broadcasting system comprising in combination, a distributing network, a plurality of amplifiers, each effective over a band of frequencies and each associated with one of the channels, a plurality of impedance-matching networks each interposed between the output of one of said amplifiers and said distributing network, a plurality of bandpass filters each interposed between one of said amplifiers and said distributing network and a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, each designed to present a high impedance to a band of frequencies, the effective reactance of each of said rejector circuits constituting part of the reactance of the corresponding band-pass filter.

2. A multi-channel high frequency wire-broadcasting system comprising in combination, a distributing network, a plurality of amplifiers each effective over a band of frequencies and each associated with one of the channels, a plurality of step-down transformers each interposed between the output of one of said amplifiers and said distributing network, a plurality of band-pass filters each interposed between one of said amplifiers and said distributing network and a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, each designed to present a high impedance to a band of frequencies, the effective reactance of each of said rejector circuits constituting part of the reactance of the corresponding band-pass filter.

3. A multi-channel high frequency wire-broad casting system comprising in combination a distributing network, a plurality of amplifiers each effective over a band of frequencies and each associated with one of the channels, a plurality of step-down transformers each interposed between the output of one of said amplifiers and said distributing network, a plurality of band-pass filters each interposed between one of said amplifiers and said distributing network and a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, and each designed to present a high impedance to a band of frequencies, said band-pass filters comprising pairs of series tuned circuits, the components of each pair of said series tuned circuits being selected so that the point of connection between them is a minimum point of potential.

4. A multi-channel high frequency wire-broadcasting system comprising in combination, a distributing network, a plurality of amplifiers each effective over a band of frequencies and each associated with one of the channels, a plurality of step-down transformers each interposed between the output of one of said amplifiers and said distributing network, a plurality of band-pass filters each interposed between one of said amplifiers and said distributing network and a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, and each designed to present a high impedance to a band of frequencies, the effective reactance of each of said rejector circuits constituting part of the reactance of the corresponding band-pass filter, said band pass filters comprising pairs of series tuned circuits, the components of each pair of said series tuned circuits being selected so that the point of connection between them is a minimum point of potential.

5. A multi-channel high frequency wire-broadcasting system comprising in combination a distributing network, a plurality of amplifiers each effective over a band of frequencies and each associated with one of the channels, a plurality of step-down transformers each interposed between the output of one of said amplifiers and said distributing network, a plurality of band-pass filters each interposed between one of said amplifiers and said distributing network and a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, and each designed to present a high impedance to a band of frequencies, the effective reactance of each of said rejector circuits constituting part of the reactance of the corresponding band-pass filter, said channels having carrier frequencies chosen near to the cut-01f frequencies of said band-pass filters.

6. A multi-channel high frequency wire-broadcasting system comprising in combination a distributing network, a plurality of amplifiers each effective over a band of frequencies and each associated with one of the channels, a plurality of step-down transformers each interposed between the output of one of said amplifiers and said distributing network, a plurality of band-pass filters each interposed between one of said amplifiers and said distributing network, a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, each designed to present a high impedance to a band of frequencies, a plurality of low-pass filters each interposed between the output of one of said amplifiers and said distributing network and designed to prevent harmonic frequencies generated in said amplifier from passing to said distributing network, the effective reactance of each of said rejector circuits constituting part of the reactance of the corresponding band-pass filter.

7. A multi-channel high frequency wire-broadcasting system comprising in combination, a distributing network, a plurality of amplifiers each effective over a band of frequencies and each associated with one of the channels, a plurality of step-down transformers each interposed between the output of one of said amplifiers and said distributing network, a plurality of bandpass filters each interposed between one of said amplifiers and said distributing network, and a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, and each designed to present a high impedance to a band of frequencies, the effective reactance of each of said rejector circuits constituting part of the rea/ctance of the corresponding band-pass filter, said rejector circuits each consisting of reactively-coupled parallel-tuned circuits. v

8. A multi-channel high frequency wire-broadcasting system comprising in combination a distributing network, a plurality of amplifiers each effective over a band of frequencies and each associated with one of the channels, a plurality of impedance matching networks each interposed between the output of one of said amplifiers and said distributing network, a plurality of bandpass filters each interposed between one of said amplifiers and said distributing network and. a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, and each designed to present a high impedance to a band of frequencies, said band-pass filters comprising pairs of series tuned circuits, the components of each pair of said series tuned circuits being selected so that the point Olf connection between them is a minimum point of potential.

9. A multi-channel high frequency wire-broad.-

casting system comprising in combination, a distributing network, a plurality of amplifiers each eflective over a band of frequencies and each associated with one of the channels, a plurality of impedance matching networks each interposed between the output of one of said amplifiers and said distributing network, a plurality of bandpass filters each interposed between one of said amplifiers and said distributing network and a plurality of rejector circuits interposed between one of said amplifiers and said distributing network, each designed to present a high impedance to a band of frequencies, the effective reactance of each of said rejector circuits constituting part of the reactan-ce of the corresponding band-pass filter, said band-pass filters comprising pairs of series tuned circuits, the components of each pair of said series tuned circuits being selected so that the point of connection between them is a minimum point of potential.

PETER PENDLE'ION ECKERSLEY. RUPERT EVAN HOWARD CARPENTER.

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