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1947-48 Theatre Catalog, 6th Edition, Page 365 (351)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 365
Page 365

1947-48 Theatre Catalog, 6th Edition, Page 365

to the impedance of the transmitting conductor at low frequencies-for example, 100 cycles-although in this case a greater power output rating will be necessary'to obtain adequate signal volume than where it can be matched at a higher frequency and the loss of bass ignored. In some cases it may be necessary to connect an inductance in series with the transmitting conductor to render the load impedance substantially inductive down to low frequencies. Where a transmitting conductor offering an inductive load is connected to an amplifier that also supplies loudspeakers offering a resistive load, the transmitting conductor may be shunted with a resistance and connected in series with the speakers.

In cases where the output stage of the power amplifier feeding the transmitting conductor comprises one or more pentodes, it is desirable to shunt the output circuit with a resistance shown at 59, Figure 1, thus giving a reflected impedance of the same order as the optimum working impedance of the pentode, or an equivalent effect may be obtained by known methods of negative reaction between the plate or plates or grids of the pentodes.

Where the installation is in a cinema and an independent amplifier or output stage is employed, the input to the amplifier or output stage may be taken from some point in the speech amplifiers of the talking film reproducing equipment. Thus, for example, the input to a low gain amplifier feeding the transmitting conductor may be taken from the low impedance output to a monitor speaker of the talking film reproducing equipment.

It may sometimes be desirable to provide automatic volume compression. This may be done by connecting a metal filament lamp 60 (Figure 1) whose resistance rises considerably with temperature, in series with the transmitting conductor. Under normal circumstances, the Current in the transmitting conductor should be sufficient to bring the filament of the lamp 60 intermittently to red heat and its resistance at such a temperature should be of the order to that of the impedance of the transmitting conductor at middle speech frequencies. The effect of the variable resistance will be greatest at the low frequencies for which the impedance of the transmitting conductor is lowest; and this effect may be advantageous as it reduced masking at high sound volumes. Alternatively or additionally any known method of automatic volume control can be applied to the amplifier or its input.

Where the system is installed in a theatre, it is usually desirable to employ an amplifier with a high degree of automatic volume control obtained in any known way and with its input coupled to several microphones in different positions on or near the stage.

At distances outside the loops of the transmitting conductor equal to a few times the diameter or width of the said loops, the induced field will be in most cases weaker than the electrical interference due to power cables which usually exist in towns.

The inductive field will not in general interfere with radio-receiving apparatus


or telephones, even if they are situated Within the loop. The spread of the field may, however, be reduced by arranging the transmitting circuit to consist of one or more loops with the current flowing clockwise and one or more loops enclosing substantially an equal total area in which the current flows counterclockwise, as shown in Figures 1 through 5.

Where the current in the transmitting conductor is substantially independent of frequency and a uniform response is required, the voltage set up across the pick-up coil is also made independent of frequency. To do this, a resistance 33 (Figure 6) is connected across the pickup coil or the secondary of a trans former, whose primary is connected to a pick-up coil, such resistance having a value less than the impedance of the pick-up coil or its reflected inductive impedance across the transformer secondary respectively, over the material range of frequencies. The pick-up coil may in some cases be provided with an iron core and be of cylindrical form.

To ensure a reasonably uniform response between 300 and 3,000 cycles, for example, the impedance of the transmitting conductor up to 3,000 cycles must be primarily resistive and not inductive. The resistance of the pick-up coil or the reflected resistance across the secondary of the transformer at 300 cycles must be primarily inductive and be shunted by a fixed resistance at least not greater than this impedance, and the impedance of this coil at 3,000 cycles, which in the case of a coil of a great number of turns, may be largely dependent on its self-capacity, must be not less than the shunting resistance.

Where the ohmic resistance of the pick-up coil may be considerable compared with the inductive impedance thereof at some frequencies within a range for which a linear response is required instead of shunting the coil by a resistance alone it should be shunted by a resistance and a condenser connected in series with one another. Thus a condenser 61 can be connected in series with the resistance 33, Figure 6, the value of the resistance being determined in the same manner as previously and the condenser being of such value that its impedance bears the same ratio to the shunting resistance as the ohmic resistance to the coil does to the inductive impedance of the coil.

If the ohmic resistance of the pick-up coil is considerable compared with its inductive impedance over the entire frequency range concerned, then it should be shunted with a condenser alone of such value that its impedance over the material frequency range is low compared with the resistance of the coil.

It may be found desirable in some cases to increase the effective resistance of the pick-up coil by winding it with resistance wire or by connecting a resistance in series.

Volume and tone control may be obtained by varying the resistance and/or the condenser shunting said coil and/or the resistance in series with the coil, and/or by varying the inductance of the coil by means of tapping or any other known method. Alternatively deviations from linear response may be corrected by any known method of tone correction

in the transmitter or receiver irrespective of where they arise.

Where-the intensity of the current in the transmitting conductor falls with frequency, a receiving apparatus whose pick-up rises with the frequency should be employed. As in most cases of nerve deafness and in the case of normal hearing at high volumes, a rising response curve is advantageous. A receiving apparatus having this characteristic may sometimes be used with advantage even where the current in the transmitting conductor is substantially independent of frequency.

Such a receiving apparatus may consist of a pick-up coil, whose natural resonance is above the range of speech frequency efficiently reproduced by headphones, coupled to the input of a thermionic amplifier. The sensitivity of the receiving apparatus will increase with the size of the pick-up coil and the number of turns therein, but increasing the size of the number of turns in the coil, in general lowers the frequency of its natural resonance due to its inductance and self-capacity.

For maximum sensitivity the pick-up coil is wound so as to have its natural resonance near the top limit of the frequency range over which a fairly uniform response is required and the coil is shunted by a resistance. This resistance should, at the resonance frequency, be approximately equal to the impedance of a pure inductance whose inductance equals that of the pick-up coil. l

The presence of the shunt resistance not only improves the frequency response but greatly reduces any tendency to instability on the part of the receiving apparatus.

Where it is desired to extend the area in which receptioh of signals may be obtained throughout a town or other desired locality, the transmitting conductor may consist of a telephone, telegraph, or electric power lines already existing or cables installed especially for use in connection with the invention. Radio receiving apparatus, modified by incorporating therein or connecting thereto a suitable pick-up coil connected for example to the gramophone pick-up terminals of the apparatus, can be employed to receive the signals transmitted. In the case of a main set, the pick-up coil should, however, be located at a short distance away from the radio receiving apparatus to reduce interference picked up from the mains transformer or smoothing choke of the set.

In this case the coil may be of larger dimensions than where it is employed with portable apparatus and may be mounted so that it can be turned to obtain maximum pick-up of speech or other sound with minimum pick-up of interference. It may conveniently be of one or a few turns of heavy gauge wire coupled to the set through a suitable step-up transformer. In this case, it will generally be found desirable to earth the primary.

A receiving apparatus according to this invention will also function in stray inductive fields such as exist in proximity to most electromagnetic reproducers such as loudspeakers and the usual communication telephones or their associated transformers.
1947-48 Theatre Catalog, 6th Edition, Page 365