Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS2252641 A
Publication typeGrant
Publication dateAug 12, 1941
Filing dateJun 30, 1938
Priority dateJul 5, 1937
Publication numberUS 2252641 A, US 2252641A, US-A-2252641, US2252641 A, US2252641A
InventorsBarber Sneath Oswald, Joseph Poliakoff
Original AssigneeBarber Sneath Oswald, Joseph Poliakoff
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method of and apparatus for the transmission of speech and other sounds
US 2252641 A
Abstract  available in
Images(2)
Previous page
Next page
Claims  available in
Description  (OCR text may contain errors)

1941- J. POLIAKOFF ET AL 2,252,641

METHOD AND APPARATUS FOR THE TRANSMISSION OF SPEECH AND OTHER SOUNDS Filed June 39, 1938 2 Sheets-Sheet l moms JOSEPH POLIAKOFF osvmm B. SIEATH ATTORHEYS Aug. 12, 1941- J. P-OLIAKOFF ETAL METHOD OF AND APPARATUS FOR THE TRANSMISSION OF SPEECH AND OTHER SOUNDS Filed June 30, 1938 2 Sheets-Sheet 2 IIVEITDRS arosxrn POLIAKOFF osvmLn B. 31mm ATTORIEYS Patented Aug. 12, 1941 METHOD OF AND APPARATUS FOR THE .TRANSMISSION OF SPEECH AND OTHER SOUNDS Joseph Poliakol! and Oswald Barber Sneath, London, England Application June 30, 1938, Serial No. 216,816

- In Great Britain July 5, 1937 '23 Claims.

The invention relates to a method of transmitting speech and other sounds,-including recorded speech and sounds, within a limited space such as the auditorium of a cinema or theatre, or a sports arena; and to transmitting apparatus and receiving apparatus for carrying the method into practice. The invention is particularly intended for assisting people who are deai= or hard of hearing to receive speech and other sounds through headphones or bone conductors by the aid of a portable apparatus. It has thus the advantage over the systems of deal aid commonly installed in cinemas and theatres, that the deaf person can hear with a small portable instrument or receiving apparatus from any seat or can move about and does not have to plug it into one of a number of fixed points.

According to this invention the transmission takes place by audio-frequency induction between a conductor carrying an undulatorycurrent and a pick-up coil energising a portable receiver. The transmitting apparatus comprises a power amplifier such as is commonly used for public address "and allied purposes, the output of which is delivered to a single length or network of conductors disposed about the hall or auditorium. The limited space within which the transmitted signals can be efliciently received lies within and around the coil formed by the transmitting con-. ductor or it may be restricted to a region adjacent to the said conductor when the size of the coil or loop is very large. The receiving apparatus comprises a thermionic amplifier having combined therewith a pick-up or receiving coil which may be connected by suitable flexible leads to the apparatus, and in the case where the amplifier is a deaf aid appliance having a microphone, may be arranged so that it can be switched in to replace the microphone or it can be effective simultaneously with the microphone.

The invention is illustrated by way of example in the accompanying drawings in which:

Figure 1 shows diagrammatically one form of transmitting apparatus according to this invention,

Figures 2 to 5 show modified arrangements of the transmitting conductor.

Figure 6 shows diagrammatically one form of receiving apparatus, and

Figures 7 to 9 show modifications of the receiving apparatus.

The output from a thermionic amplifier l is connected to a transmitting conductor 4 that forms loops 5, 6 which enclose areas that are approximately equal although not necessarily of similar shape. Such loops may for example, be arranged in the balcony of the cinema and have the current flowing therein in opposite directions. Thus the direction of flow of the current in the loop 5 is clockwise whilst in the loop 6 it'is counterclockwise, whereby a substantial neutralisation or the fields of the two loops beyond a desired distance, i. e. outside the cinema, is obtained. Loops I, 8, 9 and ID in series with the loops 5 and 6, are arranged in another part of the cinema, for example the ground floor. The transmitting conductor is preferably earthed as shown at I I to prevent the said conductor from having a potential to earth other than that due to the small A. C. potential across the'output terminals of the amplifier. Moreover, the parts of the transmitting conductor that form leads between the amplifier and the loops and between the several groups of loops, should be kept parallel to one another and close together so as not to produce a substantial field in themselves, such leads being, if desired, arranged in conduits in a manner similar to mains wiring. The amount of power necessary for the satisfactory working will, where there is no outside electrical interference, be determined by the amplification of the receiving apparatus, but it will in practice frequently depend upon the power needed to mask interference by other electrical apparatus.

Figure 2 shows a modified arrangement of the loops of a transmitting conductor in which loops l2-l9 are in series. The-loops l2, ll, 16 and I8 have the current flowing therein in a clockwise direction whilst in the loops [3, l5, l1 and IS the current flows counterclockwise. The area enclosed by the loops l2, ll, l6 and I8 is made substantially equal to the area enclosed by the loops l3, i5, i1 and IQ, for the reason above set forth.

In some cases however the loops, or series of loops forming the transmitting conductor, are arranged in parallel. In this case, to obtain neutralisation at a distance, the aggregate of the products of the currents flowing clockwise by the areas enclosed by the loops conducting such currents, must equal the aggregate of the products of the currents flowing counterclockwise by'the areas enclosed by the loops conducting such currents.

Figure 3 also shows an mitting conductor in which the areas enclosed by loops which are in series and in which the current flows respectively in clockwise and counterclockwise directions are substantially equal. In this case the transmitting conductor is formed into a large loop 20, in which the direction of flow is arrangement of transclockwise, and loops 2|, 22, 2'2 and 24 in which the direction of flow is counterclockwise, all of the said loops 2. to'"24 being in series. Furthermore, the loops are so disposed that the median point or centre 25 of the loop 20 coincides with the median point or common centre about which the loops 2| to 24 are arranged.

Figures 4 and 5 show further arrangements of transmitting conductors in the form of loops, arranged in series and presenting the same features as those disclosed in Figure 3. By thus having the loops arranged about a common median point or centre, the neutralisation of the effects at a distance of the loops is obtained to a further degree than is possible if the series of loops are disposed for example as shown at '|lll in Figure 1 in which the centre of the field formed by the loops 1 and 8 does not coincide with the centre of the field formed by the loops 9 and I0.

As shown in Figure 4, the conductor forms four loops 4a, 4b, 4c and 4d of which the intermediate loops 4b, 4c have current flowing in the same direction. Thus the field produced between them will be weak and they should be placed close to each other. This arrangement is particularly advantageous where reception is more particularly required at the outer part of an enclosed area as is the case with sports arenas, e. g. a football ground, where the central part of the enclosed space is occupied by the players.

As shown in Figure 6 a receiving apparatus suitable for deaf aid purposes may comprise a thermionic amplifier having three thermionic valves 25, 21 and 28. In the input circuit of the grid of the thermionic valve 26 is a pick-up coil 29 which is screened against electro-static reaction and interference by metallic discs 30, 3|. The said screening discs, unless of low conductivity, should be perforated or slotted to reduce induced currents. A potentiometer 32 is used for volume control. It is generally desirable to apply a slight bias to the grid of the valve 26 to which the pick-up coil is connected, as otherwise the grid current may cause the impedance of the pick-up circuit to drop sufliciently to alter adversely the frequency response curve of the amplifier. Bias for the first and second stages of amplification is obtained by the voltage drop across an electric flash lamp 34 arranged in the cathode circuit and bias for the output valve 28 is obtained by superimposing on the voltage drop across the electric flash lamp 34, a voltage drop across a resistance 35 which is shunted by a condenser 36. A headphone 31 is arranged in the anode circuit of the thermionic valve 28. With a receiving apparatus as shown, it has been found that decoupling of the high tension current for the thermionic valve 21 is unnecessary. Instead of applying bias as set forth above, the grid of the valve 26 may be made self biasing by incorporating in the grid circuit a condenser 62 shunted by a high resistance 63, in which case the electric lamp 34 may in some circumstances be dispensed with.

Figure 7 showsdiagrammatically an arrangement of deaf aid appliance including a capacity microphone adapted for use as a receiving apparatus in conjunction with a transmitting apparatus as shown in any of the Figures 1 to 5. By means of a switch 38 a microphone 39 or a suitably screened pick-up coil 40 can be connected to the grid of a valve 31 as desired. The pick-up coil 49 is shunted by a resistance 4|, whilst the'said coil and resistance are in series with a condenser 42. With this arrangement the receiving apparatus can be employed for receiving the signals transmitted through a transmitting conductor 4 by connecting the pick-up coil 40 to the grid of the thermionic valve 31, or it may be employed as a normal deaf aid appliance by connecting the microphone to the grid. It is preferable that the condenser 42 shall have a capacity similar to that of the microphone 39 so that when the pick-up coil is connected to the grid, the grid load may be substantially capacitative over all or a substantial part of the frequency range to be received, a condition that is desirable, for example, when negative reaction is applied to the grid for volume or tone control.

Figure 8 shows a further arrangement of receiving apparatus. A switch 44 is adapted to connect the primary winding of a step-up transformer 45 either to a carbon microphone 46 or to a pick-up coil 41 as desired. The secondary winding of the transformer 45 is connected to the grid of the thermionic valve 31 and also to the cathode of the valve through a condenser 48 that is shunted by a resistance 49 whereby bias is acquired by the grid of the valve 31. On account of low impedance of the coil in this case, the screening is generally unnecessary.

Figure 9 shows a modification of the receiving apparatus in which the microphone and the coil can act simultaneously in variable degrees controlled by a potentiometer 51 on the grid of a thermionic valve 5| respectively through resistances 52 and 53 and through resistances 55 and 59. The resistances 52 and 55 should be of such values that the loads of the microphone 50 and of the pick-up coil 54 are substantially resistive over the range of frequencies to be received whilst the resistances 53 and 56 should be of a like value and that of the potentiometer 51 somewhat higher. In order to obtain volume and tone control one or more of the resistances 52, 53, 55 and 56 may be made variable or shunted by a condenser to modify the tone. Thus for example, condensers 58 and 58 may be arranged to shunt respectively the resistances 53 and 55.

In order to obtain the most efiicient reception it is usually preferable that the pick-up coil shall be parallel to the plane or planes of adjacent loops forming the transmitting conductor but in some cases, as when the receiving apparatus is situated immediately above or below the conductor, reception is very poor or negligible until the pick-up coil is tilted out of parallelism to the plane or planes of the said loops of the transmitting conductor.

Where wire of a light gauge is used for the transmitting conductor the ohmic resistance of the wire will be large compared with its inductance over the range or part of the range of frequencies to be received and the current flowing in the coil will be substantially independent of frequency. Where the resistance of the transmitting conductor is insufllcient in itself, a fixed or variable resistance may be connected in series to render the load resistive over the whole frequency range. Tone control may be effected by varying such resistance. The use of such a resistive load is particularly applicable where no separate output stage is employed but the transmitting conductor is connected in parallel with a resistive load such as loudspeakers. A receiving apparatus as hereinafter described with amplification independent of frequency must be used in conjunction with such transmitter if a uniform frequency response is required. n the other hand when the transmitting conductor consists of a heavy cable. or copper foil its inductive ductor is greater than that of the impedance of the output circuit to which it is coupled, the current produced in the transmitting conductor by a given output voltage falls off with rising frequency and tends to become inversely proportional to frequency when the inductance is large compared with the impedance of the said output circuit and the receiving apparatus and pick-up coil may be arranged to have an amplification proportional to frequency. In this latter case, where as uniform a frequency response as possibleis required, the output of the amplifier should be adjusted according to the impedance of the transmitting conductor at low frequencies, e. g. 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 base 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 loud speakers offering a resistive load, the transmitting conductor may In this case for frequencies at which the be shuntedwith a resistance and connected in V 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 and grid 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 fromgthe 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, whose resistance rises considerably with temperature, in series with the transmitting conductor. Under normal circumstances the current in the transmitting conductor should be suflicient to bring the filament of the lamp 60 intermittently to red heat and its resistance at such a temperature should be of the order of 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 impedanceof the transmitting conductor is lowest; and this effect may be advantageous as it reduces 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 ob tained in any known way and with its input coupied 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 induce 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 to 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 pick-up coil or the secondary of a transformer whose primary is connected to a pick-up coil, such resistance having a value which is 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. I

To ensure a reasonably uniform response between 300 and 3,000 cycles, for example, the impedancev 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 6| 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 ofthe 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 current in the transmitting conductor falls with frequency, a receiving apparatus whose pick-up rises with 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 characte tic ay 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 efllciently 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 or 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 pickup coil.

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 eflicient reception of signals may be obtained throughout a town or other desired cality the transmitting conductor may consist of 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 then be employed to receive :the signals transmitted. In the case of a mains 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 itcan 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 and be 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.

We claim:

1. In an apparatus for transmitting speech or other sounds throughout a limited space by means of electro-magnetic induction, a thermionic amplifier and a transmitting conductor coupled to the output of said thermionic amplifier and arranged to form a plurality of loops in series with certain loops arranged in clockwise relation and other loops in counterclockwise relation, the area enclosed by the series of loops in which the current flows in a clockwise direction being substantially equal to the area enclosed by the series of loops in which the current flows in a counterclockwise direction, whereby the intensity of the field at a distance is reduced.

2. A transmitting apparatus according to claim 1, wherein the median point of the series of loops in which the current flows in a clockwise direction coincides with the median point of the series of loops in which the current flows in a counterclockwise direction.

3. An apparatus for transmitting speech and other sounds throughout a limited space by means of electro-magnetic induction, comprising a thermionic amplifier, a transmitting conductor coupled to said amplifier and consisting of a plurality of loops with certain loops arranged so that the flow of current therein is in a clockwise direction and the other loops arranged so that the flow of current therein is in a counterclockwise direction, and a portable receiver comprising a pick-up coil, a thermionic amplifier and a sound reproducing device.

4. The apparatus claimed in claim 3 wherein, for the purpose of volume compression, a resistance, such as an electric lamp, whose resistance rises considerably with current, is arranged in series with the transmitting conductor.

5. The apparatus claimed in claim 3, wherein, for the purpose of volume compression, the primary winding of a transformer is placed in series with the transmitting conductor, and a resistance, such as an electric lamp, whose resistance rises considerably with current, is connected across the secondary winding of the transformer.

6. A modification of the transmitting apparatus as claimed in claim 1, wherein said series of loops forming the transmitting conductor are arranged in parallel.

7. An apparatus for transmitting speech and other sounds throughout a limited space by electromagnetic induction, comprising a thermionic amplifier, a looped conductor that extends within the area over which transmission is to take place and consists of a plurality of loops with certain loops arranged so that the flow of current therein is in a clockwise direction and the other loops arranged so that the flow of current therein isin a counter-clockwise direction, and a portable receiving device comprising a thermionic amplifier, a pick-up coil arranged in a low frequency input stage of said thermionic amplifier, and a sound reproducing device.

8. A receiving apparatus according to claim 7, having a response rising with frequency, wherein the pick-up coil has a natural resonance at or beyond the upper frequency of the range of frequencies where a rising response is required.

9. A receiving apparatus as claimed in claim 7, wherein the pick-up coil is adapted to be connected to the audio frequency amplifier of a wireless set.

10. A receiving apparatus according to claim 7, wherein the pick-up coil is shunted by a resistance of such value as to give a substantially uniform frequency response.

11. A receiving apparatus according to claim '7, wherein the pick-up coil has a natural resonance at a frequency near the upper limit of the frequencies over which a rising response is required and a resistance is shunted across the said coil of the order or value of the impedance at the resonance frequency of a pure inductance of the same inductance as that of the pick-up coil.

12. A receiving apparatus as claimed in claim 7, wherein the pick-up coil is connected in series with the primary winding of a transformer whose secondary winding is arranged in an input stage of the receiving apparatus, and a resistance is connected across the secondary winding of the transformer.

13. A receiving apparatus as claimed in claim 7, wherein means is provided to screen the pickup coil against electrostatic reaction and interference.

14. The receiving apparatus claimed in claim 7, wherein the pick-up coil is shunted by a resistance of such value as to give a substantially uniform frequency response.

15. The receiving apparatus claimed in claim 7, wherein the pick-up coil is shunted by a resistance and a condenser in series, of such values as to give a substantially uniform frequency.

16. The receiving apparatus claimed in claim 7, wherein a transformer has connected in series with its primary winding said pick-up coil, and a resistance, whose reflected impedance across the coil is of the value necessary to give a linear response with frequency, is connected across a winding of the transformer, the secondary winding being arranged in an input stage of said thermionic valve.

17. The receiving apparatus claimed in claim 7, wherein a transformer has said pick-up coil connected in series with its primary winding, and a condenser and resistance in series, whose combined reflected impedance across the coil is of the value necessary to give a linear response with frequency, are connected across a winding .1

of the transformer, the secondary winding thereof being arranged in an input stage of the thermionic amplifier.

18. An apparatus for transmitting speech and other sounds in a limited space by electro-magnetic induction, comprising a thermionic amplifier, a looped conductor that extends within the area over which transmission is to take place and consists of a plurality of loops with certain loops arranged so that the flow of current therein is in a clockwise direction and the other loops arranged so that the flow of current therein is in a counterclockwise direction, and a portable receiver comprising a microphonic speech amplifying apparatus, which normally serves as a deaf aid appliance, a pick-up coil which can be plugged-in in place of or in addition to the microphone, and a sound reproducing device.

19. A receiving apparatus as claimed in claim 18, wherein the pick-up coil is adapted to be switched in to replace the microphone of a microphonic speech amplifying apparatus.

20. A receiving apparatus as claimed in claim 18, wherein signals that are received by the microphone of said microphonic speech amplifying apparatus and the pick-up coil are simultaneously fed to an input stage of the amplifier.

21. The receiving apparatus claimed in claim 18, wherein signals that are received by the microphone of the microphonic speech amplifying apparatus and the pick-up coil are simultaneously fed toseparate input stages of the amplifier.

22. Means for signalling throughout a limited space by electro-magnetic induction, comprising a transmitting apparatus and a transmitting conductor connected to the output of said transmitting apparatus and consisting of a series of loops in different planes, certain of said loops being arranged so that the flow of current therein is in a clockwise direction and others of said loops being arranged so that the flow of current therein is in a counter-clockwise direction. and a receiving apparatus that includes a pickup coil, a thermionic amplifier andv a signal reproducing device.

23. An apparatus for transmitting speech and other sounds throughout a limited space, comprising a transmitting system consisting of a signalling device, an amplifier, a looped conductor of metallic foil arranged in a plurality of loops disposed within the area over which transmission takes place, and a freely portable re ceiver situated within said limited spaceand consisting of a pick-up coil, a thermionic valve amplifier and a sound reproducer.

JOSEPH POLIAKOFF. OSWALD BARBER SNEATH.

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2419833 *Dec 12, 1945Apr 29, 1947Grimes Harlan EAntenna arrangement for induction communication systems
US2567431 *May 5, 1947Sep 11, 1951Halstead William SCommunications system of restricted-range type
US2628275 *Feb 25, 1948Feb 10, 1953Parker Louis WRadio and television distribution system for hotels and apartment houses
US2721896 *Jul 20, 1950Oct 25, 1955Albert Foot VictorSound-reproducing systems
US2853557 *Jan 28, 1955Sep 23, 1958Dictograph Products Company InSound distribution system
US3162726 *Jul 14, 1958Dec 22, 1964RosebergAudio systems for drive-in theatres
US3673334 *Mar 19, 1971Jun 27, 1972Educational Service Programs IAntenna for use with an audio induction communication system
US4016553 *Jun 27, 1975Apr 5, 1977Knogo CorporationArticle detection system with near field electromagnetic wave control
US4135184 *Aug 31, 1977Jan 16, 1979Knogo CorporationElectronic theft detection system for monitoring wide passageways
US4908869 *May 9, 1989Mar 13, 1990Norman LedermanFor addressing hearing-impaired persons within an audience
DE969603C *Aug 22, 1950Jun 26, 1958Victor Albert FootVerbesserung an einer Tonwiedergabevorrichtung
DE1038116B *Mar 13, 1956Sep 4, 1958Siemens AgLeiterschleife fuer induktiven Empfang mit Schwerhoerigengeraeten od. dgl.
WO1990013953A1 *Mar 12, 1990Nov 15, 1990Norman LedermanInduction-based assistive listening system
Classifications
U.S. Classification381/79
International ClassificationH04B5/00
Cooperative ClassificationH04B5/00
European ClassificationH04B5/00