US2785226A - Binaural sound reproduction - Google Patents

Description

March 12, 1957 E. e. cooK BINAURAL SOUND REPRODUCTION 2 Sheets-Sheet 1 Filed March 16, 1953 lll lli INVENTOR [MO/9y 6. (00K BY CLAC4.

ATTORNEYS March 12, 1957 E. G. cooK BINAURAL SOUND REPRODUCTION 2 Sheets-Sheet 2 Filed March 16. 1953 INVENTOR EMORY a 600K BY daddy MM? n"@/M ATTO RN EYS L mited States Patent C.

BINAURAL SOUND REPRQDUCTION Emory G. Cook, Stamford, Conn.

Application March 16, 1953, Serial N 342,342

Claims. (Cl. 179-1) This invention relates to improvements in sound reproduction systems, and particularly to a sound reproducing system of the so-called binaural type.

in a binaural sound system of the type with which the present invention is concerned, sound waves are reproduced from electrical signals derived from two substantially identical sound recordings. The recordings are made in the first instance from sound waves picked up by a pair of suitably spaced microphones. When these recordings are played back simultaneously through similarly spaced loudspeakers, the reproduction has a marked depth or dimensional quality not obtainable with the more conventional reproduction from a single recording. In fact, the listener is given the distinct impression of being present at an original performance rather than merely hearing a reproduction; an efiect that is particularly appealing in the reproduction of recorded music.

While binaural sound reproduction has this recognized superiority over its unidimensional counterpart, one of the main obstacles to widspread acceptance and use of the binaural method has been the relative complexity of binaural reproduction apparatus. The conventional approach has been to have a completely separate signal reproduction channel for each of the recordings being played, which obviously puts the binaural system at a considerable disadvantage from the standpoint of size, cost and weight. Accordingly, it is a general object of the present invention to provide a binaural sound reproduction system in which similar parts in separate signal channels are consolidatedv and arranged to serve in dual capacities, thereby reducing duplication of parts with attendant savings in system size, weight and cost.

As is well known, the output transformer that supplies driving power to the loudspeaker or other transducer in a sound reproduction system ordinarily is one of the heaviest and most expensive components, due primarily to the relatively bulky, heavy transformer core required. in accordance with one feature of the present invention, a lighter, more compact and less expensive binaural sound reproduction system is made possible by utilizing a common core for the output transformers in two separate signo.1 channels. in accordance with a further feature of the invention, additional consolidation of parts is effected by winding the system power supply choke coil on the same core with the two output transformers.

A more complete understanding of the invention can be a by reference to the following description of illustrative embodiments thereof, when considered in connection with the accompanying drawing, wherein Figure l is a schematic diagram illustrating electrical signal amplifiers with push-pull output in a binaural sound reproduction system embodying the present invention, and

Figure 2 is a diagram similar to Figure 1 showing single-sided or unbalanced amplifiers in a system embodying the invention.

As was previously stated, an important feature of the present inventionis in the use of a single core for two outice put transformers in a binaural sound system. Consider, for example, the double window transformer core 10 shown in Figure 1. Core structures of this type are Well known per se, and readily available, customarily comprising E and l shaped laminations of iron or the like that are stacked and interleaved to form the structure shown. In accordance with the invention, on one end leg 12 of the core it? there is placed a primary winding 14 and a secondary winding 16 forming a first transformer, while on the opposite end leg 18 there is placed a similar pair of windings 2i) and 22 forminga second transformer.

The core 10 provides two similar pairs of magnetic circuits for the magnetic fields that will be produced by current flow through the primary windings l4 and 20. Considering the winding 14, for example, one magnetic circuit includes the end leg 12, the adjacent portions 24a and 25a of the side legs, and the center leg 28. A second magnetic circuit for the same winding 14 includes the end leg 12, the side legs 24:: and Zeb, and 26a and 26b, and the remote end leg 18. A similar pair of magnetic circuits, encompassing the same or corresponding components of the core lil, can be traced for the fields of the other primary winding 24 beginning in each instance with the end leg 18.

With the foregoing magnetic circuits in mind, it can be seen that magnetic fields produced by suitably phased alternating current flow through the two primaries 14. and 2% will oppose each other in the end legs 12 and 18 of the core 10. However, under the same circumstances, these fields will be additive in the center leg 28.

If these fields are produced by currents of substantially equal intensity, flowing through similar windings, then the portion of each field that traverses the opposite end leg of the core will be substantially cancelled out.

by the stronger opposing field established by the winding on that particular leg. in other words, the resultant field in each end leg will be substantially entirely representative of the current flowing through the winding on that particular end leg. Accordingly, it is entirely feasible to have the output transformers for independent channels 3% and 32 of a binaural sound system wound on the same core structure ill.

In practice, it is deemed preferable to arrange the circuits of the system to guard against cross-talk, or interference due to channel-to-channel signal transfer that may take place through the core 16 due to imperfect circuit balancing or similar unpredictable factors. Such an arrangement may comprise a negative feedback loop for suppressing in a given channel any interfering signals introduced from an adjacent channel through the medium of the common transformer core. Such negative feedback connections for interference suppression are included in the'circuits of Figure l, which will now be described.

For simplicity, only one of the channels 30 is shown in detail, it being understood that the two channels 30 and 32 ordinarily will correspond throughout.

The channel 3% includes input connections 34 to conduct signals from a phonograph pickup or the like (not shown) to a first amplifier stage 33. From the anode circuit of the amplifier 38, a coupling capacitor 40 is connected to carry the amplified input signal to a phase splitter stage 42. The phase splitter 42 will supply opposite phase signals through coupling capacitors 46 and 48 to the two sections of a push-pull amplifier 44.

The circuits of the amplifier and phase splitter stages 38 and 42 include resistors 59 through which anode voltage is applied from a source designated 3+, and a cathode load resistor 52 for the phase splitter. Also included in the phase splitter circuit is a volume control comprising a. potentiometer 56 which is connected across the 3-!- supply in series with a voltage divider resistor 53. The potentiometer arm 56;; is connected to the grid circuit of the phase, splitter, through an.isolating. resistor 66 to control the bias voltage on the phase splitter stage. A resistor voice coil 64 of a loudspeaker-type transducer The V circuit of. the amplifier 44" is completed by a cathode biasing network 68 and grid return resistors 54. V j

' The feedback arrangement. for suppressingfcross talk comprises a connection 69 from-the transformer secondary winding 16 to the cathode circuitof the input amplifier stage 38; This connection will introduce into the cathode circuit a small signal that will be reproduced intheoutput circuit of the push-pull amplifier 44 as a signal of proper phase to suppress cross-talk.

it mightbe noted that. evenif. some-small amount of cross-talkisnot completely eliminated, it will ordinarily be. indistinguishable in the loudspeaker outputs. The signals reproduced in the loudspeakers willbe substantially identicaltoybegin with,.so that'a slight amount of cross-talk will be most difiicul-t to detect andwill not materially affect the desired"dimen'sional quality of thereproducedisignals. V

Depending on the. power "output of the final amplifier stage 44 in. the, Figure 1 system, it is possible that the additive magnetic fields produced in the center leg 28 of the core. by the transformer primary currents may saturate the magnetic circuits that include this center leg, with resultant signal distortion. in any case where this possibility exists, the corestructure can be altered slightly to-avoidi the difficulty by introducing a small air gap in that portion of the. magnetic circuit wherein the magnetic.

fieldsarje additive. Such an arrangement is shown in are ure', 2', wherein there. is shown the applicability of the features of the invention to amplifiers of the single-sided output type. Also illustrated in Figure 2 is a further feature of the invention; the utilization of the transformer core as a core for a power-supply choke.

Referring-to Figure 2', there is shown a combined trans- {ormer-a'nd-choke core 70' having end legs.72' and 74, a

center leg 76, and side legs 78 and 80. In this case; the

center leg 76icon-tacts only one side leg 78; and whil'e'extending nearly entirely across the central open area ofthe core, -terminates j ust short of the other side leg 89. This A5 before, only one channel 94 is shown in detail Figure 2, and the description will be limited tothis single channel, it being understood that the two channels as and 96 included within the broken line blocks ordinarily will be identical throughout. 7

In channel 94, a pair of input leads 98 areprovided to conduct signals from a phonograph pickup or the like (not shown) to a first amplifier stage 1%. A coupling capacitor 162 connectsthe anode-circuitofrthefirst ampliher 16% to the grid circuitv of an outputamplifier stage 16- 3. The amplifier-stages 199; i394 include the usual grid 7 return resistors 1%, as. well as a cathode resistor for the leaves a slight'air gap. 32, between the side leg 89 and the center leg 76, to increase the reluctance of the magnetic circuits that traverse the center leg 76.1

As, in the Figure 1 system, a pair oftransformer primary Windingsi84 and 86 are wound on the end legs 72 and'74 together with their. associated secondary windings 88' and 99'; In addition, however, there is wound on the center 1eg'76a coil Q2 which serves as the filteringin ductor or-choke'coil in a power'supply filter circuit described hereinafter. V

It will be understood that the core' 74 in Figure'2 will provide similar pairs of magnetic circuits for'the transformers 84, 88 and-35, it, corresponding to them-agnetic.

circuits previously described with'respect to the core 10 in the Figure 1 system. One such circuit for each of the transformers will include the center leg 7 6 of the core 7 0 in Figure- 2', and, as in the Figure- 1- system, it is'c'ontem-- plated that additive magnetic'field's will. beestablished in this center leg. However; the reluctance increase provided by the gap 82 inthe Figure 2 structure willrlieep down the flux density in the center leg 76" and thereby avoid saturating the important magnetic circuits that traverse this center leg. Additional protection against saturation will be provided by the field resulting from current flow through the choke coil 92, aswill now be described connection with a description of theamplifier' and power supply circuitsin-Figure 2.

inputstage'ltlfi across which to apply negative feedbac. voltage and a cathode biasing network 110 for the output stage 104.

Anode voltage is supplied for all stages in both channels of the Figure 2 system by a power supply circuit 114 that is shown-in detail in order to illustrate atypical connection for the choke coil: 92; The power supply circuit 114 comprises a transformer 116 havin g'a primary winding 118', adapted to. be'connected to the usual alternating voltage supply source (not shown), and a secondary winding I26 connected for fullwave rectification to a duo'- diode rectifier 120. The output circuit of the'power' supply I14 includesa filter network comprising a resistor 124 in 's'eries' with the choke-92, in combination with a shunt capacitor 126'. a 1

From the choke '92, B'+ voltage is applied to the anode circuits' of the amplifier stages 100 and 104' through the load-resistors 112 and through the transformer primary windings 84, 88 respectively;

As inthe'circuit' of" Figure l, ar -feedback connection 126 is provided in Figure 2fron1the transformer secondary winding 86m the cathode circuit of the first'amplifier stage lllilto suppress interfering signals that may be introduced fronr theother channel 94 through the medium of the common. transformer core170. 7

In the Figure 2 system, it is seen that the unidirectional currentfiow through the choke coil 92 can readily bean. ranged to set up a magnetic field that will'oppose the fields the: center leg 76' established by similar current flow through the transformer. primaries 84 and 88. This will help to avoid saturation of the center. leg, as previously stated. 7

In addition to the partsv consolidation and anti-satura tion features of'the choke coil arrangement shown in Figure 2, Ya further advantage of this arrangementis in the fact that the choke coil 92' will serve as a hum-bucking element; 7

Residual ripple in the power.- supply outputsometimes is reproduced as power hum? in therloudspeakers 66. 7

However, with the choke coil wound onthe transformer core center leg 76,. any such ripple-will set up afield in' thecofe in a direction such. as. to oppose and cancel 7 outthe'hum-producing fieldsv that might otherwise exist, and thereby will eliminate an'y such: difiiculty.

It will, of course, bevunderstood that the choke coil arrangement; shown in Figure). canas well be used in a system such as is shown inrFigure l. Similarly,.the core structures'10'and70 of Figureslandl will be understood to be interchangeable in the circuits shown in the two figures. a e

What is-,clairned is; e

1. In abinauraL soundreproducing system for converting/into sound. waves substantially identical electrical signals. derived from; sound. recordings,. tl1e combination of. a. pair. ofelectrical; signal channels each comprising means. for'amplifyingsaidsignals and each having an output circuit, a transformer circuit ineach said channel hav ing a. primary winding; and. a secondary winding, said primary windingsbeing connected in. said: output circuits, a transformer corecomprisin g a structure. ofImagnetic ma.

terial'having' end'l'egs;sidelegscormecting said end legs and aeenter leg-extending betwecnsaid side legs, the=wind.-

avsaaas ings of said transformers being wound on separate ones of said end legs, and means connected in said channels for suppressing in each said channel interfering signals introduced therein from the other of said channels through the medium of said core.

2. In a binaural sound reproducing system, two electrical signal channels each comprising (1) signal amplifying means including an output circuit, (2) a transformer circuit having a primary winding connected in said output circuit and having a secondary winding, and (3) a transducer for converting electrical signals to sound waves and connected to said secondary Winding to receive electrical signals therefrom, and a unitary core structure of magnetic material on which said windings are wound, said structure including members forming a first magnetic circuit common to both said primary windings and in which magnetic fields due to current flow through said primary windings oppose each other, and said structure also including a further member forming with said first named members second and third magnetic circuits having a common portion traversing said further member and independent portions traversing said first-named members.

3. In a binaural sound reproducing system for converting into sound waves substantially identical electrical signals derived from sound recordings, the combination of a pair of electrical signal channels each comprising means for amplifying said signals and each having an output circuit, a transformer circuit in each said channel having a primary winding and a secondary winding, said primary windings being connected in said output circuits, a unitary core structure of magnetic material on which said windings are wound, said structure including members forming a first magnetic circuit common to both said primary windings and in which opposing fields are generated by current flow through said primary windings, said structure also including a further member forming with said first named members second and third magnetic circuits having a common portion traversing said further member and independent portions traversing separate ones of said first-named members, and means connected in said channels for suppressing in each said channel interfering signals introduced therein from the other of said channels through the medium of said core.

4. A system as defined in claim 3 wherein said further member comprises an elongated element extending between separated ones of said first named members and is joined only to one of said separated members whereby said second and third magnetic circuits are interrupted by an air gap between the other of said separated members and the free end of said further member.

5. In a binaural sound reproducing system, two electrical signal channels each comprising (1) signal amplifying means having input and output circuits, (2) a transformer circuit having a primary winding connected in said output circuit and having a secondary winding, and (3) a transducer for converting electrical signals to sound waves and connected to said secondary winding to receive electrical signals therefrom, and a unitary core structure on which said windings are placed, said core comprising magnetic material forming magnetic circuits having independent portions and having a common portion, the primary windings of said transformers being wound on separate ones of said independent portions of said core structure in such directions as to generate aiding magnetic fields in said common circuit portion and opposing fields in said independent circuit portions upon energization of said primary windings, and means associated with each said channel for substantially suppressing signals coupled thereinto from the other channel through said transformer core.

6. A system as defined in claim 5 wherein said lastnamed means comprises a connection from each said transformer secondary winding to the input circuit of the associated amplifying means.

7. In a binaural sound reproducing system for convert- '6 inginto sound waves substantially identical electrical signals derived from sound recordings, the combination of a pair of electrical signal channels each comprising means.

for amplifying said signals and each having an output cir cuit, a transformer circuit in .each said channel having a pr mary winding and a secondary winding, said primary windings being connected in said output circuits, a transformer core comprising a structure of magnetic material having end legs, side legs connecting said end legs and a center leg extending between said side legs, the windings of said transformers being wound on separate ones of said end legs, means connected in said channels for suppressing in each said channel interfering signals introduced therein from the other of said channels through the medium of said core, a power supply circuit for converting alternating current to direct current for energizing said amplifying means, and a filter network in said power supply circuit including a choke coil, said output circuits being connected to said power supply circuit to draw unidirectional current therefrom through said transformer primary windings, and said choke coil being so wound on said transformer core center leg that opposing magnetic fields are set up in said core structure by current flow through said choke coil and through said primary windings.

8. In a binaural sound reproduction system for converting into sound waves substantially identical electrical signals derived from sound recordings, two electrical signal channels each comprising 1) signal amplifying means including an output circuit, (2) a transformer circuit having a primary winding connected in said output circuit and having a secondary winding, and (3) a transducer for converting electrical signals to sound waves and connected to said secondary winding to receive amplified electrical signals therefrom, a unitary core structure of magnetic material on which said windings are wound, said structure including members forming a first magnetic circuit common to both said primary windings and in which magnetic fields established by current flow through said primary windings oppose each other, said structure also including a further member forming with said first named members second and third magnetic circuits having a common portion traversing said further member and independent portions traversing said first named members, a power supply circuit for converting alternating current to unidirectional current for energizing said amplifying means, and a filter network in said power supply circuit including a choke coil, said choke coil being wound on said further core member.

9. in a binaural sound reproducing system for converting into sound waves substantially identical electrical signals derived from sound recordings, the combination of a pair of electrical signal channels each comprising an electrical amplifier and each having an input circuit and an output circuit, a transformer circuit in each said channel having a primary winding and a secondary winding, said primary windings being connected in said output circuits, a transformer core comprising a structure of magnetic material having end legs, side legs connecting said end le s, and a center leg extending between said side legs but contacting only one of said side legs and thereby leaving an air gap between said center leg and the other of said side legs, the windings of said transformer circuits being wound on separate ones of said end legs, a connection from each said transformer secondary winding to the input circuit of the associated amplifier to suppress in each said channel interfering signals introduced therein from the other of said channels through the medium of said core, a power supply circuit for converting alternating current to unidirectional current for energizing said amplifiers, and a filter network in said power supply circuit including a choke coil, said output circuits being connected to said power supply circuit to draw unidirectional current therefrom through said transformer primary windings, and said choke coil being so wound on said transforrncreore center leg that imposing magnetic fields are .set up in said core structure by cur-' rent flow through said choke coil and through said primary 7 windings.

10. In a binaural sound reproducing system, 'twe CiSC? trical signal channels each comprising (1) signal amplify- 5 ing meanshaving input and output circuits, (2) a transformer circuit having a primary Winding connected in said output 'circuit and having a secondary winding, and .(3)

a transducer for converting electrical signals to sound .51

'Waves and connected to said secondary winding, and

unitary core structure on which said windings are placed,

said core comprising magnetic material forming magnetic crate aiding magnetic fields in said common circuit portion and opposing fields'in said independent circuit portions upon energization of said primary windings.

References Cited in the file of this patent UNITED STATES PATENTS 2,393,885 Claasen Jan. ,29, 1945' 7 2,505,535

Silent Apr. 25,, 1950

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