Patent GB-394325-A (GB)
Title: Improvements in and relating to sound-transmission, sound-recording and sound-reproducing systems
IPC: H04R9/16
Filed: 1931-12-14
Abstract: 394,325. Transmitting, recording, and reproducing music. BLUMLEIN, A. D., 57, Earl's Court Square, London, and ELECTRIC & MUSICAL INDUSTRIES, Ltd., Blyth Road, Hayes, Middlesex. Dec. 14, 1931, No. 34657. [Class 40 (iv).] In a binaural transmission system, the transmission circuits comprise modifying arrangements whereby the relative loudness of a number of loud-speakers is made dependent on the direction from which sounds arrive at the transmitting microphones or; directionally sensitive microphones are used with or without such circuit arrangements. The system may comprise a recording and reproducing link which may be used in conjuction with motion pictures, or a wireless link in which case modifying arrangements may be provided in the high frequency circuits. Directional hearing sense is due to phase and intensity differences between sounds reaching the two ears, phase differences being more effective for the lower frequencies and intensity differences for higher frequencies. As phase differences in two loud-speakers (both heard by both ears) do not produce the required effect and normally reproduced intensity differences are not sufficiently marked, the modifying arrangements translate low frequency phase differences into intensity differences and amplify the higher frequency intensity differences. In some systems described below, the modifying arrangements deal only with intensity or phase differences. In recording, separate traces of the sound channels may be made on disc, cylinder or optical records, or a combined trace, e.g. lateral and hill and dale cuts or variable density and variable width optical records, may be made of both channels. The modifiying arrangements may occur entirely at one stage in the system or partial modifications may be made at a number of stages. Systems providing full directional (horizontal and vertical) effect are described. Modifying arrangements (audio frequency). Sound from source a, Fig. 1, impinges on two microphones a1, a2 separated by a baffle b, representing the human head, and the two channels after passing through amplifiers b1, b2 are combined in circuit c and re-separated into channels with original phase differences represented as intensity differences and original intensity differences amplified. The two channels then pass to amplifiers d1, d2 and loudspeakers e1, e2. The circuit c may be as shown in Fig. 5, wherein the left hand rectangle represents a transformer so wound that the current in the upper output channel is half the sum of the two inputs whereas that in the lower output channel is half the difference in the inputs. Assuming the inputs differ in phase only, the two outputs now differ in phase by II/2 and after passage through the networks shown, one of which has a shunt condenser f, become in phase and are again submitted to a sum and difference process. By choosing the values of the shunt resistance i and condenser f any degree of amplitude difference between the two channels may be obtained. For dealing with the higher frequencies, where intensity differences are important, a resistance k is connected in series with condenser f and is of a value equal to the reactance of the condenser at the frequency above which it is not desired to convert phase differences into intensity differences. When directionally sensitive mircophones are used and intensity differences are dealt with at all frequencies, a similar modifying circuit may be used without the shunt condenser, but if the mircophones are very close together, there may be phase differences which are dealt with as in Fig. 5. For modifying intensities only, a tapped choke may be used in place of the transformer. When two small-pressure microphones are used very close together the differences may be almost entirely in phase and modification is made accordingly. The relative intensities required in the sum and difference channels is dependent on the layout at the transmitting and reproducing ends of the system. and the Specification gives a formula for determining the relationship. Directionally sensitive mircrophone arrangements ; complete directional effects. Directionally sensitive microphones may be of the light moving strip type, and two elements may be mounted vertically in line in a common casing and with a common magnetic system. For horizontal directional effects, there are no phase differences and the intensity differences may be sufficient without the previously described modification. In one arrangement, one strip faces the sound source field, the other being at right-angles thereto. In another arrangement, the strips may face directions at 45‹ on each side of the centre line to the sound source field or the angle may be variable to accommodate different layouts of the field. Vertical displacement of the sound source gives rise to phase differences so that the two channels from the microphone elements provide differences giving full directional significance (movement towards and away from the microphones varies the combined intensity). The phase and intensity differences of the two channels are subsequently separated to provide four channels, the intensity pair and the phase pair being subjected to modification as previously described and the outputs fed to four or more suitable spaced loud-speakers. With pressure microphones, a similar effect may be obtained by using more than two microphones. Wireless link. The two channels may be transmitted separately, or as an amplitude modulation and as a phase or frequency modulation of the same carrier, or as amplitude modulations of two carriers 90‹ out of phase. In the latter case, one of the sum and difference processes described in a preceding paragraph may be be effected at the receiving end by demodulating with carriers 90‹ out of phase and each 45‹ out of phase with the modulated carriers. Recording and reproducing link. When the two sound versions are optically recorded as a combined variable density and variable width record, reproduction may be made by passing light from the same slit through the combined record and separating the beam into two constituents by prisms &c. and supplying the consequent outputs from two light cells to loud-speakers one on each side of a picture screen. When cylinder or disc recording is used, pure hill and dale and lateral cut recording may be done in one groove where modification of the two sound channels has previously been made, but where this has not been done, the two versions may be cut in tracks in one groove at 45‹ to the recording surface on opposite sides of the normal. Such a record if reproduced as a combined hill and dale and lateral cut gives a sum and difference effect for modifications a described in a previous paragraph. The two versions may be cut at any two angles to the normal and cellulose acetate may be the material used for the record. Recording and reproducing instruments. One form of cutter is shown in Fig. 8 and comprises two driving elements 1, 2 normally adapted for lateral cuts. These elements impart vibrations in the plane of the paper to arms 3, 4 which are connected by ligaments 5, 6 to one end of a reed 7 carrying a sapphire 8 and extending normal to the plane of the paper to a fixed support. Another form, Fig. 9, comprises pairs of main and compensating windings 11-14 on pair of laminated polepieces each circuit comprising a main winding on one pole and a compensating winding on the other, the compensating winding serving to prevent movement of the reed away from the pole on which it is mounted due to decrease of flux over this pole by current in the main winding. The reed may be operated by torsion for one cut and flexure for the other. A moving coil instrument, Fig. 10, comprises two speech coils 19, 20 mounted on a light T-member 17 supported at 18 so that it may rotate and move vertically but resist horizontal movement. The movement may be damped as described in Specification 350,998. A moving iron instrument may have an armature mounted in a manner similar to the member 17 and operated by two fixed speech coils. Reproducers may be designed on similar lines.