ACOUSTIC COUPLER CAVITY LINEARISER

Abstract: An acoustic coupler (18) for transmitting and/or receiving encoded data signals over telephone lines to which, in use, a telephone handset (10) is acoustically coupled. A cavity (32) is formed between the handset (10) and an acoustic seal (30), surrounding a transducer (34). A plug (36) of material is located in the cavity (32) and, in use, is deformed between the handset (10) and the coupler (18) to fill the cavity, thereby linearising the data transmission in audio frequency sound form across the cavity (32). An aperture (40) may be provided in the plug to increase sensitivity.

Description: TITLE: ACOUSTIC COUPLER.CAVITY LINEARISER This invention relates to improvements in acoustic couplers for the transmission of computer or other data over telephone lines. Acoustic couplers are used for the transmission and reception of encoded data signals by coupling the encoded signal in audio frequency sound form from a suitable transducer to the telephone handset microphone, and from the telephone handset receiver by a suitable microphone. In a conventional acoustic coupler, the coupling transducers are held in a suitable mechanical arrangement and surrounded by acoustic seals usually formed from rubber or a rubber like substance, and a telephone.handset is mechanically held in contact with the arrangement for transference of acoustic signals. There is normally an air cavity between the telephone handset transducers and the acoustic coupler transducers. Such cavities are usually present to improve the coupling efficiency and frequency response of the overall system, included in which are the telephone handset transducers. It is also necessary to have some air cavity, as the acoustic seal around the coupling transducers must be made of a resilient material having enough compliance to make an efficient acoustic seal with a wide range of telephone handset designs. In data transmission using FSK transmission techniques, conventional acoustic couplers perform satisfactorily up to speeds in the vicinity of 300 to 6 bits per second (BPS) . Similarly, acoustic couplers used for frequenc modulated analog signals such as facsimile or medical telemetry signals perform quite well. However, when da transmission speeds are higher, for example, in the vic of 1200 BPS, and phase modulated and wide band FM signa are used, the limitations of conventional acoustic coup become more apparent. These limitations are caused in by uneven phase, frequency, and amplitude response of t coupler transducers and cavities, as well as acoustic t delays which become appreciable with higher speed and w bandwidth signals. An object of the present invention is to substantially overcome the deficiencies of acoustic cou at high data transmission speeds. According to the invention there is provided a acoustic coupler (18) for transmitting and/or receiving encoded data signals over telephone lines, said acousti coupler including data transmitting and/or receiving elements (20,28) to which a telephone handset (10) is acoustically coupled in audio frequency sound form, and which in use a cavity (32) exists between respective telephone handset transducers and data transmitting and receiving transducers, characterized in that a plug (36 of material is provided in said cavity. An embodiment of the invention will be describ hereinafter, with reference to the accompanying drawing in which:- Fig. 1 is a diagrammatic partly-sectioned view the essential features of an acoustic coupler, together with a separated telephone handset. The telephone handset 10 is of conventional design, being connected by a cord 12 to a control termi -_ O The handset includes a receiving end 14, which includes a microphone, and a transmitting end 16, which includes an acoustic transducer. The duplex acoustic coupler structure 18 includes a transmitting element 20, which has an annular acoustic seal 22, preferably formed from a resilient material such as rubber, which seal is adapted to co-operate with telephone handset receiving end 14 to define, in use, an acoustic cavity 24. A transmit transducer 26 is located within the seal to transmit, in audio frequency form, encoded data to the receiving microphone in receiving end 14. The receive/transmit handset/coupler arrangement 14/20 is not shown with a plug of material in accordance with the invention, for clarity, and because the acoustic coupler problems described herein are not so apparent in this mode. The features of the coupler structure which are not shown may include a flexible central portion to enable the relative angle of the coupler elements to be altered to accommodate various telephone handset types. The coupler 18 also includes a receiving element 28, which has an acoustic seal 30 which is substantially identical, in construction and operation, to seal 20, and forms with end 16, in use, an acoustic cavity 32. A receiver microphone 34 is located within seal 30. A plug 36 is located within seal 30. The plug is preferably formed from resilient semi-open pore material such as a foamed plastics material, although the structure and degree of compressibility may be altered to change operating parameters. The plug is shown in a cylindrical form, with a curved top edge 38. An axial aperture 40 extends through the plug. The plug is preferably formed in a size greater than the expected cavity size, so that in use, when end 16 is located to abut seal 30, the end also deforms plug 36 to fill the entire cavity 32, although leaving aperture 40, in a truncated form. The plug 36 acts as a damping device in the air cavity 32, and as a consequence reduces the resonances within the cavity. The plug also modifies the time delay of sound travelling across the cavity, and reduces the difference in time delay between two or more given frequencies within the bandwidth used. Depending on the degree -of the compressibility of the plug material, the damping effect on the air cavity will vary. The apertured embodiment shown is considered superior in that the aperture 40 provides direct transducer/ transducer communication, through the air, thus not decreasing, sensitivity, whilst maintaining the damping effect by filling the remainder of the cavity 32. It is clear that if plugs of different sizes and/ or materials used, it would be possible to alter the degree and effect of the modification to the response of an acoustic coupler. Furthermore, plugs of different sizes, shapes, and/ or materials may be fitted with double sided adhesive pads or discs so that they may be added to, removed from, or changed in acoustic coupler air cavities for the purpose of optimising the performance, or equalizing the response, so that the acoustic coupler could be used with, for instance, different types of telephone handsets, or adapt to different line conditions.

Claims: CLAIMS 1. An acoustic coupler (18) for transmitting and/or receiving encoded data signals over telephone lines, said acoustic coupler including data transmitting and/or receiving elements (20,28) to which a telephone handset (10) is acoustically coupled in audio frequency sound form, and in which in use a cavity (32) exists between respective telephone handset transducers and data transmitting and/or receiving transducers, characterized in that a plug (36) of material is provided in said cavity. 2. An acoustic coupler according to claim 1, characterized in that the material is semi-open pore material. 3. An acoustic coupler according to claim 2, characterized in that the material is foamed plastics material. 4. An acoustic coupler according to claim 3, characterized in that the material is resilient. 5. An acoustic coupler according to claim 4, characterized in that when said handset (10) is coupled to said acoustic coupler (18) , said plug (36) of material is deformable to fill said cavity (32) . 6. An acoustic coupler according to any preceding claim, characterized in that said plug (36) of material has an aperture (40) extending therethrough. 7. An acoustic coupler according to claim 6, characterized in that said aperture (40) is adapted to provide a passage between the respective handset and data terminal transducers. 8. An acoustic coupler according to claim 7, characterized in that the plug (36) of material is substantially cylindrical, and the aperture (40) is axial. 9. A method of linearising the transmission of data in an acoustic coupler (18) , including the step of providing a plug (36) of material according to any one of claims 1 to 8 in a cavity (32) of sai ' d acoustic coupler (18). OMPI

Inventors: WHITBY D
Applicants: ELECTRO MED ENG PTY LTD, WHITBY D
Application year: 1980
Main classification: H04R1
IPC codes: H04M1