WO1999039546A1

WO1999039546A1 - Virtual surround sound headphone loudspeaker output system

Info

Publication number: WO1999039546A1
Application number: PCT/GB1998/001743
Authority: WO
Inventors: Christopher Glenn Wass
Original assignee: Christopher Glenn Wass
Priority date: 1998-02-02
Filing date: 1998-06-15
Publication date: 1999-08-05
Also published as: GB2333925B; GB2333925A; AU7926998A; GB9802223D0

Abstract

A virtual surround sound headphone loudspeaker system has one speaker (44) mounted (72) with dispersion (44a) for mono center-left channel. One speaker (46) mounted (74) with dispersion (46a) for mono center-right channel. One speaker (40) mounted (76) with dispersion (40a) for stereo front-left channel. 44, 48 and 40 are mounted in the left earphone enclosure (52). One speaker (42) mounted (78) with dispersion (42a) for stereo front-right channel. One speaker (48) mounted (80) with dispersion (48a) for stereo rear-left channel. One speaker (50) mounted (82) for stereo rear-right channel. 42, 46 and 50 are mounted in the right earphone enclosure (54). Infra-red transmission on three separate frequencies received at 30 and 34. Associated amplification preferably mounted (60, 62, 84, 88, 86, 90, 28 and 32) and power supply (151, 153 (not shown)) and receiver circuitry (56, 58) in the retractable headband (26).

Description

VIRTUAL SURROUND SOUND

HEADPHONE LOUDSPEAKER

OUTPUT SYSTEM

BACKGROUND OF INVENTION

1. Field of the invention.

The mvention relates to headphone loudspeakers. More particularly the invention relates to a headphone loudspeaker system incorporating a plurality of speakers mounted on opposite sides of the headphones.

2. State of the art.

The art of headphone design involves many variables which relate to stereo and mono production only. Traditionally headphones cover the ears of the listener, giving mono or stereophonic sound from a source such as a compact disc. Prior art Figure 1 shows the sound dispersion characteristic from this type of headphone loudspeaker system (2),(4). The most memorable feature of the headphone loudspeaker system is the placement of two enclosed loudspeakers wliich are individually directed at each ear of the listener. Prior art Figure 2 shows the sound dispersion set-up of a Dolby Pro-Logic Surround Sound processor and associated surround sound speaker system in an OFF mode comprising of Source in Dolby Digital stereo or THX (6) Dolby Pro- Logic Sound Processor (8) Front-Left surround speaker (10) Front-Right surround speaker (12). Rear-Left surround speaker (14) Rear-Right surround speaker (16), Mono-Center speaker (18). Surround Sound works on a principle of gated timing between the front to rear to center speakers. Prior Art Figure 2 a and Figure 7 indicate this gated tuning by showing the Front/Left speaker sound dispersion (10) moving to the Front/Right speaker (12) of Prior art Figure 2b. This process continues through to Prior Art Figures 2c and 2d whereby the sound dispersion moving to rear-left surround speaker (14) and then moving to the rear-right surround speaker (16). The center channel (18) remaining on for the duration of the programme. The source must contain the relevant encoded audio infoπnation (for example THX) with which a Dolby Pro-logic Surround Sound Processor then translates and transfers into the respective speaker channels.

1 Thus producing the surround sound effect. Prior art Figure 3 shows the Infra- Red transmitter and receiver pair setup comprising of a source in stereo (20) Infra-Red transmitter (22) which encodes the source signal and transmits it in Infra-Red at a predeteπnined wavelength. An Infra-Red receiver (24) wliich decodes the coded infoπnation at the same predetennined wavelength. Modified Prior art Figure 3a shows . A test setup comprised of a video casette recorded in THX and a hi-fi stereo video casette recorder. The output being an analogue stereo signal which was fed into a Dolby Digital Processor. The processor then split up this stereo analogue signal into five seperately digitised signals upon which are placed a systematizability according to the programme material. The processor then performs a Digital to Analogue conversion resulting in a stereo Front analogue output, a mono center analogue output and a stereo Rear analogue output. These were independantly fed to three Infra-Red transmitters made by Phillips, Matsui and Sanyo companies. This enabled the programme to be encoded and then transmitted via Infra-Red in the Surround Sound foπnat. Referring now to Figure 3b, the Infra-Red sources 22 22a and 22b make referance to these individual transmitters. Each one of these transmitters works at a different signal wavelengths thus ensuring that cross-talk or channel to channel interferance as it is otherwise known is kept to an absolute ininimum. The transmitters are placed in the close proximity of the source resulting in transmission across a distance which would enable remote reception by the listener. This distance has a practical upper limit of the Infra-Red transmitter power and receiver sensitivity.

Referring now to Figure 3c, the Infra-Red receivers shown also as 24, 24a and 24b of Figure 3b, work at the same corresponding wavelengths. The receivers then decode the encoded surround sound infoπnation and this output is amplified by one individual stereo amplifier and fed to the front left and front right speakers. This is repeated for the rear left and rear-right speakers and one individual mono amplifier for center left and center-right speaker. SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a virtual surround sound headphone loudspeaker output system which emulates the Surround Sound Speaker dispersion pattern.

It is also an object of the invention to provide a virtual surround sound headphone loudspeaker output system wliich incorporates stereo front-left, mono-center-left and stereo rear-left channels in a single left headphone enclosure.

It is another object of the invention to provide a virtual surround sound headphone loudspeaker output system which incorporates stereo front-right, mono-center-right and stereo rear-right channels in a single right headphone enclosure.

The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with referance to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of the dispersion pattern of a first prior art headphone loudspeaker receiver system.

Figure 2 is a schematic view of the dispersion pattern of a second prior art Dolby Surround Sound speaker system setup with speakers indicated as off.

Figure 2a is a schematic view of part of second prior art showing the front- left and center speakers activated. All other speakers are indicated as off.

Figure 2b is a schematic view of part of second prior art showing the front- right and center speakers activated. All other speakers are indicated as off.

Figure 2c is a schematic view of part of second prior art showing the rear-left and center speakers activated. All other speakers are indicated as off.

Figure 2d is a schematic view of part of second prior art showing the rear- right and center speakers activated. All other speakers are indicated as off.

Figure 3 is a schematic view of a third prior art Infra-Red transmission and reception setup utilising one frequency.

Figure 3 a is an electrical block diagram view of a preferred embodiment of the headphone loudspeaker transmitter system utilising Infra-Red transmission on three separate frequencies.

Figure 3b is a schematic view similar to Prior art Figure 3 of the headphone loudspeaker transmitter and receiver system utilising Infra-Red transmission and reception on three separate frequencies.

Figure 3c is an electrical block diagram view of the headphone loudspeaker receiver system.

Figure 4 is a transparent vertical rear elevation view of a preferred embodiment of a headphone loudspeaker receiver system according to the invention.

Figure 4a is a transparent vertical front elevation view of a preferred embodiment of a headphone loudspeaker receiver system according to the invention.

Figure 5 is a transparent left end view of the headphone loudspeaker reciever system of Figure 4. figure ^a is a transparent right end view of the headphone loudspeaker receiver system of Figure 4.

Figure 5b is a transparent left side view indicating the front speaker on the center line and indicating the center speaker angled according to the generally ovaloid nature of the enclosure of the headphone loudspeaker receiver system of Figure 4.

Figure 5c is a transparent left rear end view indicating the front speaker on the center line and indicating the center speaker angled according to the generally ovaloid nature of the enclosure of the headphone loudspeaker receiver system of Figure 4.

Figure 5d is a transparent left side view indicating the front speaker on the center line and indicating the rear speaker angled according to the generally ovaloid nature of the enclosure of the headphone loudspeaker receiver system of Figure 4.

Figure 5e is a transparent left rear end view indicating the front speaker on the center line and indicating the rear speaker angled according to the generally ovaloid nature of the enclosure of the headphone loudspeaker receiver system of Figure 4.

Figure 5f is a transparent right end view indicating the front speaker on the center line and indicating the center speaker angled according to the generally ovaloid nature of the enclosure of the headphone loudspeaker receiver system of Figure 4.

Figure 5g is a transparent right side view indicating the front speaker on the center line and indicating the center speaker angled according to the generally ovaloid nature of the enclosure of the headphone loudspeaker receiver system of Figure 4.

Figure 5h is a transparent right end view indicating the front speaker on the center line and indicating the rear speaker angled according to the generally ovaloid nature of the enclosure of the headphone loudspeaker receiver system of Figure 4.

Figure 5i is a transparent right side view indicating the front speaker on the center line and indicating the rear speaker angled according to the generally ovaloid nature of the enclosure of the headphone loudspeaker receiver system of Figure 4.

Figures 6 and 7a is a schematic representation of the mono-center-left and mono-center-right dispersion patterns and of the front-left dispersion pattern of the headphone loudspeaker receiver system of Figure 6d. All other speakers are indicated as off. Figure 6a and 7a is a schematic representation of the mono-center-left and mono center-right dispersion patterns and the front-right dispersion pattern of the headphone loudspeaker receiver system of Figure 6d. All other speakers are indicated as off.

Figure 6b and 7a is a schematic representation of the mono-center-left and mono-center right dispersion patterns and the rear-left dispersion pattern of the headphone loudspeaker receiver system of Figure 6d. All other speakers are indicated as off.

Figure 6c and 7a is the schematic representation of the mono-center-left and mono-center right dispersion patterns and the rear-right dispersion pattern of the headphone loudspeaker receiver system of Figure 6d. All other speakers are indicated as off.

Figure 6d is a transparent vertical rear elevation view of the preffered embodiment of the headphone loudspeaker receiver system.

Figure 7 is a timing diagram of Prior art Figures 2a, 2b, 2c and 2d.

Figure 7a is a timing diagram of Figures 6, 6a, 6b and 6c.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accord with these objects will be discussed in detail below the headphones loudspeaker system of the present mvention with the following configuration which follows the configuration of the outputs on a typical Surround Sound processoπ-

The FRONT speakers of the following description are actually mounted in the CENTER of the headphones.

The CENTER speakers of the following description are actually mounted in the TOP-FRONT of the headphones.

The REAR speakers of the following description are actually mounted at the BOTTOM-REAR of the headphones.

Figure 4 and Figure 5 show a full-range headphone loudspeaker 44 mechanically affixed to the top-left 72 of the left headphone enclosure 52 of an injection moulded plastic headphone. The headphone enclosure 52 is preformed with an internally protruding cylindπcal supports 51 (not shown)

6 spaced equidistantly from the front speaker 40 of Figure 5 at a minimum angle of twenty degrees deviation to the left of the center line as indicated by Figure 5c. Figure 5b shows the headphone loudspeaker 44 tilting by a minimum of forty degrees towards the center line of the front speaker 40 following the internal peripheral curvature of the ovaloid nature of the headphone enclosure 52. This arrangement enables the soimd dispersion of speaker 44 namely the MONO CENTER-LEFT speaker 44 to appear to have forward moving to rear characteristics when coupled with the acoustics of the ovaloid headphone enclosure 52 and viewed audibly.

Figure 4 and Figure 5 also show a full-range headphone loudspeaker 46 mechanically affixed to the top-right 74 of the right headphone enclosure 54 of an injection moulded plastic headphone. The headphone enclosure 54 is pre-foπned with an internally protruding cylindrical supports 53 (not shown) spaced equidistantly from the front speaker 42 of Figure 5 a at a minimum angle of twenty degrees deviation to the right of the center line as indicated by Figure 5f. Figure 5g shows the headphone loudspeaker 46 tilting by a minimum of forty degrees towards the center line of the front speaker 42 following the internal peripheral curvature of the ovaloid nature of the headphone enclosure 54. This arrangement enables the sound dispersion of speaker 46 namely the MONO CENTER-RIGHT speaker 46 to appear to have forward moving to rear characteristics when coupled with the acoustics of the ovaloid headphone enclosure 54 and viewed audibly.

Figure 4 and Figure 5 also show a full-range headphone loudspeaker 40 mechanically affixed to the center 76 of the left headphone enclosure 52 of an injection moulded plastic headphone. The headphone enclosure 52 is preformed with an internally protruding cylindrical supports 55 (not shown) spaced at a preferred distance from the center speaker 44 of Figure 5 and on the center line of Figure 5. This arrangement enables the sound dispersion of the front speaker 40 namely the STEREO FRONT-LEFT speaker 40 to appear to show the Haas effect characteristic when coupled with the acoustics of the ovaloid headphone enclosure 52 and the center speaker 44 and the rear speaker 48 of the left headphone enclosure 52 when viewed audibly. The psycho-acoustic Haas effect gives dominance to the front speaker 40 during Surround Sound playback. Figure 4 and Figure 5 also show a full-range headphone loudspeaker 42 mechanically affixed to the center 78 of the riglit headphone enclosure 54 of an injection moulded plastic headphone. The headphone enclosure 54 is pre- foπned with an internally protruding cylindrical supports 57 (not shown) spaced at a prefeπed distance from the center speaker 46 of Figure 5a and on the center line of Figure 5a. This aπangement enables the sound dispersion of the front speaker 42 namely the STEREO FRONT-RIGHT speaker 42 to appear to show the Haas effect characteristic when coupled with the acoustics of the ovaloid headphone enclosure 54 and the center speaker 46 and the rear speaker 50 of the right headphone enclosure 54 when viewed audibly. The psycho-acoustic Haas effect gives dominance to the front speaker 42 during Surround Sound playback.

Figure 4 and Figure 5 also show a full-range headphone loudspeaker 48 mechanically affixed to the bottom-left 80 of the left headphone enclosure 52 of an injection moulded plastic headphone. The headphone enclosure 52 is pre-formed with an internally protruding cylindrical supports 59 (not shown) spaced at a prefeπed distance from the front speaker 40 of Figure 5 at a minimum angle of twenty degrees deviation to the left of the center line as indicated by Figure 5e. Figure 5d shows the headphone loudspeaker 48 tilting by a minimum of forty degrees towards the center line of the front speaker 40 following the internal peripheral curvature of the ovaloid nature of the headphone enclosure 52. This aπangement enables the sound dispersion of speaker 48 namely the STEREO REAR-LEFT speaker 48 to appear to have rear moving to front characteristics when coupled with the acoustics of the ovaloid headphone enclosure 52 and viewed audibly.

Figure 4 and Figure 5 also show a full-range headphone loudspeaker 50 mechanically affixed to the bottom-left 82 of the right headphone enclosure 54 of an injection moulded plastic headphone. The headphone enclosure 54 is pre-fonned with an internally protruding cylindrical supports 61 (not shown) spaced at a prefeπed distance from the front speaker 42 of Figure 5 a at a minimum angle of twenty degrees deviation to the left of the center line as indicated by Figure 5h. Figure 5i shows the headphone loudspeaker 50 tilting by a minimum of forty degrees towards the center line of the front speaker 42 following the internal peripheral curvature of the ovaloid nature of the headphone enclosure 54. Tins aπangement enables the sound dispersion of

8 speaker 50 namely the STEREO REAR-RIGHT speaker 50 to appear to have rear moving to front characteristics when coupled with the acoustics of the ovaloid headphone enclosure 54 and viewed audibly.

The speaker 44 of the left enclosure 52 shown in Figure 4 is soldered on the positive side 63 (not shown) to a single core multistrand, polarity indicated wire 65 (not shown) and fed up towards and through the hollow headband 26 and soldered to the positive side 67 (not shown) of the speaker 46 of the right enclosure 54. Another single core multistrand, polarity indicated wire 69 (not shown) is fed back from the positive side 71 (not shown) of the speaker 46 of the right enclosure 54 towards the hollow headband 26 and soldered to the positive side 73 (not shown) of a mono audio amplifier circuit 60 in the headband 26. The negative side 75 (not shown) of the speaker 44 of the left enclosure 52 shown in Figure 4 is soldered to a single core multistrand wire 77 (not shown) and fed up towards and across the headband 26 and soldered to the negative side 79 (not shown) of the speaker 46 of the right enclosure 54. Another single core multistrand wire 81 (not shown) is fed back from the negative side 83 (not shown) of the speaker 46 of the right enclosure 54 towards the hollow headband 26 and soldered to the negative side 85 (not shown) of the mono audio amplifier circuit 60 in the headband 26.

The speaker 40 of the left enclosure 52 shown in Figure 4 is soldered on the positive side 87 (not shown) to the positive side of a two core multistrand, polarity indicated wire 89 (not shown) and the negative side 91 (not shown) of speaker 40 is soldered to the negative side of a two core multistrand, polarity indicated wire 93 (not shown). Both wires 89 and 93 (not shown) are fed up towards and through the hollow headband 26. The wire 89 (not shown) is soldered to the positive side 95 (not shown) of the left output side of the stereo audio amplifier circuit 84. The wire 93 (not shown) is soldered to the negative side 97 (not shown) of the left output side of the stereo audio amplifier circuit 84.

The speaker 42 of the riglit enclosure 54 shown in Figure 4 is soldered on the positive side 99 (not shown) to the positive side of a two core multistrand, polarity indicated wire 101 (not shown) and the negative side 103 (not shown) of speaker 42 is soldered to the negative side of a two core multistrand, polarity indicated wire 105 (not shown). Both wires 101 and 105 (not shown) are fed up towards and through the hollow headband 26. The wire 101 (not shown) is soldered to the positive side 107 (not shown) of the right output side of the stereo audio amplifier circuit 86. The wire 105 (not shown) is soldered to the negative side 109 (not shown) of the left output side of the stereo audio amplifier circuit 86.

The speaker 48 of the left enclosure 52 shown in Figure 4 is soldered on the positive side 111 (not shown) to the positive side of a two core multistrand, polarity indicated wire 113 (not shown) and the negative side 115 (not shown) of speaker 40 is soldered to the negative side of a two core multistrand, polarity indicated wire 117 (not shown). Both wires 113 and 117 (not shown) are fed up towards and through the hollow headband 26. The wire 113 (not shown) is soldered to the positive side 119 (not shown) of the left output side of the stereo audio amplifier circuit 88. The wire 117 (not shown) is soldered to the negative side 121 (not shown) of the left output side of the stereo audio amplifier circuit 88.

The speaker 50 of the riglit enclosure 54 shown in Figure 4 is soldered on the positive side 123 (not shown) to the positive side of a two core multistrand, polarity indicated wire 125 (not shown) and the negative side 127 (not shown) of speaker 50 is soldered to the negative side of a two core multistrand, polarity indicated wire 129 (not shown). Both wires 125 and 129 (not shown) are fed up towards and through the hollow headband 26. The wire 125 (not shown) is soldered to the positive side 131 (not shown) of the right output side of the stereo audio amplifier circuit 90. The wire 129 (not shown) is soldered to the negative side 133 (not shown) of the left output side of the stereo audio amplifier circuit 90.

The output impedance of the stereo audio amplifier 84 88 must match the input impedance of the speakers 44 46 according to maxwells power transfer theorem. The output impedance of the stereo audio amplifier 86 90 must match the input impedance of the speakers 48 50 according to maxwells power transfer theorem. The output unpedance of the mono audio amplifier 60 62 must match the input impedance of the speakers 40 42 according to maxwells power transfer theorem.

Figure 4 and Figure 5 show the left Infra-Red reciever positioned 34 of the left enclosure 52 positioned with an Infra-Red peπneable injection moulded

10 plastic covering 135 (not shown) affixed to the headband 26. Within which are moimted six Infra-Red receiver Liglit Emitting Diodes 137 (not shown) of prefeπed wavelengths A,B and C of Figure 3c. The diodes are soldered as pairs onto a securely fastened printed circuit board 139 (not shown). In a hexagonal aπangement 141 (not shown).

Figure 4 and Figure 5a show the riglit Infra-Red reciever positioned 30 of the riglit enclosure 54 positioned with an Infra-Red permeable injection moulded plastic covering 143 (not shown) affixed to the headband 26. Within which are moimted six Infra-Red receiver Liglit Emitting Diodes 145 (not shown) of prefeπed wavelengths A,B and C of Figure 3c. The diodes are soldered onto a securely fastened printed circuit board 14 (not shown). In a hexagonal aπangement 149 (not shown).

The Diodes of wavelength A, B and C of Figure 3c (not shown) and Figure 4 of the left enclosure 52 are hard-wired to the Infra-Red receiver circuit headphone reciever 56. Polarity is observed on the Light Emitting Diodes whilst soldering.

The Diodes of wavelength A, B and C of Figure 3 c (not shown) and Figure 4 of the riglit enclosure 54 are hard-wired to the Infra-Red receiver circuit headphone reciever 58. Polarity is observed on the Light Emitting Diodes whilst soldering.

The headband 26 also contains power supply circuitry 151 (not shown) which will enable sufficient power to be supplied to the amplifiers, Infra-Red circuitry and speakers. This will be in the order of 6 volts Direct Cuπent, rechargeable Nickel-Cadmium battery pack 153 (not shown).

The transmitter 155 (not shown has phono jacks to the rear which are labelled Front-left, Front-right, Mono-Center, Rear-left and Rear-right. These are typically connected up to a surround sound system as indicated on the rear of the processor.

The transmitter 155 (not shown) has an auxilliary +6 volt regulated power supply output 157 (not shown) with which the headphone receiver is plugged into jack-to-jack in order to recharge the Nickel-Cadmium battery pack in the headphone receiver.

11 The transmitter 155 (not shown) has a row of transmitter Light Emitting Diodes 159 (not shown) behind an Infra-Red transparent visor 161 (not shown) with which transmission into free space can occur. The transmitter 155 (not shown) requires a regulated +12 volt power supply 163 (not shown) with winch it can operate properly. The transmitter 155 (not shown) contains the transmitter circuitry 165 (not shown) which is hard wired to the Transmitter Light Emitting Diodes 159 (not shown) and the transmitter power supply 163 (not shown).

Figure 6 and Figure 7a indicate a specific embodiment according to the invention whereby sound dispersion begins with the mono center-left 44b and mono center-right 46b both indicated as ON and the front-left speaker 40b is also ON. Providing mono center and front-left acoustics shown in a moving time frame.

Figure 6a and Figure 7a indicate a specific embodiment according to the invention whereby sound dispersion continues with the mono center-left 44b and mono center-right 46b both indicated as ON and the front-right speaker 42b is also ON. Providing mono center and front-right acoustics shown in a moving time frame.

Figure 6b and Figure 7 a indicate a specific embodiment according to the invention whereby sound dispersion continues with the mono center-left 44b and mono center-riglit 46b both indicated as ON and the rear-left speaker 48b is also ON. Providing mono center and rear-left acoustics shown in a moving time frame.

Figure 6c and Figure 7a indicate a specific embodiment according to the invention whereby sound dispersion continues with the mono center-left 44b and mono center-riglit 46b both indicated as ON and the rear-right speaker 50b is also ON. Providing mono center and rear-right acoustics shown in a moving time frame.

Figure 6d shows another embodiment of the invention whereby the Surround Sound programme material is activating all speakers 44, 40, 48, 46, 42 and 50 and showing the dispersion patterns 44a, 40a, 48a, 46a, 42a and 50a.

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Claims

1. A virtual su╧Çound sound headphone loudspeaker system for use with a surround sound effects processor that produces centre, front left, front riglit, rear left and rear right signals comprising a pair of headphones in wliich

each of the left and riglit earpieces of the headphones supports, when in use, three loudspeakers directed towards the ear canal, an upper front loudspeaker above and in front of the ear canal, a lower rear loudspeaker behind and below the ear canal and a centre loudspeaker facing the ear canal, and wherein

the centre signal from the processor is directed to both the upper front loudspeaker of the left earpiece and the upper front loudspeaker of the right earpiece,

the front left signal from the processor is directed to the centre loudspeaker of the left earpiece,

the front right signal from the processor is directed to the centre loudspeaker of the riglit earpiece,

the rear left signal from the processor is directed to the lower rear loudspeaker of the left earpiece, and

the rear right signal from the processor is directed to the lower rear louspeaker of the right earpiece.

2. A headphone system according to claim 1 in which each of the left and riglit earpiece of the headphones includes a generally ovaloid enclosure.

3. A headphone system according to either claim 1 or claim 2 in which the headphones include an electronic power supply.

4. A headphone system according to any preceding claim in which the headphones include electronic amplification means.

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5. A headphone system according to claim 1 in which the signals produced by the processor are passed to the headphone system by infra-red transmitter and receiver means.

6. A headphone system according to claim 5 in which the infra-red transmitter and receiver means makes use of tliree separate communication channels.

7. A headphone system according to claim 6 in which the first coimnimication channel ca╧Çies the centre signal, the second communication channel ca╧Çies the front left and front riglit signals and the third channel ca╧Çies the rear left and rear riglit signals.

,

8. A headphone system according to claim 5 in which the infra-red transmitter means includes means to charge the headphone power supply.

9. A system hereinbefore described with referance to figures 3 a to 6d and 7a.

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