GB2502983A - Electronic host device with flexible interface for accessory connection - Google Patents

Abstract

An electronic host device, such as a cellular telephone or music player, comprises an interface for connecting an accessory device, such as earphones and headsets, which are connected by a TRRS(4-pole) 3.5 mm jack plug and socket connector 10. The interface is switchable to selectively configure the connections of the TRRS connector either to convey unidirectional analogue signals between the host and an analogue accessory or to convey bidirectional digital signals between the host and a connected digital accessory and to carry power as required from the host to the accessory. The switching may either be triggered manually, or implemented automatically in response to electronic detection of the connection of a digital accessory device to the host device, and may utilise, for example, FET transistor switches or FET-based integrated switches. The accessory can be a headset with ambient noise cancelling or a digital signal processor.

Description

An Electronic Host Device with Flexible Interface for Accessory Connection The present invention relates to mobile electronic devices, such as cellular telephone handsets and music players, (hereinafter generically referred to as "host devices") and it relates more particularly to the provision of flexible interfacing means to facilitate their connectability to external accessory devices.

Cellular phone and "smart-phone" handsets are now highly sophisticated and can store, for example, large music libraries that can be accessed via high-quality in-ear earphones using thin rubber ear-bud flanges to locate and seal the earphone into the user's ear-canal. These are now widely used for portable communications and entertaimnent applications and, in conjunction with cellular telephone handsets, for hands-free calls and conversations, in this latter example, it is now commonplace to use a "headset", comprising a pair of earphones in conjunction with a microphone that is located on the earphone cable near the mouth of the listener.

For practical convenience, the earphone/headset connector plug which has now evolved as the industry standard is the 3.5 mm jack, in both 3-pole and 4-pole formats, characterized by the number of connector rings present on the shaft, and hence referred to as "TRS" (tip-ring-sleeve) and "TRRS" (tip-ring-ring-sleeve), respectively. The 3.5 nn jack plug does not require rotational axial aligmnent for insertion (unlike a IJSB socket, for example), it clicks positively into place, and hence it is quick and easy to insert and remove whilst "on the move". Figure 1 (a) shows a typical TRRS-type 3.5 miii plug 1 and Figure 1 (b) shows the plug inserted into its associated socket 2; these figures also indicating the connector terminology. All of the connector regions I, Rl, R2 and S on the plug 1 are mutually insulated by intervening insulator bands such as 3. The terminals of the socket 2 are spring-loaded against the corresponding connector regions of the plug 1.

A second, related, feature that has evolved is the actual positioning of the 3.5 mm socket on the host device. Whilst travelling and using earphones or a headset, the host device is usually carried in a chest-pocket, in vertical orientation, and hence the 3.5 mm socket is located on an upwards-facing edge of the host device (usually the uppermost edge of the host device), thus allowing quick and easy access for engaging and removing the earphones.

However, because the host devices often carry significant music collections, they are also widely used, in conjunction with so-called "docking stations", as home hi-ti music playback systems. A typical docking station comprises a base-plate, featuring a multi-way electrical connector and mechanical mounting arrangement on to which the host device is plugged, in upright orientation so that its display can be read and manipulated, together with an amplifier and stereo loudspeaker pair, all in one unit. Consequently, a multi-way electrical connector must be provided on the lowermost edge of the host device for transmitting audio to the docking station and receiving power from it to re-charge the n internal batteries. In addition, a rnicro-USB connector is also often present and can provide the same functionality.

In suimnary, the typical mobile host device has evolved such that it features a multi-way electrical socket with comprehensive connectivity and power supply on the lowermost edge, and a limited connectivity (3-or 4-poles only) analogue socket on an upwards-facing edge, usually the uppermost edge.

It is an object of this invention to provide an improved upwards-facing connection interface with enhanced flexibility to accommodate the connection of digital as well as analogue accessories.

One example of a situation requiring such flexibility is the increasing requirement for accessories such as earphones and headsets to incorporate an ambient noise-cancelling (ANC) system, so as to improve intelligibility and/or the general listening experience against background noise, and moreover so that the listener is not required to increase the listening volume to an excessively high level in order to overcome the

background noise.

Ambient noise-cancellation type earphones are well-known, and include models manufactured by, for example, Audio-Technica (ATH-ANC23) and Sony (MDR-NC33). Further background information about ANC in-ear earphones can be found hi GB-A-2,475,526, which is incorporated herein by reference.

A common format for ANC earphones is to provide the associated electronics and its battery in a small "pod", linked to the earphones via thin, flexible cable, about 0.5 m in length; the pod bearing a second, short cable, terminated by a 3.5 nun jack plug, which plugs into the host device. The battery is usually either an AAA type cell, which lasts typically 20 hours, or a small, rechargeable Li-polymer cell lasting about 7 hours. The former is expensive for the user in terms of frequent cell replacement, and the latter is costly in terms of the initial cost of the Li-polymer battery as a component in the ANC system.

The pod unit, being located mid-way between the earphones and a chest-pocket, and typically bearing "on-off' and "mute" switches for the ANC control, is often clipped temporarily to the user's clothing, and so ideally it is made as lightweight and inconspicuous as possible. However, the battery adds considerable weight and bulk to the pod, which is undesirable. In addition, a battery might expire during a long journey, and so a spare is usually carried. Alternatively, when a re-chargeable cell is fitted in the ANC pod, battery replacement is not possible, in which case it must be re-charged ininiediately prior to extended use.

These inconveniences of weight, bulk, expense and sudden loss of fruiction could be eliminated if the ANC pod unit were to be powered from the host device. In principle, this is possible via the multi-way connector or via a micro-USB connector. However, because, as previously described and for fruictional reasons, it is situated on the lowermost edge of the cell-phone or music player, the multi-way connector is not conveniently accessible when the handset is carried in a shirt pocket, and so this option is not practical for use "on the move".

According to the invention there is provided an electronic host device comprising an interface for accessory devices temporarily connectable thereto by way of a TRRS 3.5 mm jack plug-and-socket connector, the interface comprising switchable means incorporated into the host device and means for switching the switchable means to selectively configure the connections of the TRRS connector either to convey unidirectional analogue signals between the host and a connected analogue accessory or to convey bidirectional digital signals between the host and a connected digital accessory and to convey power as required for one or more components of the digital accessory.

By this means, and counter to the logic that digital interfacing and power supply means is already present (in the form of the IJSB socket) in many existing host devices, the invention addresses the practical problems described above by providing bi-directional digital and power connections via the 3.5 nim analogue socket on an upwards-facing edge of the device, whilst retaining tile ability to utilise instead the conventional analogue functions of the socket. This enables the host device to both provide power for, and communicate with, an external accessory device, such as an ANC earphone system. In addition, the 3.5 nijn socket remains fully functional with all of the conventional analogue earphones, headsets and other accessories that might be used with it. In particular, the invention provides a flexible interface means for transferring digital data and supplying power via a 3.5 mm TRRS analogue jack socket, which has become the standard analogue output socket fonnat for host devices such as cellular phones and music players.

With particular reference to the usage of accessories capable of providing ANC, and in addition to the value of supplying power to the ANC electronics, the bi-directiona! digital interfacing provided by the present invention confers several further benefits.

I. The ANC processing can be carried out on the main CPU of the host device, using digital signals from digital microphones in the ANC earphones.

2. A cellular phone handset can be used to control and "personalise" some of the ANC functionality, such as ANC filter selection or ANC level, choice of which might vary froni user to user.

3. A handset can be used to monitor noise exposure levels, registered by the ANC microphones.

4. The host device can be used to select and control various automatic frmnctions, such as ANC off/on when the ambient noise levels are low (to preserve power).

5. The host device can interrogate the accessory device in order to acquire, for example, calibration data relevant to ANC signal levels, for subsequent use in ANC processing.

In one preferred embodiment of the invention, the switchable means can be manually actuated by a user.

In another preferred embodiment of the invention, actuation of the switchable means can be carried out automatically in response to electronic detection of the connection of a digital accessory device to the host device.

In order to provide a "legacy" compatibility with existing analogue accessory devices, such as headsets and earphones, it is preferred that the default start-up mode of the interface is in the anahgue-mode, rather than the digital-mode, and that provision is made for a safe "power-up" procedure in respect of the voltage source which is present hi digital-mode.

In some preferred embodiments of the invention, the analogue signals comprise left and right analogue output signals conveyed from the host device to a headset accessory device, and microphone signals conveyed from the accessory device to the host device; all of these signals being conveyed along respective channels via separate connections of the TRRS connector.

In some preferred examples of the invention, the digital signals comprise two bi-directional input/output (I/O) channels; said digital signals and said power-supply into the accessory device from the host all being conveyed via separate connections of the TRRS connector.

Preferably the switchable means comprises FET transistor switches or FET-based integrated switches. In preferred embodiments of this kind,

S

tile switchable means is configured as a piurahty (preferably two) of single pole, double throw switches, and it is particularly preferred that means are provided to synchronously toggle the switches on and off in response to a digital "1" or "0" signal.

In some preferred embodiments of the invention, a digital accessory connected to the plug of the plug-and-socket connector incorporates a digital signal processor configured to communicate directly with the plug.

Although the interface of the invention is robust against tile erroneous connection to the host device of an accessory comprising a load that is inconsistent with the setting of the switchable means, it is preferred that the analogue/digital mode selection is carried out automatically.

In one preferred embodiment of the invention, this is effected by detecting the insertion of an accessory's TRRS plug into the 3.5 mm socket on the host device, and then briefly switching to digital mode and interrogating the load for the presence of a digital system. In these circumstaiices, if a digital system is present, the digital niode is maintained. However, if no digital "handshake" occurs with the accessory device, within a predetermined brief time interval, the switching system reverts to analogue mode.

The host device may typically comprise the handset of a cellular telephone or a music playing device. In either event, it is preferable that the accessory incorporates ambient noise cancelling components.

In some embodiments of the invention, it is preferred to configure the switchable means and the said means for switching the switchable means into a headphone driver IC which also incorporates conventional stereo headphone driver amplifier circuitry, thereby to provide a headphone driver IC with both analogue-mode and digital-mode capability.

In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described with reference to the accompanying drawings, of which: Figures 1 (a) and 1 (b) have already been referred to as showing the frmndaniental components of a 3.5 nmi TRRS jack plug-and-socket connector; Figure 2 shows, in block schematic form, an existing TRRS connection between a host device and an accessory device; Figure 3 shows in block diagrammatic form the basic concept behind the invention; Figure 4 shows how a digital accessory can be configured to utilise the interface provided by the invention; Figure 5 shows how the interface of the invention can connect a host device to a typical analogue accessory device; Figure 6 shows how the interface of the invention can connect a host device to a typical digital accessory device; Figure 7 is a flow diagram showing a procedure capable of automatically detecting the nature (analogue or digital) of an accessory device connected to the interface; Figure 8 is another flow diagram showing a procedure capable of flagging an accessory to the host device; and Figure 9 shows, in schematic form, how the interface provided by the invention can be incorporated into the driver IC of the host device.

Referring now to the additional drawings, Figure 2 shows the typical, conventional circuitry that is used at the present time for the connection of a stereo headset to a cellular phone handset (or other mobile host device) via its 3.5 mm TRRS coirnector 10. The handset ("host") circuitry is represented above the dashed lines 12, and the headset ("accessory") circuitry is represented below the lines 12. The sleeve-connector ("S") of the TRRS system is always used for the common ground connection. The tip-connector ("T") is used to connect the hell-channel analogue audio output 14 from the host to the headset, and the first rhig-connector ("Ri") is used for the right-channel audio output 16; both channels connecting to respective microspeakers 1 8, 20 hi the headset earphones. The second ring-connector ("R2") is used to connect the headset microphone 22 to the handset. The microphone 22 is typically a sub-miniature electret type, featuring an internal JFET buffer transistor which requires a suitable bias voltage to be supplied, followed by AC coupling of the generated audio signal in to the following amplifier stages, as shown in Figure 2. The microphone JFET operates in saturation mode, typically at a drain current of about 250 RA. in series with its bias resistor 24, connected to a positive voltage source shown as +v.

With a bias resistor 24 of about 6 kO and a supply voltage of 3 V, there is a DC voltage on the output (microphone 22 -resistor 24) node of about 1.5 V, which is AC modulated by the microphone signal (typically 22 mY peak-to-peak at 94 dBSPL). This node is coirnected to the microphone input of the handset host via a coupling capacitor, 26 (typically 5 iF), with an optional pull-down resistor 28 to define the time-constant of the couple. The node is also connected to a level detector means 30, such as a voltage comparator, in order to monitor its DC voltage, and this enables a crude signalling method to be achieved by connecting a push switch 32 in parallel with the microphone 22, and located on the headset or headset cable. When the switch 32 is pressed shut by the user, the DC voltage on the microphone output node is short-circuited to ground and the associated voltage change, from I.5 V to 0 V, is detected by the level detector 30. Naturally, the out-bound microphone signal becomes muted during this operation, and the current through the switch 32 is limited by bias resistor 24, until the switch is released again. Such signals are used to accept and terminate incoming calls to the handset, for example.

Variations on this arrangement include the use of multiple switches, arranged to switch different value resistors in parallel with the microphone, hi order to provide a plurality of functions such as "fast-forward" and "volume level increase" during music playback.

The present invention preserves the above functionality, whilst also providing a bi-directional digital and power-supply interface between a host and an accessory device. Figure 3 shows the invention in its simplest conceptual form, comprising a TRRS 3.5 mm jack socket connected to both analogue and digital transmission means; either of said transmission means, but not both simultaneously, being selectable, together with a power-supply source that can be enabled simultaneously with selection of the digital transmission means. The analogue transmission means comprise both left and right analogue output channels 14, 16 respectively, from host to headset, and a single microphone-to-host channel 34. The digital transmission means comprises two bi-directional input/output (I/O) channels, 36 and 38. A "mode select" terminal 40 is used to select either the digital-mode or analogue-mode of operation, and there is a +V power-supply provided, as will be described below, and also a ground connection.

The analogue load detail for existing TRRS connections has been described above, and in Figure 2. In addition to compatibility with the analogue format, the invention enables a host device to be used alternatively with, and to supply power to, digital peripheral circuits. For example, a digital ambient-noise-cancelling (ANC) system can be powered from a host, and at the same time receive a digital audio stream from it, such that the user can listen to the music from the host device with the added benefit of ambient-noise-cancellation, provided by the accessory, to suppress loud background noise of the kind that may be encountered whilst travelling.

The general coiiiiection requirements for a digital signal-processing load of this type are shown in Figure 4, where the T and Ri connections of the connector 1 0 are assigned to two, bi-directional input/output (1/0) channels, Dl and D2, of a digital signal processor 42. The R2 connection of the connector 10 is used for the power supply and the S connection of the connector 10 provides the ground terminal. The digital signal processor 42 drives the earphone microspeakers 18 and 20, and it is also connected to the headset microphone 22 and one or more switches 32'.

These components enable basic digital headset operation. For an additional ambient noise-cancellation fttnction, there are two additional microphones required but, for clarity, they are not shown here.

It will be appreciated that the invention, having two data lines, is capable of operating with a wide variety of well-known digital data-transmission formats, including j2 ("Integrated Interchip Sound"). 12C ("Inter-Integrated Circuit") and the various Universal Serial Bus (USB) formats, and so in some instances one of the data lines would be allocated to a clock (CLK) signal, and the other to a data signal. For simplicity, however, the two digital data lines will be referred to here as bi-directional input/output (I/O) channels.

Referring again to Figure 3, the analogue-digital mode-switching can be achieved by switchable means which can take the form of various electronic means, preferably incorporating FET transistor switches or FET-based integrated switches, such as the Texas Instruments I.C. type TS5A23 157 utilising a dual 100 SPDT analogue switcl1 to provide a double-pole, double throw (DPDT) switch 44. The two single-pole double-throw (SPDT) FET switches each have a low (10 0) "ON" resistance, suitable for connecting the analogue audio output to the low-impedance (16 0) microspeakers 18 and 20. The switches are synchronously toggled on and off by a digital "1" or "0" signal applied to the appropriate terminals on the analogue switch I.C. This particular device is used as an example in the following description and in the associated Figures 5 and 6.

Figures 5 and 6 show the present invention connected to analogue and digital loads, respectively, in the same format as Figure 2 with host-side (handset) circuitry above the dashed lines 12, and accessory-side (headset) circuitry below the lines 12.

Figure 5 shows an arrangement in accordance with one embodiment of the present invention connected to a conventional analogue load. A non-inverting digital buffer 50 is driven with a high ("1") signal, which drives both a resistor 52 and a mode-select control pin CTL on the switch 44 high, thus selecting analogue-mode for the interface. This switches the "analogue hi" terminals of the switch 44 to the T and Rl terminals on the TRRS socket, which, in turn, connect the left and right analogue output lines 14 and 16 respectively to the left and right niicrospeakers 1 8, 20 in the analogue load. The high signal on resistor 52 turns the pnp bipolar transistor 54 off, and so TRRS terminal R2 is effectively connected only to the bias voltage (+V) via resistor 24, and to the AC coupling circuitry (capacitor 26 and resistor 28) and the level detector comparator 30. This, hi turn, supplies the correct bias voltage to, and signal coupling from, the headset microphone 22, and also connects to any parallel switches such as 32 that might be present. Accordingly, the switching interface system provides ftill connectivity between the host and a conventional analogue accessory. both for signals and voltage biasing.

Figure 6 shows the present invention connected to an example digital load. The non-inverting digital buffer 50 is driven with a low ("0") signal, which drives both resistor 52 and the mode-select control pin (CTL) on the DPDT analogue switch 44 low, thus selecting digital-mode for the interface. This switches the "digital I/O" terminals of the analogue switch 44 to the T and RI terminals on the TRRS socket, which, hi turn, connect to the Dl and D2 I/O data terminals of the digital load (such as the DSP 42 shown here). The low signal on resistor 52 turns pnp bipolar transistor 54 on, which connects the power supply voltage (+V) to the R2 terminal on the TRRS connector, thus supplying power to the digital load. There is no adverse effect on the biasing and coupling circuitry (resistor 24, capacitor 26 and resistor 28) and the level detector 30 registers the presence of the supply voltage, +V, on the R2 terminal.

Accordingly, the switching interface system provides full connectivity between the host and a digital accessory, both for bi-directional digital signals and for a power supply.

The mode-select signal derives from the central processor unit of the host device, and can be manually initiated by the user. For example, a srnartphone host can feature a touch-screen selection icon to toggle the 3.5 mm jack function between analogue-mode and digital-mode peripherals. In this case, it is preferred that the analogue-mode, being the prevailing standard, is the default mode. The system is intrinsically safe in terms of incorrect settings; if an analogue headset were to be erroneously plugged into a 3.5 mm socket that was set in digital-mode, it would tolerate the +V supply on its microphone 22 (instead of a current-limited bias voltage), and the microspeakers 18, 20 will tolerate being driven by digital data (although this would be audible and undesirable).

A current-limiting modification can be added to transistor 54 to prevent excessive current flowing if analogue switches were present and activated during this "incorrect mode" operation. This is also useful in the event that a TRS (rather than TRRS) accessory were to be used, which would short-circuit the R2 terminal to the S terminal (ground).

Similarly, there would be no adverse effect in erroneously plugging a digital load into a 3.5 mm socket that is switched into analogue-mode: the DSP 42 would not power up from the current-limited bias-voltage on R2, and the digital I/O terminals on the DSP 42 would not be harmed by the application of analogue signals intended for the microspeakers 18 and 20.

It is preferred, however, that the analogue/digital mode selection is carried out automatically. This can readily be achieved by detecting the insertion of an accessory's TRRS plug into the host 3.5 mm socket, and then briefly switching to digital mode and interrogating the load for the presence of a digital system; if a digital system is present, then the digital mode is maintained. However, if no digital "handshake" occurs with the accessory device, then the switching system reverts to analogue mode.

As an example, Figures 7 and 8 show flow-diagrams for the above digital "handshake", which is silent to an analogue headset, in that digital signals are hot transmitted from the host to the accessory unless it is confirmed as a digital one, and so there are no unpleasant audible digital signals sent to the microspealcers 18, 20. Figure 7 shows an example flow diagram for the detection and interrogation of an accessory by a host device. On detection of the insertion of a TRRS plug (as described below) at stage 101, a tinier is started at 102 and the host sets the interface to digital-mode at stage 103. This sends power to the accessory. The host hiterrogates the incoming clock data line at stage 104 and, if there is a clock signal present from a newly-connected accessory, the host initialises the digital accessory at stage 105 and digital operation commences at 106. This continues until plug withdrawal is detected at stage 107, at which time the host reverts to the default analogue mode at 108.

If, at stage 104, no clock data is received within a predetermined time period (in this example. 0.2 ms) determined at stage 109, the analogue mode is selected at 108. The process is ended at 110.

Figure 8 shows an example flow diagram for an accessory to flag its presence to a host device. When the accessory is plugged hi, the host detects the event and switches the interface to digital-mode, thus sending power to the accessory at step 111. When the accessory's microcontroller powers up, it sends its cloclc signal out at 112 to the host device, reads its input buffer at step 113 and checks at 114 for an initialisation code from tile host. When an initiahsation code is received and found at step 114, the niicrocontroHer initiahses the accessory at 115 and begins digital-mode operation at I I 6. If no initialisation code is found at step 114, the sequence steps back to repeat step 113.

The detection of the insertion of an accessory plug can be achieved simply by using the contact switches present in the TRRS socket.

Usually, the spring-conductors of the 3.5 nun socket that contact the tip, ring(s) and sleeve contacts of the inserted plug, contact a separate set of conductors on the socket when there is no plug present. The connection to these conductors is therefore "nornrnlly closed" (NC). Consequently, when there is no plug present in the 3.5 mm socket, the voltage that is present on the R2 terminal (present for both analogue and digital modes), is connected to its associated "NC" conductor on the socket, and this can be monitored by the host CPU. When a plug is inserted, this connection is broken, and the voltage is removed. By providing a pull-down resistor, connected between the "NC" terminal and ground, the voltage on the tenninal switches from -i-V to 0 V when a plug is inserted into the socket, and then reverts to +V when the plug is removed.

Alternatively, the level detector comparator can be used to monitor the voltage present on R2, which will normally be the power supply voltage, +V, and which will drop when either an analogue or digital accessory load is connected.

Because, in use, the invention directly links the host device's headphone driver amplifiers to its 3.5 mm audio socket, it is advantageous to provide a headphone driver IC which incorporates both circuitry to implement the invention and conventional stereo headphone driver amplifier circuitry, thereby providing a headphone driver TC with both analogue-mode and digital-mode capability. An example of such au IC is showii at 60 in Figure 9, with the conventional headphone drive amplifiers being identified as Al and A2 respectively. The additional cost, compared to a conventional headphone driver amplifier IC, is trivial.

Claims (17)

1. Claims: 1. An electronic host device comprising an interface for accessory devices temporarily connectable thereto by way of a TRRS 3.5 imn jack plug-and-socket connector, the interface comprising switchable means incorporated into the host device and means for switching the switchable means to selectively configure the connections of the TRRS connector either to convey unidirectional analogue signals between the host and a connected analogue accessory or to convey bidirectional digital signals between the host and a connected digital accessory and to convey power as required for one or more components of the digital accessory.
2. 2. A device according to claim I, wherein said means for switching the switchable means is configured to be manually actuated by a user.
3. 3. A device according to claim 1, wherein said means for switching the switchable means the switchable means is adapted to switch automatically hi response to electronic detection of the connection of a digital accessory device to the host device.
4. 4. A device according to any preceding claim configured to provide the analogue mode as its default start-up mode.
5. 5. A device according to any preceding claim, wherein the analogue signals comprise left and right analogue output signals to be conveyed from the host device to a headset accessory device, and microphone signals to be conveyed from the accessory device to the host device; all of these signals being conveyed along respective channels via separate connections of the TRRS connector.
6. 6. A device according to ally preceding claim, wherein the digital signals comprise two bi-directional input/output (T/O) channels; said digital signals and said power-supply into the accessory device from tile host all being conveyed via separate connections of the TRRS connector.
7. 7. A device according to any preceding claim, wherein the switchable means comprises EEl transistor switches or EEl-based integrated switches.
8. 8. A device according to claim 7, wherein the switchable means is configured as a plurality of single pole, double throw switches.
9. 9. A device according to claim 8, wherein said means for switching comprises means to synchronously toggle the switches on and off in response to a digital "1" or "0" signal.
10. 10. A device according to any preceding claim, wherein said interface is adapted to communicate directly with a digital signal processor incorporated into an accessory device and connected to the plug of said plug-and-socket connector.
11. 11. A device according to any preceding claim, frirther comprising control means to automatically select analogue or digital mode operation.
12. 12. A device according to claim 11, wherein the control means comprises means for detecting tile insertion of an accessory's TRRS plug into tile host 3.5 mm socket, and fttrther means for then briefly switching to digital mode and interrogating the load for the presence of a digital system whereby, if a digital system is present, the digital mode is maintained but otherwise the switchable means reverts to analogue mode.
13. 13. A device according to any preceding claim incorporated into the handset of a cellular telephone.
14. 14. A device according to any of claims 1 to 12, incorporated into a music storage and replaying device.
15. 15. A device according to any preceding claim adapted to interface with an accessory incorporating ambient noise-cancelling components.
16. 16. A device according to any preceding claim, wherein the switchable means and the said means for switching the switchable means, together with associated circuitry and components, are incorporated into a headphone driver IC which also includes conventional stereo headphone driver amplifier circuitry, thereby to provide a headphone driver IC with both analogue-mode and digital-mode capability.
17. 17. An electronic host device comprising an interface for accessory devices temporarily connectable thereto by way of a TRRS 3.5 nn jack plug-and-socket connector, substantially as herein described with reference to and/or as shown in Figures 3 to 9 of the accompanying drawings.