EP1536663A1

EP1536663A1 - Headset interface circuit

Info

Publication number: EP1536663A1
Application number: EP03292966A
Authority: EP
Inventors: Cor Voorwinden; Ludovic Oddoart
Original assignee: Freescale Semiconductor Inc
Current assignee: NXP USA Inc
Priority date: 2003-11-28
Filing date: 2003-11-28
Publication date: 2005-06-01

Abstract

An interface circuit (200) for a headset (100) comprising one or more loudspeakers (SP_L,SP_R) and a microphone (M) makes use of a phantom ground approach for supplying a reference ground potential to the loudspeakers and microphone in the headset. The headset interface circuit (200) outputs for the microphone (M) within the headset (100) a stacked biasing voltage (V_MBiasSt) that is substantially equal to the sum of the phantom ground potential and the normal biasing voltage (V_MBias) required across the microphone (M). When the headset amplifier and interface circuit (200) is provided within a radiotelephone, for example, the radiotelephone's audio ground (VAG) can be used to provide the phantom ground voltage.

Description

Field of the invention

The present invention relates to an interface circuit for a module, typically a headset, which comprises one or more loudspeakers (earphones) and a microphone.

Background of the invention

For simplicity, in what follows the module comprising one or more loudspeakers and a microphone will be referred to as a "headset". However, it is to be understood that the invention is applicable even to modules which are not specifically adapted for wearing on the head. Furthermore, the interface circuit of the present invention will generally, but not exclusively, be an amplifier and interface circuit. It is to be understood that amplifier and interface circuits are intended to be covered by the references in this document to interface circuits. Moreover, although in the present document reference will be made to headsets and headset interface circuits being interconnected using "three wires" or "four wires", these wires will generally be bundled in a single cable.
The headset comprising one or more loudspeakers and a microphone is typically an accessory that is detachable from the interface circuit. Headsets of this kind are often used with mobile telephones (radiotelephones) to enable hands-free operation, or with a range of other devices, for example, portable MP3 players having a voice-control feature.
The headset interface circuit receives audio signals from the microphone in the headset, supplies audio signals to the loudspeaker(s) in the headset, and supplies voltage references to the headset. In the case of a stereo headset, the headset interface circuit supplies stereo audio signals to the headset and conventional accessories of this type use four wires to interconnect the stereo headset and the interface circuit (usually via a cable terminated in a four-contact jack plug). In the case of a mono headset, the headset interface circuit supplies mono signals to the headset, and accessories of this type will generally use three wires to interconnect the mono headset and the headset interface circuit (usually via a cable terminated in a three-contact jack plug).
A problem addressed by the present invention will now be explained with reference to conventional stereo headsets and associated conventional amplifier and interface circuits, interconnected using four wires. However, it is to be understood that a comparable problem arises in the case of using conventional mono headsets and associated conventional amplifier and interface circuits, interconnected using three wires.
Fig.1 of the accompanying drawings shows the main components of a typical conventional stereo headset and of a typical conventional stereo headset amplifier and interface circuit.
As shown in Fig.1, the stereo headset 10 includes left and right loudspeakers (or earphones), SP_L and SP_R, and a microphone, M. The left loudspeaker, SP_L, is supplied with a first audio signal, A_L, via a first wire, W₁, and is supplied with a ground reference voltage V_G via a second wire, W₂. The right loudspeaker, SP_R, is also connected to the second wire, W₂, so as to have a ground reference, as well as to a third wire, W₃, via which it receives a second audio signal A_R. The first and second audio signals, A_L and A_R, constitute stereo audio signals.
Large-value capacitors C₁ and C₂ are connected in series with the first and third wires, W1 and W3, respectively, so as to avoid large DC currents (e.g. 40-50mA, for loudspeakers having 32Ω impedance) flowing from the output of the amplifier and the ground connection of the headset.
Typically, the microphone, M, requires a biasing voltage of 2.1 volts to be applied across it. As shown in Fig.1, conventionally, one terminal of the microphone, M, is connected to the ground reference voltage, V_G, via the second wire, W₂. More particularly, the microphone, M, and both loudspeakers, SP_L and SP_R, are connected to the second wire, W₂, via a common connection point, N. A 2.1V biasing voltage, V_MBias, required by the microphone, M, is supplied to its other terminal via a fourth wire, W₄.
In the conventional arrangement illustrated in Fig.1, the headset amplifier and interface circuit 20 connected to the stereo headset 10 includes an audio processing section 30 (typically implemented using an integrated circuit), a ground reference, G, circuitry 40 to produce the 2.1 volt bias voltage, V_MBias, required by the microphone, M, as well as two large-value capacitors C₁ and C₂. The circuitry 40 used to produce the microphone biasing voltage is conventional and so will not be described in detail at this point. Typically, the two large-value capacitors, C₁ and C₂, are in the 100µF to 220µF range. Such capacitors are expensive and occupy a relatively large area on the circuit board on which the interface circuit 20 is mounted.
An attempt has been made to eliminate the two large-value capacitors, C₁ and C₂. In particular, a proposal has been made to employ the so-called "phantom ground" technique in order to render the large-value capacitors, C₁ and C₂, unnecessary. Fig.2 illustrates how a phantom ground approach has been used when interconnecting a stereo headset 10' to a stereo headset amplifier and interface circuit 20'. in Fig.2, parts which have substantially the same structure and function as in the arrangement of Fig.1 are indicated using the same reference numerals as in Fig.1.
As shown in Fig.2, when the phantom ground approach is used, the two stereo loudspeakers, SP_L and SP_R, are connected via the second wire, W₂, not to a separate ground reference potential, G, but with the 1.4 volt audio reference (or "signal ground"), VAG, that already exists in the audio processing IC 30' of the headset amplifier and interface circuit 20'. This VAG, or signal ground, typically is at 1.4 volts for headsets used with radiotelephones. In other applications there will be other signal ground voltages used within the apparatus to which the headset is to be connected. Generally the signal ground level will be half the supply voltage to the apparatus in question. For example, in an MP3 player powered by two NiCd batteries, the signal ground level will be 1.2 volts (or even lower). In some applications the signal ground level may be adaptive, based on a varying supply voltage (typically when batteries powering the device become depleted). These other signal ground levels can be used as the phantom ground potential supplied to the second wire, W₂.
The VAG voltage is supplied to the non-inverting input of an operational amplifier, 50, which is connected in a voltage follower configuration (unity gain buffer) and so has an output having low output impedance and taking a stable value of 1.4 volts.
When using the phantom ground approach as in Fig.2, because the second wire, W₂, no longer provides a potential at 0 volts, the microphone, M, is not connected to the second wire, W₂, but to a separate, fifth wire, W₅, which is connected to a ground reference at 0 volts. Thus, although this approach has the advantage of dispensing with the large-value capacitors, C₁ and C₂, it has the disadvantage of introducing a requirement for a fifth wire interconnecting the stereo headset and the stereo headset amplifier and interface circuit. This approach is not compatible with standard four-wire equipment.
In a similar way, when it has been attempted to use a phantom ground approach for supplying mono headsets, it has been considered necessary to use a fourth wire supplying a ground reference to the microphone, in addition to the normal three wires used for interconnecting the mono headset and its interface circuit.
There is a need for a headset that can be connected to a headset interface circuit without the need for large-value capacitors and without increasing the number of wires required for interconnecting the headset with the interface circuit.

Summary of the invention

The present invention provides an interface circuit for a module comprising one or more loudspeakers and a microphone, the interface circuit comprising means for outputting audio signals at one or more audio output terminals, means for establishing a phantom ground potential at a ground output terminal, and means for providing a reference potential to a further terminal, said reference potential taking a value substantially equal to the sum of the phantom ground potential and the biasing voltage required across the microphone.
The present inventors have realised that it is not essential for the microphone to have one terminal at ground potential and the other at the normal microphone biasing voltage. As long as the microphone has a potential difference across it that is substantially equal to the normal microphone biasing voltage, that potential difference can be established by connecting one microphone terminal to a phantom ground potential and the other to a reference potential which is substantially equal to the sum of the phantom ground potential and the normal microphone biasing voltage. In other words, the microphone is biased relative to the phantom ground potential. This can be considered to be a stacked biasing approach.
Thus the present invention involves the combination of use of a phantom ground technique with use of stacked microphone biasing in order to enable a loudspeaker/microphone module to be connected to an interface circuit using a small number of wires yet avoiding use of large-value capacitors. Moreover, using this approach good noise performance is maintained on the audio microphone biasing.
In preferred embodiments of the invention, the reference potential used for microphone biasing is created using components that are already present in the interface circuit, or present in the overall apparatus in which the interface circuit is provided. For example, in the case where the interface circuit is incorporated into a radiotelephone, the reference potential used for microphone biasing can be generated using a DC/DC converter that is already present in the telephone system. As another example, in some other applications the reference potential can be generated using a charge pump that is already present in the apparatus that incorporates the interface circuit.
The present invention further provides a system comprising a module including one or more loudspeakers and a microphone, and an interface circuit, as defined above, for connection to said module.
The present invention still further provides apparatus comprising an audio signal supply section, and an interface circuit as defined above. This apparatus can be, for example, a radiotelephone or portable audio player (e.g. an MP3 player, CD player, etc.).
The present invention yet further provides a package including a module including one or more loudspeakers and a microphone, as well as apparatus as defined above comprising an interface circuit for connection to said module.

Brief description of the drawings

Fig.1 shows the main elements of a conventional arrangement in which a stereo headset including loudspeakers and a microphone is connected to a headset amplifier and interface circuit;
Fig.2 shows the main elements of another known arrangement in which a phantom ground technique is used and a stereo headset is connected to a headset amplifier and interface circuit using five wires;
Fig.3 shows the main elements of an embodiment of the present invention in which a phantom ground technique is used and a stereo headset is connected to a headset amplifier and interface circuit using four wires;
Fig.4 shows a possible implementation of the headset amplifier and interface circuit of Fig.3 provided in a mobile telephone; and
Fig.5 shows the main elements of an embodiment of the present invention in which a phantom ground technique is used and a mono headset is connected to a headset amplifier and interface circuit using three wires.

Detailed description of the preferred embodiments

Preferred embodiments of the present invention will now be described with reference to Figs. 3 to 5. Figs. 3 and 4 relate to an embodiment in which stereo audio signals are used, whereas Fig.5 relates to a mono embodiment.
A first preferred embodiment of the present invention will now be described with reference to Figs. 3 and 4. The first preferred embodiment relates to a headset amplifier and interface circuit 200 which is provided within a radiotelephone, for use with a stereo headset. However, it is to be understood that the present invention is more widely applicable to headset interface circuits in general, regardless of whether or not they are incorporated into another device of whatever sort it may be.
As shown in Fig.3, the headset amplifier and interface circuit 200 according to the first preferred embodiment of the invention is adapted to be compatible with a conventional stereo headset 100. The headset amplifier and interface circuit 100 is connectible to the stereo headset 200 via a four-wire connection, in the usual way. In the conventional stereo headset 100 the two loudspeakers, SP_L and SP_R, and the microphone, M, are all connected to a common point, N, which is provided with a voltage from the second wire, W₂. Normally, the common connection point, N, would be connected to a ground reference potential, G, via the second connecting wire, W₂. However, according to the present invention the second wire, W₂, is connected to a phantom ground.
Preferably, when the headset amplifier and interface circuit 200 is provided in a radiotelephone, the phantom ground potential is VAG, that is, the audio reference (signal ground) used within the radiotelephone. In other applications, it is convenient for the phantom ground voltage to be constituted by the signal ground applicable in the application in question.
In the first preferred embodiment of the invention, as shown in Fig.3, the phantom ground potential is supplied to the second wire, W₂, from an operational amplifier, 500, which is connected in a voltage follower configuration. The non-inverting input of the operational amplifier 500 is connected to the signal ground potential which, in this example, is VAG at, for example, 1.4 voits. Thus, in this example, a 1.4 volt potential is supplied to the second wire, W₂.
Now, the microphone, M, present in the conventional stereo headset 100 requires a specific potential difference across it for proper functioning (typically, 2.1 volts, as mentioned above). In view of the fact that one terminal of the microphone is connected to the phantom ground, via connection point, N, the headset amplifier and interface circuit 200 of the present invention is adapted to provide the other terminal of the microphone, M, via wire W₄, with a biasing potential which is "stacked" on the phantom ground level. In other words, the headset amplifier and interface circuit 200 of the present invention outputs on the fourth wire, W₄, a voltage which is substantially equal to the sum of the phantom ground voltage and the normal microphone biasing voltage.
In the present example, if the microphone normally requires a voltage of 2.1 volts across it, and the phantom ground corresponds to the 1.4 volt audio ground provided in a radiotelephone, then the stacked microphone biasing potential, V_MBiasSt, will be at 3.5 volts.
The present invention is not limited with respect to the way in which the stacked microphone biasing potential, V_MBiasSt, can be generated: numerous different approaches can be used. However, it is advantageous if the stacked microphone biasing potential is generated without adding a significant number of components to the headset amplifier and interface circuit 200.
In fact, one very simple method for providing the required stacked microphone biasing potential, V_MBiasSt, is illustrated in Fig.3 and consists in making a small modification to the microphone biasing circuit 40 known from the prior art and shown in Figs.1 and 2. The known microphone biasing circuit includes two operational amplifiers, O₁ and O₂, resistors, R₁ to R₅, and capacitors, C_A and C_B, interconnected so as to output a steady microphone biasing potential to the wire W₄. The modification consists in applying a boost voltage, V_B, to the operational amplifier O₁, resulting in the output of an increased potential on the wire W₄.
In the stacked microphone biasing circuit 400 shown in Fig.3, the operational amplifier O₁ amplifies VAG with a view to producing a microphone biasing voltage. The operational amplifier O₂ serves to amplify the low-level audio signals picked up by the microphone in the connected headset (typically having an amplitude of 1 mV r.m.s.) to an acceptable voltage level for the audio processing circuits (typically 100 mV r.m.s.). The amplified microphone output that is supplied to the audio processing circuits is labelled Mic o/p in Fig.3. Because O₂, as well as the capacitor C_B and the resistors R₃ and R₄, are involved in processing the microphone signals, rather than in generating the stacked microphone biasing voltage of the present invention, they will not be described here in further detail.
The boost voltage V_B that is supplied to operational amplifier O₁ according to the present embodiment, serves as +Vcc for O₁ (-Vcc being ground). Operational amplifier O₁ is capable of rejecting noise that is present on the V_B supply. The inverting input of O₁ is supplied with the same voltage as the phantom ground that is being used, in this case VAG (at 1.4 volts). The non-inverting input of O₁ is connected to ground via the resistor R₁. The output of O₁ is connected to the inverting input thereof via the resistor R₂. As mentioned above, the operational amplifier O₁ amplifies VAG with a view to producing the stacked microphone biasing voltage, V_MBiasSt. More particularly: VMBiasSt = VAG (1+R2/R1)
Taking as an example the case where the normal microphone biasing voltage is 2.1 volts, a phantom ground potential of 1.4 volts is applied to wire W₂, and a stacked microphone biasing potential, V_MBiasSt, of 3.5 volts is required on wire W₄, a suitable boost voltage, V_B, to apply to operational amplifier O₁ could be, for instance, 5 volts. Suitable values for the passive components of the stacked microphone biasing circuit 400 could then be, as follows: R₁ = 20kΩ; R₂ = 30kΩ; R₃ = 100kΩ; R₄ = 10kΩ; C_A =100 nF; and C_B = 100 nF.
If the interface circuit 200 were to be used with a headset requiring a microphone biasing voltage different from 2.1 volts, then the appropriate stacked microphone biasing voltage (phantom ground + microphone biasing voltage) could be produced by altering the ratio R₂/R₁. In this case, where the audio processing unit 300 is an integrated circuit, it is simplest to change the vaiue of the resistor R₁ that is connected to ground.
Similarly, if the phantom ground voltage took a value different from 1.4 volts then this could also be accommodated by suitable setting of the ratio R₂/R₁. In a case where the phantom ground is provided using a signal ground level that is adaptive, it would be necessary to use additional circuitry in order to ensure that the stacked microphone biasing voltage correctly followed changes in the signal ground level.
The first preferred embodiment, illustrated in Fig.3, is not limited with regard to the origin of the boost voltage, V_B. Once again, many different techniques can be used for generating this boost voltage. Special circuitry can be added specifically in view of generation of the boost voltage. However, the number of components in the headset interface circuit (or in the apparatus in which that circuit is incorporated) can be kept to a minimum, thus reducing costs, if the boost voltage is derived from circuit components that are already present in the headset interface circuit (or present in the apparatus in which that circuit is incorporated).
Fig.4 illustrates one embodiment of headset amplifier and interface circuit 200' adapted for use within a radiotelephone and in which existing circuit components are used to generate the boost voltage required in the embodiment of Fig.3.
In the embodiment of Fig.4, the audio processing IC 300' incorporates a boost DC to DC converter 600 which is adapted to generate a 5 volt potential from the battery voltage powering the radiotelephone. This DC/DC converter 600 is required in a radiotelephone for purposes such as supplying a white backlight for the display, supplying the USB "on the go" bus, etc. In the embodiment of Fig.4, the 5 volt potential produced by the boost DC/DC converter is fed additionally to operational amplifier O₁ to serve as the boost voltage, V_B. The load on the DC/DC converter 600 is not thereby unduly increased because the load thereon is typically already 200 mA, whereas the added load is typically approximately 200 µA
Fig.4 illustrates just one example where existing circuit components in a headset interface circuit, or in a device incorporating such a circuit, are re-used in order to produce a boost voltage employed in generating the stacked microphone biasing voltage used in the present invention. As mentioned above, the example of Fig.4 concerns the case where a headset amplifier and interface circuit 200' is used in a radiotelephone and the "re-used" component is a DC/DC converter. There are other applications of a headset interface circuit in which a comparable DC/DC converter will be present (for example, highly featured MP3 players with colour display and USB on the go), and these DC/DC converters can be "re-used" to produce the boost voltage, V_B, in a comparable manner to Fig.4.
There are other applications of a headset amplifier and interface circuit in which no such DC/DC converter is present, notably, cheaper MP3 players having lower technical specifications. However, in such MP3 players there will generally be a voltage boost device of some sort (for example, a charge pump) whose output can be tapped to produce the boost voltage, V_B.
Fig.5 illustrates a second preferred embodiment of the present invention, in which the stacked microphone biasing technique of the present invention is applied in a mono context.
More particularly, Fig.5 illustrates a headset amplifier and interface circuit 200_M adapted for supplying mono audio signals, A_M, and a voltage reference to a mono headset 100_M as well as to receive microphone signals therefrom, via three wires, W₁ to W₃. The mono headset 100_M includes one loudspeaker, SP, and a microphone, M.
The headset amplifier and interface circuit 200_M supplies mono audio signals, A_M, to the loudspeaker, SP, of the mono headset, 100_M, via a first wire, W₁. The headset amplifier and interface circuit 200_M also supplies a phantom ground potential to the mono headset, 100_M, via the second wire, W₂. Once again, it is convenient for the phantom ground to be constituted by the signal ground, SG, of an apparatus in which the headset amplifier and interface circuit 200_M is incorporated (or with which it is associated).
The headset amplifier and interface circuit 200_M also supplies the mono headset, 100_M, with a stacked microphone biasing potential V_MBiasSt, which is substantially equal to the sum of the phantom ground potential (here SG) and the potential difference required across the microphone, M. This stacked microphone biasing voltage can be produced using circuitry 400_M which is substantially the same as that used in the first preferred embodiment (circuit 400 of Fig.3). The boost voltage can be produced in a variety of ways, including by the approach illustrated in Fig.4.
Although the present invention has been described above with reference to certain particular preferred embodiments, it is to be understood that the invention is not limited by reference to the specific details (quoted voltage values, specific circuit components, etc.) of those preferred embodiments. More specifically, the person skilled in the art will readily appreciate that modifications and developments can be made in the preferred embodiments without departing from the scope of the invention as defined in the accompanying claims.
It is to be understood that references to "first wire", "second wire", etc. do not imply any particular order of preference or hierarchy among the wires interconnecting the headset and the headset interface circuit according to the present invention. The numbering is arbitrary and used solely to render the explanation more intelligible.

Claims

An interface circuit (200/200'/200_M) for a module (100/100_M) comprising one or more loudspeakers and a microphone, the interface circuit (200/200'/200_M) comprising:

means for outputting audio signals (A_L,A_R/A_M) at one or more audio output terminals;

means for establishing a phantom ground potential at a ground output terminal; and

means for providing a reference potential (V_MBiasSt) to a further output terminal, said reference potential taking a value substantially equal to the sum of the phantom ground potential and the biasing voltage required across the microphone.
An interface circuit (200/200'/200_M) according to claim 1, wherein said means for providing a reference potential comprises a microphone bias voltage generating circuit (400/400_M) and means for applying a boost voltage (V_B) to the microphone bias voltage generating circuit (400/400_M).
An interface circuit (200'/200_M) according to claim 2, adapted for use in a radiotelephone, wherein said means for applying a boost voltage (V_B) to the microphone bias voltage generating circuit (400) comprises a DC/DC converter (600) adapted to supply a DC voltage to at least one other component of the radiotelephone
An interface circuit (200/200'/200_M) according to any previous claim, adapted for using an audio ground potential (VAG), wherein the means for establishing a phantom ground potential at the ground output terminal is adapted to output the audio ground potential (VAG) at the ground output terminal.
A method of biasing a microphone (M) in a module (100) comprising one or more loudspeakers and a microphone, the method comprising the steps of:

connecting one terminal of the or each loudspeaker (SP_L,SP_R/SP) and of the microphone (M) to a phantom ground potential; and

applying a stacked microphone biasing potential (V_MBiasSt) to another terminal of the microphone (M), said stacked biasing potential (V_MBiasSt) taking a value substantially equal to the sum of the phantom ground potential and the biasing voltage required across the microphone.
A microphone biasing method according to claim 5, and comprising the step of generating the stacked microphone biasing potential (V_MBiasSt) by applying a boost voltage (V_B) to a microphone bias voltage generating circuit (400/400_M).
A microphone biasing method according to claim 6, wherein said boost voltage (V_B) is generated by a DC/DC converter (600).
A microphone biasing method according to claim 7, wherein said DC/DC converter (600) is adapted to supply a DC voltage to at least one other module.
A microphone biasing method according to claim 5, 6, 7 or 8, wherein:

the connecting step comprises connecting one terminal of the or each loudspeaker (SP_L,SP_R/SP) and of the microphone (M) to an audio ground potential (VAG); and

the stacked biasing potential (V_MBiasSt) is substantially equal to the sum of the audio ground potential (VAG) and the biasing voltage required across the microphone.
A system comprising a module (100/100_M) including one or more loudspeakers and a microphone, and an interface circuit (200/200'/200_M) according to any one of claims 1 to 4 for connection to said module (100/100_M).
Apparatus comprising an audio signal supply section, and an interface circuit (200/200'/200_M) according to any one of claims 1 to 4.
Apparatus according to claim 11, wherein the apparatus is a radiotelephone.
Apparatus according to claim 11, wherein the apparatus is a portable audio player.
A package including a module (100/100_M) including one or more loudspeakers and a microphone, as well as apparatus according to claim 11, 12 or 13 comprising an interface circuit (200/200'/200_M) for connection to said module (100/100_M).characterised in that.