Mixer apparatus

Abstract

The present invention relates to a mixer apparatus (30) for mixing audio signals from a musical instrument (10). The mixer apparatus (30) comprises plural input circuits (34) and an audio signal mixer (36). Each of the plural input circuits (34) has an audio signal input which, in use, is coupled electrically with and thereby receives an audio signal from a different one of plural musical instrument pickups (32) comprised in the musical instrument. The audio signal mixer (36) receives audio signals from the plural input circuits (34) and mixes the received audio signals with one another. Each of the plural input circuits (34) comprises a linear active circuit in an audio signal path between the audio signal input and the audio signal mixer (36).

Claims

1. A mixer apparatus for mixing audio signals from a musical instrument, comprising: plural input circuits, each of the plural input circuits having an audio signal input which, in use, is coupled electrically with and thereby receives an audio signal from a different one of plural musical instrument pickups for the musical instrument; and an audio signal mixer receiving audio signals from the plural input circuits and mixing the received audio signals with one another, wherein each of the plural input circuits comprises a linear active circuit in an audio signal path between the audio signal input and the audio signal mixer, wherein each of the plural input circuits comprises a first input circuit amplifier operative as a buffer amplifier to present a high impedance to the different one of the plural musical instrument pickups, the first input circuit amplifier comprising the linear active circuit, wherein each of the plural input circuits further comprises a second input circuit amplifier which receives an audio signal from the audio signal input and provides a second audio signal output which is the other of inverted and not inverted when the first input circuit amplifier provides a first audio signal output which is one of inverted and not inverted, and wherein each of the plural input circuits further comprises a phase mixer circuit which receives the first and second audio signal outputs and provides a phase mixer circuit output signal which is a combination of the first and second audio signal output signals, the audio signal mixer receiving the phase mixer circuit output signal from each of the plural input circuits and mixing the plural received phase mixer circuit output signals with one another.

2. The mixer apparatus of claim 1, wherein the mixer apparatus comprises the plural musical instrument pickups.

3. The mixer apparatus according to claim 2, wherein each of the plural musical instrument pickups comprises a current output sensor and is a one port device having first and second output terminals, the first output terminal being electrically connected to the audio signal input and the second output terminal being electrically connected to a circuit common defined by the mixer apparatus, each input circuit comprising an input circuit impedance element between the audio signal input and the circuit common, a current signal provided by the current output sensor being developed as a voltage signal across the input circuit impedance element, an impedance of the input circuit impedance element being digitally controlled.

4. The mixer apparatus of claim 1, wherein the linear active circuit of each of the plural input circuits has a substantially linear transfer function.

5. The mixer apparatus of claim 1, wherein the first input circuit amplifier is configured as a non-inverting amplifier having substantially unity gain, the second input circuit amplifier is configured as an inverting amplifier having substantially unity gain, and the second input circuit amplifier receives an audio signal from an output from the first input circuit amplifier whereby the second input circuit amplifier receives the audio signal from the audio signal input by way of the first input circuit amplifier.

6. The mixer apparatus of claim 1, wherein the phase mixer circuit comprises a digital potentiometer which is digitally controlled to change the relative proportions in the phase mixer circuit output signal of the audio signals from the first and second input circuit amplifiers.

7. The mixer apparatus of claim 6, wherein one end of the digital potentiometer is connected to the output from the first input circuit amplifier and another end of the digital potentiometer is connected to the output from second input circuit amplifier, a wiper of the digital potentiometer providing the phase mixer circuit output signal.

8. The mixer apparatus of claim 1, wherein the audio signal mixer comprises a summing circuit which receives the phase mixer circuit output signal from each of the plural phase mixer circuits and sums the plural received phase mixer circuit output signals to thereby mix the plural received phase mixer circuit output signals with one another.

9. The mixer apparatus of claim 8, wherein the summing circuit comprises a summing circuit amplifier operative as a buffer amplifier to thereby present a high impedance to each input circuit.

10. The mixer apparatus of claim 8, further comprising a gain stage which receives an audio signal from the summing circuit, the gain stage comprising a first variable gain circuit comprising an amplifier and having a digitally controlled gain and which receives the audio signal from an output of the summing circuit.

11. The mixer apparatus of claim 10, wherein the gain stage further comprises a second variable gain circuit which receives an audio signal from an output of the first variable gain circuit, the second variable gain circuit comprising a digitally controlled variable voltage divider.

12. The mixer apparatus of claim 10, further comprising a peak detector circuit, an analogue-to-digital converter and a processor, an output from the audio signal mixer being received as an input by the peak detector circuit, an output from the peak detector circuit being received as an input to the analogue-to-digital converter to thereby provide a digital representation of a sampled peak value, the digital representation of the sampled peak value being processed in the processor to determine if the output from the audio signal mixer should be amplified or attenuated, and the gain of the gain stage being digitally controlled in dependence on the determination made by the processor.

13. The mixer apparatus according to claim 1 further comprising an audio signal output connector electrically coupled to an output from the mixer apparatus, the audio signal output connector being configured to connect the mixer apparatus to a power amplifier by way of a lead.

14. The mixer apparatus according to claim 1 further comprising a hand-portable computing apparatus providing a software interface which digitally controls the mixer apparatus, digital control of the mixer apparatus being effected by wireless communication between the hand-portable computing apparatus and electronic circuitry of the mixer apparatus.

15. A musical instrument comprising the mixer apparatus according to claim 1 and the plural musical instrument pickups.

16. A method of mixing audio signals from a musical instrument, the method comprising: receiving in each of plural input circuits an audio signal at an audio signal input of the input circuit from a different one of plural musical instrument pickups for the musical instrument; receiving audio signals from the plural input circuits in an audio signal mixer; and mixing the received audio signals with one another in the audio signal mixer, wherein each of the plural input circuits comprises a linear active circuit in an audio signal path between the audio signal input and the audio signal mixer, wherein each of the plural input circuits comprises a first input circuit amplifier operative as a buffer amplifier to present a high impedance to the different one of the plural musical instrument pickups, the first input circuit amplifier comprising the linear active circuit, wherein each of the plural input circuits further comprises a second input circuit amplifier which receives an audio signal from the audio signal input and provides a second audio signal output which is the other of inverted and not inverted when the first input circuit amplifier provides a first audio signal output which is one of inverted and not inverted, and wherein each of the plural input circuits further comprises a phase mixer circuit which receives the first and second audio signal outputs and provides a phase mixer circuit output signal which is a combination of the first and second audio signal output signals, the audio signal mixer receiving the phase mixer circuit output signal from each of the plural input circuits and mixing the plural received phase mixer circuit output signals with one another.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Further features and advantages of the present invention will become apparent from the following specific description, which is given by way of example only and with reference to the accompanying drawings, in which:

(2) FIG. 1 is a representation of an electric guitar comprising a mixer apparatus according to the present invention;

(3) FIG. 2 is a block diagram representation of the mixer apparatus of FIG. 1;

(4) FIG. 3 is a schematic of an input circuit, audio signal mixer and gain stage of the mixer apparatus;

(5) FIG. 4 is a schematic of a peak detector circuit;

(6) FIG. 5 is a schematic of an alternative embodiment of input circuit;

(7) FIG. 6 is a schematic of a connector arrangement for the mixer apparatus; and

(8) FIG. 7 is a representation of a display of the local computing apparatus.

DESCRIPTION OF EMBODIMENTS

(9) A representation of an electric guitar 10 comprising a mixer apparatus 12 according to the present invention is shown in FIG. 1. The mixer apparatus 12 is mounted within the electric guitar 10. In the embodiment shown in FIG. 1 the mixer apparatus 12 is received in an existing space in the guitar, although the mixer apparatus may be mounted at any other location on the guitar which presents substantially no impediment to playing of the electric guitar. The mixer apparatus 12 is electrically powered by way of a rechargeable battery. Apart from the mixer apparatus 12, the electric guitar 10 is of conventional form and function. For example, the electric guitar 10 comprises three magnetic pickups 14 with each magnetic pickup at a respective one of bridge, middle and neck locations. Although not shown in FIG. 1, the mixer apparatus 12 is electrically connected by way of copper connections to each of the three magnetic pickups 14 whereby the mixer apparatus receives audio signals from all three of the magnetic pickups. Where the mixer apparatus 12 is retrofitted to the guitar, the mixer apparatus is electrically connected to the magnetic pickups 14 by re-soldering or re-wiring. As shown in FIG. 1, the mixer apparatus 12 is electrically connected by way of a lead 16 to a preamplifier and power amplifier 18 which in turn is electrically connected to a loudspeaker arrangement 20. The preamplifier and power amplifier 18 and the loudspeaker arrangement 20 are of conventional form and function. The mixer apparatus 12 comprises a Bluetooth or WiFi transceiver which provides for short range wireless communication with local computing apparatus 22 in the form of a tablet computer, smartphone or the like. As described below, the local computing apparatus 22 provides for control of the mixer apparatus 12. A central computing apparatus 24 is in wireless communication with the local computing apparatus 22 such as by way of the Internet. The central computing apparatus 24 is operated by or on behalf of a vendor, such as the vendor of a software interface, such as a dedicated App, that runs on the local computing apparatus 22.

(10) A block diagram representation of the mixer apparatus of FIG. 1 is shown in FIG. 2. The mixer apparatus 30 of FIG. 2 comprises four magnetic pickups 32 which are comprised in an electric guitar such as the electric guitar 10 of FIG. 1. A first end of the coil of each of the four magnetic pickups 32 is connected to a circuit common and in the present configuration to a zero volt line. The mixer apparatus 30 also comprises four input circuits 34 which are each electrically connected to a second end of the coil of a respective one of the four magnetic pickups 32 whereby the input circuit receives an audio signal generated by the magnetic pickup when the electric guitar is played. As described in more detail below, each input circuit 34 provides two audio output signals. The mixer apparatus 30 further comprises an audio mixer which comprises a summing circuit 36. The mixer apparatus 30 yet further comprises a voltage to current converter 38 and a switch arrangement 40 for each of the four input circuits 34. As described in more detail below, the summing circuit 36 receives one of the two audio output signals from each of the four input circuits 34 and sums the received four audio output signals. Each voltage to current converter 38 receives the other of the two audio output signals from a respective one of the four input circuits 34. FIG. 2 shows in inset a detailed schematic for the voltage to current converter 38. The detailed schematic for the voltage to current converter 38 is described below with reference to FIG. 3. Each switch arrangement 40 receives a current signal from a respective one of the four voltage to current converters 38 and selectively applies the received current signal to the second end of the coils of the four magnetic pickups 32.

(11) The mixer apparatus 30 also comprises an equalizer switch 42 which is operable under digital control to switch a parametric equalizer in and out of operation. The parametric equalizer is of known form and function with its parameters being set by digital potentiometers either directly or by way of the I2C bus mentioned below. The mixer apparatus 30 further comprises a gain stage 44 which is operative directly on the output from the summing circuit 36 or is operative on the output from the summing circuit after processing by the parametric equalizer when the parametric equalizer has been selected for operation by the equalizer switch 42. The gain stage 44 is described in more detail below with reference to FIG. 3. The mixer apparatus 30 yet further comprises a peak detector circuit, an analogue-to-digital converter 46, an embedded microcontroller 48 and a Bluetooth or WiFi transceiver 50. The analogue-to-digital converter 46, the embedded microcontroller 48 and the Bluetooth or WiFi transceiver 50 are of conventional form and function except as described herein. The peak detector circuit is shown in FIG. 4 but not shown in FIG. 1 although it is comprised in the mixer apparatus 30. The output from summing circuit 36 is received as an input by the peak detector circuit. An output from the peak detector circuit is received in the analogue-to-digital converter 46 to thereby provide a digital representation of a sampled peak value. The digital representation of the sampled peak value is received in the embedded microcontroller 48 and is processed to determine if the output from the audio signal mixer should be amplified or attenuated by way of the gain stage 44. The gain of the gain stage 44 is controlled digitally by the embedded microcontroller 48 in dependence on the determination. Digital control of the input circuits 34, the switch arrangements 40, the equalizer switch 42 and the gain stage 44 is by the embedded microcontroller 48 either directly or by way of a bus, such as an I2C bus. The embedded microcontroller 48 is further operative to control the Bluetooth or WiFi transceiver 50 to provide for communication between the circuitry of the mixer apparatus 30 and the local computing apparatus 22.

(12) A schematic of part of the mixer apparatus 30 of FIG. 2 containing one of the input circuits, the audio signal mixer and the gain stage is shown in FIG. 3. Each input circuit comprises a first input circuit amplifier 62 and a second input circuit amplifier 64. The first input circuit amplifier 62 comprises an op-amp 66 in a non-inverting amplifier configuration having substantially unity gain. The first input circuit amplifier 62 is therefore operative as a voltage buffer. The input circuit also comprises an audio signal input 68 which is connected to one end of the coil of a magnetic pickup 32 with the current return path connected to the other end of the coil and defining a zero volt circuit common. The input circuit further comprises an input circuit resistor 70 in parallel with the coil of the magnetic pickup 32 and a series capacitor 72 and parallel resistor 74 (which constitutes an input circuit impedance element) at the input to the op-amp 66. The live end of the coil (i.e. the end other than the end at circuit common) is connected to the non-inverting input of the op-amp 66. The first input circuit amplifier 62 thus provides a linear active circuit in a signal path between the live end of the input circuit resistor 70 and the input to the second input circuit amplifier 64. The input circuit resistor 70, the series capacitor 72 and the parallel resistor 74 place both input terminals of the op-amp 66 at zero volts but deliver the signal from the magnetic pickup to the op-amp 66 referenced to 2.5 volts. The parallel resistor 74 is the main determinant of input impedance although input circuit resistor 70 does contribute to input impedance. This is because the input circuit resistor is of much higher value than the parallel resistor. The input circuit resistor 70 is present to charge the series capacitor 72 to 2.5 volts at switch on whether or not a magnetic pickup 32 is connected whereby the end of the magnetic pickup at circuit common is at the same potential as circuit common of the mixer apparatus. The return path of the power supply to the op-amp 66 is connected to the zero volt circuit common with the inputs to the op-amp being referred to a circuit common at a voltage mid-way between the power supply voltage and zero volts.

(13) The second input circuit amplifier 64 comprises an op-amp 76 in a unity gain inverting amplifier configuration. The second input circuit amplifier 64 receives the output from the first input circuit amplifier 62 at its inverting input. The input circuit therefore provides two buffered voltage signals which each correspond substantially to the audio signal received at the audio signal input with a first one of the two buffered voltage signals being non-inverted and a second one of the two buffered voltage signals being inverted.

(14) Each input circuit yet further comprises a first digital potentiometer 78 (which constitutes a first phase mixer circuit) which is connected at one end to the output from the first input circuit amplifier 62 and is connected at the other end to the output from second input circuit amplifier 64. The ‘wiper’ of the first digital potentiometer 78 provides an input to the summing circuit. The first digital potentiometer 78 is digitally controlled by the embedded microcontroller 48 to move the ‘wiper’ whereby the ‘wiper’ provides an output which corresponds at one end of movement to the non-inverted output from the first input circuit amplifier 62 and at the other end of movement to the inverted output from the second input circuit amplifier 64. When the ‘wiper’ is at its mid-point, the outputs from the first and second input circuit amplifiers 62, 64 cancel each other whereby a signal from the magnetic pickup 32 connected to the present input circuit is not applied to the summing circuit. As the ‘wiper’ moves above its mid-point location, the ‘wiper’ provides an output of changing proportions of outputs from the first and second input circuit amplifiers 62, 64 with the output from the second, inverting circuit amplifier 64 predominating. As the ‘wiper’ moves below its mid-point location, the ‘wiper’ provides an output of changing proportions of outputs from the first and second input circuit amplifiers 62, 64 with the output from the first, non-inverting circuit amplifier 62 predominating.

(15) The summing circuit 36 comprises an op-amp 82 in a unity gain summing amplifier configuration. The summing circuit 36 receives an input from the ‘wiper’ of the first digital potentiometer 78 of each of the plural input circuits whereby the output from the summing circuit is a voltage analogue of the summed current signals generated by the coils of the magnetic pickups 32. The mixer apparatus 30 thus provides a series emulation mode.

(16) Each input circuit further comprises a second digital potentiometer 80 (which constitutes a second phase mixer circuit) which is connected at one end to the output from the first input circuit amplifier 62 and is connected at the other end to the output from second input circuit amplifier 64. The ‘wiper’ of the second digital potentiometer 80 provides an input to the voltage to current converter 38 shown in FIG. 2. Otherwise, the second digital potentiometer 80 is configured as described above with reference to the first digital potentiometer 78. The ‘wiper’ of the second digital potentiometer 80 therefore provides an output comprising changeable proportions of outputs from the first and second input circuit amplifiers 62, 64. The voltage signal at the output from the second digital potentiometer 80 is converted to a floating current signal analogue by way of the voltage to current converter 38. The voltage to current converter 38 is in the form of a Howland current pump having a two op-amp configuration. A first op-amp of the Howland current pump is in the forward path with the second op-amp of the Howland current pump being in the feedback loop. Each input circuit also comprises the switch arrangement 40 shown in FIG. 2 to which the output from the voltage to current converter 38 is applied as an input. The switch arrangement 40 is digitally controlled by the embedded microcontroller 48 to apply the current signal to none or a selected one of the other input circuits. The current signal is applied at the live end 84 of the input circuit resistor 70 of an input circuit whereby current signals from two or more magnetic pickups are summed. The second digital potentiometer 80, the voltage to current converter 38 and the switch arrangement 40 of each of the input circuits therefore provide a parallel emulation mode.

(17) The gain stage 44 of the mixer apparatus 30 will now be described. The gain stage 44 receives an audio voltage signal from the output of the summing circuit 36. The gain stage 44 comprises a first variable gain circuit 84 and a second variable gain circuit 86. The first variable gain circuit 84 comprises an op-amp 88 configured as an inverting amplifier. The first variable gain circuit 84 further comprises a third digital potentiometer 90 with a first end of the third digital potentiometer in a feedback loop of the op-amp 88, a second end of the third digital potentiometer connected to a resistor in series with an inverting input of the op-amp 88, and a ‘wiper’ of the third digital potentiometer connected to the inverting input of the op-amp 88. The third digital potentiometer is digitally controlled by the embedded microcontroller 48. The first variable gain circuit 84 is configured by way of selection of resistance values to provide for fine volume control, such as between −12 dBV and +12 dBV.

(18) The second variable gain circuit 86 receives an audio voltage signal from the first variable gain circuit 84. The second variable gain circuit 86 has the form of a variable voltage divider. The variable voltage divider comprises a fourth digital potentiometer 92 which is digitally controlled by the embedded microcontroller 48. A first end of the fourth digital potentiometer 92 is connected to an output from the first variable gain circuit 84 and a second end of the fourth digital potentiometer is connected to the mid-rail common. A ‘wiper’ of the fourth digital potentiometer 92 provides an output from the variable voltage divider. The variable voltage divider further comprises a voltage divider resistor 94 between the ‘wiper’ of the fourth digital potentiometer 92 and the mid-rail common to thereby make operation of the variable voltage divider non-linear. The second variable gain circuit 86 is configured by way of selection of appropriate resistance values to provide for coarse volume control. The mixer apparatus 30 further comprises a voltage follower 96 at the output from the second variable gain circuit 86.

(19) The mixer apparatus 30 comprises the peak detector circuit 100 shown in FIG. 4. The mixer apparatus 30 receives an output from the summing circuit 36 as an input. The peak detector circuit 100 is digitally controlled by the embedded microcontroller 48 to sample and hold peak signals and to reset at an interval of milliseconds appropriate to the bandwidth of the audio signal. The peak detector circuit 100 is of known design. FIG. 4 shows an example of peak detector circuit. The output from the peak detector circuit 100 is received in the analogue-to-digital converter 46 to thereby provide a digital representation of a sampled peak value. The digital representation of the sampled peak value is processed in the embedded microcontroller 48 to determine if the output from the summing circuit 36 should be amplified or attenuated and, if so, an extent of amplification or attenuation to thereby provide automatic signal level control. The embedded microcontroller 48 then controls the gain of the gain stage 44 by way of control of the third and fourth digital potentiometers 90, 92. As described above, control is either direct from the embedded microcontroller 48 or by way of a bus, such as an I2C bus.

(20) The mixer apparatus 30 comprises active electronic components, such as the op-amps 66, 76, 82, 88, which are put selectively into a low power state by either cutting their power supply or by digital control where such active electronic components are configured of themselves to enter a low power mode. The mixer apparatus is configured to put such active electronic components into the low power state when no signal or when a signal no greater than a predetermined threshold is received from any musical instrument pickup over a predetermined period. Although not shown in the drawing, the mixer apparatus 30 comprises a wake-up circuit which changes such active electronic components from the low power state to an operative state. The wake-up circuit comprises an amplifier, such as an op-amp based amplifier, which receives an output from at least one of the musical instrument pickups 32 as an input and provides an output to the embedded microcontroller 48. The embedded microcontroller 48 performs threshold detection on the wake-up circuit input with active electronic components which have been put into the low power state being powered up under control of the embedded microcontroller in dependence on an outcome of the threshold detection.

(21) A schematic of an alternative embodiment of input circuit 110 is shown in FIG. 5. The input circuit 110 of FIG. 5 is as described above with reference to FIGS. 2 and 3 except as will now be described. The first input circuit amplifier 112 of the embodiment of FIG. 5 is a unity gain non-inverting amplifier. The input circuit resistor 70 of the embodiment of FIG. 3 is replaced in the embodiment of FIG. 5 with a digitally controlled variable resistor 114. The variable resistor 114 is controlled by the embedded microcontroller 48 to change the input impedance and thereby change the damping characteristic. Changing the damping characteristic changes the tonal characteristics of the signal from the magnetic pickup 32.

(22) A schematic of a connector arrangement for the mixer apparatus is shown in FIG. 6. As shown in FIG. 1, the mixer apparatus 12, 30 is electrically connected by way of a lead 16 to the like of a power amplifier 18. The connection is made by way of a jack socket comprised in the mixer apparatus 12, 30 and a mono jack plug on the end of the lead 16. The jack socket 120 comprised in the mixer apparatus 12, 30 is of stereo form with the chassis terminal 122 connected to the zero volt circuit common, the tip terminal 124 connected to the audio output signal from the mixer apparatus (i.e. the output from the gain stage 44) and the ring terminal 126 providing a battery charging current for recharging the battery comprised in the mixer apparatus 12, 30. The jack socket 120 therefore provides an outgoing path for the audio output signal and incoming path for the battery charging current with the two paths sharing a circuit common return path.

(23) The connector arrangement also comprises a first adaptor 130. The first adaptor 130 is terminated with a stereo jack plug 132 which is received in the stereo jack socket 120. The tip terminal 134 of the stereo jack plug 132 is connected to the tip terminal of a mono jack socket 136 and the chassis terminal 138 of the stereo jack plug is connected to the chassis terminal of the mono jack socket. The mono jack plug on the end of the lead 16 is plugged into the mono jack socket 136 of the first adaptor 130. The ring terminal 140 of the stereo jack plug 132 is connected to a first terminal of a micro-USB socket 142 and the chassis terminal 138 of the stereo jack plug is connected to a second terminal of the micro-USB socket. A micro-USB plug on a lead from a power supply is plugged into the micro-USB socket 142 with the corresponding terminals of the micro-USB plug carrying a charging current. The first adaptor 130 thus provides for recharging of the battery while the mixer apparatus 12, 30 is being used.

(24) The connector arrangement also comprises a second adaptor 150. The second adaptor 150 is terminated with a stereo jack plug 152 which is received in the stereo jack socket 120 of the mixer apparatus. The tip terminal 154 of the stereo jack plug 152 is not connected. The ring terminal 156 of the stereo jack plug 152 is connected to a first terminal of a micro-USB socket 158 and the chassis terminal 160 of the stereo jack plug is connected to a second terminal of the micro-USB socket. A micro-USB plug on a lead from a power supply is plugged into the micro-USB socket 158 with the corresponding terminals of the micro-USB plug carrying a charging current. The second adaptor 150 thus provides for recharging of the battery while the mixer apparatus 12, 30 is not being used.

(25) As described above, the mixer apparatus is in different respects under digital control by the embedded microcontroller 48. The embedded microcontroller 48 also interfaces by way of a Bluetooth or WiFi communication channel with the local computing apparatus 22 shown in FIG. 1. The local computing apparatus 22 runs a software interface in the form of an App which provides user control of the mixer apparatus 12, 30 and feedback from the mixer apparatus to the user. The App provides for control of the first and second digital potentiometers 78, 80, the switch arrangements 40 and the variable resistors 114. Control of the mixer apparatus is by way of an interactive graphical user interface provided on a touchscreen display of the local computing apparatus 22. A representation of the touchscreen display 180 is shown in FIG. 7.

(26) A user of the local computing apparatus 22 creates a user account with the central computing apparatus 24 to provide full access to the App and functionality of the mixer apparatus 12. 30. Creation of a user account and access to the user account is in accordance with known practice. Thereafter, the user has access to plural different electric guitar configurations which are stored in the central computing apparatus 24. The user selects an appropriate one of the plural different electric guitar configurations from a drop-down list shown on the display 180. The user then takes a photograph of the actual electric guitar 10 which is to be used with the mixer apparatus 12, 30 by way of a camera comprised in the local computing apparatus 22. The taking of a photograph is aided by the App providing an outline of the selected electric guitar on the display 180 of the local computing apparatus whereby the user can position the local computing apparatus such that the displayed outline is in registration with an image of the actual electric guitar acquired by the local computing apparatus and displayed on the display. When the user is happy with the composition of the image acquired by the camera, the user operates the local computing apparatus to store an image of the actual electric guitar. The stored image is processed by the App to abstract a colour scheme of the guitar and to incorporate the abstracted colour scheme in a representative image of the selected electric guitar which is received in the local computing apparatus 22 from the central computing apparatus 24. The representative image is thus personalised.

(27) As shown in FIG. 7, the App is then operative to present the representative image 182 of the elected electric guitar on the display 180. The pickups 184 comprised in the representative image are shown as larger relative to the rest of the guitar than in the actual guitar. Furthermore, the pickups 184 are displayed such that they float above their location on the actual guitar as indicated by the arrows present in FIG. 7. Each of the pickups 184 in the representative image is an object which is manipulable by a user independently of the rest of the representative image. The App translates different forms of manipulation of the pickup objects into different control data for controlling the mixer apparatus 12, 30. For example, tapping a pickup object switches between a pickup being selected and deselected. By way of further example, changing the shape of the pickup object, such as stretching or contracting the object along at least one of two orthogonal axes, controls gain at the input to the corresponding input circuit. By way of yet further example, rotation of a pickup object controls at least one of the first and second digital potentiometers 78, 80. Series emulation, parallel emulation and gain for each pickup may thus be controlled.

(28) A guitar usually comprises plural manually operable controls, such as a switches and control knobs. When the mixer apparatus is brought into use, the manually operable controls are electrically connected to inputs to the mixer apparatus. Where a manually operable control is a switch, the input to the mixer apparatus is received at a digital input to the embedded microcontroller 48. Where a manually operable control is a control knob, the input to the mixer apparatus is received by an analogue-to-digital converter and the converted digital input is then received in the embedded microcontroller 48. The inputs received in the embedded microcontroller 48 are used by way of firmware running on the embedded microcontroller to provide for control of the mixer apparatus, such as in respect of master volume and master tone.

(29) In a form of the invention, the mixer apparatus comprises an illuminated rotary encoder (which constitutes a primary manually operable control). The illuminated rotary encoder is an EC12PLRGBSDVBF-D-25K-24-24C from Top-Up Industry Corp. of 8F, No. 189, Yung An Road, Taoyuan, Taiwan 33054. In use, the illuminated rotary encoder replaces a control knob comprised in the guitar upon manufacture. Alternatively, the illuminated rotary encoder is incorporated in the guitar upon manufacture. The illuminated rotary encoder is electrically coupled to the embedded microcontroller 48 with the embedded microcontroller being configured by way of firmware to make selections from predetermined configurations (or patches) for the guitar in dependence on the position of the illuminated rotary encoder.

(30) The mixer apparatus provides for a wide range of different settings. The App provides for storage and naming of each setting. Furthermore, stored settings are conveyed by way of the above described user account for storage on the central computing apparatus 24 from where they may be retrieved by the user for subsequent use. According to another application, the operator of the central computing apparatus 24 forms a library of reference audio files for different makes and models of electric guitar. Each reference audio file is formed by recording the sound of a particular make and model of electric guitar when, for example, the guitar is strummed on all strings concurrently and open chord. The user records in a personal audio file the sound of his or her guitar when strummed in the same fashion. The user also selects a particular make and model of electric guitar by way of the App. The App is operative to compare, such as by way of correlation, the reference audio file for the elected guitar and the personal audio file and to determine how to make the audio signal from the user's guitar sound like the selected guitar. The App then forms a reference configuration in dependence on this determination and downloads the reference configuration to the embedded microcontroller 48 which then controls the mixer apparatus 12, 30 accordingly.

(31) In a further application, user account data comprised in the central computing apparatus 24 comprises a unique code, such as a MAC address or IMEI number, for the local computing apparatus 22 of each of plural mixer apparatus 12, 30 operating with the central computing apparatus. The unique code provides for identification of each of the mixer apparatus 12, 30. The user account data further comprises information relating to the electric guitar with which the mixer apparatus 12, 30 is operable. The information relating to the electric guitar comprises the like of a photograph of the electric guitar and a photograph of documents serving as proof of ownership of the electric guitar, such as a purchase receipt. The user account data therefore serves to provide proof of ownership of the electric guitar. The central computing apparatus 24 is configured to regulate transfer of ownership of the electric guitar by controlling access to the user account by the present and future owners. Such an approach is useful where the seller and buyer are remote from each other. More specifically, the process involves the seller sending the electric guitar to the buyer and the seller retaining control of the user account until payment for the electric guitar clears whereupon control of the user account is passed to the buyer.

(32) In a further application, the mixer apparatus provides a ‘find my guitar’ function. As described above, communication between the local computing apparatus 22 and the electronic circuitry of the mixer apparatus 12, 30 is by way of a Bluetooth or WiFi communication channel. In this further application, the local computing apparatus 22 and the electronic circuitry of the mixer apparatus 12, 30 are configured for periodic wireless communication between the local computing apparatus and the electronic circuitry of the mixer apparatus whereby presence of the electronic circuitry of the mixer apparatus and hence the electric guitar can be determined. If the electric guitar and hence the electronic circuitry of the mixer apparatus is moved, the local computing apparatus is operative to determine that there has been no periodic wireless communication and to generate an alarm in dependence on this determination. The alarm may be output by at least one of the local computing apparatus 22 and the central computing apparatus 24. The App provides for selection between generation of an alarm being enabled and generation of an alarm being disabled with, for example, the former being an ‘away from home’ mode and the latter being an ‘at home mode’.

(33) The App provides security of access features. The App is optionally configured by a user to require full access to the App upon entry of a password. The password is entered by manipulation of the image objects 184 of FIG. 7 in a predetermined fashion. Further to this, the App generates an unauthorised access attempt notification after a predetermined number of failed attempts to enter the password whereby the App is locked and the central computing apparatus 24 is notified accordingly. Unlocking of the App is by way of communication by the user with the operator of the central computing apparatus 24 in accordance with procedures of known form.