AUDIO SIGNAL GENERATING SYSTEM

Abstract: An audio signal processing system for use with a stringed musical instrument, for exemple a guitar, includes a transducer assembly (1) comprising a respective transducer (6, 7) for each string, the outputs of the transducers being independent of each other. Signal processing means for each transducer output includes a loop of effects devices (20), such as reverberation, flanger, echo or delay devices, which can be switched in or out of circuit as desired by the musician. The outputs corresponding to the notes played on the respective strings can therefore be processed entirely independently of each other. The processed signals from the various transducers are then combined, as desired, into two stereo channels (29, 30). The signals in the stereo channels are amplified and further processed, if desired, by another set of effects devices (40). They can also be fed through respective bucket brigade delay devices (54) and through voltage-controlled amplifiers the gain of which can be varied to adjust the stereo balance and/or to cause panning of the stereo image. The panning may be cyclic, in accordance with a triangular or sinusoidal waveform. The routing of the signals, the connection of effects devices, the length of delay imparted to the signals, the control of the stereo balance and/or the control of the panning is determined by a microprocessor which is controlled by routines contained in an EPROM and by instructions entered into a RAM by the user.

Description: AUDIO SIGNAL GENERATING SYSTEM This invention relates to an audio signal generating system, and particularly to a system for generating electrical signals in response to notes played on the strings of a musical instrument, for example an electric or acoustic guitar or a piano. The electric guitar and the electric bass guitar are two of the most commonly used musical instruments in the field of non-traditional (i.e. "popular") music. Such instruments are provided with transducers which produce electrical signals in response to plucking of the strings. The transducer output is fed out of the instrument via a single channel, so that the output from the instrument is a composite signal representing the notes of all the four or six strings together. There is a continuing desire by the musicians to create new effects by the use of devices which modify the composite signal produced by the instrument. Equipment manufacturers are continually striving to produce new devices for this purpose. Such devices, including reverberation or echo producing circuits and mechanisms, delay lines and "flanger" and "chorus" devices, are all well-known and are commonly used in the popular music field to create illusions of space and movement in the music. The range of such effects is limited by the fact that the modifying devices can be used only to modify the single composite output of the instrument. The sound from each string cannot be modified independently of the others. It is an object of the present invention to provide an audio signal generating system which will make possible a very considerable extension of the available range of effects. According to the invention there is provided an audio signal generating system, comprising a plurality of transducers each coupled, or to be coupled, to a respective string of a musical instrument and each operative to produce, in a respective signal channel, an audio electrical signal corresponding to notes played on the respective string; and respective audio signal modifying means coupled to the signal channels and operable to act on the electrical signal in the respective channel, whereby the electrical signal in any of the channels can be modified, as desired, independently of the other channels. The transducers may, for example, be electromagnetic or piezoresistive transducers, and the signal modifying means may comprise, for the respective channel, a signal distorting device or group of distorting devices. An embodiment . of the invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 is a schematic plan view of a transducer assembly forming part of a system according to the invention; Figure 2 is a cross section taken on a line II-II of Figure 1; Figure 3 is a block schematic diagram of an input section of the system; Figure 4 is a block schematic diagram of a master effects section of the system; Figure 5 is a block schematic diagram of an audio output section of the system; Figure 6 is a block schematic diagram of a data processing section of the system; and Figure 7 is a block schematic diagram of interfacing circuitry of the system. Referring to Figures 1 and 2, a transducer assembly 1 for a 6-stringed guitar comprises six pole pieces 2 mounted in a line across a base plate 3 and spaced-apart to align with the strings 4 of the guitar. Each pole piece is mounted on the base plate by means (not shown) for adjusting the gap between the tip of the pole piece and the respective string. A bar magnet 5 is mounted on the base plate, in line contact with the periphery of each pole piece. A corresponding second set of pole pieces 6 is similarly mounted at the opposite side of the magnet 5. On each of the pole pieces 2 is mounted a respective main coil 7 and on each of the pole pieces 6 is mounted a respective hum-bucking coil 8. The main coil and the corresponding hum-bucking coil for each string are interconnected, and the lead-out wires therefrom are connected to a corresponding pair of tags 9, 10.. In some circumstances the hum-bucking coils and their pole pieces could be omitted. Each pair of tags 9 and 10 carries signals produced by plucking the respective string, quite independently of the signals produced by the transducers related to the other strings. The transducer assembly 1 is mounted at any suitable position along the length of the strings, but it is preferable to mount it fairly close to the bridge, where the amplitude of the oscillations of the strings is not too great. This reduces the likelihood of the movement of a string affecting the pairs of transducers associated with the adjacent strings, and hence reduces the likelihood of cross-talk being generated. The transducer assembly or "pick-up" may form part of an electric guitar o it may be used as an addition to an acoustic guitar. Furthermore, it may be provided as part of a kit for modifying an existing guitar. Conventional guitars are commonly provided with one or two monophonic pickups, i.e. pickups which each provide a single output signal which is a composite signal for all of the strings. If two such pickups are provided, one will be near the bridge and the other further along the strings, i.e. closer to the finger board. If the present invention is to be used in respect of an existing electric guitar, the transducer assembly 1 will preferably be constructed such that it is a direct replacement for that one of the monophonic pickups which is closer to the bridge, i.e. so that the assembly 1 can be substituted for the conventional pickup without effecting any changes in the fixings, etc. on the guitar. No modification of an existing electric guitar, which may be a very valuable instrument, is therefore required. Instead of " using an electromagnetic transducer assembly as described above, an assembly of piezoresistive transducers may be mounted in the bridge of the guitar. In this case, each piezoresistive element will have a layer of copper bonded to each of two opposite sides to provide the pair of signal contacts, the pair of contacts for each element being completely isolated from those of every other element. Although a guitar having six strings is described above, clearly the transducer assembly may be made in- a suitable configuration for use on an instrument having fewer or more strings. Such instrument might, for example, be a violin, a 'cello, a double bass, or any other stringed instrument. A signal processing system for modifying the signals from the transducers will now be described. Figure 3 shows an input section of the system. There will be n such input sections, where n is the number of strings on the instrument. Each input section comprises an input buffer amplifier 11 which effects impedance matching. The respective transducer output signal is fed from the terminals 9, 10 (Fig.l) to the input section via a line 13, which is a core of a ulticore screened cable 12 connecting the transducer assembly to the signal processing system. The signal is fed to the non-inverting input of the amplifier 11. The output of the amplifier is fed via a capacitor 14 to a pre-emphasis circuit comprising a resistor 15 and a capacitor 16 connected in parallel. The latter circuit gives a lift to the treble end of the frequency spectrum to reduce the effect of noise produced in subsequent stages of the system. The pre-emphasised signal is fed to a field effect transistor (FET)- 17 which acts as a switch under the control of a gate voltage from a line 19. The signal from the amplifier 11 is also fed to one end of a chain of sound processing (effects) devices 20, such as a reverberation unit, a flanger, an echo unit, etc., selected as preferred by the musician. The other end of the processing chain is fed to a second FET switch 21 via an amplifier 22 and a pre-emphasis circuit 23, 24. The switch 21 is controlled by a voltage from a line 25. The outputs of the switches 17 and 21 are fed in parallel, via an amplifier 26, to two voltage-controlled amplifiers 27 and 28, the outputs of which are fed to a "left" signal bus 29 and a "right" signal bus 30, respectively. All of the input sections for the other strings can be similarly connected to a respective chain of effects devices, as desired. The output voltage-controlled amplifiers of those sections are also connected to the left and right signal buses 29, 30. The gain of the amplifier 28 is controlled by a voltage from an amplifier 31, which is itself controlled by a voltage from a "pan" line 32 or a voltage from an "autopan" line 33. The output of the amplifier 31 is fed to an inverter 34, the output of which controls the gain of the amplifier 27. Each of the amplifiers 27 and 28 has a respective circuit 35, 36 for controlling the offset of the amplifier. If desired, the gain of either of the amplifiers 27 and 28 may be reduced to zero, so that the signal from the amplifier 26 is not fed by that amplifier 27 or 28 to the respective bus 29 or 30. Figure 4 shows a master effects section of the system. There will be two such output sections, one for "left" signal processing and the other for "right" signal processing. All of the "left" signals from the n input circuits are fed over the bus 29 and are applied to the input of an amplifier 37, which acts as a virtual earth mixer. The output of the amplifier 37 is fed via a FET switch 38 to an amplifier 39, the output of which is fed to another chain of effects devices 40, such as reverberation, echo or harmoniser devices, which can be used as well as, or instead of, the devices 20 in the input sections. These devices will generally be foot-operated devices controlled by the musician. The other end of the chain is fed to an amplifier 41 and thence to an FET switch 42. If, as mentioned previously, a monophonic pickup is provided on the guitar, in addition to the hexaphonic transducer assembly 1, the output of that pickup is fed over a line 43 to the input of the amplifier 39, via an FET switch 44. The switches 37 and 44 are controlled by voltages on respective lines 45 and 46, so that a high-level signal on the line 46 causes the switch 44 to feed the output of the monophonic pickup to the amplifier 39, and a high-level signal on the line 45 causes the switch 38 to feed the output of the amplifier 37 to the amplifier 39. Both of the switches may be on at the same time, if desired. The output of the amplifier 39 is also fed, via an FET switch 47, to the input of an amplifier 48. The output of the switch 42 is also connected to that input. By applying a control voltage to either a line 49 or a line 50, the switch 42 or the switch 47 can be turned on. Hence, the signal from the amplifier 39 can be fed either directly, through the switch 47, or via the effects loop, through the switch 42, to the amplifier 48. The output of the amplifier 48 is fed to a "left" : output line 49 via one or the other of two paths, as controlled by FET switches 50 and 51, respectively, which are operated by gate signals applied to lines 52 and 53, respectively. If the switch 50 is turned on, the signal from the amplifier 48 will be fed directly to the output line 49, but if the switch 51 is turned on, that signal will be fed to the output line via a "bucket brigade" delay circuit 54 comprising a chain of charge-coupled devices. An integrated circuit 55 to which the output of the amplifier 48 is fed performs two functions. Firstly, it acts as a current-controlled oscillator for generating clock pulses for the delay device 54. A variable current for controlling the oscillator frequency is produced by a circuit 56 in response to a variable voltage applied thereto over a line 57. The voltage may, if desired, be ' modulated so that it has, for example, a sawtooth or sinusoidal waveform, whereby corresponding modulation of the delay provided by the circuit 54 is effected. Secondly, the circuit 55 provides an anti-aliasing function by filtering out from the audio-frequency signal passed to the delay circuit 54 the higher frequency components which might otherwise beat with the sampling frequency of the circuit 54. The cut-off frequency of the filter varies with the clock pulse rate produced by the circuit 55. The clock pulses from the circuit 55 are fed to a divide-by-eight circuit 58, and the divided clock is fed to a circuit 59 which produces the two antiphase clock signals required for shifting the audio signal through the delay circuit 54. -8- A number of different effects can be produced by the circuit of Fig. 4. If the direct route for the audio signal from the amplifier 48 to the output 49 is switched off, modulation of the delay period produced by the delay circuit 54 will result in a pitch-changing or vibrato effect. If both the direct route and the delay route are switched on via the switches 50 and 51, two different effects can be obtained. If the delay is constant, a stereo "spacial enhancement" effect (i.e. apparent widening of the • stereo image) can be achieved. If the delay is variable, a "chorus" effect is provided. Further effects can be achieved by feeding the signals from the monophonic pickup and the signals from the hexaphonic transducer assembly simultaneously to the amplifier 48 via the switches 44 and 38, using or omitting the effects devices 40, as desired. Figure 5 shows an audio output section of the system. The line 43 from the monophonic pickup, if provided, is coupled to an amplifier 60 and thence to a monophonic output jack 61 or to an output line 62 if the jack is not in use. The line ' 62 may be coupled to a power amplifier for feeding centrally-placed loudspeakers. The "left" output line 49 (from Fig. 4) feeds an amplifier 63, the output of which is coupled to a voltage-controlled amplifier 64. The gain of the amplifier 64 is controlled by a voltage from an amplifier 65, which receives a control signal over a "left balance" line 66. The offset of the amplifier 64 is controlled by a variable resistance network 67. The output of the amplifier 64 is fed to an operational amplifier 68 which has a frequency-sensitive feedback circuit 69, 70 for providing de-emphasis to cancel the pre-emphasis which was applied in the input sections (Fig.3). The output of the amplifier 68 is fed to a "left output" jack 71 and also to a phase-splitting circuit 72 which provides an output signal on lines 73 and 74 which is balanced relative to a ground line 75. A "right" signal from a circuit identical to that of Fig. 4 is fed over a line 76 to an audio channel 77 comprising components 78-90 which are identical to the components 63-75, respectively, of the left-hand channel. A master volume control 91, which is provided on the guitar, controls the dc voltage appearing at the input of an amplifier 92. The output voltage from that amplifier is fed to the inputs of the amplifiers 65 and 80, to control the gain of the voltage-controlled amplifiers 64 and 79 simultaneously. The outputs of the amplifiers 68 and 83 are both fed to the input of an amplifier 93, which combines the left and right channel signals to give an output on a jack 94 for use, for example, when tuning the instrument. The left and right channel outputs from the jacks or the balanced output lines can be fed to a mixing and/or signal recording equipment (not shown) , and/or to stereo power amplifiers and loudspeakers (not shown) . By adjustment of the voltages on the lines 66 and 81, balancing of the left and right channels can be effected to centralise the stereo image, or to move it towards one side or the other, as desired. A comparator 95 produces an output signal on a line 96 for switching the line 45 (Fig. 4) in dependence upon whether there is an open-circuit or a ground connection on a line 97. Referring to Fig. 6, the switching of the various audio paths, the panning and the variable delay functions are controlled by a central process unit (CPU) 98. The CPU 98 operates in accordance with data stored in an EPROM 99 and a RAM 100. The EPROM 99 is programmed, as desired, so that it contains routines which have to be executed by the CPU 98. The RAM 100 holds data which are fed in by the musician, and also acts as a scratch pad memory for the CPU. Because of the volatile nature of the RAM 100, a battery 106 is provided as a back-up power supply. An oscillator circuit 101 is controlled by a crystal 102 to produce a clock signal at, for example, 4.0 MHz. This clock signal is divided, by circuits 103 and 104, down to approximately 1 kHz, and the divided clock is fed to an interrupt line 105 for the CPU. This causes the CPU to scan continuously through the tasks set by the program in the EPROM 99. Address outputs 107 of the CPU 98 are connected to the EPROM 99 and to the ROM 100 over an address bus 108. When the system has been switched off, resetting of the CPU 98 to its correct initial state when the " supply is switched back on is ensured by a Schmitt trigger 109 which is connected to the RESET input of the CPU. Control of the functions is effected via an integrated circuit 110 which has three data ports. A, B and C. Data from and to the CPU 98, the EPROM 99, the ROM 100 and the IC 110 are distributed over a data bus 111. Data for selecting the various functions are fed out from the port B of the IC 110 over a data bus 112. Data for producing analog values for the panning, left and right balance and left and right delay functions are fed from the port A over a data bus 113. Control signals from a keypad 114, and from a jack 115 to which a footswitch can be connected, are fed over a control bus 116 to the port C. The CPU 98 controls a liquid crystal display (LCD) unit 117 to display instructions, and menus of possible settings for the various functions, for use by the musician. The musician can instruct the CPU, in accordance with the displayed data, by operation of the relevant switches in the keypad 114 and/or by operation of the footswitch connected to the jack 115. The LCD 117 preferably displays two rows of characters with sixteen characters per row. The CPU 98 and the associated components are capable of operating with the standard musical instrument digital interface (MIDI) system to drive synthesisers, drum machines and other MIDI-operated instruments. For this purpose a parallel-to-serial converter 118 converts the parallel data from the CPU 98 into serial data for transmission over a line 119 to a MIDI output 120. The converter also receives input data, over a line 121, from a MIDI input 122. The input 122 is coupled to the line 121 via an optocoupler 123, the use of which avoids a possible source of ground loops. A decoder 124 applies enable signals to the components 99, 100, 117 and 118 selectively, as commanded by the CPU 98. The data outputs from the ports A and B of the IC 110 are fed over the buses 113 and 112, respectively, to interfacing circuitry which is shown in Fig. 7. The digital data on the bus 113 are converted to analog levels by a digital-to-analog converter (DAC) 125, and the analog signal is fed, via an amplifier 126, to two decoders 127 and 128. Selection data from the bus 112 are fed to the decoder 127, which determines therefrom which of a set of output lines 129-136 is to be selected for application thereto of the analog level. The lines 129-136 are connected to respective sample-and-hold buffer circuits 137-144 having respective capacitors 145-152 and respective outputs 153-160. Selection data are also fed from the bus 112 to the decoder 128, which operates in a similar manner to the decoder 127 to feed the analog value to a selected one of a set of lines 161-163, to which are connected respective sample-and-hold buffers 164-166 having respective capacitors 167-169 and respective outputs 170-172. The output 153 is connected to the pan control line 32 of Fig. 3, and the outputs 154-158 are connected to the corresponding pan control lines of the other five input sections. The outputs 159 and 160 are connected, respectively, to the "left balance" line 66 and the "right balance" line 81 of Fig. 5. The outputs 170 and 171 are connected, respectively, to the "left delay" line 57 of Fig. 4 and the corresponding "right delay" line of the other master effects section. The output 172 is connected to the "autopan" line 33 of Fig. 3. The CPU 98 (Fig. 6) causes the port A of the circuit 110 to produce a digital value which changes, from instant to instant, to represent the required pan, balance, delay and autopan levels in sequence, as the CPU scans through the various parameters which have been set up in the system. By operation of the DAC 125 and the decoders 127 and 128, the instantaneous digital values are converted to analog values, and these values are directed to the corresponding control lines. The capacitors 145-152 and 167-169 of the sample-and-hold buffers retain the analog values for only a relatively short time, but are continuously refreshed by the cycling of the CPU. The autopan levels provided by the CPU may vary in accordance with software-generated waveforms, such as triangular, sawtooth and sinusoidal waves. The bus 112 also carries digital signals for operating the various FET switches in the system. These are fed to decoders 173-175 which direct corresponding switching signals over control buses 176-178, respectively. The signals on the bus 176 are fed, as required, to the lines 46, 52 and 53 of the master effects section of Fig. 4, and to the corresponding lines of the other master effects section. The signals on the bus 177 are fed, as required, to the line 19 of the input section of Fig. 3 and to the corresponding lines of the other five input sections, to the line 50 of the master effects section of Fig. 4 and to the corresponding line of the other master effects section. The signals on the bus 178 are fed, as required, to the line 25 of the input section of Fig. 3 and to the corresponding lines of the other five input sections, to the line 49 of the master effects section of Fig. 4 and to the corresponding line of the other master effects section. It will be apparent that a very considerable range of effects can be produced by switching into circuit groups of effects devices selected by the musician, each such group of devices acting independently on the signals derived from a respective string of the instrument. Further effects devices may be switched in and out of circuit automatically under microcomputer control, and automatic control of delay, pan, autopan and balance can be achieved. Even without any special effects, the stereo image of the instrument may be greatly enhanced by the use of the separate signals from the six transducers. As the notes of a chord are played, the image can appear to move from left to right, or vice versa. If the transducer signals are re-allocated so that they alternate between the left and right channels, a different illusion of movement can be created. The transducer assembly and the electronics of the system may be provided with the instrument when originally marketed, or may be provided as a kit for updating an existing instrument.

Claims: CLAIMS 1. An audio signal generating system, comprising a plurality of transducers each coupled, or to be coupled, to a respective string of a musical instrument and each operative to produce, in a respective signal channel, an audio electrical signal corresponding to notes played on the respective string; and respective audio signal modifying means coupled to the signal channels and operable to act on the electrical signal in the respective channel, whereby the electrical signal in any of the channels can be modified, as desired, independently of the other channels. 2. A system as claimed in claim 1, further comprising means to produce from the channels, after modification of the signals therein by the modifying means, two stereo channels. 3. A system as claimed in claim 2, including further audio signal modifying means selectively connectable into the stereo channels. 4. A system as claimed in claim 3, wherein each audio signal modifying means comprises one or more effects devices, such as reverberation, echo, or other devices. 5. A system as claimed in claim 2, claim 3 or claim 4, further comprising first voltage-controlled amplifier means in each stereo channel; and means to apply variable voltages to the first voltage-controlled amplifier means to vary the condition of balance of the stereo channels. 6. A system as claimed in any one of claims 2-5, further comprising second voltage-controlled amplifier means in each signal channel; and means to apply variable voltages to the respective second voltage-controlled amplifier means to cause panning of the stereo image produced by the stereo channels. 7. A system as claimed in claim 6, wherein the means to apply variable voltages to the respective second voltage-controlled amplifier means comprises means to vary the voltages cyclically in accordance with a 55 triangular, sawtooth or sinusoidal waveform. 8. A system as claimed in any one of claims 2-7, further comprising variable audio signal delay means selectively connectable into the stereo channels. 9. A system as claimed in claim 8, wherein the 10 " ' delay means comprises a chain of charge-coupled devices; and means to cause the audio signal to progress stepwise through the chain. 10. A system as claimed in any preceding claim, further comprising data processing means for controlling 15 " ; parameters of paths through which the audio signals pass. 11. A system as claimed in claim 10, further comprising a read-only memory for controlling the data processing means in accordance with program routines stored in the read-only memory. 20 ' 12. A system as claimed in claim 10 or claim 11, further comprising a random-access memory for controlling the data processing means in accordance with data entered into the random-access memory from a keypad. 13. A system as claimed in any one of claims 10-12, 255 further comprising display means for displaying instructions and/or other data relating to the operation of the data processing means. 14. A system as claimed in claim 13, wherein the display means comprises an array of liquid-crystal •30 devices. 15. A system as claimed in any one of claims 10-14, wherein the data processing means scans cyclically through data relating to desired " values of the parameters which are to be controlled, and continuously- up-dates 35 command signals for controlling the parameters. 16. A system as claimed in any one of claims 10-15, wherein the controlled parameters include gain levels of amplifiers in the system. 17. A system as claimed in any one of claims 10-16, 5 wherein the controlled parameters include routing of audio signals in the system. 18. A system as claimed in any one of claims 10-17, wherein the controlled parameters include periods of cfelay to which audio signals in the system are subjected. KB 19. A system as claimed in any one of claims 2-18, further comprising transducer means coupled to all of the plurality of strings to produce a composite audio output signal; and means operable to feed the composite. signal ϋrtto one or both of the stereo channels or to disconnect 15 the composite signal therefrom. 20 25 30 35

Inventors: EVANS JEREMY MARK, EVANS, JEREMY, MARK
Applicants: EVANS JEREMY MARK, EVANS, JEREMY, MARK
Application year: 1986
Main classification: G10H3
IPC codes: G10H3