The present disclosure relates to a system for identification of a note played by a musical instrument.
Note identification by acoustic identification in reed woodwind instruments has already been described in GB 1513036. However, the larger instruments in this reed woodwind family pose a particular challenge for note identification because their lower acoustic wavelengths require stimulation of the instruments at lower frequencies and longer analysis frames.
A further technical problem in note identification using speakers to input stimulus signals to musical instruments and microphones to receive the stimulus signals modified by the transfer functions of the musical instruments is that the note identification method is not immune to acoustic interference. This can mean that such methods are not available for performance purposes.
The present disclosure provides a system and method for identification of a note played by a musical instrument.
The disclosure uses a transmitted electromagnetic signal to determine a configuration of a resonant chamber in the musical instrument from a sensed reflected wave. The configuration of the resonant chamber may include one or more of a state of openings of the resonant chamber, a state of valve positions of the resonant chamber, a length of the resonant chamber, or some other property of the resonant chamber that influences the musical note selected to be played by a player of the musical instrument.
The system and method of the disclosure can provide for instruments with an electrically conductive surface a real-time system for musical note identification with complete immunity to acoustic interference. Instruments with an electrically conductive surface include the following instruments: saxophones, labrasones (brass instruments), edge-blown aerophones (flutes) and metal clarinets. Additionally, it is feasible to coat the inside surface of traditionally wooden instruments to provide a conductive surface which would allow use of the disclosure. Ideally the instrument would have metal key caps, but the disturbance caused by a player's fingers covering holes could prove sufficient to make a measurable difference to the reflected signal. The disclosure can be used with instruments with a wide variety of internal bore profiles including conical bore profiles (saxophone family) and cylindrical bore profiles (the edge-blown aerophone (e.g. flute) and labrasone (e.g. brass instruments) families).
Embodiments of the present disclosure will now be described with reference to the accompanying drawings, in which:
The current disclosure makes use of signals in the electromagnetic spectrum and recognises that for higher (radio) frequencies within this spectrum the wave nature of an alternating current must be taken into account.
The disclosure treats a metal-bodied (i.e. electrically conducting) instrument, e.g. a tenor saxophone 10 (see
The system of the disclosure includes an antenna 11 which by transmitting radio waves allows a resonant chamber of the musical instrument 10 to form an electromagnetic resonant cavity at electromagnetic wavelengths which are similar to the normally played acoustic wavelengths.
The saxophone family of instruments have conical bores with relatively small (in comparison with other musical instruments) initial dimensions. For instance, the entrance bore into the crook of a tenor saxophone is about 15 mm in diameter. The lowest ‘cut-off’ frequency for a circular waveguide to sustain a TE01 wave is defined:
Thus for mode TE01, the cut-off frequency is 11.72 GHz (although it should be mentioned that since the bore of a saxophone is conical, this figure will not be a precisely accurate figure). However, it will suffice for the present disclosure, which recognises that it is necessary to be above the cut-off frequency in order to sustain the wave in the waveguide. The TE01 (transverse electric) mode signifies that all electric fields are transverse to the direction of propagation and that no longitudinal electric field is present.
The implementations described below and supported by
Furthermore, it may be that excitation electromagnetic radiation may be polarised. For example, the antenna may generate a circularly polarised electromagnetic signal. Alternative, the antenna may generate a linearly polarised electromagnetic signal. Alternative, the antenna may generate an unpolarised electromagnetic signal.
The antenna 11 of the present disclosure may be a single probe antenna with a shorted back-stop provided to broadcast a radio frequency electromagnetic signal in a resonant chamber of the instrument 10, as shown in
Alternatively, multiple probe antennae may be used, typically being arranged equally around a plane orthogonal to the bore which provides the resonant chamber of the instrument. This could be conveniently be realised as a microstrip circuit with 4 orthogonal probes (or any number of orthogonal probes spaced around the resonant chamber, equally spaced in terms of angle of separation, when viewed in a plane perpendicular to a longitudinal axis of the resonant chamber). It is important to make a good ohmic connection between the shorted back-stop body and the instrument, if necessary connecting to the internal surface of the bore of the instrument with a sprung connection.
Saxophones have conical bores opening out very considerably from the initial approximate 15 mm radius (tenor saxophone) to approximately 140 mm at the bell. The sustainable wavelength is directly proportionate to the bore radius. As with an acoustic wave, an element of the radio frequency wave will be reflected at the impedance discontinuity of the opening of the bell into free-air. The reflected energy can advantageously be increased by attaching a conducting plane reflector 12 over a bell end 13 of the instrument 10, as shown in
The system of the disclosure depends upon stimulating the bore of the instrument with an accurately repeatable range of frequencies and monitoring the reflected energy. The stimulated frequencies may be continuously scanned or individually stepped such that the reflected wave is measured with repeatable frequencies. Measuring the reflected energy across a range of frequencies will produce a ‘frame’ of data, with a data point per frequency of interest. A programmable network analyser (e.g. the Keysight 5225B™ analyser) can carry out a scan of 1600 points in a few milliseconds (ms). Practical realisations used by the system of the present disclosure generate a sufficient few hundred points in 10 ms.
The stimulus waveform is generated by the system in one or both of the following ways:
There can be seen in
Thus the transmitted stimulus waveform can resemble a classical ‘chirp’ waveform and either move smoothly between frequencies or be stepped. For instance, the microprocessor 27 can step the broadcast frequencies by way of a control output to the digital to analogue converter 28; the microprocessor 27 knows what frequency has just been broadcast, so it will know the next frequency to be broadcast in the series. The direct synthesis steps may be chosen linearly or exponentially depending upon the range of the scanning frequency or at spot frequencies chosen to maximise the difference responses.
When the stimulus waveform is applied to the instrument 10, being transmitted by the antenna 11, it is modified by the reflected waveform dependent upon the keyholes which are currently closed.
In
Other standard microwave circuits to measure the magnitude and/or phase of the reflected wave are possible, e.g. a homodyne circulatory mixer supplied with the analysis waveform and the reflected waveform as input signals to the mixer.
In
In both the
Further schemes are possible combining
In implementing the system, there is an initial training phase in which the system operates in a training mode in which every possible outcome which it is desirable to recognise is generated and the frame of data for each outcome is acquired and stored in a memory of the microprocessor 27, e.g. being digitised by the microprocessor 27 and committed to the memory as representing the respective outcome. So, for each musical instrument there is a training phase when each note is played at least once and the magnitude spectral outcome for each note is captured by the system. Measured spectra for the notes D3 and A3 on a tenor saxophone are shown in
Subsequent to the training phase, the system runs in a note recognition mode while the instrument is played normally. In the note recognition mode, live frames of data are acquired and then compared by the microprocessor 27 with those collected in the memory during training. The closest match with the training date is used to determine the ‘played’ note. A variety of statistical techniques may be applied to determine the closeness of the match. The signal processing and matching process can be completed typically in under 10 ms, depending upon processing power.
Once a played note has been determined by the system, the system can use a synthesizer unit of the system (not shown) to synthesize and to output the detected musical note for transmission to e.g. headphones, so the player can hear a synthesized musical note in response to a change of fingering with a typical worst-case latency of under 20 ms.
A pressure sensor (not shown) can be incorporated in the system to measure the breath pressure of the player and thereby the timing of the starting of generation of the synthesized musical notes and/or their volume can be controlled by the system with reference to a pressure signal generated by the pressure sensor, in order to provide a realistic playing experience. The pressure sensor can be incorporated in a replacement mouthpiece, integral with the end cap 15 or mountable thereon, used to replace the regular mouthpiece of the instrument. The replacement mouthpiece could have a passage directing the breath of the player of the instrument through an outlet provided in the replacement mouthpiece or a small aperture could be provided in the end cap 15 for the passage of breath and a tube could be connected to such an aperture to lead the breath through the instrument to a tube outlet at or beyond the outlet of the instrument. When the system of the disclosure is used with an Aerophone or for a Labrasone, a breath sensor could be provided or a lip vibration sensor, e.g. as described in published PCT applications WO2018/138504A1 and WO2018/138591A3, and a signal from such a breath senor or lip vibration sensor sent to the microprocessor 27 and used thereby to control the starting of generation of the synthesized musical notes and/or their volume. When a breath sensor is used e.g. with a flute, then the breath sensor can send signals to the microprocessor 27 indicating the direction and the velocity of breath and these signals can be used by the microprocessor e.g. to select the correct octave or register for the musical note to be synthesized.
The transmission and measurement of an electromagnetic wave (as opposed to the acoustic wave) has the distinct advantage that it the system is immune to acoustic interference. With suitable amplification a musical instrument fitted with the system of the disclosure may be played in a performance ensemble with other instruments or in a solo capacity.
The analysis waveform power requirement is very small, typically 0 dBm (1 mW), and is within international safety standards for electromagnetic radiation. Advantageously the whole system may be battery powered, with a battery possible being contained within the bell of the instrument. A power amplifier and loudspeaker may also be contained within the bell of the instrument for local performance. Alternatively for performance to a large audience the instrument may be linked to an off-instrument synthesiser/amplifier/speaker arrangement by means of a digital radio connection, e.g. Bluetooth™.
The synthesizer unit of the system can run a user-controllable musical synthesis algorithm to allow the player to choose synthesized signals which synthesize the musical notes of a different type of instrument, e.g. so that an experience saxophonist can play his/her saxophone yet hear musical notes output via headphones or speakers which sound like notes played on a piano.
The system has been described above in use with a saxophone, which includes metal keycaps fingered by the player to open and close the holes spaced along the resonant chamber. This is ideal for the disclosed system and method since the position of the metal keycaps will significantly affect the electromagnetic transfer function of the resonant cavity. However, there will be some change to the electromagnetic transfer function of the resonant chamber with just the player's fingers opening and closing the holes, so the system and method of the disclsoure can be used also with metal flutes and metal clarinets, which can have a mixture of rings/holes covered by the finger and metal caps. Also instruments that are traditionally wooden could be provided with a metal coating on the surface defining the resonant cavity, in order to allow use of the system and method of the disclosure. It should also be mentioned that some instruments (e.g. labrasones such as trumpets) do not have openings but rather valves changing tube lengths and others (e.g. labrasones such as trombones) have sliding elements altering the length of the resonant chambers; such changes to the resonant cavity and would be detected by the system of the disclosure, which is therefore of use with such instruments.
Number | Date | Country | Kind |
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1914588.7 | Oct 2019 | GB | national |
This application is a national stage entry application under 35 U.S.C. 371 of PCT Patent Application No. PCT/GB2020/052517, filed 9 Oct. 2020, which claims priority to GB Patent Application No. 1914588.7 filed 9 Oct. 2019, the entire contents of each of which are incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
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PCT/GB2020/052517 | 10/9/2020 | WO |