GB2218527A

GB2218527A - Determining the fundamental frequency of a signal

Info

Publication number: GB2218527A
Application number: GB8908748A
Authority: GB
Inventors: Paul Wisdom
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-04-19
Filing date: 1989-04-18
Publication date: 1989-11-15
Anticipated expiration: 2009-04-18
Also published as: GB8809189D0; GB8908748D0; GB2218527B

Abstract

A method for identifying the fundamental frequency of a signal, e.g. a note from a vibrating string (1) comprises providing (23) a signal corresponding to the note; detecting (4a,b) when a parameter of the signal changes from a positive to a negative value and vice versa so as to provide corresponding pulses; and passing the pulses to a bistable switch (5) which produces pulses whose frequency corresponds to the fundamental of the note being monitored. Detectors (4a, 4b) are positive and negative peak detectors. Pulses from the bistable (5) are applied to gates (8a, b) responsive to positive and negative half waves. The gates control clock pulses (7) to counters (9a, b) whose counts are converted to frequency using look-up devices (10a, b). The frequency values are transferred to registers (11a, b), and are compared (12) for validity checking prior to output (14). The method may be applied to acoustic, electrical and optical signals. <IMAGE>

Description

TITLE: METHOD FOR EXTRACTING THE PITCH OF A NOTE The present invention relates to a method for extracting the pitch of a note, notably to a method for extracting the fundamental pitch (frequency) of a note from a single musical instrument source.

BACKGROUND TO THE INVENTION: Many forms of electronic organs or music synthesizers exist i.n which a note is created electronically. In such apparatus, the identity of the note being generated is known from the identity of the key or other note input device which is being actuated by the player. It is known to use the note generation information in such an instrument to generate an accompaniment, eg. timpani or a chord, to the note being played.

It would be desirable to be able to provide that faci.lity for a free-standing musical instrument, for example for a guitar or a wind instrument, where the note is not being generated electronically and hence the identity of the note has to be establi.shed before the appropriate accompaniment can be generated. However, thi.s raises problems in that the notes generated by most musical instruments comprise a fundamental note accompanied by a number of harmonics of that note and i.t i.s necessary to extract the frequency of the fundamental note in order to be able to synthesise the appropri.ate chords or other accompaniment.

It is known to use an inductance device to detect the vibration of a metal stringed instrument and to extract the frequency of the fundamental note from the information provided from that device. However, present methods for extracting the identity of the fundamental note are slow, notably for low frequency notes, and this can give rise to dis torti on and mi.s-synchroni.sation of the accompaniment.

I have now devised a method and apparatus for extracting the frequency of the fundamental note from a single instrument which reduces the above problems.

SUMMARY OF THE INVENTION: Accordingly, the present invention relates to a method for i.dentifyi.ng the fundamental frequency of a note which comprises the steps of: a. monitoring the note and providing a wave form signal corresponding to the note; b. detecting when the wave form signal changes from a positive to a negative value and vice versa so as to provide a series of pulses corresponding to the frequency of change of polarity of the wave form signal that is a series of pulses whose frequency corresponds approximately to the frequency of the note bei.ng monitored; and c. passing the pulses from stage b to a means which responds to changes in the polarity of the signals as opposed to the size of the signals, for example a bistable switch means, so as to derive a third series of pulses whose frequency corresponds substantially to the frequency of the fundamental or carri.er wave form of the note being monitored.

The term polarity is used herein with respect to the wave form signal to denote some component of the signal whose value changes from a posi.ti.ve value to a negative or opposite value. Thus, the component of the signal which is monitored can be the slope of the curve of the wave which changes from a positive value for the rising portions of the wave to a negative value for the falling slopes of the curve. Alternatively, the point at which the curve crosses the x abscissa and the wave changes from its positive half wave to its negative half wave can be monitored.

In an ideal state where there are no harmonics or noise or other i.nterference with the wave form from stage a, the pulses from stage b will correspond to the fundamental frequency of the note being monitored. However, this i.s usually not the case and it is preferred to monitor the presence of signals having at least a predetermined value in stage b so as to monitor only the top or peak values of the wave signal from stage a. This reduces the amount of harmonics or noise or other extraneous signals which are fed into later stages and enhances the accuracey of the.method of the invention.It is therefore preferred in stage b to detect the existence of peak positive and negative values in the waveform of the signal from stage a so as to obtain a first and second series of pulses corresponding to the occurence of pre-determi.ned peak positive and negative values in the waveform.

It is preferred to further process the signal output from stage c above in a subsequent stage d so as to reduce any extraneous or noise signals which may have been passed through stage c of the method of the invention. This is preferably done by integrating the third series of pulses from stage c with a clock means so as to provide a measure of the duration of the wave length of the pulses; and comparing the duration of a number of the wave lengths to determine whether those wave lengths are the same to within acceptable limits, indicating a consistent note.It is particularly preferred to determine the duration of each half wave length of the pulses from stage c and to compare the duration of the i.mmediately succeeding or preceding half wave lengths to establish whether a pulse from the third series approxi.mates within predetermined limits to a 50% duty cycle so as to determine whether that pulse from the third series is an acceptable signal or not.

The comparison of the durations of the wave forms in stage d i.s conveniently done by passi.ng the duration information for each positive and negative half wave to an identification means which identifies the note corresponding to that half wave value and passing that note identity to a comparison means whereby the note identity for each positive half wave is compared against the identity of the corresponding negative half wave to determine the validity of the note dentification to within predetermined limits, for example to within a demi-semi-tone.

The frequency or identity of the note being monitored is passed from any suitable output stage in the method of the invention to means for generating an accompaniment or other signal in response to that note. It is preferred that the verifi.ed note identification from stage d of the method of the invention is passed to a memory means where the note i.denti.fi.cation i.s mai.ntained, preferably only whilst there is an input to stage a, unti.l a subsequent note identifi.cati.on is determined as being valid and displaces the fi.rst note identificati.on from the memory means. An accompaniment signal can then be generated in response to the note identification held in the memory means.

The invention further provides a device for generating the identity of the fundamental frequency of a note, which device comprises: 1. means for receiving a wave form signal from means for detecting the note to be identified; 2. means for detecting the existence of positive and negative values for the wave form of the note and for generati.ng a first and a second series of pulses corresponding to the occurence of those positive and negative values, preferably this means comprises a peak waveform detector; 3. means actuatable by the first and second series of pulses for producing a third series of pulses whose frequency corresponds predominantly to the frequency of the fundamental wave form of the note being identified, preferably this means is a bistable swi.tch means.

The apparatus preferably further comprises: 4. clock means adapted to provide a measure of the duration of a wave length, notably of each half wave length, of the pulses i.n the third seri.es of pulses; 5. means for comparing one wave length duration, notably the duration of a half wave length, for pulses from the third series of pulses with another wave length duration from that seri.es, notably the duration of an i.mmedi.ately succedi.ng or preceding half wave length, to establish whether the thi.rd series of pulses approximate within predetermined li.mi.ts to a 50% duty cycle so as to determine whether a thi.rd pulse is an acceptable signal or not; and 6. output means for transmitting the acceptable signal to a means for generating a signal i.n response to the frequency of the acceptable note.

It is preferred that. the comparative means for i tem 5 comprises an identification means incorporating memory units for alloting a note i.denti.fi.cation to each positive and negative half wave duration signal from item 4 and means for comparing the note identifications for the corresponding positive and negative half waves so as to establish the vali.di.ty of the note identification to within predetermined limits; and memory means where the i.dentifi.cation from the note i.denti.fi.cati.on means are held until di.splaced by a subsequent acceptable note identification, the output means 6 reading the identity of the note from the memory means.

It is also preferred that the identity of an acceptable note be maintained in the memory means only whilst there i.s a note input to item 1 of the apparatus, for example by feeding part of the input to item 1 to actuate the memory means.

As stated above, the output means 6 can derive its signal from any suitable output point in the apparatus of the invention.

The invention can be applied to the identification of the fundamental signal from wave form signals generated from a wide range of sources. Thus, the i.nventi.on can be applied to the analysis of signals on a carrier wave, to identification of carrier waves in radio transmissions, for example in radio tuners; in speech recognition to convert the spoken input i.nto digital signals; in tachometers, for example is vehicle speedometers, where the signal from the speedometer or engi.ne revolution detector is i.ntegrated wi.th a time signal to produce a digital output; or in general for identifying signals havi.ng a high level of harmonics. The term note is therefore used herein to denote in general wave form signals, i.ncludi.ng optical, accoustic, electrical and electromagnetic signals; and is not to be construed as limited solely to notes from musical instruments. However, the invention is of especial use in the identification of the fundamental frequency of a note from a single musical instrument. For convenience, the invention will be described with respect to this preferred application.

The note can be generated by a wi.de range of instruments.

The means for monitoring the notes emitted by the instrument can therefore be selected from a wide range of types havi.ng regard to nature of that instrument. Thus, for a metal stringed instrument such as guitar, the note can be detected by an inductance or Hall effect sensor; for a wi.nd instrument, the note can be detected by a transducer placed the wind passages of the instrument or by a microphone placed externally of the instrument. It is preferred that the monitoring means generate an electrical signal directly and not via a Pitot tube or other i.ntermediate sensor operating in another medium.

For convenience, the invention will be described hereinafter in terms of the use of an inductance sensor comprising a magnetic core having a coil wound around the core and adapted to have electrical currents induced therein by the vibration of one or more strings of a guitar in the magnetic fi.eld from the core. Such sensors are commercially available and may be used as such in the method and apparatus of the invention.

The signal from the sensor i.s typically in the form of a fundamental sine wave with the harmonics of the note and any extraneous mechanical or electrical noi.se signals, for example fingering or fret noises from the instrument or electrical interference, superimposed thereon. This signal i.s fed to a conventional circuit for detecting when the wave moves from it positive portion to the negative portion and vice versa to establish the overall frequency of the note.

This can be done by detecting when the signal crosses the zero value. However, such a detector may be triggered by low amplitude harmonics or extraneous signals and these would produce erratic pulses within the regular pulses produced by the fundamental wave form.

It is therefore preferred to detect the peak negative and positive values of the si.ne wave usi.ng conventional positive and negative peak detectors. Preferably, the peak detector is set to be actuated at a refe-rence level 5 to 10% below the maximum actual peak value expected, so as to exclude the majority of the lower amplitude (higher order) harmonics which will occur on the rise and fall slopes of the fundamental wave form. Where a substantially consistent signal amplitude i.s being monitored, the reference level can be preset or manually variable. However, it t i.s preferred that the reference level be automatically set by suitable circuitry having regard to the peak value of the preceding wave or the mean value of preceding waves.The reference level for each wave decays wi.th time and it will usually be preferred that the reference signal decays by less than 40 to 50% between consecutive wave cycles. However, the decay rate can be adjusted according to the decay rate of the note generated by the instrument as is known.

The signals corresponding to the occurence of positive and negative si.gnals exceedi.ng the reference levels are fed as a first and second series of pulses from the two peak level detectors to a means for detecting the presence of each positive or negative input. Various forms of sui table detectors are known and commercially available. For example, a zero-crossing detector could be used to provide the function of both detecting the positive and negative porti.ons of the wave signal i.nput and for providing a pulsed signal output corresponding to the frequency of the change between positive and negative.However, it is prefered to use positive and negative peak detectors and to feed the outputs from the detectors separately to a bistable multivibrator switch.

The bistable switch preferably is triggered by the leading edge of an input signal and operates to detect the first signal for each of the positive and negative peaks.

Subsequent peaks of the same polarity merely serve to hold the switch in the same state until a signal of the opposite polarity is fed to the switch to cause it to change state.

As a result, the switch does not register in its output more than the first peak of each sequence of peaks of the same polarity and the output therefore does not record the majority of any peaks due to harmonics, noise, interference or phase shifting. As a result, the output from the bistable switch is a signal, typically a square wave, having a frequency which corresponds predominantly to the actual frequency of the fundamental note being identified. For convenience, the invention will be described hereafter in terms of the use of the combination of two peak detectors and a multivibrator bistable switch.

Where the note is of high quality and substantially free from extraneous noises, such as fret squeaks or squeaks in changing fingering on the strings, the output from the bistable switch may be good enough to generate the desired accompaniment to the instrument as described below.

However, there will usually be noises and/or some distortion of the note and it is preferred to verify the third series of pulses from the bistable switch so as to reject noise or distortion. This is done by measuring the duration of the pulses (waves) in the output from the bistable switch and rejecting those signals which have durations differing markedly from the general values of the pulses being moni.tored. The duration is conveniently measured by integrating the signal with the output of a conventional clocking circuit so that the duration of the waves is measured as a number of arbi.trary ti.me pulses from the clock circuit. Preferably, the clock pulses will be generated at a frequency at least 8 times higher than the hi.ghest frequency to be identified. Typically, for musical notes, the clock pulses will have a frequency 24 times greater than the highest frequency to be identified.

The comparison of the durations of adjacent wave or half wave lengths described below may not reject interference or other unwanted signals which have substantially equal wave or half wave lengths yet which are significantly different from the fundamental frequency of the note being monitored.

It is therefore usually preferred to subject the pulses from the bistable switch to a binary di.vi.di.ng stage in which the frequency of the pulses is effectively halved. As a result, a wave which has short but substantially equal half wave lengths will be converted to a si.ngle pulse. The duration of this pulse will then be distinguishable from the longer duration pulses derived from the fundamental note and will be rejected during the pulse duration comparison stage described below.

Excessi.ve division of the signal from the bistable switch may affect the veracity of the identification of the frequency of the note being investigated and usually only one or two division stages will be acceptable. Where the output from the bistable switch i.s divided, it t will be appreciated that the speed of the clock with which the divided signals are subsequently integrated will have to be reduced accordingly so that the effective duration of the output is correctly identified.

After integration with the timer signals, the wave form for the third series of pulses i.s now identified as a series of digital values corresponding to the number of duration units for the, preferably each, wave length of the pulses. The duration of a number of waves within the seri.es is then compared using conventional ci.rcui.try to detect anomalies in the regularity of the pulses. These anomalies will correspond to noise or interference which has a different frequency from the fundamental wave, phase shifting or to changes i.n the note being investigated. Typically, the duration of one or more waves wave i.s compared to the duration of a similar number of preceding waves.However, i.n order to enhance the reaction time and selecti.vity of the method of the invention, it is preferred to compare the duration of one half wave with the duration of an immediately succeeding or precedi.ng half wave.

The comparison of the wave duration will give a measure of the regularity of the wave form of the pulses in the third series. Ideally, in each complete wave length, each half wave length should be of the same durati.on, that is the wave form should correspond to a 50% duty cycle. Usually, there wi.ll be some fluctuation i.n the fundamental wave form and it i.s possible to preset, for example wi.thi.n the software or the memories controlling the compari.son process, the variation which can be accepted between the wave durations being compared. For most cases, a variation of a demi.-semi.-tone in the note observed or a factor of 24jim between successi.ve half waves may be accepted, although in other applications other tolerances may be required. Where a wave duration does not meet the required acceptance level, for example where it is caused by a short high frequency squeak, that signal will be rejected. In this way noise of significantly different frequency from the fundamental wave can be eliminated from the output which i.s to be fed to the accompaniment generator.

The comparison of the wave durations can be carried out merely by comparing the digital information on the duration of each wave or half wave length so as to determine which si.gnals are acceptable and which can be used to drive the accompaniment generator. However, it will usually be desired to convert the digital signals into note i.denti.ties before they are used to drive the accompaniment generator.

Thus, it will usually be preferred to feed the di gi tal values for each half wave of the signal to a memory uni.t which identifiers the half wave length from the number of duration units and hence the note from the frequency as represented by that number of duration units. Thi.s i.dentifi.cation will usually be carried out in two stages, first the identificati.on of the half wave length from the number of clock units in its duration, then the indentification of the note from the half wave length. This identifi.cation can be carried out using conventional read only memories containing look up tables and these can incorporate the acceptable tolerances referred to above.It i.s preferred to process the positive and negative half waves through separate identi.fi.cation means i.n parallel since this enhances the speed of processing of the signals. The compari.son process will produce the identi.ty of each half of the i.nput wave and if the data has been correctly processed and clocked through the various regi.sters and memory units, the two identities should match, thus conforming the validity of the note identification.

During the processi.ng and cleaning up of the initial signal from the induction or other sensor, considerable portions of the original signal may have been rejected. This could lead to interruptions in the output to the accompaniment generation means. It is therefore preferred to hold the validated note identification in a register which i.s read by the accompaniment generating means and to maintain the note identity in the register until it is displaced by a susequent note validation.In order to prevent the register from being retained i.ndefinitely when the instrument bei.ng monitored is silent, it i.s also preferred that the validated note identification be maintained in the register only whilst there is an input to the original sensor from the instrument being observed.

The output from the method and apparatus of the invention i.s a signal whose frequency corresponds subtantially accurately to the frequency of the fundamental wave of the note being monitored. This output can be used via any suitable interface to drive a wide variety of tone generators to provide chords, timpani or other accompani.ment, to drive directly electronic single tone generators or to actuate electromechanical string plucking mechanisms or the like.

If desired, the output can be related to a predetermi.ned format or bank of chords, for example entered into a suitable memory unit and processor from a key pad data i.nput device, and the chord output modified according to the frequency of the output from the method or apparatus of the inventi.on.

The invention thus provides a method and apparatus for identifying the frequency of the fundamental note from an instrument which reduce lower amplitude (generally higher order) harmonics from the input signal in the peak detection stage; which reduce higher amplitude (generally lower order) harmonics and momentary phase shifting from the input signal in the bistable switch stage; which reduce momentary frequency changes and accommodate special waveforms in the bi.nary division stage; which can accommodate variati.ons in the fundamental note frequency in the selection of the pass band of the tolerance in the half wave duration/note identification stage; and which maintain continuity of the output to the accompaniment generator in the fi.nal register stage.Furthermore, the apparatus of the invention can be modified by the selecti.on of the note sensor and by varyi.ng the performance parameters, for example to accept greater deviation in the match of half wave length durations at validation of the output from the bistable switch. It is therefore possible to vary the musical character of the output from the method or apparatus of the i.nventi.on so as to reproduce more faithfully any vari-ations from a perfect sine wave in the fundamental note being monitored and thus enhance the musical acceptabi.lty of the accompaniment being generated.

DESCRIPTION OF THE DRAWING: The invention will now be described by way of i.llustrati.on only with respect to a preferred form of apparatus as shown in the accompanying Figure 1, which is a block circuit diagram of the circuit used i.n the apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT: A note is generated by the vibration of a string 1 of a guitar (not shown) by a sensor 2. The sensor can serve all the strings of the guitar, or separate sensors can be provided for each string. The sensor s an inductance sensor incorporating a coil wound around a magnetic core, so that the vibration of the string i.n the magnetic field of the core induces an electrical voltage in the coi.l.

The output from the coil is fed to two peak voltage detectors, optionally after being amplified in amplifier 3.

One peak detector 4a detects the positive half wave of the output from the coil, the other detector 4b the negative half wave. The detectors are set to detect when the voltage exceeds 90 to 95% of the peak voltage detected on a previous wave. The decay rate of the reference level signal can be adjusted to reflect the decay characteristics of the note being monitored by sensor 2. Such peak detectors and their operation are conventional and many sui.table forms are commercially avai.lable.

The output from each peak detector is fed to a bistable multivibrator swi.tch 5 which is actuated by the leading edge of an incoming signal, the negative peak signal being fed to one input and the positive peak signal being fed to another input. As explai.ned above, the switch 5 reacts to only the fi.rst signal i.n a seri.es of signals of the same polari.ty, so that the presence of secondary peaks on a basic curve are no-t recorded by the switch, thus eliminating the effect of many of the harmonics, noise or i.nterference on the operation of switch 5. The output from switch 5 is a series of pulses having a frequency corresponding predominantly to the frequency of the basic wave (that i.s the fundamental note).

This signal is fed to a cleaning and verification process via gates 8a and 8b which separate the signals into positive and negative half waves. The duration of the half waves in terms of timer pulses from a conventi.onal clock circuit 7 i.s determined by a counter 9a or 9b for each half wave.

However, where the signal carries excessive noise, interference etc, i t can be fed to one or more bi.nary dividing circuits 6 which halve the frequency of the signal to reduce the number of peaks due to noise or interference.

The divided si.gnals can then be fed through gates 8a and Sb to a veri.fication and cleaning stage. Where division of the signal is carried out, it will be appreciated that the operation of the clock circuit must be divided accordingly.

The output from the counters 9 is a digital signal corresponding to the duration of the half wave, ie to its wave length. The note corresponding to that half wave length is identified in a look up table memory 10a or 10b with the identity of the note havi.ng that half wave length and that identity is held in a register lla llb through which it is clocked in synchronisation with the other register llb or lla so that the note identities are presented simultaneously at a comparitor 12 which compares the two half wave i.denti.ties. If they are the same or within an accepted variati.on, the note identi ties are accepted as valid and are passed to a buffer memory 13 from which they are read off by an interface 14 serving the particular form of accompaniment generator (not shown). In order that memory 13 does not maintain note identities when there is no note being generated by string 1, a Schmidt tri.gger 15 detects whether there i.s an input signal to one of the peak detectors 4 and either maintains the memory in 13 if there i.s an input or allows it to clear or to produce a no signal present output to the accompani.ment generator i.f there i.s no input.

The above device enables rapid identifi.cation of the fundamental note to be established over a wide frequency range and also allows allocati.on of frequenci.es over a range of acceptable note identities, thus compensating for tuning or other errors in the instrument generating the note being identified.

Claims

WHAT I CLAIM IS:

1. A method for identifying the fundamental frequency of a note which comprises the steps of: a. monitoring the note and providing a wave form signal corresponding to the note; b. detecting when the wave form signal from stage a changes from a positive to a negati.ve value and vice versa so as to provide a series of pulses corresponding to the frequency of change of polarity of the wave form signal; and c. passing the pulses from stage b to a means which responds to changes in the polari ty of the signals as opposed to the size of the signals, so as to deri.ve a third seri.es of pulses whose frequency corresponds substantially to the frequency of the fundamental or carrier wave form of the note being moni.tored.

2. A method as claimed in claim 1 wherein the presence of signals from stage a having at least a predetermined value are moni.tored in stage b so as to monitor the existence of peak positive and negative values in the waveform of the signal from stage a so as to obtai.n a first and second series of pulses corresponding to the occurence of peak posi.ti.ve and negative values i.n the waveform.

3. A method as claimed in either of claims 1 or 2 wherein the signal output from stage c i.s processed in a subsequent stage d by integrating the third series of pulses from stage c with a clock means so as to provide a measure of the duration of the wave length of the pulses; and comparing the duration of a number of the wave lengths to determine whether those wave lengths are the same to within acceptable limits.

4. A method as claimed in claim 3 wherein the duration of each half wave length of the pulses from stage c is determined and the duration of immediately succeedi.ng or preceding half wave lengths are compared to establish whether a pulse from the third series approximates within predetermined limits to a 50% duty cycle so as to determine whether that pulse from the third seri.es is an acceptable signal or not.

5. A method as claimed in claim 3 wherein the comparison of the durations of the wave forms in stage d is carried out by passing the duration i.nformation for each posi.tive and negative half wave to an identi.fi.cation means which identifies the note corresponding to that half wave duration value and passing that note identity to a comparison means whereby the note identity for each posi.ti.ve half wave i.s compared against the identity of the corresponding negative half wave to determi.ne the validity of the note identification to within predetermined li.mj.ts.

6. A method as clai.med i.n any one of the precedi.ng claims wherein the frequency or identity of the note being monitored i.s passed from a suitable output stage in the method of the invention to means for generating an accompani.ment or other signal i.n response to that note.

7. A method as claimed i.n claim 5 wherein the verified note identification from stage d i.s passed to a memory means where the note identification is maintained, preferably only whilst there is an input to stage a, until a subsequent note identifi.cation is determined as being valid and displaces the first note identi.fi.cation from the memory means.

8. A method as claimed in claim 7 wherein a signal is generated in response to the note identification held in the memory means and is fed to an accompaniment generation device

9. A method as clai.med i.n claim 2 wherein the signals from stage a are monitored by peak detectors adapted to be actuated at a reference level 5 to 10% below the maximum actual peak value expected.

10. A method as claimed in claim 9 wherein the reference level decays by less than 40 to 50% between consecutive wave cycles.

11. A method as claimed i.n any one of the preceding clai.ms wherein the signals from stage a are fed to positive and negative peak detectors for stage b and the outputs from the detectors are fed separately to a bistable multi.vi.brator switch for stage c.

12. A method as clai.med in claim 1 wherein the signals from stage a are fed to a zero crossing detector which provides the thi.rd series of pulses from stage c.

13. A method as claimed in claim 3 wherein the clock means generates pulses at a frequency at least 8 times higher than the highest frequency of the note expected to be monitored.

14. A method as claimed in claim 13 wherein the pulses are generated at a frequency 24 times greater than the highest frequency to be monitored.

15. A method as claimed in claim 1 wherein the sereis of pulses from stage c are passed to a binary dividing stage in which the frequency of the pulses i.s effectively halved so as to enhance the distinction between high frequency noise signals and the longer durati.on fundamental note signal

16. A method as claimed i.n claim 5 wherein the note identifi.cation is carried out using conventional read only memories containing look up tables of the note values corresponding to the duration values.

17. A method as claimed i.n claim 16 wherein the duration values for the posi.tive and negative half waves are processed in parallel through separate identification means.

18. A devi.ce for generating the identi.ty of the fundamental frequency of a note, which device compri.ses:

1. means for receiving a wave form signal from means for detecting the note to be identified;

2. means for detecting the existence of positive and negative values for the wave form of the note and for generating a first and a second series of pulses corresponding to the occurence of those positive and negative values; ;

3. means actuatable by the fi.rst and second series of pulses for producing a thi.rd series of pulses whose frequency corresponds predominantly to the frequency of the fundamental wave form of the note being identified, preferably this means i.s a bistable switch means.

19. A device as claimed i.n claim 18 which further comprises

4. clock means adapted to provide a measure of the duration of a wave length, notably of each half wave length, of the pulses in the third series of pulses;

5. means for comparing one wave length duration, notably the duration of a half wave length, for pulses from the third series of pulses with another wave length duration from that series, notably the duration of an immediately succeding or preceding half wave length, to establish whether the third series of pulses approximate within predetermined limits to a 50% duty cycle so as to determine whether a thi.rd pulse is an acceptable signal or not; and

6. output means for transmitting the acceptable signal to a means for generati.ng a signal in response to the frequency of the acceptable note.

20. A device as claimed in either of claims 18 or 19 wherein the comparative means in item 5 comprises an i.denti.fi.cation means incorporating memory units for allotting a note identifi.cation to each posi.tive and negative half wave duration signal from item 4 and means for comparing the note identifications for the corresponding posi.ti.ve and negative half waves so as to establish the validity of the note identifi.cation to within predetermined li.mi.ts;; and memory means where the identificati.on from the note identifi.cation means are held until 1 displaced by a subsequent acceptable note identification, the output means 6 reading the identity of the note from the memory means.

21. A device as claimed in any either of claims 19 or 20 where the note identity to be read by the output means is retained only whilst there i.s a note i.nput to item 1.

22. A device as claimed in claim 18 which comprises an inductance device for generating a signal from the note being monitored; positive and negative peak voltage detectors for monitoring the signal from the inductance device to provide two series of signals; a bistable multivibrator switch having inputs to receive the two series of si.gnals separately and to provide a single pulsed output; gate circuits to separate the output from the switch into posi.ti.ve and negative pulses; a clock ci.rcui.t to provide ti.mer pulses and an integrator circuit to establish the duration of each positive and negative pulse n terms of the ti.mer pulses; an identi.ficati.on circuit incorporati.ng one or more look up tables for identifying the note value corresponding to the duration value of the negative and positive pulses; a comparison circuit for comparing the note values for successive pulses; and a buffer for receiving note values which have been accepted as identical to within pre-determined limits in the comparison circuit and for feeding those note values tq an accompaniment generator means.

23. A method or device substantially as hereinbefore described.

24. A method or device substantially as herei.nbefore descri.bed with respect to the accompanying drawing.