To build a Monophonic, Modular, Electronic, Music Synthesiser

7/28/2019 To build a Monophonic, Modular, Electronic, Music Synthesiser.

http://slidepdf.com/reader/full/to-builda-monophonicmodularelectronic-music-synthesiser 1/17

BrianLeach

MUSC9002:ComputerMusic2

Project2c)Apreparation/proposalfortheMastersProject(Technology

Research)

Lecturer:HughMcCarthy

Due:Friday30thMarch2012

Studentnumber:R00043544

17pages.

TobuildaMonophonicModularElectronicMusicSynthesiser.



2

Introduction

Abasicsoundsynthesisercanbemadefromthefollowing5typesofmodules:

1) Voltage-ControlledOscillator(VCO)2) Voltage-ControlledAmplifier(VCA)3) Voltage-ControlledFilter(VCF)4) EnvelopeGenerator(EG)5) LowFrequencyOscillator(LFO)

I will discuss the operation of each of these modules and propose animplementation of them in hardware electronics, along with suitable circuit

diagrams. Iwillalsoestimatethecostofbuildingthemachine.Theaimofthis

essayistogaugethefeasibilityofbuildingasynthesiserformyMastersProject

(TechnologyResearch).

Overview

Themodularnatureofthisprojectmeansthateachmoduleisseparatefromthe

othersandalsothattheycanbeinterconnectedinanyconceivablewaytocreatemanydifferenttypesofsoundswhenplayedbyakeyboardorothercontroller.I

willnowdiscussatypicalmodeofsoundproduction.

IfweconsidertheVoltage-ControlledOscillator(VCO)wemustofcoursedefineitsfrequency,aControlVoltage(CV)isusedtodothis.Thisvoltageisdesignatedby the key that the user presses on the keyboard. Thus we need a way to

transformkeyboardactivityintoControlVoltages.Fornowletsassumethatthe

synthesiserisreceivingmidimessages.Acontrolmoduleisnecessarywhichhasamidi input, and a control voltageoutput.For clarity Iwill represent control

pathswitharedline,andaudiopathswithablueline.

Figure 1. Midi messages being transformed into an audio signal with adefinedfrequency.



3

This CVoutputcanbe connected into theCV inputof aVCO. TheVCO is nowoutputtingasignalwithadefinedfrequency.AsyoucanseeinFigure1theVCO

hasaCVinputandanaudiosignaloutput.

ThissignalcanbeputintotheinputoftheVoltage-ControlledAmplifier(VCA)whichmultipliesthesignalbysomenumber≥0.Thisnumberisdefinedbythe

CVinputoftheVCA.ConsiderthatthisCVcouldbechangingovertime,creatinganamplitudeenvelope.ThisenvelopeisdefinedbytheEnvelopeGenerator(EG).

TheEGmustreceivedatafromthecontrolmoduletotellitwhentogeneratetheenvelope,triggers.

Figure2.AnEnvelopeGeneratoraffectingtheamplitudeoftheaudiosignal

viaaVoltage-ControlledAmplifier.

TheEGhas4parametersthataresetbytheuser:Attack,Decay,Sustain,Release(ADSR).

Figure3.AtypicalvoltageenvelopegeneratedbytheEnvelopeGenerator.



4

Ifthesignal iscomplex(hasmanyovertones)thenwecanpass itthroughtheVoltageControlledFilter(VCF)toshapethesound.TheVCFwillcutfrequencies

aboveor below some defined frequency. This frequency is defined by the CVinputoftheVCF.Thisfrequencycouldchangeovertime(haveanenvelope)ifwe

passtheEGintotheVCF.TheinputoftheVCOcouldalsobeaffectedinthesameway.

Figure4.TheEnvelopeGeneratorcanaffectanyofthemodulesbybeing

passedintotheirControlVoltageinput.

NotethattheVCOisnowreceivingtwocontrolvoltages.Thevoltagesneedtobeaddedtogetherinamixer.ThesimplestwaytoachievethisistohavetwoCV

inputs on theVCO, each one going through an internal resistor. This forms a

passivemixer. Anymodule can receivemultiple inputs via a built in passive

mixer.Notealso that thissimplemixercan add controlvoltagesand/or audiosignalsindiscriminatelyasallitisdoingisaddingvoltages.

Sowehavetheabilitytovarythepitch,volumeandcharacterofthetonebeing

produced.Modules could be replicated so that twooscillatorswould generateaudiosignalsfromthesamecontrolvoltage:



5

Figure5.TwoVoltage-ControlledOscillatorsbeingcontrolled

simultaneouslytoproduceadditivesynthesis.

Orthesecondoscillator’soutputcouldbeusedtocontrolanothermodule:

Figure 6. A Voltage-Controlled Oscillator being used to modulate the

FrequencybeingproducedbyasecondVoltage-ControlledOscillator.

NotethattheoutputofthesecondVCOisactingasacontrolvoltageandnotas

anaudiosignal.UsedinthiswayanoscillatorcanmodulatethecontrolvoltageofanotheroscillatortocreateFrequencyModulation(FM),orifitisaffectingthe



6

VCA,AmplitudeModulation(AM).Ifwewantafixedfrequencyofmodulation,

ratherthanonedeterminedbywhichkeywepress,wecanuseaLowFrequencyOscillator(LFO),socalledbecauseitgenerallyoperatesatfrequenciesbelowthe

audiblerangealthoughthisisnotnecessarilythecase.Thisoscillatorneedsnocontrolinputasitsfrequencyissetbyitsownonboardcontrol.

Figure7.ALowFrequencyOscillatormodulatingthecutofffrequencyofa

Voltage-ControlledFilteratafixedfrequency.

This outlines just someof themanyways thesemodules can beconnected to

generatesound.Iwillnowlookateachmoduleinfurtherdetail.



7

ControlModule

This module must sensewhich key is pressed, when the key is pressed, andwhenitisreleased.TheoutputsmusttriggertheEGandalsogivetheVCOthe

correctcontrolvoltagetoproducethedesiredfrequency.

ControlVoltage

Analogue synthesisers generally use control voltages ranging from 0V to 5V.MoogsynthesisersusetheVoltsPerOctavesystemwhichmeansthatanincrease

of1Vcorrespondstoadoublingoffrequency 1,oranincreaseofonesemitoneis

equalto1/12ofavolt.Hencethenotesandcorrespondingcontrolvoltagesare:

Table1.NotesandControlVoltagesintheVoltsPerOctavescheme.

Trigger/Gate

TheControlModulemustalsosendinformationtotheEGwhichwilltelltheVCA

toturnon,allowingthesignalfromtheVCOtobeheardonlywhenakeyisbeinghelddown.AcommonwaytoimplementthisiswithanegativeTriggeranda

positive Gate1. The Trigger signal is kept high (around 5V) and goes to 0Vmomentarilywhenakeyispressed.TheGatesignaliskeptat0Vandgoeshigh(5V)forthedurationoftimethatakeyispressed.Thisinformationisenoughto

telltheenvelopegeneratortogeneratetheattack,sustainorreleaseportionof

theenvelope7(decayhappensimmediatelyafterattackbydefinition).Inpracticethe Gate voltage could be the Control Voltage discussed above (or a suitable

multiple of it) as the CV 0V when no key is pressed, and>0Vwhen a key ispressed.

Note CV Note CV

Note CV

Note CV

Note CV

A1 0 A2 1.0000 A3 2.0000 A4 3.0000 A5 4.0000

A♯ 0.0833 A♯ 1.0833 A♯ 2.0833 A♯ 3.0833 A♯ 4.0833

B 0.1667 B 1.1667 B 2.1667 B 3.1667 B 4.1667

C 0.2500 C 1.2500 C 2.2500 C 3.2500 C 4.2500

C♯ 0.3333 C♯ 1.3333 C♯ 2.3333 C♯ 3.3333 C♯ 4.3333

D 0.4167 D 1.4167 D 2.4167 D 3.4167 D 4.4167

D♯ 0.5000 D♯ 1.5000 D♯ 2.5000 D♯ 3.5000 D♯ 4.5000

E 0.5833 E 1.5833 E 2.5833 E 3.5833 E 4.5833

F 0.6667 F 1.6667 F 2.6667 F 3.6667 F 4.6667

F♯ 0.7500 F♯ 1.7500 F♯ 2.7500 F♯ 3.7500 F♯ 4.7500

G 0.8333 G 1.8333 G 2.8333 G 3.8333 G 4.8333

G♯ 0.9167 G♯ 1.9167 G♯ 2.9167 G♯ 3.9167 G♯ 4.9167

A 5.0000



8

Oneway toimplement the controlmodulewouldbetotakeinmidimessages

whicharecomprisedofanotenumber(whichkeyispressed),anoteonmessage(whenthe key ispressed)and anoteoffmessage (whenthe key isreleased).

ThisinformationcouldbetransformedtoCVandTrigger/Gateinformation:

Figure8.BlockdiagramofaMiditoControlVoltage/Gateconverter8.

Thisisacomplicatedsystemwhichwouldbedifficulttobuildfromscratch,but

there are commercially available modules retailing around €200 which are

designed to perform exactly this function e.g. the ‘Kenton Modular Solo’http://www.kentonuk.com/products/items/m-cv/modsolo.shtml. An Arduinocouldbeprogrammedtoperformthesefunctionsandpurchasedforaround€50.

Another more fundamental way to implement this would be to modify anexisting keyboard. Each key could tap into a line of equal resistors in series

whichwoulddivide5Vinto61equallyspacedvoltages.Twoadditionalcontacts

undereachkeycouldprovidetheTriggerandGate.



9

Figure9.Eachkeyonthekeyboardcouldactuate3switchestogeneratea

Gate,TriggerandspecificControlvoltage.

Thegateandcontrolvoltageoutputisselfexplanatory.Thetriggeroutputworkslikethis:whennokeyispressedtheswitchesaretotheleftandtheoutputis5V,

whenakeyispressedthecorrespondingswitchgoestotheright,thevoltagewill

dipmomentarilyto0Vwhilethecorrespondingcapacitorischarging.Thistimeneed only be a fewmilliseconds so a small capacitancewill suffice. Once the

capacitorischargedtheoutputwillreturnto5V.Whenthekeyisreleasedtheswitchreturnstotheleftcontactandthecapacitorisdischarged,allowingthe

processtoberepeated.

Thiswouldbethecheapestoptionasanoldkeyboardcouldbesalvagedforvery

littlemoney.



10

Voltage-ControlledOscillator

ThekeytomakingamusicalVCOwasdiscoveredbyRobertMoogandoutlined

inhis1964paper‘Voltage-ControlledElectronicMusicModules’.Henoted:

“In Technical measurement and control operations, a linear control voltage –frequencyrelationshipisfrequentlyuseful.Intheproductionofmusic,however,

constant frequency differences are of little value. The fundamental subjective

qualityoffrequencychangeistheinterval,whichisaratiooftwofrequencies.Inordertobemusicallyvaluable,aVCOshouldgenerateafixedfrequencyratiofor

agivencontrolvoltagechange.Inmathematicallanguage,thefrequencyshould

beanexponentialfunctionofthecontrolvoltage.”2

TheimportanceofhavinganexponentialrelationshipbetweentheCVandtheoutput frequency is clear ifwe imagine anLFOmodulating the CV feedingan

audiooscillator.LetssaytheCVis3V,thiscorrespondstothenoteAat440Hzas

describedabove.NowletssaywewanttheLFOtovarythisnoteupanddownbyasemitone,henceitsvaluemustchangefrom-0.0833to+0.0833(±1/12V).A

linearVCOcouldbecreatedthatwouldproduceaG#(415.3Hz,24.7Hzbelow

A440)witha controlvoltageof2.9167V,but anincreaseofcontrolvoltage to3.0833Vwouldnow lead toan increaseof24.7Hz,giving 464.7Hz.The actual

valuewewantisA#(466.16Hz),soweseethatourLFOreducesthepitchbyasemitonebutfailstoincreasethepitchbythesameamount.Thisvibratoisnot

centered on A and thus does not sound as we would expect musically. The

introduction of an exponential converter for the control voltage solves thisproblem and ensures that a linear increase/decrease in CV corresponds to a

fractional increase/decreasein frequency,andhencea fixed increase/decreaseinpitch.

SoourVCOmusthavetheabilitytoacceptmultiplecontrolvoltages,addthemtogetherlinearly,andconvertthesumtoanexponentialvaluewhichaffectsthe

frequencyofoscillation.

Figure 10. BlockdiagramofamusicalVoltage-ControlledOscillatorwith

Exponentialconverter.

TheExponentialconverterisofkeyimportanceandwillrequirefurtherresearchtobeunderstoodfullyandimplementedproperly.ThecircuitsuppliedbyMoog

inhispapergivesastartingpointforfurtherresearch.HereishisVCOdesign:



11

Figure11.CircuitDiagramofRobertMoog’sVoltage-controlledoscillator

from his 1965 paper. Note the three sections: Adder, Exponential

Generator,andRelaxationOscillator(bottom).



12

Voltage-ControlledAmplifier

Ourperception of loudness followsan exponential relationshipsimilar to thatdescribed for perception of pitch: a wave with twice the amplitude is not

perceived as twice as loud3. Luckily the VCA is controlled by the envelope

generatorwhichgeneratesitsenvelopebycharging/dischargingcapacitors.Thisprocessgenerates exponential voltage increases/decreases9 andhencewe can

useaVCAthathasalinearCV-to-gainrelationship.

AsimpleopampamplifiercircuitcouldbeusedwithalinearVoltageControlled

Resistor(VCR)controllingthegain.ItisimportantthattheVCRislinearsothatthe signal is not distorted4. Remember thatwe are amplifying anAlternating

CurrentsotheVCAmustbelinearforpositiveandnegativevoltages.Ofcourse

some‘distortion’maybedesirable,asthiswillgivethesounditsowncharacter.ItwasthespecifictypeofdistortioninherentinthecircuitsdesignedbyMoog

thatmadethemsoundsodesirable5.

The VCRs value, and hence the gain of the VCA is controlled by the varying

ControlVoltagecomingfromtheEnvelopeGenerator.

Figure 12. A Voltage-Controlled Amplifier utilizing a Voltage-controlled

Resistor.

Note thatanInverting amplifiermustbeusedbecauseagainof0 isrequiredwhennonoteisbeingplayed.Thegainofthecircuitis

G=-VCR/R1

SowhennonoteisbeingplayedthevalueoftheVCRmustbe0andwhenatmaximumgainwerequireVCR=R1togiveunitygain.



13

TheVCRcouldbeaFieldEffectTransistor(FET) 4oranLEDpointedataLightDependentResistor.Someexperimentationwillberequiredtoseewhatmethod

soundsbest.

Voltage-ControlledFilterTheFiltermodulewilltakeanaudioinputandfilteroutsomefrequencieswhich

aredefinedbythecutofffrequencycontrol.Thiswillshapethetoneofthesound

beingproducedbytheVCO.Avariablequality(Q)controlcauseresonancejustbelowthecutofffrequency.Thisresultsinanamplificationofasmallfrequency

bandwhichaddscharactertothefilter.ThemoduleshouldhaveLowPass,HighPassandBandPassmodes.

Hereisasimplecircuitasastartingpoint.VaryingtheR1andR2simultaneouslywillchangethecutofffrequency.ReplacingtheseresistorswithVCRswillallow

thecutofftobechangedovertimebytheCVprovidedbytheEGorLFO.Having

variableresistors,controllablefromthefrontpanel,inserieswiththeVCRswilldefinetherangeofeffecttheCVhas.

Figure 13. SampleFilter circuitdiagramwhichcouldbeadapted tohave

Voltagecontrol6.



14

EnvelopeGenerator

AsuitablecircuitdiagramfortheEnvelopeGeneratorisgivenbelow

Figure14.EnvelopeGeneratorwithADSRcontrols7.

TheGate/Triggermessagesandcorrespondingoutputareshownbelow

Figure15.Trigger/GatemessagesandcorrespondingEnvelopes7.

TheinputsofthemodulearetheGateandTriggermessages.TheoutputisthevoltageenvelopewhichcanbesentontotheCVinputsofanyothermodule.The

amplitudeoftheenvelopecanbesetbytheoutputamplifiersectionandthustherangeortheEGcanbechangedbysettingthegainofthisstage.



15

LowFrequencyOscillator

TheLFO is the simplest module as it requires no control input. It only has asignaloutput.Initsmostsimpleform,thismodulecouldbeasinewavephase

shiftoscillatorwhichisacircuitthatIhavebuiltbefore.

Figure16.APhaseShiftOscillator.

The frequency of oscillation can be set using a variable resistor, and theamplitudeofoscillationcanbeset bypassing thephaseshiftoscillatoroutput

throughanopampamplifiercircuitwithavariableresistorcontrollingitsgain.

Othermorecomplicatedcircuitscouldbeusedwhicharecapableofgeneratingtriangular, saw toothand squarewaveforms thusgivingmoreoptions for the

typeofmodulationproduced.TheoutputoftheLFOcanbesenttotheCVinput

ofanyothermodule.

PowerSupplyUnit

Eachmoduleshouldbecapableofbeingpoweredbya15V1ADCPowersupply.

Thisunitcouldbebought,orsalvagedfromoldequipment,as15Vsuppliesare

quitecommon.



16

Estimatedcost:

Electroniccomponents

ControlModule €50

VCOModule €10VCAModule €5

VCFModule €5EGModule €5

LFOModule €5

PSU €15

Hardware

JackSockets €10

Knobs €5

PatchLeads €0(Ihavemanybrokenguitarleadsthatcanbesalvaged)

MetalHousing €0(canbescavenged)

Total €110

This is probably an over estimation as the bulk of the cost is in the controlmodule, which could be an Arduino borrowed from the college (if midi

compatibility is desired), or a modified keyboard which would require an

investment of time rather thanmoney. This project couldbe realized for lessthan€100,whichisfeasibleforme.

ConclusionIhaveoutlinedthemodeofoperation ofamodularsynthesiserand proposed

somecircuitdesigns.Whilefurtherresearchisneededtorealiseallthemodulesdiscussed(particularlytheVCO),Ibelievethatitispossibleandaffordablefor

me to build a self-contained modular synthesiser for my Masters research

project.Thesynthesiserwouldbecomprisedof:

1ControlModule2Voltage-ControlledOscillators

1Voltage-ControlledAmplifier

1Voltage-ControlledFilter1EnvelopeGenerator

1LowFrequencyOscillator

1PowerSupplyUnit



17

References

[1]Jenkins,Mary. AnalogueSynthesizers.FocalPress,Oxford,2007.p.35

[2] Moog, Robert. Voltage-Controlled Electronic Music Modules, Journal of the AudioEngineeringSociety ,Volume13,Number3.NewYork,July1965.p.1.

[3]Howard,David&Angus,Jamie. AcousticsandPsychoacoustics,fourthedition.

FocalPress,Oxford,2009.p.92.

[4]Mayes,Lawrence.TheFieldEffectTransistorasaVoltageControlledResistor ,

2002,Viewed30March2012,http://graffiti.virgin.net/ljmayes.mal/comp/vcr.htm

[5]Editor:Sinclair,IanR.Chapter20,SoundSynthesisbyJenkins,Mary. Audio

andHiFiHandbook ,3rdedition.Newnes,Oxford,1998.p.362.

[6]Steiner,Nyle.Voltagetuneableactivefilterfeatureslow,highandbandpassmodes.ElectronicDesign,vol25,SaltLakeCity,December1974.p.1.

[7]Jacky,Jonathan.TwoChipgeneratorshapessynthesizer’ssounds.Electronics,

Seatle.September1980.

[8]Rees,Phillip.LittleMCVMiditoCVconverter .2005.Viewed30March2012.

http://www.philrees.co.uk/products/miditocv.htm

[9]Mansfield,Michael&O’Sullivan,Colm. UnderstandingPhysics,Chapter15.10.

Wiley-Praxis,2008.

Bibliography

HorowitzandHill,TheArtofElectronics.CambridgeUniversityPress,1980.

Douglas,Alan&Astley,S.TransistorElectronicOrgansfortheAmateur .PitmanPress,Bath.1965.

KendallWebsterSessions.Discrete/TransistorCircuitSourcemaster .Wiley.NewYork,1978.

Huelsman, Lawrence. Active and Passive Analogue Filter Design. McGrath-Hill,Newyork,1993.

Documents

To build a Monophonic, Modular, Electronic, Music Synthesiser