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SAE Berlin
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Student Id: 18128
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Course: AED412
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Headinstructor: Boris Kummerer
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Berlin, Germany 2012
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author{
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by Karl Pannek
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}
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title{
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LARGE{Prototyping a Modular Analog Synthesizer}}
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\begin_layout Chapter
Preface
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Introduction
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This paper constitutes an attempt to summarize the most important facts
 and information on the topic of analog, modular synthesizers.
 The range of discussed subjects involves a series of various perspectives,
 including historical, theoretical, electronic and practical viewpoints.
 
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Its goal is to convey an understanding of the inner workings of electronic
 synthesizers and their components.
 Moreover the reader is guided through the process of creating a small but
 functional modular synthesizer setup that is fun to play and experiment
 with.
 The intention was to investigate the possibilities and limits in designing
 and building an analog sound device for someone, who had not been in contact
 with analog synthesizers, let alone building electronics devices before.
 The project was inspired by the film 
\emph on
moog
\emph default
 
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, a documentary about Dr.
 Robert Moog, electronic instrument pioneer and inventor.
 
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Chapter Overview
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The first chapter represents the research on the historical background of
 analog synthesizers since the beginning of the twentieth century.
 It was tried to outline important milestones in the historic development
 from the first electronic sound generating devices until a point in time
 when manufacturers of modular synthesizers have developed a profitable
 market.
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Subsequently the most important concepts of subtractive synthesis are summarized.
 A general overview over common sound generation and processing methods
 is given, whereby all concepts are applicable to both analog and digital
 synthesis.
 In chapter three these concepts are taken one step further and discussed
 in the context of electronic circuitry.
 Lastly the process of building an electronic synthesizer prototype is described.
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The process of building the prototype includes working with an oscilloscope
 to examine and verify the shape of various waveforms before and after modulatio
n.
 
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To make it playable with a keyboard, a MIDI input module is added.
 It features an Arduino microprocessor to convert digital MIDI messages
 into control voltage outputs that other modules can connect to.
 It is the only digital component of the synthesizer, while tone generation
 and processing are analog.
 
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Historic Evolution of the Synthesizer
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Early Development Milestones
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Electric instruments at that time were developed primarily to imitate and
 evolve the sounds of classical instruments and therefore satisfy traditional
 ideas of musical writing 
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Around 1900 american Thaddeus Cahill initiated a new era of music by inventing
 a 200 ton machine known as the Dynamophone or Thelharmonium 
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.
 Working against incredible technical difficulties, he succeeded to create
 an electrical sound generator, that produced alternating sine wave shaped
 currents of different audio frequencies.
 A modified electrical dynamo was used in conjunction with several specially
 geared shafts and inductors to create the signals.
 The Dynamophone could be played with a polyphonic keyboard and featured
 special acoustic horns to convert the electrical vibrations into sound
 
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.
 The timbre of the instrument was manually shaped from fundamentals and
 overtones.
 This is known as the principle of additive synthesis 
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Leon Theremin performing the Aetherophone
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In 1924 the russian inventor Leon Theremin created the Aetherophone, which
 would later be known as the Theremin.
 Unlike most electric instrument developed around that time, the Theremin
 had no keyboard.
 It was played merely by hand motion around two capacitive detecors, that
 generated electrical fields.
 These were affected by the electric capacity of the human body.
 One of these detectors was a vertical rod to control dynamics and the other
 a horizontal loop to change the pitch 
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.
 
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The theatricality of its playing technique and the uniqueness of its sound
 made the Theremin the most radical musical instrument innovation of the
 early 20th century.
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Some organ-like precursors to the synthesizer were the Ondes Martenot and
 the Trautonium, which were devised just a few years later.
 The Ondes Martenot is one of the few early electric instruments, that are
 still in concert- and theatre use in their original design today 
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The Givelet (1929) was a commercially more successful instrument, since
 it was designed as a cheap alternative to pipe organs.
 These instruments were polyphonic and unified the concepts of the Pianola
 - a self-playing piano, controlled by pre-punched tape - with electronic
 sound genaration.
 The ability to program electronic sounds should lead the way for future
 devices such as the RCA synthesizer or computer music production in general.
 However, the Givelet was about to take a back seat, when Laurens Hammond
 published his Hammond Organ in 1935.
 Its technical operation principle is reminiscent of the Dynamophone, since
 it also involved rotating discs in a magnetic field 
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Harald Bode tuning his first Melochord
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The german engineer Harald Bode contributed to the design of several new
 instruments from the 1930's on, like the warbo formant organ (1937) or
 later the Melochord (1949).
 He was primarily interested in providing tools for a wide range of musicians,
 which is why his contributions straddled between the two major design tradition
s of new sounds versus imitation of traditional ones.
 He turned out to be one of central figures in the history of electronic
 music, since he was also one of the primary engineers in establishing the
 classic tape music studio in europe 
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Bode was one of the first engineers to grasp the significance of the invention
 of the solid state transistor for sound synthesis.
 In an article published in 1961 he draws particular attention to the advanteges
 of modular design.
 
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The versatility of transistor-based electronics made it possible to design
 any number of devices which could be controlled by a common set of voltage
 characteristics.
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.
 But it was not until the early 1960's that major advances in electronic
 design took shape 
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Sakbutt (1948) Hugh LeCaine
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The First Synthesizers
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In 1955 the laboratories of the Radio Corporation of America (RCA) introduced
 a new and very advanced machine to the public named the Olson-Belar Sound
 Synthesizer, later known as the RCA Mark I Music Synthesizer.
 It combined many means of tone generation and sound modification known
 at the time and is considered the first synthesizer.
 Mark I was built with the specific intention of imitating traditional instrumen
t sounds and to reduce the costs of the production of popular music by replacing
 musicians.
 However, the machine proved unsuitable for its original intent and was
 later used completely for electronic music experimentation and composition
 
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.
 The synthesizer could not be played in the conventional sense in real time.
 Instead musical information had to be pre programmed as punched holes in
 a large paper tape.
 Harry Olson and Herbert Belar produced an improved Mark II Synthesizer
 in 1957, which the nickname 
\emph on
Victor
\emph default
 was given
\emph on
 
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Around the same time the outstanding guitarist and inventor Les Paul became
 famous with his multitrack guitar recordings.
 He stimulated many innovators not only with the success of his multitrack
 recorder, but also with his methods of electronic sound processing.
 Harald Bode was so impressed and inspired by his work, that he built a
 system consisting of a number of electronic modules for sound modification
 in late 1959 through 1960.
 His system featured ring modulator devices, envelope followers and generators,
 voltage-controlled amplifiers, filters, mixers and others 
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.
 The modular concept of his device had proven attractive due to its versatility
 and predicted the more powerful modular synthesizers that emerged in the
 early 1960's 
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In 1963 Robert Moog, a passionate inventor from Ithaca, New York, was selling
 kits of transistorized Theremins 
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.
 As he states in the movie about him 
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, he had been completely obsessed with building and later designing Theremins
 since the age of 14.
 A year later he built a transistor based voltage-controlled oscillator
 and amplifier for the composer Herbert Deutsch.
 This led moog to the presentation of a paper entitled 
\emph on
Voltage-Controlled Electronic Music Modules
\emph default
 at the sixteenth annual convention of the Audio Engineering Society, which
 had stimulated widespread interest 
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Donald Buchla with a Series 100 system in the 1960's
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 Similar developments had been taking place at the west coast of the united
 states.
 Morton Subotnick and Ramon Sender started their carreer in electronic music
 experimentation, and became increasingly dissatisfied with the severe limitatio
ns of traditional equipment at the San Francisco Tape Music Center, where
 they were working.
 They sought out to hire a competent engineer and met Donald Buchla 
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.
 Their discussions resulted in the concept of a modular voltage-controlled
 system.
 Buchla's design approach differed significantly from Moog.
 He rejected the idea of a synthesizing familiar sounds and resisted the
 word 
\emph on
synthesizer
\emph default
 ever since.
 It seemed much more interesting to emphasize new timbral possibilities
 and stress the complexity that could arise from randomness.
 At the same time Buchla was fascinated with designing control devices other
 than the standard keyboard, which Moog decided to use for playing 
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\emph on
Switched-On Bach
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 LP artwork
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 In 1966 Bob Moogs first production model was available from the business
 R.A.
 Moog Co.
 that he had founded 
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.
 At this time Walter Carlos, an audio engineer from New York who adviced
 Bob Moog while perfecting his system, worked with Benjamin Folkman to produce
 an album of titles by Johann Sebastian Bach interpreted only with Moog
 synthesizers.
 With the title 
\emph on
Switched-on Bach
\emph default
 they demonstrated the performance of the system so convincingly, that they
 hit the popmusic charts and sold a million LP's 
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By the end of the decade two other manufactrurers entered the market: ARP
 in America and EMS Ltd.
 in England.
 They had become major rivals for Moog and Buchla.
 Synthesizer production was dominated by these four companies for several
 years, whereby each firm struggled for a major share of a highly lucrative,
 rapidly expanding market 
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\begin_layout Chapter
Theory of Subtractive Synthesis
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\begin_layout Section
Sources
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Acoustic events can generally be divided in two groups: noises and tones.
 Whereas tones - as opposed to noise - are classified as sound waves, that
 oscillate in a periodic manner.
 However this is only a theoretical classification, since most natural sounds
 are a combination of the two 
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Wave Oscillation
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At the root of every artificial tone generating system there is an element
 that produces an oscillation.
 This element is mostly described as the oscillator, which represents the
 very source of what can be heard eventually.
 The oscillator produces a periodic wave, that moves between an amplitude-minima
 and -maxima.
 Its waveform (shape of the wave) determines the overtone structure and
 therefore the timbre of this basic source sound.
 Oscillators often provide several waveforms between which it is possible
 to switch back and forth.
 The pitch of the output signal is defined by the frequency of the wave
 and must oscillate between 20Hz and 20kHz in order for it to be audible
 to humans 
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 The output signal can later be processed and modulated in several ways.
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Oscillators that swing at an infrasonic frequency - meaning a frequency
 so low, that it is not hearable anymore - are called low frequency oscillators
 (LFO).
 They are used to control parameters of different components of the synthesizer
 periodically.
 For example to influence the pitch of another oscillator to get a vibrato
 - or the amplitude to get a tremolo effect.
 Some oscillators frequencies range from very low to very high, in which
 case a distinction between oscillator and LFO is unnecessary.
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Difference between poly and monophonic synthesis (voices, mono: store last
 note value) Voices
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unison
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sync
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\begin_layout Subsubsection
Characteristics of Common Waveforms
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sine The most basic waveform is the sine wave.
 It contains no overtones at all and sounds round and dull.
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sawtooth The sawtooth, also known as saw or ramp waveform sounds very bright,
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.
 It consist of a complete series of harmonics and is therefore well suited
 for subtractive synthesis.
 There are two types of sawtooth waves: rising and descending.
 
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triangle Composed of only odd harmonics, the triangle wave has a much softer,
 flute-like sound.
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square Also known as rectangle, 
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clearify difference to triangle 
\end_layout

\end_inset

 the square wave also consists of odd harmonics only.
 Its timbre reminds of woodwind instruments 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~55]{Ruschkowski1990}
\end_layout

\end_inset

.
 A true square wave has a 50 % duty cycle - equal high and low periods.
 However, oscillators often feature a pulse width parameter, trough which
 the high-low time ratio can be accessed.
 This has a distinct influence on the wave's timbre.
 In this case, the square becomes a pulse waveform.
\end_layout

\begin_layout Subsection
Noise Generation
\end_layout

\begin_layout Standard
A different approach on the creation of source audio material is resembled
 by noise generators, which generate random non-periodic frequencies.
 Therefore the signal contains no tonal information.
\end_layout

\begin_layout Subsubsection
Noise Types
\end_layout

\begin_layout Labeling
\labelwidthstring 00.00.0000
white Equal power density in any band of the frequency spectrum
\end_layout

\begin_layout Labeling
\labelwidthstring 00.00.0000
pink Power density decreases by 3dB per octave; also referred to as 1/f
 noise
\end_layout

\begin_layout Labeling
\labelwidthstring 00.00.0000
brown Power density decreases by 6dB per octave; also referred to as 1/f
\begin_inset script superscript

\begin_layout Plain Layout
2
\end_layout

\end_inset

noise
\end_layout

\begin_layout Standard
The names of these noise types were derived from the spectral distribution
 of the correspondingly colored light 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~155]{Friesecke2007}
\end_layout

\end_inset

.
\end_layout

\begin_layout Subsection
Triggering Notes
\end_layout

\begin_layout Standard
In order to use the previously discussed signal generators in a musical
 context, it is necessary to cut off their stationary signals when no note
 is being played.
 This is accomplished by routing the output signal of the generator to an
 amplifier and providing it with a gate signal.
 The source of the gate signal can be a keyboard or a sequencer, which would
 also send a pitch value to the oscillator to set its frequency 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~36]{Anwander2011}
\end_layout

\end_inset

.
\end_layout

\begin_layout Section
Signal Processing
\end_layout

\begin_layout Standard
In their raw shape the mentioned source signals sound rather underwhelming,
 since they produce fixed timbres lacking of distinctive qualities 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~49]{Manning1985}
\end_layout

\end_inset

.
 To get a more interesting sound, the signal can be manipulated in acoustic
 colour or dynamics by one or more processing units.
\end_layout

\begin_layout Subsection
Dynamic Envelopes
\end_layout

\begin_layout Standard
The most important component responsible for shaping the dynamic structure
 of a note is the envelope.
 It is triggered by the the gate on/off signal and outputs a control signal
 that fades between the different state phases of a note.
 The rapidity of these changes is adjusted by parameters, that represent
 these states.
 Its output signal can be used to control an amplifier and therefore shape
 the dynamic structure of the note.
 The most common envelope type is the ADSR, which stands for attack, decay,
 sustain, release.
 
\end_layout

\begin_layout Labeling
\labelwidthstring 00.00.0000
Attack sets how long the envelope signal rises after a note was triggered
\end_layout

\begin_layout Labeling
\labelwidthstring 00.00.0000
Decay sets how long it takes for the envelope signal to drop from its maximum
 to the sustain level after the attack phase was completed
\end_layout

\begin_layout Labeling
\labelwidthstring 00.00.0000
Sustain sets the output level for the time period after the decay phase
 and before the gate signal was terminated
\end_layout

\begin_layout Labeling
\labelwidthstring 00.00.0000
Release sets the length of the fade out after the note has endede
\end_layout

\begin_layout Standard
Envelopes can also be used to control other parameters, for example the
 cutoff frequency of a filter (see chapter 
\begin_inset CommandInset ref
LatexCommand ref
reference "sub:filters"

\end_inset

).
 
\end_layout

\begin_layout Subsection
Filtering 
\begin_inset CommandInset label
LatexCommand label
name "sub:filters"

\end_inset


\end_layout

\begin_layout Standard
The filter is the processing component responsible for the sound changes,
 that people associate with 
\emph on
the typical synthesizer sound
\emph default
 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~53]{Anwander2011}
\end_layout

\end_inset

.
 They remove a spectrum of frequencies from their input signal above or
 below the cutoff frequency and are often used in conjunction with an envelope
 or LFO modulation on the cutoff.
 This cutoff frequency is an important parameter determining the frequency
 at which the signal begins to be attenuated.
 The slew rate sets the slope of the filter - meaning how abrubt frequencies
 are being cut.
\end_layout

\begin_layout Standard
Filters can generally be devided into two categories: Low pass and high
 pass filters (also called high cut and low cut).
 To get a bandpass filter, low- and high pass are connected in series.
 When connected parallely, they become a bandstop or bandreject filter.
 Lastly the allpass filter should be mentioned, which does not change the
 frequency spectrum but merely influences the phase of the signal around
 its cutoff 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~55]{Anwander2011}
\end_layout

\end_inset

.
 
\begin_inset Note Note
status open

\begin_layout Plain Layout
COVER RESONANCE!!!
\end_layout

\end_inset


\end_layout

\begin_layout Section
Controllers
\end_layout

\begin_layout Standard
Controllers can be characterized by the way of how humans interact with
 them and how their output signal is applied in controlling other components
 of the system 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[Ch.~1A, p.~5]{Hutchins1975}
\end_layout

\end_inset

.
 A keyboard for example is a manual controller, since it is the movement
 of the players fingers which are translated into a voltage or control value
 and then used to control pitch and amplitude of a note.
 The same applies for rotary knobs and faders or touch sensitive surfaces.
 
\end_layout

\begin_layout Standard
Sequencers on the other hand are programmable controllers.
 They are not dependend upon a manual interaction except for their programming
 and activation.
 
\end_layout

\begin_layout Section
The Modular Approach
\end_layout

\begin_layout Standard
A modular synthesizer is an electronic instrument, where sound generators,
 processors and control facilities are presented in separate independent
 entities called modules.
 These modules are not wired in a preconceived way, but connected together
 with patchchords.
 The second essential aspect is the concept of intermodular controllability,
 with which modules may modulate or regulate the behaviour of other modules.
 
\end_layout

\begin_layout Chapter
Analog Synthesis
\end_layout

\begin_layout Section
General
\end_layout

\begin_layout Standard
In the context of audio electronics it is important to be know that sound
 signals are nothing but currents.
 
\end_layout

\begin_layout Standard
Voltage 
\end_layout

\begin_layout Standard
Control Voltage Audio Signal
\end_layout

\begin_layout Standard
Current
\end_layout

\begin_layout Standard
Rotary Knob
\end_layout

\begin_layout Standard
intermodular stuff like buffering 
\end_layout

\begin_layout Section
Modules
\end_layout

\begin_layout Subsection
Oscillator 
\begin_inset CommandInset label
LatexCommand label
name "sub:electronic-oscillator"

\end_inset


\end_layout

\begin_layout Subsubsection
Natural Resonance
\end_layout

\begin_layout Standard
The most basic form of an oscillator circuit producing a sawtooth signal
 can be realized with just a few parts.
 A current charges a capacitor at a certain rate.
 Between the electrodes of the capacitor a voltage potential rises.
 The voltage is constantly compared with a reference voltage by a detector
 (Schmitt trigger).
 Once it reaches the predefined threshold, an electronic switch (transistor)
 is activated.
 This switch short-circuits the capacitor and discharges it, causing the
 output voltage to drop back to its initial potential.
 This is happening continiously, wheras the rate of the repetition determines
 the frequency of the generated signal.
 
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status collapsed

\begin_layout Plain Layout
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
begin{center}
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Graphics
	filename graphics/sawtooth-oscillator-circuit.png
	display false
	width 80text%

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
end{center}
\end_layout

\end_inset


\begin_inset Caption

\begin_layout Plain Layout
Abstract circuit scheme of a basic sawtooth oscillator
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Standard
To get a triangle shaped output signal, the current source can be reversed
 in response to the detector 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[Ch.~5B, p.~3]{Hutchins1975}
\end_layout

\end_inset

.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status collapsed

\begin_layout Plain Layout
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
begin{center}
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Graphics
	filename graphics/triangle-oscillator-circuit.png
	display false
	width 80text%

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
end{center}
\end_layout

\end_inset


\begin_inset Caption

\begin_layout Plain Layout
Circuit scheme of a simple triangle oscillator
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
Either of these two waveforms can be waveshaped into to other common basic
 waveforms.
 All required waveshaping methods are fairly simple and their accuracy is
 sufficient for musical purposes.
 The sine wave is an exception, because there is no simple electronic method
 of creating a pure sine wave.
 What is generally used is a rounded triangle, which gives at least 1 %
 of harmonic distortion.
 In order to get a purer sine wave, the harmonics can be filtered out with
 a VCF following in the signal chain 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[Ch.~5B, p.~3-4]{Hutchins1975}
\end_layout

\end_inset

.
\end_layout

\begin_layout Subsubsection
Voltage Control
\end_layout

\begin_layout Standard
How fast the capacitor is charged is determined by the intensity of the
 current that it is being charged with.
 This current is being extracted from a voltage, which of course can be
 a control voltage.
\end_layout

\begin_layout Standard
However, creating a stable VCO design presents one of the toughest challenges
 for musical engineers, because the human ear is extremely sensitive to
 pitch changes.
 Also compositional processes like multitrack recording plainly reveal errors
 in pitch relationships 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[Ch.~5B, p.~1]{Hutchins1975}
\end_layout

\end_inset

.
 The voltage to pitch distribution curve of an oscillator is determined
 by its exponential response to the voltage at its CV input.
 In a volt per octave system an increase by 1 volt at the input must result
 in a doubling of the oscillation frequency at the output.
 Thus octave purity is achieved.
 
\end_layout

\begin_layout Standard
A major issue that oscillator designers have to face are temperature influenced
 variations of the electrical parameters of the system, causing the oscillator
 pitch to drift when temperature changes occur.
 To overcome this problem some kind of temperature compensation should be
 implemented.
\end_layout

\begin_layout Subsection
Filter 
\begin_inset CommandInset label
LatexCommand label
name "sub:electronic-filter"

\end_inset


\end_layout

\begin_layout Subsubsection
Passive Filter
\end_layout

\begin_layout Standard
Filter circuits are generally based upon the fact that the transfer of alternati
ng currents through a capacitor becomes increasingly weak below a certain
 frequency.
 A signal simply passed through a capacitor resembles a high pass filter.
 If the capacitor is connected to the ground the high frequencies are short
 circuited and low frequencies remain in the signal path.
 This is referred to as a resistor-capacitor (RC) element.
 This simple circuit represents a passive, first-order butterworth filter
 with a roll-off of 6 dB per octave.
 
\begin_inset Note Note
status collapsed

\begin_layout Plain Layout
 The moog filter for example concatenates four of these RC elements in a
 row and regulates the resistor values 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~57]{Anwander2011}
\end_layout

\end_inset

.
 
\end_layout

\end_inset

 To increase the slope of the filter multiple RC elements can be connected
 in series 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~57]{Anwander2011}
\end_layout

\end_inset

.
 
\end_layout

\begin_layout Standard
Nevertheless this poses some disadvantages, because the filter stages influence
 each other.
 The resistor of the first filter affects the resistance of the second.
 This can be dealt with by designing the second filter to have a much higher
 impedance.
 Unfortunately a high impedance circuit is much more prone to interferences
 and signal noise 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~97]{Sontheimer2004}
\end_layout

\end_inset

.
\end_layout

\begin_layout Subsubsection
Active Filter
\end_layout

\begin_layout Standard
\begin_inset Wrap figure
lines 0
placement O
overhang 0in
width "5.5cm"
status collapsed

\begin_layout Plain Layout
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
vspace*{-2em} 
\end_layout

\begin_layout Plain Layout


\backslash
begin{center}
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Graphics
	filename graphics/Sallen-Key-Filter-Lowpass.png
	display false
	width 5.5cm

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Active Sallen-Key low pass filter circuit scheme
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
end{center}
\end_layout

\begin_layout Plain Layout


\backslash
vspace*{-2em} 
\end_layout

\end_inset


\end_layout

\end_inset

 A better and more flexible solution is the usage of an active filter design,
 like the Sallen-Key filter.
 This is a common second-order filter, which is applicable as a low or high
 pass, whereby only resistors and capacitors have to be swapped 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~97]{Sontheimer2004}
\end_layout

\end_inset

.
 Resistor
\begin_inset script superscript

\begin_layout Plain Layout
2
\end_layout

\end_inset

 and capacitor
\begin_inset script superscript

\begin_layout Plain Layout
2
\end_layout

\end_inset

 can be identified to form a low pass section 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[Ch.~4B, p.~4]{Hutchins1975}
\end_layout

\end_inset

.
 
\end_layout

\begin_layout Standard
To be able to change the cutoff frequency via voltage control the frequency
 defining resistor could be replaced with a voltage controlled resistor
 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~198]{Lancaster1975}
\end_layout

\end_inset

.
\end_layout

\begin_layout Subsection
Amplifier
\end_layout

\begin_layout Subsection
Envelope Generator
\end_layout

\begin_layout Chapter
Building a Modular Synthesizer
\end_layout

\begin_layout Section
Introduction 
\begin_inset CommandInset label
LatexCommand label
name "sec:building-introduction"

\end_inset


\end_layout

\begin_layout Standard
It is relatively easy to find circuits to construct simple oscillators and
 filters based on the fairly comprehensible concepts of resonant circuits
 and RC elements (see chapter 
\begin_inset CommandInset ref
LatexCommand ref
reference "sub:electronic-oscillator"

\end_inset

 and 
\begin_inset CommandInset ref
LatexCommand ref
reference "sub:electronic-filter"

\end_inset

).
 However, as their flexibility and capabilities increase (e.g.
 controlling the frequency of an oscillator with 1 volt per octave), the
 circuits tend to get exceedingly complex, requiring solid expertise in
 electronics.
\end_layout

\begin_layout Standard
This is why it was decided to switch to the usage of pre-designed, professionall
y manufactured circuit boards for this project as opposed to elaborating
 all the circuits on perf boards as originally intended.
 This made the goal of intermodular controllability attainable more easyily.
 The downside of this approach are higher costs for boards and parts.
 However, the quality of the end-product is impressing.
 Also the time saving using this strategy is not to be underestimated.
\end_layout

\begin_layout Standard
During the research phase of this project the author found out about a modular
 synthesizer building workshop taking place in berlin monthly.
 It is organized by a spanish collective from barcelona called 
\emph on
befaco
\emph default
 (http://befaco.org/).
 At the workshop it was possible to acquire various module kits containing
 all necessary parts and also receive tips and support while assembling
 them.
 
\end_layout

\begin_layout Standard
Since the budget for this project was limited, it was tried to arrange a
 smaller setup that would still offer lots of sound design possibilities.
\end_layout

\begin_layout Standard
\begin_inset Note Note
status collapsed

\begin_layout Plain Layout
Befaco, help, how well it is documented is important, why the filter was
 chosen, limited budget
\end_layout

\end_inset


\end_layout

\begin_layout Section
Formats and Interfaces
\end_layout

\begin_layout Standard
There are several formats for module sizes, power supply plugs or patchchord
 connectors which emerged out of the production lines of various module
 manufacturers.
 For example Doepfer's modules are only compatible with their EuroRack cases,
 with a height of 128.5mm.
 These EuroRack modules use jack connectors for patching.
 A different size format often used in the DIY modular synth scene is the
 one the serge synthesizers use.
 They use banana jack connectors instead of mini jack for patching, which
 have the possibility of stacking banana connectors on top of each other
 and splitting the signal without having to use a multiplier module.
 For this project a combination was chosen: The modules are EuroRack size,
 but using banana plugs.
\end_layout

\begin_layout Standard
\begin_inset Note Note
status collapsed

\begin_layout Plain Layout
For tuned modules it is important to consider whether they use a volt per
 octave or volt per hertz characteristic.
\end_layout

\begin_layout Plain Layout
Audio Signals ±5Volts 
\end_layout

\begin_layout Plain Layout
Buffering 1:10 impedance ratio 
\end_layout

\end_inset


\end_layout

\begin_layout Section
Building and Testing
\end_layout

\begin_layout Standard
To get started with building electronic equipment, one has to obtain some
 tools first.
 This includes a soldering iron - best with adjustable temperature, a role
 of quality soldering tin, a desoldering pump and pliers for cutting and
 bending wire.
 
\end_layout

\begin_layout Standard
Soldering is a process of mounting electronic parts onto a circuit board
 by heating up board and component and then melting the soldering tin into
 the joint.
 A good temperature for the soldering iron is between 300° and 350° celsius.
 The iron should not be pressed onto the joint for too long, because there
 is a risk of destroying the component if it is sensitive to heat.
\end_layout

\begin_layout Standard
\begin_inset Note Note
status collapsed

\begin_layout Plain Layout
Research beginnings - Easy oscillation circuits.
 easy filters.
 how voltage controlled is the problem.
 How it had been decided not to design all circuits self, but instead use
 predesigned circuit boards in order to be able to get tuning stability
 and volt-per-octave possibilities.
\end_layout

\begin_layout Plain Layout
It had been understood how designing circuits requires years of work and
 experience.
\end_layout

\begin_layout Plain Layout
Module decicion, Getting the Circuit boards, Soldering, Getting Parts, General
 about parts (capacitors and resistors)
\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Note Note
status collapsed

\begin_layout Plain Layout
Oscilloscope, Multimeter, Tracking faults, measuring 
\end_layout

\end_inset


\end_layout

\begin_layout Section
Power Supply and Case
\end_layout

\begin_layout Standard
For the power unit a universal power supply circuit was chosen from Robert
 Sontheimers audio circuit technology book 
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
cite[p.~74]{Sontheimer2004}
\end_layout

\end_inset

.
 and mounted onto a perf board.
 Instead of the 7815 and 7915 voltage regulator ICs the 7812 and 7912 were
 used in order to get a ±12 volt power supply with a center tap for the
 ground.
 The modules can be connected to the four male 16-pin flat ribbon connectors,
 that were added to make the power supply compliant to the EuroRack standard.
 Another possibility would be to make a flying bus board by attaching those
 connectors to a flat ribbon cable that lies in the case.
 Or even just fix female connectors to the cable and plug them directly
 into the modules.
 Additionally it is planned to add an IEC socket and a power switch to it
 for more comfortable on and off switching and more steady starting current.
\end_layout

\begin_layout Standard
The case is a simple rack constructed from a few pieces of wood that are
 held together by 19 inch rails equipped with thread rails to fasten the
 modules.
\end_layout

\begin_layout Standard
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
pagebreak
\end_layout

\end_inset


\end_layout

\begin_layout Section
Frontpanels
\end_layout

\begin_layout Standard
\begin_inset Wrap figure
lines 0
placement R
overhang 0in
width "22text%"
status collapsed

\begin_layout Plain Layout
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
vspace*{-2.5em} 
\end_layout

\begin_layout Plain Layout


\backslash
begin{center}
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Graphics
	filename graphics/output-module-panel.pdf
	scale 60

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Output module template
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset ERT
status open

\begin_layout Plain Layout


\backslash
end{center}
\end_layout

\begin_layout Plain Layout


\backslash
vspace*{-2em} 
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
The panels for all modules were made from pre-cut aluminum plates with a
 white varnish.
 The labels for knobs and banana sockets are printed on the plates with
 a method, that is similar to homemade circuit board etching.
 A mirror-inverted label template is printed onto a piece of high gloss
 paper for inkjet printers - but with a laser printer.
 It is cut and placed face down onto the upper side of the panel.
 By thoroughly pressing down a hot flat iron (for ironing clothes) onto
 the panel for a few minutes, the toner cartridge particles move to the
 panel.
 The paper residues need to be removed by placing the panel in some water
 and rubbing them off with a sponge.
 Afterwards the panel is sealed with transparent lacquer.
 Once the panel is dried, the holes for the knobs, switches, etc.
 can be prepunched and drilled.
 Lastly all borholes are deburred.
 
\begin_inset Note Note
status open

\begin_layout Plain Layout
foto vom frontpanel ohne stecker
\end_layout

\end_inset


\end_layout

\begin_layout Section
BF-22 Filter
\end_layout

\begin_layout Standard
This module is an extended copy of the filter from the legendary Korg MS-20
 and is based upon the principle of the Sallen and Key filter (see chapter
 
\begin_inset CommandInset ref
LatexCommand ref
reference "sub:electronic-filter"

\end_inset

).
 It combines two linkable filter stages in one module.
 Each stage features cutoff and frequency knobs, as well as several voltage
 control inputs for cutoff frequency and resonance, whereas the cutoff frequency
 input can be attenuated and inverted with one knob representing modulation
 depth (labeled: ×-1 ...
 0 ...
 ×1).
 The HP/LP switch determines, if the filter is used in high pass or low
 pass mode.
 
\end_layout

\begin_layout Standard
When turning resonance up, at one point the filter begins to self-resonate
 at the given cutoff frequency, which means that the filter can also be
 utilized as an oscillator.
 Therefore a volt per octave input for the cutoff control voltage was added,
 to be able to control the oscillating frequency in a musical context.
 A look at the oscilloscope shows a sine like waveform with few overtones.
 Turning the resonance to the maximum, the filter goes into distortion and
 the wave becomes more square causing the sound to get more rough.
 The amount of distortion is visually represented by a red LED.
\end_layout

\begin_layout Standard
\begin_inset Note Note
status open

\begin_layout Plain Layout
beispiel patch mit foto und beschreibung
\end_layout

\end_inset


\end_layout

\begin_layout Section
Midi Input
\end_layout

\begin_layout Standard
Note Source
\end_layout

\begin_layout Section
Output
\end_layout

\begin_layout Chapter
Conclusion
\end_layout

\begin_layout Section*
Summary 
\end_layout

\begin_layout Standard
This paper constitutes an attempt to summarize what appeared to be the most
 important facts and information on the topics of history, theory and electronic
s during the research on analog synthesis.
 Of course for all of the discussed could only scratch the surface.
 Even only the topic of active filters circuits can (and does) fill entire
 books.
\end_layout

\begin_layout Standard
Accompanying this research a small synthesizer prototype has been set up,
 making use of the knowledge aquired during the research.
\end_layout

\begin_layout Section*
Results
\end_layout

\begin_layout Standard
\begin_inset Note Note
status open

\begin_layout Plain Layout
As mentioned in chapter 
\begin_inset CommandInset ref
LatexCommand ref
reference "sec:building-introduction"

\end_inset

 the process of building the prototype had not transpired as initially planned.
 
\end_layout

\begin_layout Plain Layout
cunstruction results - goals attained.
\end_layout

\begin_layout Plain Layout
what could be done better?
\end_layout

\begin_layout Plain Layout
what could be added, what are the next steps?
\end_layout

\begin_layout Plain Layout
problems of with output knob.
 maybe powersupply.
\end_layout

\begin_layout Plain Layout
describe the journey, discribe the difference and natururality of analog
 sound as opposed to the digital, which i only knew before.
\end_layout

\end_inset


\end_layout

\begin_layout Section*
A Personal Journey
\end_layout

\begin_layout Standard
As a trained programmer and web application developer the field of electronic
 engineering always seemed appealing to me.
 Hence the assignment for a research paper during the audio engineering
 course at SAE Institute seemed like a welcome opportunity to dive into
 the realm of building electronic devices in the context of sound generation
 and modification.
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\begin_layout Standard
The process of writing this paper has been an unexpectedly rewarding and
 inspiring experience, pushing the boundaries of my own musical and technical
 understanding.
 Most notably the concepts of free composition - meaning allowing randomness
 and therefore putting oneself in the position of reacting to a musical
 system, influencing it in terms of tendencies, rather than controlling
 it with a predetermed mindset - has been something that really changed
 my perseption of musical creativity.
 This for me seems much more attainable in the analog world, where electrical
 components and signal chains can be brought to their tipping points, resulting
 in an unpredictable outcome.
 That is where sound exploration begins, which is a totally different experience
 than knowing what will happen.
 Virtual digital environments, which I was familiar with on the other hand,
 generally seem to tend persuade the user to feel in control at all times.
\end_layout

\begin_layout Section*
Thanks
\end_layout

\begin_layout Standard
Thanks to Eddi, Derek, Befaco, Richard, David
\end_layout

\begin_layout Standard
\begin_inset FloatList figure

\end_inset


\end_layout

\begin_layout Standard
\begin_inset CommandInset bibtex
LatexCommand bibtex
bibfiles "synth_bibliography"
options "karl-second4"

\end_inset


\end_layout

\begin_layout Chapter*
Declaration of academic honesty
\end_layout

\begin_layout Standard
I hereby declare that in the attached submission I have not presented anyone
 else’s work, in whole or in part, as my own using only the admitted resources.
 Where I have taken advantage of the work of others, I have given full acknowled
gement.
\end_layout

\begin_layout Standard
My signature below constitutes my pledge that all of the writing is my own
 work, with the exception of those portions which are properly documented.
 
\end_layout

\begin_layout Chapter*
Appendix
\end_layout

\begin_layout Standard
\begin_inset Note Note
status open

\begin_layout Plain Layout
urs hegemann
\end_layout

\begin_layout Plain Layout
future audio workshop - cycle oder circle synthe
\end_layout

\end_inset


\end_layout

\end_body
\end_document
