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Added feedback control section to EM essay

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Experimental_Music_Assignment_2/essay/U1265119-Experimental_Music-Assignment2-Essay.tex
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nature of the feedback causes a distinct "howling" sound that builds
rapidly and is commonly considered unplesant. As a result, a great deal of
research has been carried out into methods for attenuating and controlling
this effect.~\parencite[p.1]{} However, the volatile and unpredictable
nature of acoustic feedback has been used to great effect in both popular
and avant-garde music. Pioneering guitarists such as Jimi Hendrix have
used the loop created through placing an electric guitar pickup close to
it's amplifier to compliment virtuosic guitar solos in pieces such as Foxy
Lady~\citeyearpar{} This is taken one step further in avante-garde works
such as Steve Reich's Pendulum Music, where feedback becomes the focus of
the piece entirely. This is discussed further in section~\ref{pendulum}
this effect.~\parencite[p.1]{waterschoot2010fyafc} However, the volatile
and unpredictable nature of acoustic feedback has been used to great effect
in both popular and avant-garde music. Pioneering guitarists such as Jimi
Hendrix have used the loop created through placing an electric guitar
pickup close to it's amplifier to compliment virtuosic guitar solos in
pieces such as Foxy Lady~\citeyearpar{} This is taken one step further in
avante-garde works such as Steve Reich's Pendulum Music, where feedback
becomes the focus of the piece entirely. This is discussed further in
section~\ref{pendulum}
\subsection{Electronic Feedback}
Electronic feedback takes the principal of recursively feeding an output
@@ -135,8 +136,10 @@
\subsection{Amplification}
Amplification is the process of scaling a signal by a chosen factor.
Factors $>1.$ result in an increased overall amplitude, whilst factors
$<1.$ result in an attenuated signal amplitude~\parencite[p.3-4]{kadis2012sosr}.
Factors $>1.$ result in an increased overall amplitude (a regenerative
feedback system), whilst factors
$<1.$ result in an attenuated signal amplitude (a degenerative feedback
system)~\parencite[p.3-4]{kadis2012sosr}.
This artificial modification of amplitude has a number of interesting sonic
effects in itself, as it allows for the magnification of sounds that may not
naturally be perceivable and conversely, the reduction of extremely loud
@@ -151,7 +154,7 @@
fed back into the system. Amplification is therefor a crucial element for
controlling the results of a feedback system. By attenuating an output
before feeding it back to a system, it is possible to ensure that outputs
do not grow at an exponential rate.
do not grow at an exponential rate.~\parencite[p.71-72]{zolzer2011dafx}
\begin{figure}[H]
\makebox[\textwidth]{\includegraphics[width=0.75\textwidth]{IIR_flow_diagram}}
\caption[Caption for LOF]{Basic Feedback Signal Flowchart\protect\footnotemark}
@@ -161,8 +164,8 @@
\footnotetext{Diagram adapted from:~\parencite[p.72]{zolzer2011dafx}}
\begin{figure}[H]
This can be demonstrated mathmatically using the following equation as
illustrated in figure~\ref{feed_flowchart}:
This can be demonstrated mathmatically using the following example equation
for a feedback system as illustrated in figure~\ref{feed_flowchart}:
\begin{align*}
& y(n) = x(n) + gy(n-M)\\
& \text{where:}\\
@@ -184,29 +187,193 @@
distorts, causing the typical "howling" effect.
\subsection{Mathmatical Feedback}
Rational Melody XXI - Tom Johnson
Not electronic feedback, but serves as an example that feedback is not
limited to electronics.
IIR filter example?
Mathmatical feedback is not technically a form of electronic feedback and
so will not be discussed in detail, however it does provide a good example
of the extent to which the concepts of feedback are used for creative
purposes in the context of experimental music.
Previously discussed feedback methods operate directly on a signal.
However, mathmatical feedback differs, in that it applies the mathmatical
principals of feedback to influence musical parameters of the composition.
This is demonstarted in Tom Johnson's Rational Melody XXI, where Johnson
specifies each subsequent bar as a retrograde of the previous bar, thus
causing the composition of each bar to rely on it's
predecessor.~\parencite[p.72]{weisert2010ioi} This follows the same
principles as stated in the equation with minor changes:
\begin{align*}
& y(n) = x(n) + g \cdot \text{ret}(y(n-M))\\
& \text{where:}\\
& \text{ret is a retrograde function}\\
& g = 1\\
& n_0 = \text{the initial musical phrase}\\
& n_{n\neq0} = \text{an empty phrase}
\end{align*}
\section{Musical Aspects and Implications of Feedback Systems}
\subsection{Indeterminacy}
There are a number of interesting musical implications when using feedback
and amplitude ajustment as a compositional tools. Features, such as it's
indeterministic nature and rhythmic characteristics, inherent to the nature
of feedback make it an interesting technique for musical exploration and
there are many examples of artists exploiting these qualities in
experimental compositions. Likewise, amplitude modification offers a number
of compositional possibilities for both the control of feedback and for
creative effect in of itself. This section outlines some of the key musical
aspects of feedback and artificial amplification and provides examples of
notable compositions that demonstrate these principals.
\subsection{Indeterminacy}\label{indeterminate}
Indeterminacy is related to the use of chance operations in music
composition and performance. techniques that involve a degree of
uncertainty, where external variables or unpredictable factors affect the
outcome, are defined as indeterminate. Simms describes indeterminacy as
"Any part of a musical work is indeterminate if it is chosen by chance, or
if its performance is not precisely
specified."~\parencite[p.357]{simms1986mtc} It is a topic of interest for
experimental composers for a number of reasons.
\subsubsection{Variation in Performance}\label{variance}
Using indeterministic processes as part of a composition allows for
variation in the performance of said composition on a case by case basis.
Through the addition of random factors to a composition, far greater
degrees of variance are created in the performance of a peice, allowing for
an infinite number of possible versions as opposed to the comparatively
limited interpretation of static compositions where all elements are
controlled directly by the composer.~\parencite[p.97-98, 381]{jc2009co,
holmes2012eaem}
This variance can take many forms dependant on the indeterministic factor.
An example might be the performer, which is demonstarted well through
Cornelius Cardew and Christian Wolff's compositions for the Fluxorchestra.
By composing "unambiguous, concrete proposals (which still left room for
personal idiosyncrasies in realization)", indeterminicity was created
through the skill and interpretation of the performers in pieces such as
"Stones" by Wolff or "The Great Learning" by
Cardew~\parencite[110]{nyman1999em}\\
In terms of feedback, variance in feedback will depend primarily on the
variance of the input to the system and the variance of control. due to the
non-linear fashion in which feedback effects input to produce an output,
subtle changes in these may resut in significant changes to the output. An
example of this is a guitarist using an amplifier to produce feedback from
his guitar. In each performance, the subtle changes in distance between the
guitar and amplifier may result in significant differences to feedback
tone. This property of feedback applies to compositions such as Steve
Reich's Pendulum Music, aswell as Gordon Mumma's Hornpipe, discussed in
section~\ref{pendulum} and section~\ref{hornpipe}.
\subsubsection{Bias Removal}
Indeterminacy is able to remove personal bias and ego involved in decision
making from a composition or performance. By leaving compositional
decisions to chance, it is ensured that the music produced is not created
with intent and is seperated from the composer's personal taste, as stated
by John Cage~\parencite[p.381]{holmes2012eaem}. This technique is used in
his composition "Music of Changes", where chance operations are used for
the organisation of material in such a way that "bypassed a reliance on his
aesthetic judgment". By combining this relinquished control with the
precise control of other aspects of the peice, Cage was able to create a
"balance between rational and irrational" through the combination of
control and total removal of control over compositional
elements~\parencite[p.97-98]{jc2009co}.\\
In relation to feedback directly, the ability for feedback to create
exponentially complex output from relatively simple feedback systems
create clear elements of indeterminacy through the unpredictability of
their output. This is demonstrated in Steve Reich's "Pendulum Music" as
discussed in section ~\ref{pendulum}. Parallels can be drawn between John
Cage's use of indeterminacy to dictate organizational aspects of "Music of
Changes" with Steve Reich's use of feedback to dictate sonic events in
"Pendulum Music".
\subsection{Process and Control}
The term "process" refers to the situation outlined by a composer, designed
for the creation of sound. Where popular music focuses on creating pre-defined
musical content and structure, experimental musicians focus on the creation
of a process through which sound may be generated. This may involve the
creation of rules or instructions that outline the conditions that are
needed in order to create an outcome, the content of which may differ on
each performance based on any indeterministic factors (see
section~\ref{indeterminate})~\parencite[p.4]{nyman1999em}
There are many forms of process used for the composition of experimental
music. These are observed in detail in \textit{Experimental Music - Cage
and Beyond}~\parencite[p.4-14]{nyman1999em}
\subsubsection{Feedback Process}
Feedback is concerned mainly with electronic process, where an electronic
system is defined/set up in order to facilitate the creation of sound. The
specified set-up will therefor have a direct impact on the outcome of the
peice, combined with factors such as the methods for control other
processes involved in the peices realisation. This is true of Gordon
Mumma's Hornpipe~\citeyearpar{mumma2002lem} (as detailed in
section~\ref{hornpipe}) where electronic circuitry is designed specifically
to explore the effects of custom electronic circuitry used to produce
controlled feedback~\parencite[p.8, 390]{nyman1999em}\\
\subsubsection{Feedback Control}
As stated previously, feedback can be difficult to control due to it's
indeterministic properties. The complex and intricate outputs possible with
even the simplest of feedback systems causes results to differ
significantly based on the exact conditions of the system. The two most
significant factors that affect a basic feedback system are:
\begin{itemize}
\item System input
\item System parameters
\end{itemize}
\paragraph{System Input} As with most systems, an alteration to the input
of the system will result in the alteration of the output. This is
generally true of feedback systems as, for example, providing an electronic
feedback circuit with a louder input will most likely result in a louder
output. This relationship may not be linear and depends on the design of
the feedback system, which in turn determines the indeterministic nature of
these systems~\parencite[p.19-27]{weisert2010ioi}. However, it still
provides a method for control over the output of the system. This is
apparent in Mumma's Hornpipe~\citeyearpar{mumma2002lem} where the performer
must adapt the input to the system (in this case the sound produced by the
french horn) in reaction to the electronic sound produced by the
``cybersonic console''.
\paragraph{System Parameters}
As stated above, the design of the feedback system will determine it's
reponse to a given input. A common method for controlling the design of a
feedback system is through the implementation of variable nodes, for the
dynamic ajustment of parameters.~\parencite[p.19-27]{weisert2010ioi}
A clear example of this is the use of a scaling factor in the feedback loop
to paraemtize the regenerative/degenerative nature of the system. By
altering this parameter, the degree to which a signal is
amplified/attenuated on each recursion can be modified dynamically during
performance. This would be attributed to the guitar-amplifier distance in
the typical guitar feedback example mentioned in section~\ref{variance}
\subsection{Rhythmic/Temporal Implications of Feedback}
\subsection{Spectral Implications of Feedback}
\subsection{Dynamic Implications of Artificial Amplitude Adjustment}
Used in isolation in alters the sound by a given factor.
Used as part of feedback it acts as an exponent to the output on each
iteration, creating a an dynamic alteration that changes as a function of
time.
\section{Composition Analysis}
\subsection{Acoustic Feedback}
\subsubsection{Steve Reich's Pendulum Music}\label{pendulum}
~\parencite[p.31]{reich2002wom}
\subsubsection{Robert Ashley's The Wolfman}\label{wolfman}
- ref: kyle gann - robert ashley
~\citeyearpar{ashley2003w}
\subsection{Electronic Feedback}\label{ElecFeed}
David Tutor's Untitled (1996), Toneburst (2004), and Pulsers (1996)
Gordon Mumma's Hornpipe
\subsection{Amplification}\label{amp}
\subsubsection{David Tutor's Untitled (1996), Toneburst (2004), and Pulsers
(1996)}
~\citeyearpar{tudor1996twfle}
~\citeyearpar{tudor2004lem}
~\citeyearpar{tudor1996twfle}
\subsubsection{Gordon Mumma's Hornpipe}\label{hornpipe}
~\citeyearpar{mumma2002lem}
A combination of electronic and person process due to the sound produced by
the electronic circuitry's interplay with the human improvisation in
reaction to create the final result.
Indeterministic due to the relative unpredictability of the electronic
circuitry.
\subsection{Amplification}\label{amp}
\subsubsection{John Cage's Cartridge Music}
~\citeyearpar{cage2013cm}
\subsubsection{Stockhausen's Mikrophonie}
~\citeyearpar{stockhausen1995mmt}
\section{Conclusion}