- ...
^{1} - This section was added in the third printing of this book (September 2013).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
numbers.
^{1.1} - Physicists and mathematicians use
instead of
to denote
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
unknowns.
^{2.1} - ``Linear'' in this context means that the unknowns
are multiplied only by constants--they may not be multiplied by each
other or raised to any power other than
(
*e.g.*, not squared or cubed or raised to the power). Linear systems of equations in unknowns are very easy to solve compared to*nonlinear*systems of equations in unknowns. For example, Matlab and Octave can easily handle them. You learn all about this in a course on*Linear Algebra*which is highly recommended for anyone interested in getting involved with signal processing. Linear algebra also teaches you all about*matrices*, which are introduced only briefly in Appendix H.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
numbers
^{2.2} - (multiplication, addition, division, distributivity
of multiplication over addition, commutativity of multiplication and
addition)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...field.
^{2.3} - See,
*e.g.*, Eric Weisstein's*World of Mathematics*(`http://mathworld.wolfram.com/`) for definitions of any unfamiliar mathematical terms such as a*field*(which is described, for example, at the easily guessed URL`http://mathworld.wolfram.com/Field.html`).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... tool.
^{2.4} - Proofs for the
fundamental theorem of algebra have a long history involving many of
the great names in classical mathematics. The first known rigorous
proof was by Gauss based on earlier efforts by Euler and Lagrange.
(Gauss also introduced the term ``complex number.'') An alternate
proof was given by Argand based on the ideas of d'Alembert. For a
summary of the history, see
`http://www-gap.dcs.st-and.ac.uk/~history/HistTopics/Fund_theorem_of_algebra.html`

(the first Google search result for ``fundamental theorem of algebra'' in July of 2002).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
^{3.1} - That the rationals are dense in the reals
is easy to show using decimal expansions. Let
and
denote
any two distinct, positive, irrational real numbers. Since
and
are distinct, their decimal expansions must differ in some
digit, say the
th. Without loss of generality, assume
.
Form the rational number
by zeroing all digits in the decimal
expansion of
after the
th. Then
, as needed.
For two negative real numbers, we can negate them, use the same
argument, and negate the result. For one positive and one negative
real number, the rational number zero lies between them.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... as
^{3.2} - This was computed via
`N[Sqrt[2],60]`in`Mathematica`. Symbolic mathematics programs, such as`Mathematica`,`Maple`(offered as a`Matlab`extension),`maxima`(a free, GNU descendant of the original`Macsyma`, written in Common Lisp, and available at`http://maxima.sourceforge.net`),`GiNaC`(the`Maple`-replacement used in the*Octave Symbolic Manipulation Toolbox*), or`Yacas`(another free, open-source program with similar goals as`Mathematica`), are handy tools for cranking out any number of digits in irrational numbers such as . In`Yacas`(as of Version 1.0.55), the syntax is`Precision(60)``N(Sqrt(2))`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... rule,
^{3.3} - We will use the chain rule from
calculus without proof. Note that the use of calculus is beyond the
normal level of this book. Since calculus is only needed at this one
point in the DFT-math story, the reader should not be discouraged if
its usage seems like ``magic''. Calculus will not be needed at all for
practical applications of the DFT, such as spectrum analysis,
discussed in Chapter 8.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{3.4} - Logarithms are reviewed in
Appendix F.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
number
^{3.5} - A number is said to be
*transcendental*if it is not a root of any polynomial with integer coefficients,*i.e.*, it is not an*algebraic number*of any degree. (Rational numbers are algebraic numbers of degree 1; irrational numbers include transcendental numbers and algebraic numbers of degree greater than 1, such as which is of degree 2.) See`http://mathworld.wolfram.com/TranscendentalNumber.html`for further discussion.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... by
^{3.6} - In
`Mathematica`, the first 50 digits of may be computed by the expression`N[E,50]`(``evaluate numerically the reserved-constant`E`to 50 decimal places''). In the*Octave Symbolic Manipulation Toolbox*(part of Octave Forge), one may type ```digits(50); Exp(1)`''.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
unity.
^{3.7} - Sometimes we see
, which is
the complex conjugate of the definition we have used here. It is
similarly a primitive
th root, since powers of it will generate all
other
th roots.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
geo\-metry.
^{3.8} - See, for example,
`http://www-spof.gsfc.nasa.gov/stargaze/Strig5.htm`.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... oscillator.
^{4.1} - A mass-spring oscillator analysis is given at
`http://ccrma.stanford.edu/~jos/filters/Mass_Spring_Oscillator_Analysis.html`(from the next book [70] in the music signal processing series).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... (LTI
^{4.2} - A system
is said to be
*linear*if for any two input signals and , we have . A system is said to be*time invariant*if implies . This subject is developed in detail in the second book [70] of the music signal processing series, available on-line at`http://ccrma.stanford.edu/~jos/filters/`.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
soundfield,
^{4.3} - For a definition, see
`http://ccrma.stanford.edu/~jos/pasp/Mean_Free_Path.html`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...combfilter.
^{4.4} - Technically, Fig.4.3 shows the
*feedforward comb filter*, also called the ``inverse comb filter'' [77]. The longer names are meant to distinguish it from the*feedback comb filter*, in which the delay output is fed*back*around the delay line and summed with the delay input instead of the input being fed*forward*around the delay line and summed with its output. (A diagram and further discussion, including how time-varying comb filters create a*flanging effect*, can be found at`http://ccrma.stanford.edu/~jos/pasp/Feedback_Comb_Filters.html`.) The frequency response of the feedforward comb filter is the inverse of that of the feedback comb filter (one can cancel the effect of the other), hence the name ``inverse comb filter.'' Frequency-response analysis of digital filters is developed in [70] (available online).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... name.
^{4.5} - While there is no reason it should be obvious
at this point, the comb-filter gain varies in fact sinusoidally
as a function of frequency.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
dc
^{4.6} - ``dc'' means ``direct current'' and is an electrical
engineering term for ``frequency 0
''.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... dB.
^{4.7} - Recall that a gain factor
is converted to
*decibels*(dB) by the formula . See §F.2 for a review.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
practice.
^{4.8} - An important variant of FM called
*feedback FM*, in which a single oscillator phase-modulates itself, simply does not work if true frequency modulation is implemented.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... section.
^{4.9} - The mathematical derivation of FM
spectra is included here as a side note. No further use will be made
of it in this book.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...Watson44,
^{4.10} - Existence of the
Laurent expansion follows from the fact that the generating function is a
product of an exponential function,
, and an
exponential function inverted with respect to the unit circle,
. It is readily verified by direct differentiation
in the complex plane that the exponential is an
*entire function*of (analytic at all finite points in the complex plane) [16], and therefore the inverted exponential is analytic everywhere except at . The desired Laurent expansion may be obtained, in principle, by multiplying the one-sided series for the exponential and inverted exponential together. The exponential series has the well known form . The series for the inverted exponential can be obtained by inverting again ( ), obtaining the appropriate exponential series, and inverting each term.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...phasor),
^{4.11} - Another example of phasor analysis can be found at
`http://ccrma.stanford.edu/~jos/filters/Phasor_Analysis.html`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... signal|textbf.
^{4.12} - In complex variables, a function
is ``analytic'' at a point
if it is differentiable of
all orders at each point in some neighborhood of
[16]. Therefore, one might expect an ``analytic signal''
to be any signal which is differentiable of all orders at any point in
time,
*i.e.*, one that admits a fully valid Taylor expansion about any point in time. However,*all*bandlimited signals (being sums of finite-frequency sinusoids) are analytic in the complex-variables sense at every point in time. Therefore, the signal processing term ``analytic signal'' refers instead to a signal having ``no negative frequencies''. Equivalently, one could say that the spectrum of an analytic signal is ``causal in the frequency domain''.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... shift.
^{4.13} - This operation is actually
used in some real-world AM and FM radio receivers (particularly in digital
radio receivers). The signal comes in centered about a high ``carrier
frequency'' (such as 101 MHz for radio station FM 101), so it looks very
much like a sinusoid at frequency 101 MHz. (The frequency modulation only
varies the carrier frequency in a relatively tiny interval about 101 MHz.
The total FM bandwidth including all the FM ``sidebands'' is about 100 kHz.
AM bands are only 10kHz wide.) By delaying the signal by 1/4 cycle, a good
approximation to the imaginary part of the analytic signal is created, and
its instantaneous amplitude and frequency are then simple to compute from
the analytic signal.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...demodulation
^{4.14} *Demodulation*is the process of recovering the modulation signal. For amplitude modulation (AM), the modulated signal is of the form , where is the ``carrier frequency'', is the amplitude envelope (modulation), is the modulation signal we wish to recover (the audio signal being broadcast in the case of AM radio), and is the modulation index for AM.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
^{4.15} - The notation
denotes a single
*sample*of the signal at sample , while the notation or simply denotes the*entire signal*for all time.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... projection
^{4.16} - The coefficient of projection of a signal
onto another signal
can be thought of as a measure of how much of
is present in
. We will consider this topic in some detail
later on.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...vector
^{5.1} - We'll use an underline to emphasize the vector
interpretation, but there is no difference between
and
. For
purposes of this book, a
*signal*is the same thing as a*vector*.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... hear,
^{5.2} - Actually,
two-sample signals with variable amplitude and spacing between the
samples provide very interesting tests of pitch perception, especially
when the samples have opposite sign [57].
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... numbers.
^{5.3} - More generally, scalars are
often defined as members of some
*mathematical field*--usually the same field used for the vector elements (coordinates, signal samples).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... type,
^{5.4} - As we'll discuss in §5.7
below, vectors of the ``same type'' are typically taken to be members
of the same
*vector space*.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... samples:
^{5.5} - In this section,
denotes a signal, while in the previous sections, we used an underline
(
) to emphasize the vector interpretation of a signal. One might
worry that it is now too easy to confuse signals (vectors) and scale
factors (scalars), but this is usually not the case: signal names are
generally taken from the end of the Roman alphabet (
),
while scalar symbols are chosen from the beginning of the Roman
(
) and Greek (
) alphabets. Also, formulas involving signals are typlically
specified on the sample level, so that signals are usually indexed
(
) or subscripted (
).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... units.
^{5.6} - The energy of a
*pressure wave*is the integral over time and area of the squared pressure divided by the wave impedance the wave is traveling in. The energy of a*velocity wave*is the integral over time of the squared velocity times the wave impedance. In audio work, a signal is typically a list of*pressure samples*derived from a microphone signal, or it might be samples of*force*from a piezoelectric transducer,*velocity*from a magnetic guitar pickup, and so on. In all of these cases, the total physical energy associated with the signal is proportional to the sum of squared signal samples. Physical connections in signal processing are explored more fully in Book III of the Music Signal Processing Series [71], (available online).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
removed:
^{5.7} - For reasons beyond the scope of this book, when the
sample mean
is estimated as the average value of the same
samples used to compute the sample variance
, the sum
should be divided by
rather than
to avoid a
*bias*[34].. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... vector.
^{5.8} - You might wonder why the norm of
is
not written as
. There would be no problem with this
since
is otherwise undefined for vectors.
However, the historically adopted notation is
instead
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... by
^{5.9} - From some points of view, it is more elegant to
conjugate the first operand in the definition of the inner
product. However, for explaining the DFT, conjugating the second
operand is better. The former case arises when expressing inner
product
as a vector operation
, where
denotes the
*Hermitian transpose*of the vector . Either convention works out fine, but it is best to choose one and stick with it forever.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
product:
^{5.10} - Remember that a norm must be a
*real*-valued function of a signal (vector).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
^{5.11} - Note that we have dropped the underbar notation for
signals/vectors such as
and
. While this is commonly
done, it is now possible to confuse vectors and scalars. The context
should keep everything clear. Also, symbols for scalars tend to be
chosen from the beginning of the alphabet (Roman or Greek), such as
, while symbols for vectors/signals are
normally chosen from letters near the end, such as
--all
of which we have seen up to now. In later sections, the underbar
notation will continue to be used when it seems to add clarity.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
definition,
^{5.12} - Note that, in this section,
denotes an entire
*signal*while denotes the th*sample*of that signal. It would be clearer to use , but the expressions below would become messier. In other contexts, outside of this section, might instead denote the th*signal*from a set of signals.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...Noble.
^{5.13} - An excellent collection of free downloadable course videos by Prof. Strang at MIT
is available at
`http://web.mit.edu/18.06/www/Video/video-fall-99-new.html`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...).
^{6.1} - The Matlab code for generating this figure is given
in §I.4.1.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... unity.
^{6.2} - The notations
,
, and
are
common in the digital signal processing literature. Sometimes
is defined with a negative exponent,
*i.e.*, .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
by
^{6.3} - As introduced in §1.3, the notation
means the
*whole signal*, , also written as simply .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{6.4} - More
precisely,
is a length
finite-impulse-response (FIR)
digital filter. See §8.3 for related discussion.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
computed,
^{6.5} - We call this the
*aliased sinc function*to distinguish it from the*sinc function*sinc .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...dftfilterb
^{6.6} - The Matlab code for this figure is
given in §I.4.2.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{6.7} - Spectral leakage is essentially equivalent to (
*i.e.*, a Fourier dual of) the*Gibb's phenomenon*for truncated Fourier series expansions (see §B.3), which some of us studied in high school. As more sinusoids are added to the expansion, the error waveform increases in frequency, and decreases in signal energy, but its peak value does not converge to zero. Instead, in the limit as infinitely many sinusoids are added to the Fourier-series sum, the peak error converges to an*isolated point*. Isolated points have ``measure zero'' under integration, and therefore have no effect on integrals such as the one which calculates Fourier-series coefficients.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{7.1} - The notation
denotes the
*half-open interval*on the real line from to . Thus the interval includes but not .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
spectra
^{7.2} - A spectrum is mathematically identical to a signal, since
both are just sequences of
complex numbers. However, for clarity, we
generally use ``signal'' when the sequence index is considered a time
index, and ``spectrum'' when the index is associated with successive
frequency samples.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...:
^{7.3} - The set of all integers is denoted
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
convolution).
^{7.4} - To simulate acyclic convolution using cyclic
convolution, as is appropriate for the simulation of sampled
continuous-time systems, sufficient
*zero padding*is used so that nonzero samples do not ``wrap around'' as a result of the shifting of in the definition of convolution. Zero padding is discussed later in this chapter (§7.2.7).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
^{7.5} - See §8.3 for an introduction to the digital-filter point
of view.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... exponential.
^{7.6} - Normally, in practice, a
*first-order recursive filter*would be used to provide such an exponential impulse response very efficiently in hardware or software (see Book II [70] of this series for details). However, the impulse response of any linear, time-invariant filter can be recorded and used for implementation via convolution. The only catch is that recursive filters generally have*infinitely long*impulse responses (true exponential decays). Therefore, it is necessary to truncate the impulse response when it decays enough that the remainder is negligible.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{7.7} - Matched filtering is briefly
discussed in §8.4.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
place.
^{7.8} - This is the also basis of the puzzle ``multiply your
age times 7 times 1443.''
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{7.9} - The symbol ``
''
means ``is equivalent to''.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... domain.
^{7.10} - Similarly, zero padding in the
frequency domain gives what we may call ``periodic interpolation'' in the
time domain which is exact in the DFT case only for periodic signals
having a time-domain period equal to the DFT length. (See §6.7.)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... times.
^{7.11} - You might wonder why we need this
since all indexing in
is defined modulo
already. The answer
is that
formally expresses a mapping from the space
of length
signals to the space
of length
signals. The
operator could alternatively be defined
as the
*scaling operator*, , where is any positive integer (note the increase in signal length from to samples). Such a definition would better suggest the related*continuous-time*Fourier theorems regarding time/frequency scaling (see Appendix C, especially §C.2).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{7.12} - The function
is also considered odd,
ignoring the singularity at
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... transform,
^{7.13} - The discrete cosine transform (DCT)
used most often in applications is defined somewhat differently
(see §A.6.1), but this is one variant.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{7.14} - To consider
as radians per second
instead of radians per sample, just replace
by
so that
the delay is in seconds instead of samples,
*i.e.*, .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... response.
^{7.15} - See
§8.3 and/or Book II [70] of the music signal processing
book series for an introduction to digital filters.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... Transform
^{7.16} - An FFT is just a fast implementation of
the DFT. See Appendix A for details and pointers.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... FFT.
^{7.17} - These results were
obtained using the program
`Octave`, version 2.9.9, running on a Linux PC with a 2.8GHz Pentium CPU, and`Matlab`, version 5.2, running on a Windows PC with an 800MHz Athlon CPU.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...table:ffttable.
^{7.18} - These results were obtained using
`Matlab`v5.2 running on a Windows PC with an 800MHz Athlon CPU.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...dual
^{7.19} - The
*dual*of a Fourier operation is obtained by interchanging time and frequency.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... time.
^{7.20} - Signal processing with a physical
interpretation is addressed in Book III [71] of the music
signal processing series.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{7.21} - You might be
concerned about what it means when
has an
*imaginary*part. This happens when the force drives a so-called ``reactive'' load such as a mass or spring. For the details, see Book III [71] or any text on classical network theory or circuit theory. When the driving force works against a real resistance , then (see §F.3), so that the power is , and in the frequency domain, so that the spectral power is proportional to . Thus, in the case of a real driving-point impedance , the spectral power is always real and nonnegative.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... frequency
^{7.22} - The
*folding frequency*is defined as half the sampling rate . It may also be called the*Nyquist limit*. The*Nyquist rate*, on the other hand, means the sampling rate, not half the sampling rate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... FIR
^{7.23} - FIR stands for ``Finite
Impulse Response.'' Digital filtering concepts and terminology are
introduced in §8.3 and more completely in Book II [70]
of the music signal processing series.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...CooleyAndTukey65.
^{8.1} - While a length
DFT requires
approximately
arithmetic operations, a Cooley-Tukey FFT requires
closer to
operations when
is a power of 2, where
denotes the log-base-2 of
. Appendix A provides an
introduction to Cooley-Tukey FFT algorithms in §A.1.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{8.2} - Say ``
`doc fft`'' in Matlab for an overview of how a specific FFT algorithm is chosen.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...SASP
^{8.3} `http://ccrma.stanford.edu/~jos/sasp/Classic_Blackman.html`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... Octave
^{8.4} `http://www.octave.org/`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... Toolbox,
^{8.5} `http://www.mathworks.com/access/helpdesk/help/toolbox/signal/signal.shtml`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
phoneme,
^{8.6} - A
*phoneme*is a single elementary sound in speech, such as a single vowel sound like the `a' in ``bat'' or the `s' in ``sit''.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
system
^{8.7} - Linearity and time invariance are introduced in
the second book of this series [70].
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...estimator
^{8.8} - In signal processing, a ``hat'' often denotes an
*estimated*quantity. Thus, is an estimate of .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
value
^{8.9} - For present purposes, the expected value
may be found by averaging an infinite number of
cross-correlations
computed using different segments of
and
. Both
and
must be infinitely long, of course, and
all stationary processes
*are*infinitely long. Otherwise, their statistics could not be time invariant.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... PSD:
^{8.10} - To clarify, we are using the word ``sample'' with
two different meanings. In addition to the usual meaning wherein a
continuous time or frequency axis is made discrete, a statistical
``sample'' refers to a set of observations from some presumed random
process. Estimated statistics based on such a statistical sample are
then called ``sample statistics'', such as the sample mean, sample
variance, and so on.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{8.11} - See
Eq.
(7.1) for a definition of
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{8.12} - Since phase information is discarded
(
),
the zero-padding can go before or after
,
or both, without affecting the results.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... kernel;
^{8.13} - By the convolution theorem dual, windowing
in the time domain is convolution (smoothing) in the frequency domain
(§7.4.6). Since a triangle is the convolution of a
rectangle with itself, its transform is
sinc
in the
continuous-time case (cf. Appendix D). In the discrete-time
case, it is proportional to
sinc
.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
composite
^{A.1} - In this context, ``highly composite'' means ``a
product of many prime factors.'' For example, the number
is highly composite since it is a power of 2. The
number
is also composite, but it
requires prime factors other than 2. Prime numbers
are not composite at all.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
^{A.2} - See
`http://en.wikipedia.org/wiki/Cooley-Tukey_FFT_algorithm`.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...DuhamelAndVetterli90.
^{A.3} `http://en.wikipedia.org/wiki/Split_radix`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...Good58,Thomas63,KolbaAndParks77,McClellanAndRader79,BurrusAndParks85,WikiPFA.
^{A.4} - See
`http://en.wikipedia.org/wiki/Prime-factor_FFT_algorithm`for an introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
^{A.5} `http://en.wikipedia.org/wiki/Prime-factor_FFT_algorithm#Re-indexing`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...GoldAndRader69.
^{A.6} - See
`http://en.wikipedia.org/wiki/Bluestein's_FFT_algorithm`for another derivation and description of Bluestein's FFT algorithm.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
unity,
^{A.7} - Note that
is the complex-conjugate of
used
in §A.1.1 above.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
length.
^{A.8} - Obtaining an exact integer number of samples per
period can be arranged using
*pitch detection*and*resampling*of the periodic signal. A time-varying pitch requires time-varying resampling [74]--see Appendix D. However, when a signal is resampled for this purpose, one can generally choose a power of 2 for the number of samples per period.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... gain.
^{A.9} - This result is well known in the field of
image processing. The DCT performs almost as well as the optimal
Karhunen-Loève Transform (KLT) when analyzing certain Gaussian
stochastic processes as the transform size goes to infinity. (In the
KLT, the basis functions are taken to be the eigenvectors of the
autocorrelation matrix of the input signal block. As a result, the
transform coefficients are
*decorrelated*in the KLT, leading to maximum energy concentration and optimal coding gain.) However, the DFT provides a similar degree of optimality for large block sizes . For practical spectral analysis and processing of audio signals, there is typically no reason to prefer the DCT over the DFT.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
DCT-II)
^{A.10} - For a discussion of eight or so DCT variations,
see the Wikipedia page:
`http://en.wikipedia.org/wiki/Discrete_cosine_transform`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... variable,
^{B.1} - We
define the DTFT using
*normalized radian frequency*.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
seconds,
^{B.2} - A signal
is said to be
*periodic*with period if for all .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... is
^{B.3} - To obtain precisely this
result, it is necessary to define
via a limiting pulse
converging to time 0 from the
*right*of time 0, as we have done in Eq. (B.3).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... principle|textbf.
^{C.1} - The Heisenberg uncertainty principle in quantum physics
applies to any dual properties of a particle. For example, the
position and velocity of an electron are oft-cited as such duals. An
electron is described, in quantum mechanics, by a probability wave
packet. Therefore, the
*position*of an electron in space can be defined as the midpoint of the amplitude envelope of its wave function; its*velocity*, on the other hand, is determined by the*frequency*of the wave packet. To accurately measure the frequency, the packet must be very long in space, to provide many cycles of oscillation under the envelope. But this means the location in space is relatively uncertain. In more precise mathematical terms, the probability wave function for velocity is proportional to the spatial Fourier transform of the probability wave for position.*I.e.*, they are exact Fourier duals. The Heisenberg Uncertainty Principle is therefore a Fourier property of fundamental particles described by waves [20]. This of course includes all matter and energy in the Universe.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... filter.
^{C.2} - An allpass filter has unity gain and
arbitrary delay at each frequency.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... public.
^{D.1} `http://cnx.org/content/m0050/latest/`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
position
^{D.2} - More typically, each sample represents the
instantaneous
*velocity*of the speaker. Here's why: Most microphones are transducers from*acoustic pressure*to*electrical voltage*, and analog-to-digital converters (ADCs) produce numerical samples which are proportional to voltage. Thus, digital samples are normally proportional to*acoustic pressure deviation*(force per unit area on the microphone, with ambient air pressure subtracted out). When digital samples are converted to analog form by digital-to-analog conversion (DAC), each sample is converted to an electrical voltage which then drives a loudspeaker (in audio applications). Typical loudspeakers use a ``voice-coil'' to convert applied voltage to electromotive force on the speaker which applies pressure on the air via the speaker cone. Since the acoustic impedance of air is a real number, wave pressure is directly proportional wave*velocity*. Since the speaker must move in contact with the air during wave generation, we may conclude that digital signal samples correspond most closely to the*velocity*of the speaker, not its position. The situation is further complicated somewhat by the fact that typical speakers do not themselves have a real driving-point impedance. However, for an ``ideal'' microphone and speaker, we should get samples proportional to speaker velocity and hence to air pressure. Well below resonance, the real part of the radiation impedance of the pushed air should dominate, as long as the excursion does not exceed the linear interval of cone displacement.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{D.3} - Mathematically,
can be allowed to be nonzero over points
provided that the set of all such points have
*measure zero*in the sense of Lebesgue integration. However, such distinctions do not arise for practical signals which are always finite in extent and which therefore have continuous Fourier transforms. This is why we specialize the sampling theorem to the case of continuous-spectrum signals.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... pulse,
^{E.1} - Thanks to Miller
Puckette for suggesting this example.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... energy).
^{E.2} - One joke along these lines, due, I'm
told, to Professor Bracewell at Stanford, is that ``since the
telephone is bandlimited to 3kHz, and since bandlimited signals cannot
be time limited, it follows that one cannot hang up the telephone''.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... bel
^{F.1} - The ``bel'' is named after Alexander
Graham Bell, the inventor of the telephone.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... intensity,
^{F.2} -
*Intensity*is physically*power per unit area*. Bels may also be defined in terms of*energy*, or*power*which is energy per unit time. Since sound is always measured over some*area*by a microphone diaphragm, its physical power is conventionally normalized by area, giving intensity. Similarly, the*force*applied by sound to a microphone diaphragm is normalized by area to give*pressure*(force per unit area).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... Scale
^{F.3} - This section was added in the third printing of this
book (September 2013).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... Scale
^{F.4} - This section was added in the third printing of this
book (September 2013).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...LobdellAndAllen07.
^{F.5} - For other software implementations, see,
*e.g.*,`http://kokkinizita.linuxaudio.org/linuxaudio/`,`https://github.com/x42/meters.lv2`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... is
^{F.6} - The bar was originally defined as one ``atmosphere'' (atmospheric pressure at sea level),
but now a microbar is defined to be exactly one
dynecm
,
where a dyne is the amount of force required to accelerate a gram by one centimeter
per second squared.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{F.7} - Standard International (SI) units were formerly
called MKS units (meters, kilograms, and seconds).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
phons.
^{F.8} - See
`http://en.wikipedia.org/wiki/A-weighting`for more information, including a plot of the A weighting curve (as well as B, C, and D weightings which can be used for louder listening levels) and pointers to relevant standards.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... by
^{F.9} - ibid.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... weighting|textbf
^{F.10} `http://en.wikipedia.org/wiki/ITU-R_468_noise_weighting`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
distortion'').
^{F.11} - Companders (compressor-expanders) essentially
``turn down'' the signal gain when it is ``loud'' and ``turn up'' the
gain when it is ``quiet''. As long as the input-output curve is
monotonic (such as a log characteristic), the dynamic-range
compression can be undone (expanded).
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... 0.
^{G.1} - Computers use bits, as opposed to the more
familiar decimal digits, because they are more convenient to implement
in digital hardware. For example, the
*decimal numbers*0, 1, 2, 3, 4, 5 become, in binary format, 0, 1, 10, 11, 100, 101. Each*bit*position in binary notation corresponds to a power of 2,*e.g.*, ; while each*digit*position in decimal notation corresponds to a power of 10,*e.g.*, . The term ``digit'' comes from the same word meaning ``finger.'' Since we have ten fingers (digits), the term ``digit'' technically should be associated only with decimal notation, but in practice it is used for others as well. Other popular number systems in computers include*octal*which is base 8 (rarely seen any more, but still specifiable in any C/C++ program by using a leading zero,*e.g.*, decimal = 111,101,101 binary), and*hexadecimal*(or simply ``*hex*'') which is base 16 and which employs the letters A through F to yield 16 digits (specifiable in C/C++ by starting the number with ``0x'',*e.g.*, 0x1ED = decimal = 1,1110,1101 binary). Note, however, that the representation within the computer is still always binary; octal and hex are simply convenient*groupings*of bits into sets of three bits (octal) or four bits (hex).. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... processors.
^{G.2} - This information is subject to change
without notice. Check your local compiler documentation.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... feedback
^{G.3} - Normally, quantization error
is computed as
, where
is the signal being
quantized, and
is the quantized value, obtained by
rounding to the nearest representable amplitude. Filtered error
feedback uses instead the formula
,
where
denotes a filtering operation which ``shapes'' the
quantization noise spectrum. An excellent article on the use of
round-off error feedback in audio digital filters is
[18].
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... most-significant).
^{G.4} - Remember that byte
addresses in a big endian word start at the big end of the word, while
in a little endian architecture, they start at the little end of the
word.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
``endianness'':
^{G.5} - Thanks to Bill Schottstaedt for help with this table.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...,
^{G.6} - The notation
denotes a
*half-open interval*which includes but not .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ....
^{G.7} - Another term commonly heard for ``significand''
is ``mantissa.'' However, this use of the term ``mantissa'' is not the same
as its previous definition as the fractional part of a logarithm. We will
therefore use only the term ``significand'' to avoid confusion.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... bias.
^{G.8} - By choosing
the bias equal to half the numerical dynamic range of
(thus effectively
inverting the sign bit of the exponent), it becomes easier to compare two
floating-point numbers in hardware: the entire floating-point word can be
treated by the hardware as one giant integer for numerical comparison
purposes. This works because negative exponents correspond to
floating-point numbers less than 1 in magnitude, while positive exponents
correspond to floating-point numbers greater than 1 in magnitude.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...CODEC|textbfCODECs
^{G.9} - CODEC is an acronym for
``COder/DECoder''.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... processing.
^{G.10} - The
first by Gray and Davisson is available free online. The second by
Papoulis is a classic textbook. The two volumes by Kay provide
perhaps the most comprehensive coverage of the field. The volumes by
Sharf and Kailath represent material used for many years in the
authors' respective graduate level courses in statistical signal
processing. All of the cited authors are well known researchers and
professors in the field. It should also perhaps be noted that Book IV
[72] in the music signal processing book series (of which this
is Book I) contains a fair amount of introductory material in this area.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...PapoulisRV:
^{G.11} `http://en.wikipedia.org/wiki/Probability_density_function`. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... element
^{H.1} - We are now using
as an
integer counter, not as
. This is standard notational
practice.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ...
argument.
^{H.2} - Alternatively, it can be extended to the complex case by
writing
, so that
includes a conjugation of the elements of
. This
difficulty arises from the fact that matrix multiplication is really
defined without consideration of conjugation or transposition at all,
making it unwieldy to express in terms of inner products in the complex
case, even though that is perhaps the most fundamental interpretation
of a matrix multiply.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... columns.
^{I.1} - For consistency with
`sum`and other functions, it would be better if`length()`returned the number of elements along dimension 1, with the special case of using dimension 2 (``along rows'') for row-vectors. However, compatibility with early Matlab dictates the convention used.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

- ... matrix|textbf.
^{I.2} - Going back into ``math mode'', let's show that the
projection matrix

yields orthogonal projection onto the column-space of (when the matrix is invertible, and here denotes the Hermitian (conjugating) transpose of ). That is, if is the projection of onto , we must have, by definition of orthogonal projection (§5.9.9), that lies in the column-space of , and that , or for any . We may say that projects onto the*orthogonal complement*of the column-space of .That is a linear combination of the columns of is immediate because is the leftmost term of the definition of in Eq. (I.1). To show orthogonality of the ``projection error'',

*i.e.*, that for all , note that in matrix notation we must show for all , which requires , or . Since , it is Hermitian symmetric, so that the orthogonal projection requirement becomes , which is easily verified for as defined in Eq. (I.1).The general property defines an

*idempotent*square matrix . Intuitively, it makes sense that a projection should be idempotent, because once a vector is projected onto a particular subspace, projecting again should do nothing. All idempotent matrices are projection matrices, and vice versa. However, only Hermitian (symmetric) idempotent matrices correspond to orthogonal projection [48].. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .