Signal Representation and Notation
Below is a summary of various notational conventions used in digital signal processing for representing signals and spectra. For a more detailed presentation, see the elementary introduction to signal representation, sinusoids, and exponentials in .A.1
In this book, time is always in physical units of seconds (s), while time or is in units of samples (counting numbers having no physical units). Time is a continuous real variable, while discrete-time in samples is integer-valued. The physical time corresponding to time in samples is given by
For frequencies, we have two physical units: (1) cycles per second and (2) radians per second. The name for cycles per second is Hertz (Hz) (though in the past it was cps). One cycle equals radians, which is 360 degrees (). Therefore, Hz is the same frequency as radians per second (rad/s). It is easy to confuse the two because both radians and cycles are pure numbers, so that both types of frequency are in physical units of inverse seconds (s ).
The term sinusoid means a waveform of the type
Thus, a sinusoid may be defined as a cosine at amplitude , frequency , and phase . (See  for a fuller development and discussion.) A sinusoid's phase is in radian units. We may call
In this book, we think of filters primarily in terms of their effect on the spectrum of a signal. This is appropriate because the ear (to a first approximation) converts the time-waveform at the eardrum into a neurologically encoded spectrum. Intuitively, a spectrum (a complex function of frequency ) gives the amplitude and phase of the sinusoidal signal-component at frequency . Mathematically, the spectrum of a signal is the Fourier transform of its time-waveform. Equivalently, the spectrum is the z transform evaluated on the unit circle . A detailed introduction to spectrum analysis is given in .A.2
We denote both the spectrum and the z transform of a signal by uppercase letters. For example, if the time-waveform is denoted , its z transform is called and its spectrum is therefore . The time-waveform is said to ``correspond'' to its z transform , meaning they are transform pairs. This correspondence is often denoted , or . Both the z transform and its special case, the (discrete-time) Fourier transform, are said to transform from the time domain to the frequency domain.
We deal most often with discrete time (or simply ) but continuous frequency (or ). This is because the computer can represent only digital signals, and digital time-waveforms are discrete in time but may have energy at any frequency. On the other hand, if we were going to talk about FFTs (Fast Fourier Transforms--efficient implementations of the Discrete Fourier Transform, or DFT) , then we would have to discretize the frequency variable also in order to represent spectra inside the computer. In this book, however, we use spectra only for conceptual insights into the perceptual effects of digital filtering; therefore, we avoid discrete frequency for simplicity.
When we wish to consider an entire signal as a ``thing in itself,'' we write , meaning the whole time-waveform ( for all ), or , to mean the entire spectrum taken as a whole. Imagine, for example, that we have plotted on a strip of paper that is infinitely long. Then refers to the complete picture, while refers to the th sample point on the plot.
This section gives a summary of some of the more useful mathematical identities for complex numbers and trigonometry in the context of digital filter analysis. For many more, see handbooks of mathematical functions such as Abramowitz and Stegun .
The symbol means ``is defined as''; stands for a complex number; and , , , and stand for real numbers. The quantity is used below to denote .
It is an important and fundamental fact that a sum of sinusoids at the same frequency, but different phase and amplitude, can always be expressed as a single sinusoid at that frequency with some resultant phase and amplitude. An important implication, for example, is that
That is, if a sinusoid is input to an LTI system, the output will be a sinusoid at the same frequency, but possibly altered in amplitude and phase. This follows because the output of every LTI system can be expressed as a linear combination of delayed copies of the input signal. In this section, we derive this important result for the general case of sinusoids at the same frequency.
We want to show it is always possible to solve
for and , given for . For each component sinusoid, we can write
Applying this expansion to Eq.(A.2) yields
Equating coefficients gives
where and are known. We now have two equations in two unknowns which are readily solved by (1) squaring and adding both sides to eliminate , and (2) forming a ratio of both sides of Eq.(A.4) to eliminate . The results are
which has a unique solution for any values of and .
Thus, equality holds when we define
Since is just the polar representation of a complex number, there is always some value of and such that equals whatever complex number results on the right-hand side of Eq.(A.5).
As is often the case, we see that the use of Euler's identity and complex analysis gives a simplified algebraic proof which replaces a proof based on trigonometric identities.
Phasor Analysis: Factoring a Complex Sinusoid into Phasor Times Carrier
The heart of the preceding proof was the algebraic manipulation
For an arbitrary sinusoid having amplitude , phase , and radian frequency , we have
Elementary Audio Digital Filters