Sampling & Aliasing

Sampling

Impulse Train

The representation of signals through equidistant, quantized values is the fundamental principle of digital sound processing and has an influence on several aspects of sound synthesis. Mathematically, a continuous signal $x(t)$ is sampled by a multiplication with an impulse train $\delta_T(t)$ (also referred to as Dirac comb) of infitite length:

$x[n] = x(t) \delta_T (t) = \sum\limits_{n=-\infty}^{\infty} x(n T) \delta (t-nT)$

This impulse train can be expressed as a Fourier series:

$\delta_T = \frac{1}{T} \left[1 +2 \cos(\omega_s t) 2 \cos(2 \omega_s t) + \cdots \right]$, with $\omega_s=\frac{2\pi}{T}$

$\delta_T = \frac{1}{T} + \sum \left( \frac{2}{T} \cos(n \omega_s t) \right)$

Fourier Transform of the Impulse Train

The Fourier transform of a time-domain impulse train is a frequency-domain impulse train:

$\mathfrak{F}(\delta_T) = \mathfrak{F}(\sum C_k e^{j k \omega_0 t})$

$\mathfrak{F}(\delta_T) = \sum\limits_{m = -\infty}^{\infty} \delta(T f)$

Fourier Transform of the Sampled Signal

The Fourier transform of the sampled signal is periodic:

$X[i] = \frac{1}{T} + \sum\limits_{n=-\infty}^{\infty} X(\omega -n \omega_s)$

Aliasing

Since the spectrum of a sampled signal is periodic, it must be band-limited, in order to avoid misinterpretations, known as aliasing. Since the spectrum is periodic with $\omega_s$, the maximum frequency which can be represented - the Nyquist frequency - is:

$f_N = \frac{f_s}{2}$

As soon as components of a digitally sampled signal exceed this boundary, aliases occur. The following example can be used to set the frequency of a sine wave beyond the Nyquist frequency, resulting in aliasing and ambiguity, visualized in the time domain. The static version of the following example shows the time-domain signal of a $900 \ \mathrm{Hz}$ sinusoid at a sampling rate $f_s = 1000 \ \mathrm{Hz}$:

Aliasing Frequency for a Sinusoid

For pure sinusoids, aliasing results in a sinusoid at the mirror- or folding frequency $f_m$:

$f_m = \Big| f - f_s \Big\lfloor \frac{f}{f_s} \Big\rfloor \Big|$

With $\lfloor x \rfloor$ as round to next integer.

At a sampling rate $f_s = 1000 \ \mathrm{Hz}$ and a Nyquist frequency $f_N = 500 \ \mathrm{Hz}$, a sinusoid with $f = 900 \ \mathrm{Hz}$ will be interpreted as one with $f = 100 \ \mathrm{Hz}$:

f_m = 100


The following example can be used interactively as a Jupyter notebook, by changing the frequency of a sinusoid and listening to the aliased output. When sweeping the range up to $2500 \ \mathrm{Hz}$, the resulting output will increase and decrease in frequency. In the static version, a sinusoid of $f = 900 \ \mathrm{Hz}$ is used, resulting in an audible output at $f_m = 100 \ \mathrm{Hz}$:

Interactive Example

Pitch (Hz):

Output Gain:

Time Domain:

Frequency Domain:

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