The McGill Physiology Virtual Lab

Biomedical Signals Acquisition

Sampling rate and Aliasing
  Now we will examine the frequency content of signals and how this must also be taken into account when setting the acquisition parameters of a signal. If a signal is not sampled properly (that is with a high enough sampling rate) a phenomenon called aliasing occurs.



A 10 Hz signal is sampled. The trace to the left shows the waveform correctly represented with a sampling rate of 1000 Hz. The trace to the right shows the same signal, but collected at a sampling frequency of 10Hz.

Let's look at the power spectrum of the correctly represented waveform:

The resulting spectrum analysis is shown below:

The dominant frequency is close to 10 Hz.

Now, let's look at the aliased signal:

With the resulting power spectrum

Note that the dominant frequency is around 0.58Hz, which gives an erroneous estimation of the true signal.


Click here to view an HTML5 movie showing sampling rate and aliasing.

Click here to continue with the EEG section