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Preventing Problems
Several problems can arise when using the PowerLab system for recording biological
signals. It is important to understand the types of problems that can occur, how they
manifest themselves, and what can be done to remove them or to minimize their eff ect.
These are usually problems of technique, and should be addressed before you set up
your equipment.
Aliasing
Recordings of periodic waveforms that have been undersampled may have misleading
shapes and may also have artifacts introduced by aliasing. Aliasing occurs when a
regular signal is digitized at too low a sampling rate, causing the false appearance of
lower frequency signals. An analogy to aliasing can be seen in old films: spoked wagon
wheels may appear to stop, rotate too slowly or even go backwards when their rate of
rotation matches the film frame speed – this is obviously not an accurate record.
The Nyquist–Shannon sampling theorem states that the minimum sampling rate (f
)
s
to accurately describe an analog signal must be at least twice the highest frequency in
the original signal. Therefore, the signal must not contain components greater or equal
to f
/2. The term f
/2 is known as the Nyquist frequency (f
) or the 'folding frequency'
s
s
n
because frequencies greater than or equal to f
fold down to lower frequencies about
n
the axis of f
.
n
When aliasing of noise or signals is seen, or even suspected, the first action you should
take is to increase the sampling rate. The highest available sampling rates are 100k /s or
200k /s, depending on your PowerLab. To view the frequencies present in your recorded
signal open the Spectrum window in LabChart. For more information about Spectrum,
see the LabChart Help Center.
If unwanted high-frequency components are present in the sampled signal, you will
achieve better results by using a low-pass filter to remove them. The best kind of filter
for this purpose is the Anti-alias filter option available in the front-end-specific Input
Amplifier... dialog. This is a special low-pass filter that is configured to automatically
remove all signals that could alias; i.e., those whose frequency is greater or equal to half
the sampling rate.
For certain PowerLabs, the Anti-alias filter option is not available. Therefore you
should select an appropriate low-pass filter to remove any unwanted signals (or noise)
occurring at frequencies greater or equal to half the sampling rate.
Frequency Distortion
Frequency distortion will occur if the bandwidth of your recording is made smaller
than the bandwidth of the incoming signal. For example, if an ECG was measured with
a sampling rate of 100 samples per second (100 Hz) and the Bio Amp had a low-pass
filter applied at 50 Hz, the fast-changing sections of the waveform (the QRS complex)
may appear smaller and 'blunted', while the slower T-wave sections remain relatively
unchanged. This overall eff ect is called frequency distortion.
It can be eliminated by increasing the frequency cut-off of the low-pass filter in the front-
end-specific Input Amplifier... dialog to obtain an undistorted waveform.
Owner's Guide
14
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