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Owner’s Guide
15
the mains power. The filtered signal is obtained by subtracting the template from the
incoming signal.
In comparison with a conventional notch filter, this method produces little waveform
distortion. It attenuates harmonics of the mains frequency as well as the 50 or 60 Hz
fundamental and therefore eff ectively removes non-sinusoidal interference, such as
that commonly caused by fluorescent lights.
The filter should not be used when:
•
the interference changes rapidly. The filter takes about 1 second to adapt to
the present level. If interference is present and then is suddenly removed,
interference in the filtered signal will temporarily worsen.
•
your signal contains exact factors or harmonics of frequencies close to the mains
frequencies, for example, a 30 Hz signal with 60 Hz mains frequency.
•
your signal is already free from interference. If the signal-to-noise ratio is greater
than about 64 the mains filter introduces more noise than it removes.
•
you are recording at close to maximum sampling rates. The mains filter uses
some of the PowerLab’s processing power and therefore reduces the maximum
rate at which you can sample.
Electrode Contact
Occasionally one of the lead wires connecting the subject to the front-end may become
disconnected, or an electrode contact may become poor. If this should happen,
relatively high voltages (potentials) can be induced in the open wire by electric fields
generated by power lines or other sources close to the front-end or the subject. Such
induced potentials will result in a constant amplitude disturbance in the recorded
waveform at the power line frequency (50 or 60 Hz), and loss of the desired signal. If
the problem is a recurring one, one of the leads may be faulty. Check connections and
replace faulty leads, if necessary.
Motion Artifacts
A common source of artifacts when recording biological signals is due to motion of
the subject or equipment. Oft en applying a high-pass filter can help to remove slowly
changing components in a recorded signal.
•
Muscular activity generates its own electrical signals, which may be recorded
along with an ECG, say, depending on the location of the electrodes.
•
If an electrode is not firmly attached, impedance (and hence the recorded signal)
may vary as the contact area changes shape owing to movement.
•
Movement of patient cables, particularly bending or rubbing together
(triboelectric eff ects) may generate artifacts in a signal.
•
Subject respiration can also generate a signal; breathing can result in a slowly
changing baseline corresponding to inspiration and expiration.
If the subject is liable to move during recording, then special care needs to be taken
when attaching the electrodes and securing the patient leads. Make sure the skin is
cleaned and lightly abraded before attaching the electrodes.