Selecting the Mains filter checkbox allows you to turn the mains filter on and off. The mains filter is an adaptive filter used to remove interference that is related to the mains frequency (normally 50 or 60 Hz). It is particularly useful when you are recording small electrical signals such as biopotentials (ECG, EEG etc).
It works by tracking the input signal for approximately one second, and creating a template of any interference from the mains frequency. The template is then subtracted from the incoming signal. Because the template is not fully generated until about one second after sampling starts, however, the mains filter is not fully effective in the first second of each data block. Compared to using the notch filter, little waveform distortion results from this method. The filter is only available for certain types of PowerLab.
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.|
|·||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.|
NB: In general it is better to prevent interference at its source than to filter it