EEG is no different than the quote above…our recordings would be chaos without them.
In simple terms:
a filter is a device or process that removes some unwanted components or features from a signal.
Mental Note: 30%
Thats the % to remember when asked about filters…
When a low-frequency filter encounters a sine wave that happens to be exactly at its cutoff frequency, it cuts down the amplitude of that wave by approximately 30%.
So since for EEG our LFF (low frequency filter) is 1 Hz – the amplitude of a 1 Hz will be reduced by 30%.
Key Point to Note:
Waves at frequencies below the LFF are reduced by somewhat more than 30%—Also, waves at frequencies somewhat above the LFF are also reduced in size, but by less than 30%.
So its more like a roll off than a cut-off.
See graph below:
The same is true on the other end of the range…when talking about the HFF (high frequency filter):
Since our HFF is 70 Hz – the amplitude of a 70 Hz will be reduced by 30%.
But this time:
Waves at frequencies higher than the HFF are reduced by somewhat more than 30%—and waves at frequencies somewhat slower than the cutoff frequency are also reduced in size by the filter, but by less than 30%.
Understanding filters is key to understanding what you are looking at on the tracing…it is also important to understand how your use of filters can alter the display.
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