Signal filtering consists of processing a signal to remove a certain band of frequencies
within it. The band of frequencies removed can be either at the low-frequency end of
the frequency spectrum, at the high-frequency end, at both ends, or in the middle of
the spectrum. Filters to perform each of these operations are known respectively as
low-pass filters, high-pass filters, band-pass filters and band-stop filters (also known
as notch filters). All such filtering operations can be carried out by either analogue or
digital methods.