There are four fundamental filters. They are
-
Lowpass -- blocks high frequencies, allowing low frequencies through
-
Highpass -- blocks low frequencies, allowing high frequencies through
-
Bandpass -- blocks all frequencies except those within a certain range
-
Bandstop -- blocks only the frequencies within a certain range, allowing
all others to pass through
Another term one may come across in the study of filters is an "allpass"
filter. This is one that allows all frequencies through. The only
meaningful effect an allpass filter can have is on the phase of the
signal.