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High-pass filter
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A high-pass filter is a filter that passes high frequencies well, but attenuates (or reduces)
frequencies lower than the cutoff frequency. The actual amount of attenuation for each frequency
varies from filter to filter. It is sometimes called a low-cut filter; the terms bass-cut filter or
rumble filter are also used in audio applications. A high-pass filter is the opposite of a low-pass
filter. See also bandpass filter.
It is useful as a filter to block any unwanted low frequency components of a complex signal
while passing the higher frequencies. Of course, the meanings of 'low' and 'high' frequencies are
relative to the cutoff frequency chosen by the filter designer.
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Implementation
A passive, analog, first-order high-pass filter, realized by an RC circuit
The simplest electronic high-pass filter consists of a capacitor in series with the signal path in
conjunction with a resistor in parallel with the signal path. The resistance times the capacitance
(R×C) is the time constant; it is inversely proportional to the cutoff frequency, at which the
output power is half the input (−3 dB):
Where f is in hertz, R is in ohms, and C is in farads.
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Applications
Such a filter could be used to direct high frequencies to a tweeter speaker while blocking bass
signals which could interfere with or damage the speaker. A low-pass filter, using a coil instead
of a capacitor, could simultaneously be used to direct low frequencies to the woofer. See audio
crossover.
High-pass and low-pass filters are also used in digital image processing to perform
transformations in the frequency domain.