The ideal low pass filter will pass all signals below a given frequency (the cutoff frequency) and not pass any signals above the cutoff frequency. A high pass filter does the opposite. The ideal is not achievable, but complex filters can come close. A problem arises when the phase response of the filter is considered. Complex waveforms (pulses) can be considered as composed of many harmonics. If the phase of the component waves is changed relative to one another, the pulse form is changed. In applications where pulse shape is important (television), phase response is an important parameter. The best results are obtained when the phase varies linearly with frequency. Linear phase filters is a subject by itself and is not covered here. As you might expect, sharp cutoff filters have more nonlinearity in the phase response than filters with a more gradual attenuation verses frequency.
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