How does constant frequency response relate to equiripple? The teacher says As a consquence, you can think of the frequency response of a filter as a shark. It must always move and it can never be rest. An important case is equiripple.



I am very confused about the abrupt conception..


Why constant frequency response can never be rest?
2.how does constant frequency response relate to equiripple? 


Thanks you!!
What your teacher tried to show is that a transfer function which is the ratio of two polynomials can never be constant (excluding the trivial case where both polynomials are of degree zero). This means that if your desired frequency response is a piecewise constant function (as is the case for an ideal frequency selective filter), you can only approximate that piecewise constant function by a realizable transfer function, and one popular approximation is an equi-ripple approximation, where the maximum deviation from the desired constant value is minimized.