Cost function to analyze the passband and the stopband of Digital Biquadratic Filters

Additionally, it was shown that the cost function can be easily computed using a polynomial approximation. Therefore, instead of using uniformly distributed frequency points, the frequency analysis is performed at the Chebyshev nodes improving accuracy while using very few terms in the approximation. This approximation can be used to speed up the computation of the cost function, and consequently reduce optimization time for complex filters composed of several biquadratic sections.