frequency response to phase and magnitude representation
[db,phi] =dbphi(repf)
vector of gains (db) and phases (degrees)
vector of complex frequency response
db(k) is the magnitude of repf(k) expressed in dB i.e.
db(k)=20*log(abs(repf(k)))/log(10) and phi(k) is the phase
of repf(k) expressed in degrees.