Patent ID: 7215762

Claim:
The method of selecting optimum impedance for the trans-hybrid echo path of a loop start trunk line, comprising: successively applying respective ones of a plurality of impedance settings to said loop start trunk line; for each of said impedance settings measuring the impulse response of said trans-hybrid echo path to obtain a time-domain trans-hybrid transfer function and deriving a frequency-domain spectrum representative of trans-hybrid loss, and then calculating figure of merit of the transfer function based on the power spectrum, said calculating of the figure of merit including determining a maximum peak B in the power spectrum, averaging the trans-hybrid loss to obtain an average value A, calculating the flatness of the frequency-domain power spectrum as a variance V from the average value A, and generating said figure of merit merit = { - 10000 , if ⁢ ⁢ ( B > 0 ) A 2 - ( w3 × B ) - ( w4 × V ) , else , where w 3 and w 4 are weight values equal to one; and selecting as said optimum impedance one of said plurality of impedance settings associated with the highest calculated figure of merit.