the selected performance criteria is selected as the configuration for all subsequent transmissions again alternatively if during the testing phase a hybrid circuit configuration exceeds a predetermined line performance that hybrid circuit configuration under consideration can be selected as the hybrid circuit configuration for all subsequent transmissions preferably the performance and selection analysis is performed only when the modem is power cycled in this way any disturbances that cause a retrain such as a line disconnect will not result in a hybrid selection change referring now to fig4 there is shown a selectable hybrid circuit according to another embodiment of the present invention the configuration shown in fig4 is an analog front end topology which supports an alcatel chip set analog front end for an adsl receiver unit for such a configuration z 2 represents an impedence equal to 787 ohms 100 nf zin represents the input impedence of the downstream receive path high pass filter drivers 100 are receive path downstream drivers zline represents the input impedence of the two wire transmission line drivers 102 represent the transmit path upstream drivers the hybrid circuitry comprises an analog switch 104 and corresponding plurality of impedence values zh 1 zh 2 zh 3 zh 4 preferably the switch 104 is an analog switch such as model number adg409 available from analog devices inc the adg409 switch has a resistance of approximately 40 ohms and this must be taken into account when selecting the corresponding hybrid impedances zh 1 through zh 4 asic 1 and asic 2 are binary select lines for the switch 104 these binary select lines are controlled by the processor which cycles through a binary count on successive hybrid circuit selections and performance determinations to select the best hybrid circuit configuration preferably the values for zh 1.