Servo channel noise limits are defined through Viterbi decisions based on servo gate signals. Y values are used to produce a first Viterbi decision at each servo gate. Viterbi decisions and Y values are used to produce ideal Y values. Y values and ideal Y values are used to produce an error value which is adjusted by a noise factor based on estimated channel characteristics. The noise value is combined with Y values and used to produce a second Viterbi decision at each servo gate.