An automatic gain control (AGC) controls the signal amplitude at the input to an analog to digital converter (ADC) input by applying a gain that produces a desired overall amplitude resolution of the patterns actually presented by the signal delivered by the ADC converter. Short RLL patterns will have sufficient resolution for reliable extraction as a result of having sufficient overall amplitude, which thereby strengthens the ability of the read channel to correctly extract data. Moreover, the system determines correct AGC settings responsive to measurements of user data parameters. The system also detects and corrects for DC offsets in the signal whose gain is controlled.