Source : Free On-Line Dictionary of Computing

Partial Response Maximum Likelihood
     
         (PRML) A method for converting the weak {analog}
        signal from the head of a {magnetic disk} drive into a digital
        signal.  PRML attempts to correctly interpret even small
        changes in the analog signal, whereas {peak detection} relies
        on fixed thresholds.  Because PRML can correctly decode a
        weaker signal it allows higher density recording.
     
        For example, PRML would read the magnetic flux density pattern
        70, 60, 55, 60, 70 as binary "101", and the same for 45, 40,
        30, 40, 45.  A peak detector would decode everything above,
        say, 50 as high, and below 50 as low, so the first pattern
        would read "111" and the second as "000".
     
        (1996-12-27)