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Partial Response Maximum Likelihood

 
Computer Desktop Encyclopedia: PRML
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(Partial Response Maximum Likelihood) A technique used to differentiate a valid signal from noise by measuring the rate of change at various intervals of the rising waveform. Bits generated by a modem or by reading a hard disk have uniform characteristics, whereas random noise does not. PRML uses digital signal processing (DSP) to reconstruct the data.

On magnetic disks, PRML increases the number of bits that can be recorded over earlier methods. It uses an RLL encoding sequence of 0,4,4 and provides an 8:9 ratio of user data to recorded data. See RLL.

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Wikipedia: Partial Response Maximum Likelihood
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In computer data storage, Partial Response Maximum Likelihood (PRML) is a method for converting the weak analog signal from the head of a magnetic disk or tape 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 (where 60 is the baseline signal) as binary "101", and the same for 45, 40, 30, 40, 45 (baseline of 40) whereas 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".

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This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.


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