Partial Response/Maximum Likelihood

Partial response / maximum likelihood ( PRML ) is a read method to read data from magnetic media, such as hard drives to higher -density read.

Method

To write data to a magnetic disk, this data is first encoded using a line code. This coding is based for example on the group of RLL codes that represent the data in the form of different length pulse sequences on the storage medium. Wherein the reading of these data, by varying the material properties of the magnetic medium, high recording densities, or by random noise, the height of the individual pulses may vary, so that the conventional fixed threshold value decisions to form a digital signal having higher error rates than PRML.

With a fixed threshold value, for example, analog signal values ​​of the read head with 50 % can be interpreted as a logic -1 signal values ​​of less than 50 % as a logic -0. In PRML, however, the threshold value is dynamically shifted depending on the waveform and thus made ​​dependent on the temporal environment in the framework of a maximum likelihood method. This can only improve the noise suppression on magnetic media, higher write densities are obtained, since the individual write pulses to a certain degree or less " into each other " can flow and be appreciated for the neighborhood.

For example, an analog waveform with the following percentage History

Read by the read head, it is interpreted as a logic for PRML bit sequence 101, while a fixed threshold value is read at a constant sequence 111. The situation is similar in signal values ​​as:

Which is interpreted as a bit string of PRML in 010, however, is read with a fixed threshold value as a constant 000.

Regardless of the PRML thus obtained digital data sequence by following the line decoder (RLL ) is decoded, usually close then set about further steps such as error correction procedures.

Removed PRML is similar to the Viterbi algorithm, although in PRML no convolutional codes must be employed.