PRML: Seagate Uses Space Age Technology Today
Partial Response Maximum Likelihood (PRML) was originally developed for data communications such as deep space probes. The technology may be best known for its work with the Viking Lander, whose quest has been to gather scientific information from the planet Mars. Used to communicate radio signals generated millions of miles from Earth, the space age PRML technology allows engineers to keep data clear from background interference on its long trip back home.
Today, PRML allows Seagate disc drives to increase areal density (disc capacity), boost transfer speed, improve yield through the drive's ability to self-calibrate, and lower disc noise.
Until recently, most drives used a read technique called peak detection. Peak detection focuses on peak voltage levels for interpreting data from the drive head. But as bits are packed more densely on the disk, it becomes harder to distinguish data from background noise or to detect separate peaks for individual bits. As bit density increases, so does the possibility of inter-symbol interference (ISI). ISI results from the overlap of analog signal peaks now streaming through the read/write head at higher and higher rates. This occurrence has traditionally been combated by encoding the data as a stream of "symbols" as it is written, in order to separate the peaks during read operations. The problem has been that the encoding requires more than one symbol per bit, exerting a negative effect on both disk capacity and drive performance.
PRML technology first converts the heads' analog signal to a digital signal, then uses the digital signal to detect data bits. The principal effect is that PRML can handle more tightly packed bits than can peak detection, while improving noise rejection. PRML read channels do not have to separate the peaks during read operations, using, instead, advanced digital filtering techniques to manage ISI. ISI facilitates a greater packing essensce by allowing discs to store more information in some forms of PRML. PRML employes digital processing and maximum likelihood detection to determine the sequence of bits that are most likely to be written on the disc.
PRML should be viewed as a two-part process:
Partial Response, a detection or sampling of the signal. PRML prefers a specific and constant pulse shape, such as those generated by MR heads. PRML does not directly provide any user output information (O or 1 bit). Additional decoding of output is required (called Viterbi detection). Magnetoresistive (MR) head technology compliments PRML by eliminating the overhead associated with wave undershoot filtering. The undershoots result from the outside edges of an inductive head as it passes over the disc to read a transition. A PRML channel requires that these undershoots be removed in one way or another. Typically, electronic equalization (undershoot filters) is applied to the problem, but that adds to the expense and complexity of the PRML channel. Conversely, rounding the sharp outside edges of the pole tips can help minimize the size of undershoots, but this may add complexity and cost. Responses from an MR head are rough in shape, but are relatively easy to shape for sampling. The result: by combining MR and PRML technologies, the overhead associated with undershoot filtering is eliminated. It should be noted that overhead can consume as much as 5 percent of a PRML chip's area.
Maximum Likelihood, converting the waveform to data. Viterbi detection represents an algorithm that checks all possible combinations of data and looks for the best match of least error with incoming data. The pattern that has the least error (difference) is the one with the maximum likelihood to be correct.
Not surprisingly, the remarkable efficiency of PRML contributes to faster transfer of data. PRML contributes to faster data transfer by its adept Run Length Limited (RLL) coding scheme. The (1,7) RLL coding used by traditional peak detection read channels is slow. The "1" means that there must be at least one digital 0 between every pair of digital 1s in the data sequence, meaning at least two "symbol" periods between every pair of magnetic transition on the disc. The "7" means there can be no more than eight symbol periods between transitions. The more efficient (0, k). PRML coding in many Seagate drives in a more compact 16 to 17 ratio, with data rates of up to 128 megabits per second. The high data rate is achieved at a clock speed of 76.5 MHz.