U. Glaeser

The various components of (34.3) are also known as branch metrics.The Viterbi detector accomplishes
the minimization in an efficient manner using a trellis-based search rather than an exhaustive search.
The search is effectively performed over a finite window known as the decision delay or path memory
length of the Viterbi detector. Increasing the window length beyond a certain value leads to only insignificant
improvements of the bit detection reliability or bit error rate (BER).
Despite the efficient nature of the Viterbi algorithm the complexity of a Viterbi detector increases
exponentially with the channel memory of the PR target. A target with channel memory of L − 1 requires
for example a 2 L−1 state Viterbi detector trellis. For a fully parallel Viterbi implementation, each Viterbi
state contains an add-compare-select (ACS) computational unit, which is used to sum up the branch
metrics of (34.4) and keep the minimum metric paths for different bit sequences. Also required for the
hardware is a 2 L−1 ⋅P bit memory to keep a history of potential bit sequences considered across the finite
decision delay window.
Timing Recovery
A phase-locked loop (PLL) is used to regenerate a synchronous clock from the data stream. The PRML
detector use decision directed timing recovery typically with a digital loop filter. The digital loop filter
parameters can be easily controlled using programmable registers and changed when read channel
switches from acquisition to tracking mode. Because a significant pipelining is necessary in the loop logic
to operate at high speeds, the digital loop filter architecture exhibits a relatively large amount of latency.
It can affect considerably the acquisition time when the timing loop must acquire significant phase and
frequency offsets. To ensure that only small frequency offsets are present, the synchronizer VCO is phaselocked
to the synthesizer during nonread times. For fast initial adjustment of the sampling phase, a
known preamble is recorded prior to user data. The time adjustment scheme is obtained by applying the
stochastic gradient technique to minimize the mean squared difference between equalized samples and
data signal estimates. To compensate for offset between the rate of the signal received and the frequency
of the local timing source the loop filter design allows for a factor ∆T n to be introduced, so that the
sample at discrete time n is taken T + ∆T n seconds after the sample at discrete time n − 1. In acquisition
mode, in order to quickly adjust the timing phase, large values for loop gains are chosen. In tracking
mode, the loop gains are lowered to reduce loop bandwidth.
Read Channel Servo Detection
In an embedded servo system (introduced in the previous article), the radial position of the read head is
estimated from two sequences recorded on servo wedges: track addresses and servo-bursts. The track
address provides a unique number for every track on a disk, while a servo-burst pattern is repeated on
each track or on a group of tracks. Determining the head position using only the track number is not
sufficient because the head has to be centered exactly on a given track. Therefore, the servo-burst
waveform is used in conjunction with the track address to determine the head position. Using the servoburst
pattern, it is possible to determine the off-track position of a head with respect to a given track
with a high resolution. While positioning the head over a surface, the disk drive can be in either seeking
or tracking operation mode. In a seeking mode, the head moves over multiple tracks, trying to reach the
track with a desired address as quickly as possible, while in a tracking mode, the head tries to maintain
its position over a track. The track addresses are therefore used mostly in the seeking mode, while servoburst
information is usually used in the tracking mode [25,30].
In read channels, periodic servo-burst waveforms are detected and used to estimate radial position.
The radial position error signal is calculated based on the current estimated position and the position
of the track to be followed, and then used in an external head positioning servo system. Generally, two
types of position estimators are in use: maximum likelihood estimators based on a matched filtering and
sub-optimal estimators based on averaging the area, or the peaks, of the incoming periodic servo-burst
waveform. A variety of techniques have been used to demodulate servo bursts including amplitude, phase,
© 2002 by CRC Press LLC