U. Glaeser

High-frequency noise is then removed with a continuous-time low pass filter to permit a sampling of
the signal without aliasing of high-frequency noise back into the signal spectrum. The filter frequently
includes programable cut-off frequency, which can be used to shape the signal to optimize data detection.
A programmable cut-off frequency is essential since the disk rotates with constant angular velocity, and
data rate varies by approximately a factor of two from the inner to outer radius of the disc. It is also
important for the analog filter bandwidth to be switched to allow for low cut-off frequencies when
processing servo sector information.
Precompensation
Nonlinear bit shift in magnetic recording is the shift in position of a written transition due to the
demagnetizing field from adjacent transitions. In a PRML channel, the readback waverofm is synchronously
sampled at regular intervals, and the sample values depend on the position of written transitions.
Therefore, nonlinear bit shift leads to error in sample values which, in turn, degrades the channel
performance. The write precompensation is employed to counteract the nonlinear bit shift. However,
determining the nonlinear bit shift is not simple and straightforward especially when one tries to fine
tune each drive for its optimum precompensation. The precompensation circuit generates the write clock
signal whose individual transition timing is delayed from its nominal position by the required precompensation
amount. The amount of precompensation and the bit patterns requiring precompensation can
be found using the extracted pulse shape [10,18]. Another approach is a frequency-domain technique
that offers simple measurement procedure and a possible hardware implementation using a band-pass
filter [32] or using PRML sample values [33].
Partial-Response Signaling with Maximum Likelihood Sequence Estimation
After sampling with a rate 1/T, the read signal is passed through an analog or digital front end filter and
detected using a maximum likelihood sequence detector. The partial-response signaling with maximum
likelihood (PRML) sequence estimation is proposed for use in magnetic recording by Kobayashi 30 years
ago [15,16]. In 1990 IBM produced the first disk drives employing PRML detection. Today’s all read
channels are based on some version of the PRML. Cideciyan et al. [3] described a complete PRML systems
including equalization, gain and timing control, and Viterbi detector. All basic functions of a PRML
system have remained practically unchanged, until the introduction of a postprocessor that performs a
special type of soft error correction after maximum likelihood sequence detection. Also, significant
improvements in all the subsystems have been made during last 10 years. The term “partial response”
comes from the fact that the sample of the equalized signal at, say, time nT (T is a signaling interval),
contains information not only on data bits at time nT, but also on neighboring bits, i.e., magnetic
transitions. The number of adjacent bits that determine the sample at nT is referred to as channel memory.
The channel memory is a parameter that can be selected in the process of defining a read channel
architecture. The channel memory and the details of the partial response selection are made based on
an attempt to have the partial response be as close a match to the channel as possible. Since the complexity
of a maximum likelihood detector is an exponential function of a memory, it is desirable to keep the
memory low, but, the equalization required to achieve this might boost the high-frequency noise, which
result in decrease of a signal-to-noise ratio, called equalization loss. The typical value of channel memory
in today’s read channels is 4. The value of an equalized sample at time nT, y n can be written as
where xn is a user-data bit recorded at time n (xn ∈ {−1, +1}), and Lh is a channel memory. The coefficients
Lh hk form, h(D) = Σk=0 hk⋅D , a partial response polynomial or partial response target (D is a formal, timedelay
variable). The main idea in partial response equalization is to equalize the channel to a known and
short target that is matched to the channel spectrum so that noise enhancement is minimum. Therefore, the
k
© 2002 by CRC Press LLC
y n
=
L h
∑
k=0
h k x n−k