7 research outputs found
An Adaptive Partial Response Data Channel for Hard Disk Magnetic Recording
An adaptive data channel is proposed which is better able to deal with the variations in
performance typically found in the recording components of a hard disk drive. Three such
categories of variation were investigated in order to gain an understanding of their relative
and absolute significance; variations over radius, along the track length, and between
different head / media pairs. The variations were characterised in terms of their effects on
the step-response pulse width and signal-to-noise ratio. It was found that in each of the
categories investigated, significant variations could be found in both longitudinal and
perpendicular recording systems which, with the exception of radial variations, were nondeterministic
over different head / media pairs but were deterministic for any particular head
/ media pair characterised.
Conventional data channel design assumes such variations are non-deterministic and is
therefore designed to provide the minimum error rate performance for the worst case
expected recording performance within the range of accepted manufacturing tolerance. The
proposed adaptive channel works on the principle that once a particular set of recording
components are assembled into the disk drive, such variations become deterministic if they
are able to be characterised. Such ability is facilitated by the recent introduction of partial
response signalling to hard disk magnetic recording which brings with it the discrete-time
sampler and the ability of the microprocessor to analyse signals digitally much more easily
than analogue domain alternatives.
Simple methods of measuring the step-response pulse width and signal to noise ratio with
the partial response channel's electronic components are presented. The expected error rate
as a function of recording density and signal to noise ratio is derived experimentally for the
PR4 and EPR4 classes of partial response. On the basis of this information and the
recording performance it has measured, the adaptive channel is able to implement either
PR4 or EPR4 signalling and at any data rate. The capacity advantage over the non-adaptive
approach is investigated for the variables previously identified. It is concluded on the basis
of this investigation that the proposed adaptive channel could provide significant
manufacturing yield and capacity advantages over the non-adaptive approach for a modest
increase in electronic complexity
EQUALISATION TECHNIQUES FOR MULTI-LEVEL DIGITAL MAGNETIC RECORDING
A large amount of research has been put into areas of signal processing, medium design,
head and servo-mechanism design and coding for conventional longitudinal as well
as perpendicular magnetic recording. This work presents some further investigation in the
signal processing and coding aspects of longitudinal and perpendicular digital magnetic
recording.
The work presented in this thesis is based upon numerical analysis using various simulation
methods. The environment used for implementation of simulation models is C/C + +
programming. Important results based upon bit error rate calculations have been documented
in this thesis.
This work presents the new designed Asymmetric Decoder (AD) which is modified to
take into account the jitter noise and shows that it has better performance than classical
BCJR decoders with the use of Error Correction Codes (ECC). In this work, a new method
of designing Generalised Partial Response (GPR) target and its equaliser has been discussed
and implemented which is based on maximising the ratio of the minimum squared
euclidean distance of the PR target to the noise penalty introduced by the Partial Response
(PR) filter. The results show that the new designed GPR targets have consistently
better performance in comparison to various GPR targets previously published.
Two methods of equalisation including the industry's standard PR, and a novel Soft-Feedback-
Equalisation (SFE) have been discussed which are complimentary to each other.
The work on SFE, which is a novelty of this work, was derived from the problem of Inter
Symbol Interference (ISI) and noise colouration in PR equalisation. This work also shows
that multi-level SFE with MAP/BCJR feedback based magnetic recording with ECC has
similar performance when compared to high density binary PR based magnetic recording
with ECC, thus documenting the benefits of multi-level magnetic recording. It has been
shown that 4-level PR based magnetic recording with ECC at half the density of binary PR
based magnetic recording has similar performance and higher packing density by a factor
of 2.
A novel technique of combining SFE and PR equalisation to achieve best ISI cancellation
in a iterative fashion has been discussed. A consistent gain of 0.5 dB and more
is achieved when this technique is investigated with application of Maximum Transition
Run (MTR) codes. As the length of the PR target in PR equalisation increases, the gain
achieved using this novel technique consistently increases and reaches up to 1.2 dB in case
of EEPR4 target for a bit error rate of 10-5
CONVERGENCE IMPROVEMENT OF ITERATIVE DECODERS
Iterative decoding techniques shaked the waters of the error correction and communications
field in general. Their amazing compromise between complexity and performance
offered much more freedom in code design and made highly complex codes, that were
being considered undecodable until recently, part of almost any communication system.
Nevertheless, iterative decoding is a sub-optimum decoding method and as such, it has
attracted huge research interest. But the iterative decoder still hides many of its secrets,
as it has not been possible yet to fully describe its behaviour and its cost function.
This work presents the convergence problem of iterative decoding from various angles
and explores methods for reducing any sub-optimalities on its operation. The decoding
algorithms for both LDPC and turbo codes were investigated and aspects that contribute
to convergence problems were identified. A new algorithm was proposed, capable of providing
considerable coding gain in any iterative scheme. Moreover, it was shown that
for some codes the proposed algorithm is sufficient to eliminate any sub-optimality and
perform maximum likelihood decoding. Its performance and efficiency was compared to
that of other convergence improvement schemes.
Various conditions that can be considered critical to the outcome of the iterative decoder
were also investigated and the decoding algorithm of LDPC codes was followed
analytically to verify the experimental results
New structures and algorithms for adaptive system identification and channel equalization
The main drawback of the ADF is that it takes lot of iteration and fails to identify nonlinear systems. BAF converges fast while maintaining the same performance as ADF but its performance degrades at nonlinear conditions.In this thesis we propose an ANN, which provides better and faster converges when employed for identifying nonlinear systems. This network employs chebyschev based nonlinear inputs updated with the RLS algorithm. Through extensive computer simulation it is demonstrated that CFLANN updated with RLS is a better candidate compared to FLANN and MLP in terms of less complex structure, less number of input simple needed and does accurate identification
REAL TIME MICROPROCESSOR TECHNIQUES FOR A DIGITAL MULTITRACK TAPE RECORDER
Transport properties of a standard compact - cassette tape
system are measured and software techniques devised to configure
a low - cost,direct digital recording system.
Tape - velocity variation is typically ± 10% of standard speed
over tape lengths of 5 µm.with occasional variations of ±40%.
Static tape - skew can result due to axial movement of the tape
reel when it spools.Dynamic tape skew occurs and is primarily
caused by tape - edge curvature with a constant contribution
due to the transport mechanism.Spectral skew components range
from 0.32 Hz to 8 Hz with magnitude normally within one 10 kbit/
sec- bit cell.The pinch roller works against the friction of the
tape guides to cause tape deformation.Average values of tape
deformation are 0.67 µm,0.85 µm and 1.08 µm for C60,C90 and
C120 tape respectively.
Parallel,software encoding / decoding algorithms have been
developed for several channel codes.Adaptive software methods
permit track data rates up to 3.33 k bits/sec in a rnultitrack
system using a simple microcomputer.For a 4 - track system,raw
error rates vary from 10ˉ⁷ at 500 bits/sec/track to 10ˉ⁵ at 3.33
kbits/sec/track.Adaptive software reduces skew - induced errors
by 50%.A skew - correction technique has been developed and
implemented on an 8 - track system at a track data rate of 10 k
bits/sec.
Real - time error correction gives a theoretical corrected
error rate of 10ˉ¹¹for a raw error rate of 10ˉ⁷. Multiple track
errors can cause mis - correction and interleaving is advised.
Software algorithms have been devised for Reed - Solomon code.
With a more powerful microprocessor this code m ay be combined
with the above techniques in a layered error-correction scheme.
The software techniques developed may be applied to N tracks
with an N - bit computer.Recording density may be increased by
using thin - film,multitrack heads and a faster computer.British Broadcasting Corporatio
Advanced Modulation and Coding Technology Conference
The objectives, approach, and status of all current LeRC-sponsored industry contracts and university grants are presented. The following topics are covered: (1) the LeRC Space Communications Program, and Advanced Modulation and Coding Projects; (2) the status of four contracts for development of proof-of-concept modems; (3) modulation and coding work done under three university grants, two small business innovation research contracts, and two demonstration model hardware development contracts; and (4) technology needs and opportunities for future missions