Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
1999-12-06
2002-05-28
Sherry, Michael J. (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
C360S046000, C360S051000
Reexamination Certificate
active
06396254
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to detection and decoding of encoded digital information stored on a storage medium or received from a communication medium. In particular the present invention relates to an improved maximum likelihood sequence detector applicable to such storage or communication applications.
2. Discussion of Related Art
Storage systems store and retrieve data on a storage medium. Communications systems transmit and receive data via a communication medium. Storage and communication systems utilize similar techniques to encode data for storage and retrieval or for transmission and reception. Data is encoded into a form in which it may be easily manipulated in such storage or communication systems. Most modern applications of such systems encode data as numeric or digital information. Discrete numeric values are used to represent user data.
The storage medium or communication medium does not directly manipulate such digital data. Rather, these media store or transmit analog signals representative of the digital data. For example, computer systems often use magnetic storage media on which encoded digital information is represented as magnetic flux changes. Certain identified sequences of flux changes, when detected, are representative of corresponding sequences of digital data. In like manner, communication systems receive information transmitted via the communication medium such as electromagnetic radiation (i.e., radio or microwave frequency radiation). Certain sequences of changes in the communication medium state are representative of corresponding digital data transmitted between a transmitter and receiver.
The data embodied in the analog signals of the medium are then received or retrieved to decode the signals and reproduce the encoded digital data. In a storage system in particular, the encoded signals are said to be read from the storage medium. In a communication system, the encoded signals are usually referred to as being received from the transmitter. A circuit that reads or receives the encoded data and reproduces the original digital data is often referred to as a read channel. In general, a read channel includes a transducer component that senses the analog signal and digital processing components that detect sequences of changes in the sensed signal that represent encoded digital data. For example, a read channel in a magnetic storage device includes a magnetic read head that senses the magnetic flux changes and produces electric current therefrom. The electric signal from the read head is a continuous analog waveform. The digital data encoded in such an analog continuous waveform must therefore be detected and decoded from the essentially continuous analog signal. In general, certain peaks in the continuous waveform represent the encoded digital data.
In an ideal theoretical environment, each magnetic flux change could be representative of a corresponding bit (binary digit—a value of zero or one). Sequences of such zeros and ones may then be recorded on and retrieved from the magnetic medium as sequences of adjacent magnetic flux changes. However, in practice as the density of such recorded information increases, the physical proximity of one recorded bit to the next adjacent bit tends to cause interference between the adjacent bits. This interference is often referred to as inter-symbol interference (or ISI). In storage applications, adjacent magnetic flux changes can cause such inter-symbol interference. Optical storage applications are also affected by such ISI concerns as the physical proximity of optically encoded bits on a storage medium is decreased to thereby increase storage density. Similar interference issues affect communication applications as distinct from storage applications. The speed of signals applied to a communication medium can cause interference problems in sensors adapted to receive the signals.
It is known in storage and communication arts to encode the digital data so that only specified sequences of transitions of the medium are permitted. These permitted sequences are known to reduce the effects of inter-symbol interference. In particular, it is common to use run-length-limited (RLL) encoding of the digital data to generate an RLL-encoded stream of bits to be read by the read channel digital processing means. This encoded stream of bits is often referred to as channel bits in that they represent the stream of bits encountered by the read channel components of the device. It may be desirable in certain applications to assure that there be no fewer than “d+1” bit times between any two transitions of the medium. This constraint can help keep interference effects among the pulses caused by the analog read channel sensing of transitions to a manageable level. On the other hand it is often the case that timing information is extracted from the read channel sensed pulses to help maintain synchronization in reading lengthy sequences of such pulses. It may also be desirable therefore that there be no more than “k” zeros between any two transitions of the medium. In other words, there must be a medium transition at least every k+1 bit times.
An RLL(d,k) code is used to encode an arbitrary set of data bits into a stream of channel bits such that the encoded channel bits satisfy the “d” and “k” constraints. A wide variety of encodings that satisfy a given (d,k) constraint may be used and several such encodings are well known in the storage and communication arts.
Read channels associated with storage devices or communication devices sense the encoded information stored or transmitted on the medium and decode the original data bits from the sensed pulses and the RLL (or other) encoding employed. In general, read channels attempt to sense pulses read from the medium by periodically sampling the present value of a transducer adapted to sense the recorded or transmitted medium state changes. Each periodic sample of the continuous waveform signal produced by a transducer is a digital value indicative of the amplitude of the continuous signal at a corresponding sample time. A sequence of such samples are then used by digital signal processing means to detect state transitions and hence the encoded channel bits and corresponding user data bits in the continuous signal represented by the sampled values.
Due to the analog nature of the waveform sensed by the read head (or receiver) and due to the inter-symbol interference problems noted above, it is a problem to accurately sense and decode the encoded user data bits. The problem is particularly exacerbated as communication speed or storage density increases.
To partially resolve such problems, it is known to use sequence detectors to sense particular expected sequences of pulses rather than attempting to detect each discrete individual pulse in the sampled waveform. In particular, Viterbi sequence detectors do not attempt to decide whether or not a particular medium transition has occurred immediately upon receipt of sample(s) that correspond to that transition. Rather, as samples are taken from the analog read signal, the Viterbi sequence detector keeps a running tally of the error between the actual sample sequence and each of one or more possible expected sequences corresponding to acceptable codes for the particular RLL encoding employed. The running error tally is maintained for each of a number of possible encoded sequences. As more samples are gathered, less likely sequences are pruned from the “tree” of possible sequences. The maximum length of possible sequences that need be tracked may be constrained to a particular value for a particular application and encoding. When so constrained, the Viterbi sequence detector may eventually select a sequence having a high degree of probability as the detected sequence of encoded channel bits.
It is recently known in the art to apply such Viterbi sequence detectors to read channels used in optical storage media. Optical storage media record encoded data using optical techniques
Bliss William G.
Feyh German Stefan Otto
Painter Christopher Lyle
Sundell Lisa Chaya
Cirrus Logic Inc.
Nguyen Jimmy
Sherry Michael J.
Shifrin Esq. Dan A.
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