Coded data generation or conversion – Digital code to digital code converters – To or from run length limited codes
Reexamination Certificate
1999-07-09
2001-03-13
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Digital code to digital code converters
To or from run length limited codes
C341S094000, C360S040000, C714S701000
Reexamination Certificate
active
06201485
ABSTRACT:
TECHNICAL FIELD
The present invention relates to channel modulation codes and methods for implementation in magnetic recording systems such as disk drives. More specifically, the present invention relates to high rate run-length limited (RLL) modulation codes for use in a PRML channel.
BACKGROUND OF THE INVENTION
Modulation codes are used in magnetic recording channels in order to limit recorded bit sequences to those that are most reliably detectable. In particular, run length limited (RLL) modulation codes have been used with partial response signaling, maximum likelihood detection (PRML) channels, decision feedback equalization (DFE) channels, and the like. Partial response systems of interest for magnetic data storage devices such as disk drives and magnetic tape include a PR4 (1−D
2
) channel and EPR4 (1+D−D
2
−D
3
) channel as well as other nonclassical polynomials. The present invention can be used with any PR or DFE channel.
In general, magnetic recording systems employ Viterbi detectors to achieve maximum likelihood detection of user data as it is played back from the recording medium. A modulation code for a PRML data recording and playback channel is selected to balance code efficiency against timing/gain loop reliability, as well as error propagation during decoding. RLL codes limit the length of the all zeros sequence, the all ones sequence, and the Nyquist sequence . . . 0101 . . . here using the NRZ notation, which corresponds to magnetization on a disk, tape or other magnetic recording medium. In channels of current interest, these sequences contain little or no timing information necessary for proper operation of the playback system. According, codes are designed to exclude them from permissible codewords.
Run length limited modulation codes are often described using the format “(rate) RLL (d,G/I)” where the “(rate)” is expressed as a ratio of the number of input bits to be encoded to the number of output bits in the resulting codeword. (The letters G and I represent global and interleave run length constraints, respectively.) For example, a rate 8/9 modulation code converts an 8-bit input byte into a 9-bit codeword. Rate 8/9 encoding is well known in the art, as described, for example, in U.S. Pat. Nos. 4,707,681 and 5,260,703. Rate 8/9 encoding for PRML data channels also is described in U.S. Pat. No. 5,196,849. As the code rate approaches unity, the code is deemed to be more efficient, in that relatively fewer code characters are required to encode user data values. Thus, a rate 8/9 code is more efficient than a rate 2/3 code.
Similarly, a rate 16/17 code is more efficient than a rate 8/9 code. A rate 16/17 code (=0.941) achieves an approximately 6% increase in recording density over a standard rate 8/9 modulation code. One example of a rate 16/17 modulation code is described in commonly assigned U.S. Pat. No. 5,635,933 incorporated herein by this reference. Another rate 16/17 code is described in U.S. Pat. No. 5,784,010 assigned to IBM.
Early PRML read channels used the well-known rate 8/9 RLL(0,4/4) channel code. In accordance with prior art, this channel code is combined with a 1/(1⊕D) modulo
2
precoder to obtain the {+1,−1} valued magnetic write-current pattern. On the decoder side, the signal is first equalized to the partial response target and then the +1/−1 write-current waveform is maximum-likelihood detected. The write current is then “unprecoded” (or postcoded) with a 1⊕D modulo
2
function. This “undoes” the precoding to regenerate a {0,1} valued sequence. The data is then RLL decoded for the user. Examples of RLL encoders and decoders are disclosed in the patents identified above.
The rate 8/9 code can be extended to a rate 16/17 code by either bit-wise or byte-wise interleaving unencoded bytes with the encoded sequence. While the G and I constraints will become considerably larger (G=12, I=8 for byte-wise interleaved case), the roughly 6% in increased code rate is often considered worthwhile. About the highest code rate described in the literature is a rate 24/25 code described in commonly-assigned U.S. Pat. No. 5,757,294.
The need remains for improvements in recording channel encoding efficiency in order to improve storage capacities in recording systems and lower costs. More specifically, the need remains to find code constraints than can be used to obtain high-rate, runlength limited (RLL) codes with optimized error propagation and simple implementation. Moreover, such codes are often used with an outer error correcting code (ECC). To ensure reliable recovery of the data, the error propagation characteristics of the RLL code with the ECC should be minimized. The new codes described below are designed to minimize error propagation with an ECC based on 8-bit symbols. The codes described in this patent application are (
0
,K) codes. The K constraint is equivalent to the G constraint. The
0
here means that consecutive ones are allowed, i.e. there is no restriction on the minimum run length of zeros.
SUMMARY OF THE INVENTION
In view of the foregoing background, a general object of the present invention is to improve the effective areal density of data recorded on a magnetic media.
Another object is to improve recording efficiency by reducing the relative number of non-data bits or “overhead” in the data encoding process.
An object of the invention is to provide very high rate modulation codes having reasonable run length limitations for use in magnetic recording and playback systems.
A further object of the invention is to minimize implementation complexity in the context of high rate RLL codes, by providing a relatively small subcode.
A further object is to improve efficiency of RLL encoding of 8-bit symbols for magnetic recording.
A still further object of the invention is to provide for implementation of various modulation codes that limit error propagation in the context of 8-bit ECC symbols.
A further object of the present invention is to provide encoding schemes having improved ratios of data bits to code word length without degrading run length limiting in encoded data.
Another object of the invention is to record data on a magnetic media so as to prevent long strings of no transitions on the magnetic media thereby allowing for reliable timing and gain recovery.
According to the invention, new constraints are applied to realize codes with rates that include 32/33, 48/49, 56/57, 72/73, and 80/81. The reader can see that such codes are highly efficient; approaching unity rates.
The invention includes several codes that are particularly well suited for use with 8-bit ECC symbols. Certain codes optimized for 4-way interleaved ECC are described. These examples have rates 32/33 and 48/49, although other combinations of code rates and ECC interleaving can be obtained in accordance with the teaching of the present invention.
A still further aspect of the invention includes methodologies for reducing error propagation caused by the precoder/postcoder in a typical magnetic storage system.
According to one aspect of the invention, methodologies and constraints are disclosed to enable the creation of a variety of high rate channel codes primarily for use in a PRML channel of a magnetic recording and playback system. The new method of designing and implementing a desired code generally includes the following steps:
First, for a desired RLL code, select a suitable base code (or “subcode”) having a rate n
+1 where n is a multiple of the ECC symbol size. Thus, for 8-bit ECC symbol size, a base code could be rate 16/17 or say, 24/25.
Second, encode one or more of the ECC symbols using the selected base code. Specifically, the number of ECC symbols to be encoded is the number of symbols necessary to provide the number of input bits appropriate to the selected base code. For example, a rate 16/17 base code will require encoding two 8-bit ECC symbols, while a rate 24/25 base code will require encoding three ECC symbols (to encode 24 input bits).
Thir
Fitzpatrick Kelly
McEwen Peter
Zafar Bahjat
Cho James H
Quantum Corp.
Tokar Michael
LandOfFree
High rate runlength limited codes for 8-bit ECC symbols does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High rate runlength limited codes for 8-bit ECC symbols, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High rate runlength limited codes for 8-bit ECC symbols will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2452120