Coded data generation or conversion – Digital code to digital code converters – To or from run length limited codes
Reexamination Certificate
2001-03-09
2002-07-09
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
06417788
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 within partial response signaling, maximum likelihood (PRML) data recording and playback 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 in any PR 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 and the Viterbi detector path memory, as well as error propagation during decoding.
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. For example, a rate 8/9 modulation code converts an 8-bit input byte into a 9-but codeword. Rate 8/9 encoding is well known in the art, as described, for example, in U.S. Pat. No. 4,797,681 and U.S. Pat. No. 5,260,703. Rate 8/9 encoding for PRML data channel 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, rate 8/9 code is more efficient than a rate 2/3 code.
Similarly, 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 channel 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
2
) 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
2
modulo 2 function. This “undoes” the precoding to generate a {0,1} valued sequence. The data is then RLL decoded for the user. Examples of RLL encoders and decoders are disclosed in the patent 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 increases code rate is often considered worthwhile. Still, the need remains for improvements in recording channel encoding efficiency in order to improve storage capacities in recording systems and lower costs. The codes in this patent application are (0,k) codes. The k constraint is equivalent to the G constraint. The 0 means that consecutive ones are allowed, i.e. there is no restriction on the minimum run length of zeros.
Another limitation of prior art is that virtually all known channel coding schemes are based on 8-bit ECC symbols, as they are historically the de facto standard. We anticipate use of a 10-bit ECC symbol and thus new methods are required to achieve improved density and error propagation performance in the context of 10-bit ECC symbols.
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 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 zero 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 of the invention is to enable effective RLL encoding of 10-bit symbols for magnetic recording.
A more specific object of the invention is to provide a 50/51 modulation code which limits error propagation in the context of 10-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 transition on the magnetic media thereby allowing for reliable timing and gain recovery.
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 rate code, selecting a suitable base code (or “subcode”), having a rate n/(n+1) where n is a multiple of the ECC symbols size. Examples are rate 10/11, 20/21, 30/31 etc. for a 10-bit ECC symbols size.
Second, encoding 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 10/11 base code will require encoding one 10-bit ECC symbol, while a rate 30/31 base code will require encoding three ECC symbols (to enable 30 input bits).
Third, partitioning the codeword produced by the base code into a plurality of m nibbles. In one version m is the number of unencoded ECC symbols. For example, if a rate 50/51 RLL code is desired, the base code rate 10/11 is selected, and one ECC symbol is encoded to form the 11-bit subcode word. That subcode word is partitioned into m=4 nibbles. Four ECC symbols remain unencoded. In one embodiment, three nibbles have three bits each, while a fourth nibble has two bits. However, other partitions are possible as described later.
The fourth step, which is optional but preferred, entails modifying the subcode nibbles in response to the values of corresponding unencoded symbols that will be positioned adjacent to the x
i
nibbles in the target codeword. Specifically, the invention forbids all zeros in a subcode nibble if the immediately preceding bit (i.e. the last bit of the preceding unencoded symbol) is zero. Conversely, we forbid all ones in a subcode nibble if the immediately preceding bit (i.e. the last bit of the preceding unencoded symbol) is a one. These constraints ensure that at least one magnetic flux transition per nibble.
Finally, the resulting modified nibbles are interleaved among the unencoded ECC symbols. The order of the unencoded symbols and the order of the subcode nibbles interleaved among them is not limited to any specific predetermined sequence. The resulting codeword can begin with either an unencoded ECC symbol. or a subcode nibble.
In one embodiment of the present invention, before int
Fitzpatrick Kelly K
McEwen Peter
Zafer Bahjat M.
Chang Daniel D.
Maxtor Corporation
Zarrabian Michael
LandOfFree
High rate runlength limited codes for 10-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 10-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 10-bit ECC symbols will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2895936