Dynamic magnetic information storage or retrieval – General processing of a digital signal – Data clocking
Reexamination Certificate
1998-12-04
2002-12-10
Holder, Regina N. (Department: 2651)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
Data clocking
C360S046000, C375S341000
Reexamination Certificate
active
06493162
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to information storage systems and, more particularly, to detection of data retrieved from storage in such systems
BACKGROUND OF THE INVENTION
Digital data magnetic recording, or storage and retrieval, systems store digital data by recording same in a moving magnetic media layer using a storage, or “write”, electrical current-to-magnetic field transducer, or “head”, positioned immediately adjacent thereto. The data is stored or written to the magnetic media by switching the direction of flow in an otherwise substantially constant magnitude write current that is established in coil windings in the write transducer in accordance with the data. Each write current direction transition results in a reversal of the magnetization direction, in that portion of the magnetic media just then passing by the transducer during this directional switching of the current flow, with respect to the magnetization direction in that media induced by the previous in the opposite direction. In one recording scheme, often termed non-return-to-zero inverted (NRZI), each magnetization direction reversal occurring over a short portion of the magnetic media moving past the transducer represents a binary number system digit “1”, and the lack of any such reversals in that portion represents a binary digit “0”.
Recovery of such recorded digital data is accomplished through positioning a retrieval, or “read” magnetic field-to-voltage transducer, (which may be the same as the storage transducer if both of these transducers rely on inductive coupling between the media fields and the transducer) or “head”, is positioned to have the magnetic media, containing previously stored data, pass thereby. Such passing by of the media adjacent to the transducer permits the flux accompanying the magnetization reversal regions in that media either to induce a corresponding voltage pulse in forming an analog output read signal for that retrieval transducer or, alternatively, change a transducer circuit parameter to thereby provide such an output signal voltage pulse. In the coding scheme described above, each such voltage pulse in the read transducer output signal due to the reversal of magnetization directions between adjacent media portions is taken to represent a binary digit “1”, and the absence of such a pulse in corresponding media portions is taken to represent a binary digit “0”.
Digital data magnetic recording systems have used peak detection methods for the detection of such voltage pulses in the retrieved analog signal as the basis for digitizing this signal. Such methods are based on determining which peaks in that signal exceed a selected threshold to determine that a binary digit “1” related pulse occurred in the retrieved signal, and also use the times between those voltage pulses to reconstruct the timing information used in the preceding recording operation in which the data were stored in the magnetic media as described above. The analog retrieved signal is provided to a phase-locked loop forming a controlled oscillator, or a phase-locked oscillator or synchronizer, which produces an output timing signal, or “clock” signal, from the positions of the detected peaks in this analog retrieved signals Absolute time is not used in operating the data retrieval system portion since the speed of the magnetic media varies over time during both the storage operation and the retrieval operation to result in nonuniform time intervals, or nonuniform multiples thereof, occurring between the voltage pulses in the analog retrieved signal.
There is always a desire in magnetic recording systems to devote less of the magnetic media along a track therein to the storage of a bit to thereby permit increasing the density of the bits stored. The use of peak detection places a limit on the density of bits along a track because increasing that density beyond some point will lead to too much intersymbol interference which in turn leads to errors in the recovery of data using such peak detection methods. Because of this limit, recent increases in bit density along a track in a magnetic media have come with the acceptance of a controlled, or known, amount of intersymbol interference which, since known, allows detection of the pulses involved despite this interference. The read transducer analog output signal generated from the binary bits or symbols stored in the magnetic media is sampled with the resulting samples being converted to digital data, and the samples are taken at a rate which leads to more than one sample per pulse rather than the single sample per pulse which would be sufficient for peak detection if sampling was used therewith. Since each individual sample reflects only part of the pulse response, this process used in a system results in referring to such a system as a partial response system.
A digital data magnetic recording system typically comprises a bandpass data retrieval channel in that it is unable to transmit very low frequencies, and in that it has an upper frequency beyond which its transmission is also quite limited, and is often termed a Lorentzian channel in view of the resulting response to an isolated pulse. Thus, the channel, including any equalizer therein, should exhibit in passing a stream of data pulses therethrough a spectral characteristic having spectral nulls at zero frequency and at a frequency equal to half the symbol rate or pulse rate. Channels having an impulse response characteristic of the form (1−D) (1+D)
k
for such a stream have been found to provide such nulls but at the cost of accepting a substantial amount of intersymbol interference leading to the description of the channel as a “partial response system.” Here, D is the unit delay operator, or D=e
j&ohgr;T
where T is the bit period and k≧I for the characteristic described. Such channel response characteristics have integer coefficients resulting in the sampled channel output symbols also having only integer values.
Although there are a number of possible alternative partial response system arrangements, there is substantial value in choosing a channel characteristic of the above form that has the smallest value for n that is possible for the data to be transmitted in the channel. Increasing the value of n leads to increased intersymbol interference resulting in more output symbol values thereby reducing the signal-to noise ratio and increasing the necessary detector complexity and performance requirements. The simplest partial response system with the desired characteristics results from k being set equal to one to yield a channel characteristic of (1−D) (1+D)=1−D
2
that is known as a class 4 partial response system, and has been typically used previously in magnetic digital data recording Systems. Such a response is obtained by providing an overall channel and filter response equal to that of the sum of two opposite polarity Nyquist channel impulse responses separated in time by two sample intervals. Such an arrangement will lead to a filter analog output signal from which ideally can be obtained three alternative possible output symbol sample values of −1, 0 and 1 for an input signal based on binary recorded data if sampled at appropriate instants.
A range of lineal bit representation densities along a track in the magnetic media, leading to a range of data pulse rates for retrieved data, can be accommodated by providing an equalizer in the channel suited to the density chosen. Such an equalizer operates by changing the effective channel characteristic as to keep the output symbol samples at the proper integer values. However, as the lineal density is increased along the tracks in the magnetic media a point is reached where the noise enhancement provided by the equalizer becomes unacceptable. This situation, along with other considerations, requires going to a greater value for n for further density increases to allow a reduction in the transmittals at frequency values.
The next higher values for k are 2 and 3, and
Holder Regina N.
Seagate Technology LLC
Westman Champlin & Kelly P.A.
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