Dynamic information storage or retrieval – Storage or retrieval by simultaneous application of diverse...
Reexamination Certificate
1999-06-02
2001-01-23
Neyzari, Ali (Department: 2752)
Dynamic information storage or retrieval
Storage or retrieval by simultaneous application of diverse...
C369S124040, C360S046000
Reexamination Certificate
active
06178144
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to digital recording systems, and more particularly to a magneto-optical digital recording system implementing linear recording and playback channels.
BACKGROUND OF THE INVENTION
Digital recording systems such as computer disk drives, audio recording/playback (DAT) systems, and video recording/playback (DVD) systems are well known. The majority of these systems use either magnetic recording or magneto-optical recording to store and retrieve data from the storage medium.
Magnetic recording (MR) systems use a magnetic medium (disk or tape) to store data. The magnetic medium contains surface ferro-magnetic particles, each having a magnetic polarity. During recording, the ferro-magnetic particles are exposed to a locally applied magnetic field. The particles become magnetized and the direction of each particle's polarity is used to represent a segment of the recorded signal. During playback, the medium is passed by a playback head which senses the direction of each particle, thereby reconstructing the originally stored data.
Magneto-optical (MO) recording is another system used to store to and retrieve data, common examples being audio and video CD systems. MO systems operate on substantially the same principle as MR systems, both using the direction of ferro-magnetic particles within a magnetic medium (disk or tape) to represent stored information. The MO medium is different from most MR media in that the ferro-magnetic particles within the medium are vertically oriented. MO systems also employ lasers to record and read data from the medium. Data is recorded onto the MO medium by laser heating the MO medium to its curie temperature point. Once the MO medium reaches it curie temperature point, the ferro-magnetic particles within the medium exhibit low coercivity and can be easily re-oriented in another direction when exposed to a magnetic field. A locally applied magnetic field orients particles in the desired direction, the direction corresponding to the data to be recorded. Once the illuminated area cools, the particles exhibit high coercivity and retain their direction even in the presence of strong magnetic fields.
Reading data from the MO medium is accomplished by again illuminating the ferro-magnetic particles with a laser, except at a lower power to avoid heating the medium. The property of the MO medium is such that the embedded particles shifts the polarization of the illuminating light. Surfaces of the MO medium are passed by the illuminating laser and the stored data therein causes a polarization shift in the reflected beam known as the Kerr effect. A detector seizes the changes in polarization and reconstructs the stored data.
The MO medium has greater storage density and retains data more reliably compared to the MR medium. Because the MO medium uses vertically oriented particles, MO media has a recording density typically 10 to 1,000 times greater than that of the MR medium. In addition, because MO systems use a laser to read data from the MO medium as opposed to a playback head, MO media lasts significantly longer than MR media (15-40 years versus 3 years). Further, since MO media is resistant to external magnetic fields at room temperature, data storage is more reliable using the MO “system compared to the MR systems.
The MO media also has a very unique beneficial side effect in that it is an amorphous film. It does not have the crystalline metallic structure of MR film which means that it has very, very fine grains in it which allows the magnetization boundary between an opposite polarity of recording saturation level to be relatively clean and noise-free, so much so that the noise does not increase as the FCI, or flux changes per inch, or flux density increases. That is in opposition to the case with conventional metallic film media where transition noise or zigzag noise as it's commonly called, increases as the FCI increases, or flux density increases.
While MO media provides higher storage density and better reliability than MR media, MR systems are still widely used. This is partly attributable to the relatively slow data rate or the speed at which MO systems can record or retrieve data from the medium. Specifically, MR systems are able to handle more data per unit time, i.e. operate with a higher bandwidth efficiency compared to MO systems.
The difference in the system's bandwidth efficiency is primarily attributable to how the two systems communicate data to and from the storage medium. MO systems conventionally operate using standard saturation or binary level recording.
FIG. 1A
shows the block diagram of a conventional MO system
100
. The MO system consists of a digital recording channel
110
for receiving an input bit stream
101
and generating a recording signal therefrom, a MO medium
120
for storing the recording signal, and a digital playback channel
130
for reading the recording signal and generating an output bit stream
102
. The recording channel
110
includes a binary encoder
111
, a writing laser
113
, and a magneto-optical recording head
114
.
The MO system operates using standard two-level saturation recording technique whereby each received bit in the input bit stream
101
is encoded using a binary encoder
111
. The resultant encoded waveform
112
is recorded onto the MO medium, bit by bit, by saturating the magnetic medium
120
to record a 1-bit, or by applying no magnetization to record a zero bit. Playback occurs bit by bit in the reverse order, using a playback head
131
, a reading laser
132
, an optical reader
134
and a binary decoder
136
. Because the recording and playback-signals are digital, the recording and playback channels are not required to be highly linear.
Introduction
Linear Data Channels are known to have considerably larger data capacity than two-level channels of similar bandwidth and signal-to-noise ratio (SNR). For example, in conventional magnetic recording modeled as a Lorentzian channel in additive white Gaussian noise (AWGN) the capacity in bits per second (bps) is at least twice as large for the linear channel (average power constrained) over the saturation channel (peak power constrained). Both channels have the same bandwidth and signal-to-noise ratio (SNR). Consequently, there is considerable competitive advantage to develop a linear magnetic recording channel. Moreover, present-day known techniques in signal processing of the saturation channel are further from the their capacity bound than those known for the linear channel; consequently, the practical difference is larger than 2:1 in potential capacity.
AC-bias Linearization
The use of AC-bias to linearize the magnetic recording channel is not new. It has been successfully used to allow “write” equalization for 2-level recording on oxide media. However, when it comes to fully linearizing the channel, past investigators have been discouraged by the 6 dB to 7 dE SNR loss suffered in linearizing the magnetic recording channel with AC-bias. A loss this large is difficult to make up even with more efficient transmission techniques.
FIG. 5
illustrates the problem with a conventional B-H loop (magnetic flux density, B, vs. magnetic field intensity, H). AC-bias removes the hysteresis leaving the zero-hysteresis line. Signal amplitude must be limited to the linear range of this curve. Recognition of this problem with conventional magnetic recording has led to efforts to find improved signaling efficiency by the composition of non-linear responses to produce a larger than two-character alphabet.
An example of a MR system utilizing linear recording is described in U.S. Pat. No. 5,124,861 to Shimotashiro. There, the system implemented quadrature amplitude modulation (QAM) to convert digital data into a QAM modulated signal. Because the modulated signal can be made to represent multiple bits, the data rate of the MR system is higher and bandwidth efficiency greater compared to the MO system.
To operate properly, MR systems requires high linearity and high signal-to-noise ratio (SNR) in the
Flehr Hohbach Test Albritton & Herbert LLP
Neyzari Ali
Seagate Technology LLC
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