Dynamic magnetic information storage or retrieval – General recording or reproducing – Specifics of equalizing
Reexamination Certificate
1999-03-08
2001-10-16
Hindi, Nabil (Department: 2651)
Dynamic magnetic information storage or retrieval
General recording or reproducing
Specifics of equalizing
C360S046000, C360S031000, C375S232000
Reexamination Certificate
active
06304400
ABSTRACT:
TECHNICAL FIELD
The present invention relates to compensating for dropout in signals read from digital magnetic tape.
BACKGROUND ART
Magnetic tape is effectively used to store digital data. Data is written onto the magnetic tape by a write head. The write head converts a current signal containing the digital information into flux patterns which are written as field transitions onto the magnetic tape. The data is retrieved when the magnetic tape is passed by a read head. The read head output may be amplified and equalized before a detector extracts a binary signal containing the data from the read signal. Magnetic tape storage may be modeled as a magnetic recording channel including the effects of converting a binary signal into magnetic tape field transitions and producing a read signal by sensing the field transitions.
One problem with magnetic tape is dropout or the decrease in the read signal amplitude envelope. Dropout is frequently caused by defects in the magnetic tape. These defects may weaken magnetic field transitions on the tape. Defects may also increase the distance between the magnetic tape and read and write heads. In addition to a decrease in the read signal amplitude envelope, a reduction in high frequency components due to dropout changes the shape of pulses in the read signal making correct detection of data more difficult.
Many designs have been proposed to deal with read signal dropout. For magnetic tape holding analog signals such as voice and video, information lost due to dropout may be replaced with substitute information. The substitute information may be obtained from other channels, may be found by averaging information prior to or following the dropout, or may be artificially generated. Provided the dropout duration is not excessive, human perception will not detect that substitute information has been provided. Substitution techniques will not work with recorded digital data, however, since each bit must be either correctly received or reconstructed using error correction schemes.
Other designs for dropout include circuitry to detect when a dropout is occurring. The output of such dropout detection circuitry is a binary signal that may be used to change filter characteristics or to signify that an error has occurred. Dropout detection circuitry may be complex and the resulting filter modification may not occur rapidly enough to prevent data loss.
Another design uses an adaptive filter preceding the detector. Filter parameters are calculated using an error signal based directly on the detector output. Detection is accomplished using the decision feedback equalization method which may result in infinite error propagation.
What is needed is signal dropout compensation that can continuously and adaptively adjust to both the decreasing amplitude and high frequency attenuation occurring during dropout. Dropout compensation should not require complex dropout detection circuitry. For increased effectiveness and applicability to a wide range of tape systems, dropout compensation should be based on magnetic recording channel properties.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide adaptive dropout compensation.
Another object of the present invention is to provide dropout compensation based on an ideal magnetic recording channel response.
Still another object of the present invention is to provide dropout compensation that continuously and adaptively adjusts to both the decreasing amplitude and high frequency attenuation occurring during dropout.
Yet another object of the present invention is to provide continuous dropout compensation without the need for dropout detection circuitry.
In carrying out the above objects and other objects and features of the present invention, a system is provided for dropout compensation. The system includes an adaptive filter receiving an actual read signal sensed from digital magnetic tape. The adaptive filter produces an output based on the actual read signal and multiplicative weighting factors, each weighting factor based on an error signal. A detector receives the adaptive filter output and generates a binary signal indicating data written onto the magnetic tape. A shaping filter receives the binary signal and produces an ideal read signal representing a signal that would result from writing the binary signal to a magnetic tape without dropout. A differencer determines the error signal as the difference between the ideal read signal and the adaptive filter output.
In an embodiment of the present invention, multiplicative weighting factors are further based on the actual read signal. The multiplicative weighting factors may be adaptively modified to minimize the difference between the actual read signal and the ideal read signal.
In still another embodiment of the present invention, the adaptive filter output includes the sum of product terms. Each product term is the product of one weighting factor and a sample of the actual read signal.
In still another embodiment of the present invention, the shaping filter implements the step response of a magnetic recording channel. The magnetic recording channel includes components for writing data on the magnetic tape and for reading the magnetic tape to produce the actual read signal. The shaping filter may be a finite impulse response filter.
A magnetic read system is also provided. The read system includes read head converting magnetic tape field transitions into a read signal. A read equalizer compensates for distortion in the read signal caused by the read head. An analog-to-digital converter digitizes the compensated read signal. The output of an adaptive filter is based on the digitized read signal and multiplicative weighting factors. Each weighting factor is based on the difference between the adaptive filter output and an ideal read signal obtained from passing the detected adaptive filter output through a shaping filter.
A method is also provided for dropout compensation. The method includes reading a magnetic tape to produce an actual read signal. Weighting factors are determined based on an error signal. The actual read signal is adaptively filtered using the weighting factors. The binary signal written on the magnetic tape is detected based on the adaptively filtered actual read signal. An ideal read signal is generated from the detected binary signal. The error signal is determined as the difference between the ideal read signal and the adaptively filtered actual read signal.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.
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Brooks & Kushman P.C.
Hindi Nabil
Neal Regina Y
Storage Technology Corporation
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