Abstract: An optical disk has groups of features (such as depressed areas) arranged in a preselected position sensing servo pattern, and a flat, non-grooved surface between a plurality of circumferentially spaced sectors. A single photodetector senses the change in amplitude of reflected light as a spot from a laser passes over one of the features and generates position sensing signals corresponding to the pattern. At least three phases of the position sensing signal are generated for each sector. Each phase is determined by edges on the disk offset circumferentially with respect to each other and separated radially from each other by a preselected critical distance that is independent of the pitch of the recording tracks on the disk to provide a track error signal that is substantially linear within a desired range to either side of a zero-crossing point.

Abstract: A magnetoresistive (MR) read transducer and method of fabricating same. A soft magnetic layer and an MR layer extend over a central active region and passive end regions of the transducer. A nonmagnetic spacer layer separates the soft magnetic film layer and MR layer and extends over the central active region and at least part of both passive end regions. An antiferromagnetic layer directly contacts the MR layer only in the end regions and produces an exchange bias field between the antiferromagnetic layer and the MR layer that is oriented at a preselected acute angle to the longitudinal direction of the transducer. This results in the MR layer being oriented at the preselected acute angle and the soft magnetic film layer in the passive end regions being transversely oriented by magnetostatic coupling to the MR layer. To fabricate the transducer, the antiferromagnetic layer is annealed in an external field above the Neel temperature to produce the canted exchange bias field.

Abstract: A data storage system comprises magnetoresistive (MR) sensing elements for sensing data from a recording medium that is referenced to ground, and an amplifier circuit including a biasing stage, an amplifying stage, and a matching stage, each referenced to a common supply voltage source and to ground. The biasing stage includes a reference current source for selectively biasing each MR element with a constant current and, in so doing, developing a single-ended voltage signal having a first dc component. The amplifying stage converts the single-ended voltage signal into an amplified single-ended output voltage signal having a second dc component but in which the first dc component is eliminated. The matching stage creates a bias and imposes said bias on the amplifying stage for converting the single-ended output voltage signal to a differential output voltage signal in which the second dc component is eliminated.

Abstract: Circuitry for eliminating magnetic instability of an inductive magnetic transducer during sensing of data from a magnetic recording medium to prevent distortion of the readback signal by hysterically moving domain walls. An inductive sensing coil has an impedance constituting the output impedance of the transducer. Means connected to the coil amplifies a data signal generated by the magnetic flux changes and produces a selected input impedance. The sum of these input and output impedances is selected to be small enough to substantially eliminate motion of the domain walls in the yoke of the transducer while the transducer is sensing data from the medium.

Abstract: Count, key, data (CKD) datas are stored on fixed block (FBA) disk recorders (DASD). A virtual track is created which emulates a physical CKD track such that the byte displacement of each CKD record on the virtual CKD track is the same as the byte displacement would be on a physical CKD track. This enables computer programs using CKD formatted data to record on the FBA recorder in an emulation mode. Each FBA block includes a header outside the addressing of the virtual CKD track which includes a byte displacement pointer to the beginning of a first CKD record stored in the FBA block, if any begins in such FBA block; otherwise the header indicates that no CKD record begins in the block. A last record indicator is included in the count field emula-tion for assisting in finding end of the virtual track.

Abstract: Decoding power is enhanced by (1) flagging only those codewords where the ECC capability has been exceeded; (2) permitting codewords to be of any byte length (n) and codeword depth (.lambda.) subject only to the requirement that n and .lambda. be relatively prime; and (3) interleaving encoded bytes of successive codewords diagonally in a single continuous sequence to form an array with a toroidal topology so that all burst errors will be continuous from codeword to codeword, irrespective of where they occur in the array. These attributes assure that there will never be a problem with burst errors affecting.ltoreq..lambda. rows because there is no "last row" and "first row".

Abstract: Writing of data on a storage disk by an inductive magnetic transducer is modulated by write driver circuitry that includes a supply voltage as low as three volts provided by a source referenced to ground, and a bias current source connected to either the supply voltage source or ground. Two bias control switches respectively direct the bias current to one or the other of two current gain circuits according to whether a positive or a negative input terminal has a preselected polarity. The gain circuits selectively respond to the bias currents supplied by the associated control switch for amplifying the bias current to remove a write current from a respective associated terminal of the transducer.

Abstract: A signal processing channel and method are described for processing digital sample values corresponding to an incoming analog signal representative of coded binary data. An eight-sample look-ahead algorithm is used to precompute the values of functional expressions for a baseline check and for a peak-position check. These precomputed values are compared against appropriate thresholds to provide respective binary decision outputs which, with state values corresponding to the current state, are used to determine state values for the next state, which become the current state values for the next iteration of the clock cycle. During each of a series of successive clock cycles, one successive bit of coded binary data corresponding to said current sample value is decoded, and at the next clock cycle, the computed next state becomes the new current state. Sensitivity to missing or extra-bit errors is minimized and full advantage of a (1,7) run-length-limited code constraint is achieved.

Abstract: An optical data storage system including an amplifier which (i) provides a differential current input having a low impedance over the full range of frequencies for which amplification is required in an optical detector that includes a photodiode that senses the intensity of an optical signal generated by data on the optical storage medium; (ii) reverse biases the photodiode; (iii) amplifies the signal current from both sides of the photodiode to improve signal-to-noise ratio; and (iv) is isolated from and unaffected by power supply noise.

Abstract: An unordered error correcting code is constructed by an encoding method and apparatus that accepts k bits of information and is capable of providing unordered ECC codewords. All pairs of these codewords are at least a distance d apart, where d.gtoreq.(2t+1) and t is the maximum number of errors correctable by the code. The k bits of information are encoded with an ECC encoding algorithm to produce ECC codewords that are at least distance d apart. The least number of tail bits required to produce the unordered ECC codewords is appended to each of the ECC codewords. The tail bits for each codeword are constructed by dividing its weight by d for determining the integer part of the resulting quotient, generating a binary representation of the value of its integer part, and complementing said binary representation. A noisy received version of the ECC codeword with tail bits truncated is decoded and a preselected error correcting algorithm is applied to correct t.sub.

Abstract: A method for processing, in a signal processing channel, digital values corresponding to a digitized incoming analog signal representative of coded binary data. A state-dependent sequence detection algorithm includes two groups of appropriate functional expressions of digital sample values, which expressions are identical but offset one sample position from each other. During each iterating step with successive pairs of clock cycles, the value of each expression in the two groups of expressions is precomputed from a preselected number of sample values ahead of a then current sample value; preselected ones of these expression values are compared against an appropriate threshold, which is the same for corresponding expressions of each group, to provide respective binary decision outputs corresponding to each of the two groups; and the current state value then advances to two next successive state values.

Abstract: A method and apparatus for detecting spectral null sequences of a spectrally-constrained code on a noisy communications channel. From a Viterbi detector with a trellis structure comprising a plurality of states and edges, predetermined ones of the states and edges in the trellis structure are deleted at preselected times modulo N, such as times .phi. modulo N and/or at intermediate times m modulo N, where m=.phi., thereby to create a time-varying trellis structure for limiting the maximum length of dominant error events. The deleted edges are generally those edges which would have entered or emanated from the deleted states if they were not deleted. The trellis structure may be a so-called systolic structure, in which case the spectral null sequences are preferably DC-free or Nyquist free.

Abstract: An algorithm for a (t.sub.1,t.sub.2)-detecting code, and a method and apparatus for decoding same, for detecting skewed transitions in a parallel asynchronous communication system without acknowledgement. Transitions sent at the same time in a parallel channel may arrive at different times, but the algorithm permits a limited degree of variation in transmission speeds between channels. A desired control operation will be initiated whenever a skewed transition occurs. The code will detect up to t.sub.1 or t.sub.2 skewed transitions. The values of t.sub.1 or t.sub.2 for the algorithm are preselected by the user. t.sub.1 is the maximum number of transitions that may be missing from a first transmitted codeword when a transition arrives from a second transmitted codeword, and t.sub.2 is the maximum number of transitions from the second codeword that may arrive before all transitions of the first codeword arrive.

Abstract: An algorithm for a (t.sub.1, t.sub.2)-tolerant code, and a method and apparatus for decoding same, for tolerating and detecting skewed transitions in a parallel asynchronous communication system without acknowledgement. Transitions sent at the same time in a parallel channel may arrive at different times, but the algorithm permits a limited degree of variation in transmission speeds between channels. Errors will be corrected and transmissions will be continuous as long as the values of t.sub.1 or t.sub.2 for the algorithm, as preselected by the user, are not exceeded. If they are exceeded, a desired control operation will generally be initiated. t.sub.1 is the maximum number of transmissions that may be missing from a first transmitted codeword when a transmission arrives from a second transmitted codeword, and t.sub.2 is the maximum number of transitions from the second codeword that may arrive before all transitions of the first codeword arrive.

Abstract: Low noise, low power, low voltage amplifier circuits with a single ended input having no common mode rejection for concurrently biasing and amplifying signals generated by magnetoresistive (MR) elements in a disk file. The amplifier circuits comprise a single (grounded) supply voltage source. One terminal of each MR element and the conductive substrate of each MR element and the conductive substrate of each disk in the disk file are grounded to minimize transient conductive asperity currents. The head/disk assembly of the disk file is completely enclosed by a highly conductive electrostatically shielded metallic enclosure that operates as a Faraday cage and isolates leads connecting the MR elements with the amplifier circuit from large, fast rise/fall time voltage transients.

Abstract: Method and circuitry for phase synchronizing an analog input signal with a clock signal by sensing clock delay error, adjusting in increments clock delay trim of a delay element that initially has an arbitrary delay setting, and stopping adjustment after differential delay between the signals has been eliminated.

Abstract: Methods and apparatus for detecting (and, if desired, imaging) the effects of magnetic resonance in a spin-containing material by measuring the magnetic force acting on a mechanical cantilever having a low damping coefficient due to modulation of the magnetic moment of the material in the polarizing field direction at a frequency substantially lower than the spin resonance frequency. Modulation of the magnetic moment in the direction of the polarizing magnetic field may be achieved by modulating (i) the magnitude of the magnetic field in the polarizing direction or (ii) the frequency or amplitude of a high-frequency magnetic field.

Abstract: A method and apparatus for generating high rate run length limited trellis codes and increasing minimum distance between output sequences of partial response channels with constrained channel inputs without requiring codes with spectral nulls. A Viterbi detector replicates a conventional trellis structure for the channel N times. The N copies of the channel response trellis are interconnected such that a preselected function associates each state in the trellis with a particular integer value modulo N. The number N is selected according to the channel detection and coding constraints so that diverging erroneous sequences of minimum distance lead to detector states which are distinct from the correct detector state. The detector trellis is time-varying such that only certain values of the preselected function are allowed every m bits. The time-variation assures there are no minimum distance extensions of erroneous sequences beyond a predetermined length.

Abstract: An optical data storage system having a highly integrated optical storage head that (1) integrates the read, write and servo paths, (2) comprises a monolithic laser-detector storage array that is integrated on a single chip an, wherein laser-detector alignment desirably is fixed at the time of fabrication, and (3) comprises a four-hologram optical element that directs light to an optical disk and divides the returning light into one track error and two focus error servo beams and two polarization component beams for data detection, and also provides a satellite beam of the incident laser beam for monitoring power of the laser beam.

Abstract: A data recording apparatus, such as a disk drive or tape drive is described wherein servo sectors written on a disk (or servo sections written on a tape) are equally spaced on a given track and read during seek, settle and track following operations. An algorithm is used to determine the allowable time separation between servo sectors (or sections) on a track and lengths of associated data sections, such as data sectors or variable length records, that may be accommodated on the disk or tape in such manner that each of the servo sectors (or sections) equally spaced on a given track is located within a data field of a data sector or within an identification region or immediately after an address indicating mark (such as address mark or index mark). The rate at which the servo sector (or section) is sampled is constant and independent of the number and lengths of the data sections.