ATA (AT Attachment) Interface
Originally developed as IDE interface, this interface became known under various names. e.g. AT-Bus interface, PC-AT interface, Task File interface.
Ablative
Method of optical recording where holes are made in the optical media. Also referred to as pit forming.
Access Time
The time required by the read/write head to reach the right position on the disk, i.e. the time required for access to specific data. Access time equals latency plus positioning time.
Actuator
Head positioning assembly consisting of a voice coil motor and head arms to position the read/write head and servo head over disk tracks.
Analogue
Analogue refers to the representation of data by a continuously changing physical state, such as intensity, frequency, or voltage.
ANSI
The American National Standards Institute.
Areal Density
A measure of the packing density of a disk drive: achieved by multiplying the figure for tracks per inch (tpi) with that for bits per inch (bpi).
ASCII
American Standard Code for Information Interchange. A convention that assigns a standard binary code to each upper and lower case character, numeral, and typographic symbol.
ATL
Automatic Tape Library.
Axial Acceleration
The acceleration of the recording layer along the fixed line normal to the disk reference plane at a specified rotational frequency (speed), as sensed by the optical system.
Axial Deflection
The deflection of any point of the recording layer from its normal position with respect to the disk reference plane, as determined by the normal optical thickness of the protective layer, in a direction normal to the reference plane.
Axial Runout, Dynamic
The peak-to-peak displacement of a track in the direction normal to the disk reference plane, measured over one revolution, as sensed by the optical system.
Bad Sector
An unwritable portion of an optical disk caused by a media defect. A bad sector map plots these locations and prevents writing to them.
Baseline Reflectance (Reflectivity)
The reflectance of an unwritten, non-grooved area of a disk through the protective layer.
Bilayer Disk
An optical disk in which the active elements consist of two layers which participate in the creation of a recorded mark when illuminated by high intensity laser light.
Bifringence
The property of a material which causes incident light waves of different polarisations to be refracted differently by the material.
Bifringence, Double-Pass
The bifringence that results after incidence and reflection through the protective layer of an optical disk.
Buffered Seek
In the buffered seek mode, the hard disk drive receives all signal pulses for a seek operation at one time and executes them in one go. In the non-buffered seek mode, the controller sends the signal pulse to the drive, the drive will execute this signal pulse, and only then, the next signal pulse may follow. This term is important for ST506 and ESDI drives; with SCSI and ATA, this mode is executed internally.
Burst Error
Carrier-To-Noise Ratio (CNR)
The ratio of the root-mean-square (RMS) power of a signal at a specified frequency to the RMS power of the noise in a specified bandwidth, expressed in decibels (dB).
CD-R
Compact Disc-Recordable.
SCSI Term. This list shows the positions of defects detected during the initialised data block verification of the FORMAT UNIT command. This list is generated internally by the IDD during execution of the FORMAT UNIT command
Composite Head
The composite head is composed primarily of a ferrite core with a wire coil and a glass-filled gap, and the material used to bond the core to the slider. Ferrite is a hard material which makes it difficult to meet today's physical and electrical characteristics required for higher areal densities. Conventional ferrite heads, such as this, have reached the limits of their performance with regard to the machining processes and magnetic characteristics in the high-frequency domain.
Cross Talk Level
The ratio of the level of a spurious signal generated by an adjacent track to the level of the signal of that adjacent track.
Curie Point
The temperature at which the polarity of a magnetic material is easily changed. Normally associated with optical disk technology.
Curie Temperature
The temperature above which a magnetic material loses its magnetic properties and can be easily magnetised by an applied magnetic field.
Closed Loop Servo System
A correction system in the form of a loop that has no exit, exchanging information about changes in the constitution of the storage medium (e.g. temperature-related expansion) and of the drive mechanics in a feedback method between a reference track on the medium and the head positioning. In this way, irregularities may be corrected more easily, which is of advantage for data integrity and which, particularly with smaller drives, renders higher data densities secure. This feedback technology is frequently accompanied by the use of linear or rotary voice coil actuator technology.
Constant Density Recording
In this technique, data is recorded at a density approximately constant on all tracks therefore, outer tracks have a larger capacity and higher data transfer rate. In conventional recording, the track capacity is based on the innermost track.
CRC
Cyclic Redundancy Check : frequently used error-checking (correctness of memory contents) in the read/write head zone.
DE
Disk enclosure, container of five sub-assemblies :
Fixed disks
Protection of the internal components from environmental contaminants requires that the DE be sealed at the source of manufacture and must not be opened. DE is sometimes referred to as the head/disk assembly (HDA).
Dedicated Servo
Positioning method where servo information is written on one disk surface only. This position information is read and evaluated permanently. The linear voice coil actuator and thus the read/write head is tracked precisely.
Defect
Some irregularity in a medium which disturbs its ability to carry recorded data. Defects in an optical disk include bumps, depressions, dirt, missing sensitive material, etc. Because of the high density of optical recording, even micrometer size defects are important.
Defect (Pre-Write)
Defect Distribution
The geographic distribution of errors on an optical disk measured along its tracks and radially across its tracks in terms of the occurrence of successive n-bit errors. This concerns the media in both an unrecorded and recorded state and does not address focus or tracking servo defects.
Defect Management
A strategy whereby defective media areas (e.g., sectors) that cannot be written due to hard (uncorrectable) errors, are automatically assigned to alternative areas which do not contain hard errors. The strategy generally includes guidelines for disk rewrite and disk retirement. Disk retirement is retention of a disk which can be read but not written.
Defect Rate, Initial
Average raw bit error rate of an optical disk when writing and reading user data for the first time.
Direct Read After Write (DRAW)
A process in which data is read for verification immediately after being written.
Direct Read During Write. (DRDW)
Describes a method of error checking data on an optical disk in which data is written and verified during the same disk rotation. Refers to the ability to read the information during the writing (recording) process.
SCSI Term. This list shows the positions of defects as specified by the INIT using the FORMAT UNIT command during disk initialisation.
ECMA
European Computer Manufacturers Association.
Eccentricity
1. The distance between the geometric centre of a circular feature on the disk and the centre which coincides with the centring feature of the disk. A more specific term is concentricity.
2. The deviation from a true circular path of circular feature of the disk expressed in the difference between the maximum and minimum distance from the geometric centre of the disk. A more specific term is circularity.
Error
The discrepancy between a computed, observed or measured value and the true, specified or theoretically correct value.
Error Correcting Code (ECC)
1. (ANSI) A code in which each acceptable expression conforms to specific rules of construction that also define one or more equivalent nonacceptable expressions, so that if certain errors occur in an acceptable expression, the result will be one of its equivalents and thus the error can be corrected.
2. An error-detecting code designed to correct certain kinds of errors is data.
Error Detection and Correction (EDAC)
A family of methods in which redundancy is added to data in known fashion and is written with the data. Upon readback, a decoder removes the redundancy and uses the redundant information to detect and correct erroneous channel systems.
Error Rate
(ANSI) The ratio of the number of data units in error to the total number of data units.
Error, Rate, Uncorrectable
This is a function of the raw bit error rate and the error detection and correction system used, and is measured by the ratio of the number of hard (uncorrectable) errors to the total user capacity.
GB
Gigabyte (one thousand million bytes).
G List
SCSI Term. This list contains the positions of defective logical data blocks specified by the INIT with the REASSIGN BLOCKS command, positions of defective logical data blocks allocated by the IDD automatic alternate block allocation. It also contains information specified by the D list and information generated for the C list. The whole list may read by the INIT through the READ DEFECTIVE DATA command.
Grooved Disk
A disk with either surface relief features within the optical depth of focus of the recording layer or reflectivity changes placed within the recording layer along the tracks to provide positional feedback to the radial tracking servo.
Head Crash
Unintentional crash of the read/write head on a disk during flight, may cause damage of the disk and/or the head and eventually result in unserviceability of the disk.
Initiator : SCSI term that describes any device that has initiated an input/output operation on another SCSI device on the bus which in turn is called TARG.
IDD
Intelligent Disk Drive. Usually refers to disk drives with either a SCSI or ATA interface.
Interleave Factor
If a program occupies several sectors, the contents of the first sector will first be transmitted from the controller to the computer system when the program is read. This sector is then saved in the memory, which requires a certain period of time. During this time, however, the disk continues to revolve and so it would be necessary to wait one complete revolution of the disk for access to the next sector. During formatting, the continuous sectors therefore are not arranged one after the other physically, but staggered to reduce this waiting time. With an interleave factor of 3, for instance, two sectors are skipped after each sector read. This means the controller has time to further process its data until the disk surface advances two sectors underneath the head. The next logical sector is physical sector 4, and so on. Modern hard disks with integrated controller and own data buffer, the interleave factor is normally 1:1.
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Kerr Rotation
When linearly polarised light passes through a thin magnetic film, the polarisation is rotated by the magnetic field in the film. The direction of rotation is used to read information from rewritable optical disks.
Kerr Effect
This describes the effect of laser light that is reflected from a magnetic surface of an optical disk, which rotates clockwise or anticlockwise depending on the polarity of the bit.
The time required for one disk revolution. The average latency is the time required for ½ a disk revolution. This is the average time for the head to reach the required sector after it has been positioned over a track.
Linear Voice Coil
Used to position the read/write heads of hard disk drives over the data tracks. There are three main types of design: balanced rotary positioner; non-balanced rotary positioner; linear positioner. All three are feedback systems and vary in the way of converting the power of the motor into a head movement. Today, the type used most is the rotary positioner.
Low-Level Format (SCSI)
Formatting which takes place during the manufacturing process. Low-level formatting checks the integrity of the entire data recording area and divides the recording surface into user and spare sector area. The locations of any defective sectors are recorded to prevent future access. Also known as media certification.
MR Head
Magneto Resistive Head : Uses similar technology to thin film heads. It comprises of a magneto resistive read element which is formed first to which an inductive write element is added. Incorporating dedicated elements in this way optimises each part of the head for performance. ~ automatically senses flux, allowing ~ to sense smaller magnetic domains.
MIG Head
Metal In Gap Head : An advancement of the ferrite head is the MIG Head (metal-in-gap) in which metal alloys (e.g. AlFeSil) are sputtered in the magnetic gap of the head. This increases substantially the magnetic field strength, allowing higher bit densities to be achieved on highly coercive data carriers.
MCLV
Modified Constant Linear Velocity : MCLV refers to the tracks on an optical disk that are separated into bands. Early Roman for 89th anniversary of William of Normandys victory over Harold (King of England) at Hastings although lots of people had forgotten by then.
Megabyte (MB)
One megabyte = 1,000,000 bytes
MTBF
Mean Time Between Failures. : Calculated by dividing the total operating time in hours at all field sites by the number of equipment failures from all field sites. Failure of the equipment means failure that requires repair, adjustments, or replacement. Mishandling by the operator, failure due to adverse environmental conditions, power trouble, host system trouble, and other failures not caused by the equipment are not considered.
MFM
Modified Frequency Modulation : Old recording method for storage products with a maximum of 17 sectors per track.
NRZ
No Return to Zero : Phase coding, used particularly with magnetic tape drives and hard disk drives.
O-ROM
Optical Read Only Memory Similar to CD-ROM in concept but applies to optical disk cartridges. The data is stamped on the disk and remains unchangeable.
ODD
Optical Disk Drive
Open Loop Stepper System
Open loop in contrast to closed loop. Conventionally driven positioning system, where the read/write head is moved over the storage medium in individual, equal and unchangeable steps, irrespective of eventual changes (without feedback), generally driven by a stepper motor.
P List
SCSI Term. This list contains the position of all the defects which were found before the disk left the factory and is stored in the system area of the disk. The defective areas in the P list are the permanent defects and accordingly must be reflected in the alternate block allocation. Can be read with the READ DEFECT DATA command. (0x37)
PR4ML
Partial Response class 4 Maximum Likelihood
P-ROM
Partial Read Only Memory Only part of the optical disk is stamped with unchangeable data, leaving the rest to be written to or erased at will.
PC AT Interface
The extension bus for IBM personal computers and their compatibles. The PC AT bus is a 16-bit data bus supporting 24-bit address bus. The address bit of the drive consists of 5 bits after decoding by the host adapter.
PCA
Printed Circuit Assembly. Also known as Printed Circuit Board (PCB)
Positioning Time
The total of the seek time and average latency time.
Pulse Code Modulation (PCM)
A method of sampling an analog signal that results in a continuous code which is produced directly from that signal. High resolution is possible with PCM, due to the reduction of required pulses in the code.
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Rare Earths
Metallic elements of the lanthanide series (atomic numbers 57 through 71) in the chemical periodic table, such as Praeseodymium, Neodimium, Samarium, Gadolinium and Dysprosium, which are used to form the thin magnetic layer in rewritable optical disks.
Read/Write Head
A head which may be shifted horizontally to the disk surface and which consists of at least one electro-magnet. The head may magnetise the disk surface (write on it), read it and erase it; erasing being a write process with a regular pattern.
Recording Density
The number of all magnetised points on the magnetic data carrier. Measured in bits per inch (bpi) and tracks per inch (tpi).
Reed-Solomon Code
An error detection code or an error correcting code or a combination of both which is particularly suited to the correction of burst errors or errors that are strongly correlated.
RLL
Run Length Limited. Recording method for hard disk drives with a high recording density.
Rotary Voice Coil Actuator
In principle, the rotary voice coil actuator consists of a movable wire coil in a stationary magnetic field (similar to the principle of a loudspeaker). When there is current flowing through the coil, the resulting magnetic field interacting with the permanent magnet causes a movement which again acts on the positioning arm. This movement may be varied infinitely (linear) with the current strength.
START/STOP UNIT Command
A SCSI device level 6 byte command for stopping or starting the platter spindle motor. Regardless of how any hardware settings have been configured the drive will respond to this command.
SCSI
Small Computer Systems Interface A parallel interface allowing the pooling of up to 56 units (computer, printer, storage devices, etc.) on a single bus although in practice only 8 units are supported.
Seek Time
The time required for moving the head from the current track to a specified track. The seek time does not include the average latency.
Skew
The actual or apparent local spacing (parallel to the reference edge) between two track elements of a row (DIN 66010), generally: shift, spacing.
Stepper Motor
Type of drive for head movement with low-cost hard disk drives.
Sputtering
The application of a magnetic layer without a binder by means of ionic bombardment in a vacuum.
ST506/412 Interface
This interface is an advancement of the floppy interface developed by Shugart. This standard interface for hard disks allows transfer rates of up to 5Mbits/second.
Target : SCSI term that describes any device that performs I/O initiated by an initiator (INIT).
Thin Film Head
Thin film heads are created using vacuum deposition, photolithographic and other similar techniques used in semiconductor wafer processing, to achieve highly controllable dimensions and characteristics. Through this process, the coil winding is deposited on a ceramic base, creating extremely small heads with equally small and precise gaps between the pole pieces of the magnetic recording material. Thin film heads perform best at lower flying heights, achieving higher areal densities and faster data transfer rates.
TPI
Tracks Per Inch.
Track
1. (ISO) The path, or one of the sets of paths, on a data medium associated with a single reading or writing component as the data medium moves past the component.
2. For an optical disk, a 360 degree segment of the path which is to be followed by the read beam during reading or the write beam during writing.
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Winchester
1. Winchester was the code name for IBM's project of developing a completely new kind of disk drive (in the early '70s). The typical feature of this technology is a permanently hermetically enclosed disk head area.
2. Cathedral city in Hampshire, UK.
Write Precompensation
With the change of the speed of the disk relative to the head (changes from track to track due to varying disk diameter), the data bit also travels within the read window margin. In order that this does not lead to read errors, the write data are shifted in time. This is done starting with a specific track.
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