Retrieving data recorded at different bit densities

Details Retrieving data recorded at different bit densities Retrieving data recorded at different bit densities

1996-08-19

1998-04-07

Psitos, Aristotelis M.

Dynamic magnetic information storage or retrieval

General processing of a digital signal

Pulse crowding correction

360 46, 360 48, 360 65, 360 67, 360 61, G11B 509

Patent

active

057371415

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to methods and apparatus for retrieving digital signals which have been recorded on a recording medium at more than one bit density, and particularly, though not exclusively, to methods and apparatus for retrieving data stored using a DDS data storage system.


BACKGROUND ART

It is already known to provide reliable storage and retrieval of large volumes of digital data, such as computer data, in particular by means of the DDS (Digital Data Storage) format defined in ISO/IEC Standard 10777:1991 E.
In a DDS read/write mechanism using the format defined in that Standard data are recorded on an elongate recording media, comprising tape coated with a magnetic medium, by a rotating drum carrying one or more electromagnetic heads. The tape is moved by a motor-driven capstan along a path extending between two spools or reels and wrapped partially around the drum. The plane of rotation of the drum is disposed at an angle to the plane of movement of the tape, so that each head traverses the tape along successive tracks extending across the width of the tape at an angle to its centreline. The mechanism includes appropriate circuitry for encoding data into signals suitable for recording on tape, including error detection and correction codes, and for conditioning those signals into a form which is optimally matched to the characteristics of the recording media; for data retrieval, additional circuitry is provided for detecting magnetic field variations stored on the tape, deriving corresponding signals, conditioning those signals into a form suitable for subsequent processing, decoding the encoded data, and detecting and correcting errors.
Data are recorded at a bit density of approximately 61 kilobits per inch (kbpi) (equivalent to approximately 24 kilobits per centimetre). At this bit density a typical DDS tape cartridge can currently store up to approximately eight gigabytes of data, using the longest practicable tape and techniques such as data compression to maximize the quantity of data the tape can accommodate.
With the steady increase in use of computer equipment and the volumes of data processed and stored by such equipment, it has become desirable to increase the capacity of a DDS tape cartridge still further. One possible approach is to increase the bit density of data stored on the tape. However, it is essential that any DDS mechanism which provides a higher capacity in this manner should also be capable of reading and writing DDS tapes which conform to the existing 61 kbpi Standard, to ensure compatibility with existing DDS mechanisms already in use.
In principle it would be expected that changing the bit density of data stored on a magnetic tape would entail corresponding changes in operating parameters throughout the DDS mechanism, e.g. in the electromagnetic heads, signal conditioning circuitry and magnetic field detection circuitry. On this basis it would appear that, to provide compatibility with 61 kbpi mechanisms, a mechanism using a higher bit density would have to incorporate two sets of much of its circuitry and possibly even of its head drum, one set for 61 kbpi operation and the other for operation at a higher density. Such a solution would clearly be very expensive and awkward to design and manufacture.


DISCLOSURE OF INVENTION

The applicant has discovered that, unexpectedly, it is possible by use of particular component parameters and decoding circuitry to provide a DDS mechanism which can read and write DDS tapes which conform to the present 61 kbpi Standard and can also, using to a large extent the same circuitry and in particular the same head drum, read and write data at the very much higher bit density of 122 kbpi. Thus the invention provides both a significant increase in bit density, and an economical way of maintaining compatibility with existing DDS equipment.
According to one aspect of this invention there is provided a method for retrieving data from a magnetic medium, comprising the steps of: magnetic medium; and partial-response maximum

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