Magnetic tape is an inexpensive and compact media for data storage, particularly useful for the storage of large volumes of data and the sequential processing thereof. Standardization has rendered magnetic tape very useful for data exchange.
Writing on magnetic tape involves producing magnetic flux reversals of a ferromagnetic material to denote binary states, usually on multiple tracks with gaps disposed between records or blocks for searching and reading. An electromechanical drive moves the tape past a read/write head in two directions under control instructions. A typical digital audio tape (DAT) recorder has magnetic tape wound on a supply reel, transported over a read/write head having a cylindrical surface contour to a take-up reel supplying torque to move the tape. The tape is threaded through and driven by a motorized capstan and pinch roller disposed downstream of the head. Tape tension is provided by spring-loaded arms disposed upstream of the head. A transducer is typically disposed between the head and the capstan to sense and control tape tension.
There are many different types of information coding used in the prior art, varying according to polarities (return to zero or not during a transition), bit train compression, and clocking capability. The most common coding schemes for high-performance tapes are non-return-to-zero-inverted (NRZI), phase encoding (PE), and group coded recording (GCR) which is a combination of NRZI and PE. A code is self-clocking if a signal pulse is generated for every stored bit.
Characters are recorded on tape by tracks with each character stored in a column across the tape with embedded parity bits for error checking. Typically, each track has one write head and usually one read head. To limit errors, information written on tape is often read immediately after being written (so-called read-after-write or RAW) by a separate read head mounted closely to the write head. On a typical tape there is a stored addressing information (SAI) section for locating a record and a data section which may also provide additional addressing information. The SAI typically includes (in sequence) a postamble immediately adjacent the previous data record, an interrecord gap (IRG) providing a space interval for tape motion changes, beginning and end of tape characters, various other markers, clocking and deskewing information, and a preamble immediately adjacent the next data record. The preamble utilizes sync marks to synchronize detection circuits for distinguishing bits. The postamble signals the end of a data record or block. To save space and access time, IRGs may be placed between blocks (IBGs) rather than records and related blocks may be grouped into a file and designated by an end of file marker. "Load point" and an "end of reel" markers indicate the beginning and end of the tape respectively and are typically reflective for detection by a photocell in the tape drive unit.
A standard format for digital data storage (DDS) using 3.81 mm digital audio tape (DAT) magnetic tape is set forth by the European Computer Manufacturers Association in the document "Flexible Magnetic Media for Digital Data Interchange" (ISO/IEC JTC 1/SC 11 N 1026, hereinafter "DDS standard", 1990-07-13).
Briefly, DDS format data has two types of separator marks indicating logical separations of the data. Separator 1 is a "file mark" and separator 2 is a "set mark". User data, separator marks, and associated information are formed into groups occupying groups of tracks in a "main zone" of the track. Additional information about the contents of the group, the location of the tracks and the contents of the tracks is recorded in two parts of each track called "sub zones". The two sub zones constitute the "sub data" area of the track. In addition, there are margin zones at the extreme ends of the tape and Automatic Track Finding (ATF) zones between the sub zones and the main zone. Each zone in a track is further segmented into blocks called margin blocks (in the margin zone), preamble, sub data, and postamble blocks (in the sub zones), spacer and ATF blocks (in the ATF zone), and preamble and main data blocks (in the main zone). A "frame" is a pair of adjacent tracks with azimuths of opposite polarity (where the azimuth is the angle between the mean flux transition line with a line normal to the centerline of the track). Data to be recorded is grouped into "basic groups" of 126632 bytes. Each basic group is identified by a running number from 1 to 65535. Data and separator marks are grouped into the basic groups starting with basic group no. 1. Error Correction Codes (ECC), termed C1 and C2, are computed bytes added into the data fields. ECC C3 is one extra frame added to the 22 frame groups and is capable of correcting any two tracks in a group which are bad.
Write data channel functions, including coding and error correction code, are typically performed by a controller operating through a write amplifier positioned near the write head. The write amplifier drives the write current through the write head.
Read data channel functions, including amplification and equalization of the read signals and data retrieval, are typically performed by automatic track-oriented gain-adjustment by a read amplifier and timing, deskewing, decoding, error detection and correction by a controller. The fundamental function of readback is to accurately convert the amplified read signal waveform into its binary equivalent. During writing, an external clock (oscillator) spaces recorded bits. An accurate readback therefore must be synchronous, and a code which inherently strobes the readback signal is desirable, such as self-clocking pulse generation in PE and GCR. One form of coding used in digital data audio tape storage is so-called 8-10 conversion GCR.
Video recorders and some audio cartridges utilize a rotating head for read and write. A typical rotating head-helical scan head is embedded between stationary upper and lower mandrels with the tape helically wrapped around the mandrels. The tape moves at a lower angular velocity than the rotating head to produce helically-written data of very high spatial density because of close track spacing. Addressing the closely-spaced tracks then requires accurate control of the linear positions of the tape around the head unit. By pressurizing the mandrels the tape is hydrostatically supported over the head by an air film at higher tape speeds. Tape speeds may be as high as 40 m/s in rotating head-helical scan systems. At such high data densities and tape speeds, accurate sensing requires precise head and tape speeds control and tape tracking control.
Digital audio tape (DAT) provides flexible, high performance storage applicable to a wide variety of tasks. However, prior art DAT drives are primarily designed for audio systems and thus do not provide the required performance, reliability, error correction, or diagnostics necessary for computer data storage. Prior art DAT systems for computer storage typically use audio mechanisms and electronics with adapter chips to operate with the computer. Computer peripheral application of tape drives requires many more start/stop and high-speed search operations than audio tape drives, which were designed primarily for the continuous play of music or voice. Because of the more strenuous and more stringent requirements of computer data storage, such prior art systems do not provide the required performance and are not sufficiently reliable for such use.