Digital Linear Tape

Digital Linear Tape (DLT; previously called CompacTape) is a magnetic-tape data storage technology developed by Digital Equipment Corporation (DEC) from 1984 onwards. In 1994, the technology was purchased by Quantum Corporation, who manufactured drives and licensed the technology and trademark. A variant with higher capacity is called Super DLT (SDLT). The lower cost "value line" was initially manufactured by Benchmark Storage Innovations under license from Quantum. Quantum acquired Benchmark in 2002.

In 2007, Quantum stopped developing DLT drives, shifting its strategy to LTO.

DEC launched the TK50 tape drive for the MicroVAX II and PDP-11 minicomputers in 1984. This used 22-track CompacTape I cartridges, storing 94 MB per cartridge. The TK50 was superseded in 1987 by the TK70 drive and the 48-track CompacTape II cartridge, capable of storing 294 MB. In 1989, the CompacTape III (later DLTtape III) format was introduced, increasing the number of tracks to 128 and capacity to 2.6 GB. Later drives into the early 1990s improved the data density of the DLTtape III cartridge, up to 10 GB. The DLTtape IV cartridge was introduced by Quantum in 1994, with increased tape length and data density, initially offering 20 GB per tape. Super DLTtape, originally capable of up to 110 GB, was launched in 2001.

DLT uses linear serpentine recording with multiple tracks on half-inch (12.7 mm) wide tape. The cartridges contain a single reel and the tape is pulled out of the cartridge by means of a leader tape attached to the take-up reel inside the drive. The drive leader tape is buckled to the cartridge leader during the load process. Tape speed and tension are controlled electronically via the reel motors; there is no capstan. The tape is guided by 4 to 6 rollers that touch only the back side of the tape. Tape material is metal particle tape (MP/AMP).

SDLT adds an optical servo system that reads servo patterns on the back of the tape to keep the data tracks on the front of the tape correctly aligned with the read/write heads. This is important for newer tape media, which have very thin, dense data tracks; 256, 384 and 768 data tracks on a 1⁄2-inch-wide (13 mm) tape are available.

DLT7000 and 8000 tilt the head forward and backward to reduce crosstalk between adjacent tracks through azimuth; this is called Symmetric Phase Recording.

All (S)DLT drives support hardware data compression. The often-used compression factor of 2:1 is optimistic and generally only achievable for text data; a more realistic factor for a file system is 1.3:1 to 1.5:1, although drive compression applied to pre-compressed data can actually make the written data larger than having compression turned off in the tape drive.

Media are guaranteed for 30 years of data retention under specified environmental conditions; they are easily damaged by mishandling (dropping or improper packaging during shipment.)