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Tape systems are often described by how they work, that is, the way they record data onto the tape. For example, although the term "streaming tape" that's appended to many tape drives may conjure images of a cassette gone awry and spewing its guts inside the dashboard of your card (and thence to the wind as you fling it out the window), it actually describes a specific recording mode that requires an uninterrupted flow of data. At least four of these terms-start-stop, streaming, parallel, and serpentine-crop up in the specifications of common tape systems for PCs.

Start-Stop Tape

The fundamental difference between tape drives is how they move the tape. Early drives operated in start-stop mode; they handled data one block (ranging from 128 bytes to a few kilobytes) at a time and wrote each block to the tape as it was received. Between blocks of data, the drive stopped moving the tape and awaited the next block. The drive had to prepare the tape for each block, identifying the block so that the data could be properly recovered. Watch an old movie with mainframe computers with jittering tape drives, and you'll see the physical embodiment of start-stop tape.

The earliest PC tape systems operated in start-stop mode. They had to. The computers and their disks were so slow that they could not move data to the drive as fast as the drive could write it to tape. Modern PCs, disks, and tape drives are all faster, and they use large memory buffers to assure that the tape-bound data forms an interrupted stream. Tape drives usually shift to start-stop mode only when an intervening circumstance-for example, an important task steals so much microprocessor time that not enough is available to prepare data for the tape-temporarily halts the data flow. The drive then will often rewind to find its place before accepting the next block of data and starting the tape in motion again.

Streaming Tape

When your PC tape drive gets the data diet it needs, bytes flow to the drive in an unbroken stream and the tape runs continuously. Engineers called this mode of operation streaming tape.

Drives using streaming tape technology can accept data and write it to tape at a rate limited only by the speed the medium moves and the density at which bits are packed-the linear density of the data on the tape. Because the tape does not have to stop between blocks, the drive wastes no time. The streaming design also lowers the cost of tape drives because the drives do not have to accelerate the tape quickly or brake the motion of the tape spools, allowing a lighter weight mechanism to be used. Nearly all PC tape drives are now capable of streaming data to tape.

Parallel Recording

Just as disk drives divide their platters into parallel tracks, the tape drive divides the tape into multiple tracks across the width of the tape. The number of tracks varies with the drive and the standard it follows.

The first tape machines used with computer systems recorded nine separate data tracks across the width of the tape. The first of these machines used parallel recording in which they spread each byte across their tracks, one bit per track with one track for parity. A tape was good for only one pass across the read/write head, after which the tape needed to be rewound for storage. Newer tape systems elaborate on this design by laying 18 or 36 tracks across a tape, corresponding to a digital word or double-word, written in parallel.

Parallel recording provides a high transfer rate for a given tape speed because multiple bits get written at a time, but makes data retrieval time consuming-finding a given byte might require fast forwarding across an entire tape. In addition, the read/write heads and electronics are necessarily complicated. The head requires a separate pole and gap for each track. To prepare the signals for each head gap, the tape drive requires a separate amplifier. These complications increase the cost of tape drives that use parallel recording.

Serpentine Recording

Most PC tape systems use multi-track drives, but do not write tracks in parallel. Instead, they convert the incoming data into serial form and write that to the tape. Serial recording across multiple tracks results in a recording method called serpentine recording.

Serpentine cartridge drives write data bits sequentially across the tape in one direction on one track at a time continuing for the length of the tape. When the drive reaches the end of the tape, it reverses the direction the tape travels and cogs its read/write head down one step to the next track. At the end of that pass, the drive repeats the process until it runs out of data or fills all the tracks.

A serpentine tape system can access data relatively quickly by jogging its head between tracks because it needs to scan only a fraction of the data on the tape for what you want. Additionally, it requires only a single channel of electronics and a single pole in the read/write head, lowering overall drive costs. Modern serpentine systems may use over 50 tracks across a tape.

Helical Recording

The basic principle of all the preceding tape systems is that the tape moves past a stationary head. The speed the tape moves and the density of data on the tape together determine how fast information can be read or written, just as the data density and rotation rate of disks controls data rate. Back in the 50s, however, data rate was already an issue when engineers tried to put television pictures on ordinary recording tape. They had the equivalent of megabytes to move every second, and most ordinary tape systems topped out in the thousands. The inspired idea that made video recording possible was to make the head move as well as the tape to increase the relative speed of the two.

Obviously, the head could not move parallel to the tape. The first videotape machines made the head move nearly perpendicular to the tape movement. Through decades of development, however, rotating a head at a slight angle to the tape so that the head traces out a section of a helix against the tape has proven to be the most practical system. The resulting process is called helical scan recording. Today two helical scan systems are popular, eight-millimeter and DAT (Digital Audio Tape).

In a helical scan recording system, the rotating heads are mounted on a drum. The tape wraps around the drum outside its protective cartridge. Two arms pull the tape out of the cartridge and wrap it about halfway around the drum (some systems, like unlamented Betamax, wrap tape nearly all the way around the drum). So that the heads travel at an angle across the tape, the drum is canted at a slight angle, about five degrees for eight-millimeter drives and about six degrees for DAT. The result is that a helical tape has multiple parallel tracks that run diagonally across the tape instead of parallel to its edges. These tracks tend to be quite fine-some helical systems put nearly 2000 of them in an inch.

In most helical systems, the diagonal tracks are accompanied by one or more tracks parallel to the tape edge used for storing servo control information. In video systems, one or more parallel audio tracks may also run the length of the tape.

Helical scan recording can take advantage of the entire tape surface. Conventional stationary head recording systems must leave blank areas-guard bands-between the tracks containing data. Helical systems can and do overlap tracks. Although current eight-millimeter systems use guard bands, DAT writes the edges of tracks over one another.

This overlapping works because the rotating head drum actually has two (or more) heads on it, and each head writes data at a different angular relationship (called the azimuth) to the tracks on the tape. In reading data, the head responds strongly to the data written at the same azimuth as the head and weakly at the other azimuth. In DAT machines, one head is skewed twenty degrees forward from perpendicular to its track; the other head is skewed backward an equal amount.