Magnetic tape

Magnetic tape is an information storage medium consisting of a magnetisable oxide coating on a thin plastic strip. Nearly all recording tape is of this type, whether used for video with a video cassette recorder, audio storage (reel-to-reel tape, compact audio cassette, digital audio tape (DAT) and other formats including 8-track cartridges) or general purpose digital data storage using a computer (specialized tape formats, as well as the above-mentioned compact audio cassette, used with home computers of the 1980s, and DAT,used for backup in workstation installations of the 1990s).

Table of contents

1 Magnetic Tape Audio Storage 2 Magnetic Tape Video Storage 3 Magnetic Tape Data Storage

Magnetic Tape Audio Storage

From the late 1940s through the 1970s, (analog) magnetic tape was the predominant and the highest quality sound recording techology available.

Magnetic recording had been demonstrated in principle as early as 1898 by Valdemar Poulsen in his telegraphone. With electronic amplification, the telegraphone evolved into wire recorders which were popular for voice recording and dictation during the 1940s and into the 1950s. The reproduction quality of wire recorders was low, however—significantly lower than that achievable with phonograph disk recording technology. Wire recorders could not prevent the wire from undergoing axial twisting, and hence could not insure that the wire was oriented the same way during recording and playback. When oriented the wrong way, high frequencies were reduced and the sound was muffled. The hysteresis of the steel material resulted in nonlinear transfer characteristics, manifesting as distortion. There were other practical difficulties, such as the tendency of the wire to become tangled or snarled. Splicing could be performed by knotting together the cut wire ends, but the results were not very satisfactory.

Magnetic tape recording as we know it today was developed in Germany during the late 1930s by the C. Lorenz company. In 1938, S. J. Begun left Germany and joined Brush Development Company in the United States, where work continued but attracted little attention. During the war, the Allies became aware of radio broadcasts that seemed to be transcriptions, but whose audio quality was indistinguishable from that of a live broadcast. After the war, the Allied capture of a number of German Magnetophon recorders from Radio Luxembourg aroused great interest. These recorders incorporated all of the key technological features of analog magnetic recording, particular the use of high-frequency "bias."

Development of magnetic tape recorders in the late 1940s and early 1950s is associated with the Brush Development Corporation and its licensee, Ampex; the equally important development of magnetic tape media itself was led by Minnesota Mining and Manufacturing corporation (now known as 3M).

The use of magnetic tape recorders in broadcasting got a significant boost from Bing Crosby, who refused to perform on radio unless his shows could be prerecorded.

7-1/2" reel of 1/4" recording tape
Typical of audiophile/consumer/educational use 1950s/60s

The typical professional tape recorder of the early 1950s used 1/4" wide tape on 10-1/2" reels, with a capacity of 2400'. Typical speeds were initially 15 inches per second (ips) yielding 30 minutes' recording time on a 2400' reel. 30 ips was used for the highest quality work.

Standard tape speeds varied by factors of two. 30 and 15 ips were used for professional audio recording; 7-1/2 ips for home audiophile prerecorded tapes; 7-1/2 and 3-3/4 ips for audiophile and consumer recordings (typically on 7" reels). 1-7/8 ips and occasionally even 15/16 ips were used for voice, dictation, and applications where very long recording times were needed (such as logging police and fire department calls).

The key electronic invention that made high-quality audio tape recording possible was the development of so-called "bias." Bias is a high-frequency signal, typically in the range of 50 to 150 kHz, which is added to the signal to be recorded before being applied to the recording head. Bias enables a linear transfer function to be obtained from the highly nonlinear magnetic recording medium. In effect, the bias causes the magnetization to be performed at levels that are in the most nearly linear portion of the medium's transfer function. As the tape leaves the head, the bias current partially demagnetizes the tape and the remaining net induction is essentially the difference between the positive and negative half-cycles of the previously recorded. This differencing operation further cancels some of the nonlinearity.

Magnetic audio tape can be easily and inaudibly spliced, a fact which revolutionized audio recording. Multiple tracks could easily be recorded simultaneously (on a wide tape), and played back with perfect synchronization; this, too was revolutionary. It became common studio recording practice to record on multiple tracks, and mix down afterwards. The convenience of tape editing and multitrack recording led to the rapid adoption of magnetic tape as the primary technology for commercial musical recordings. Although 33-1/2 rpm and 45 rpm vinyl phonograph disks were the dominant consumer format, recordings were customarily made first on tape, then transferred to disk.

Analog magnetic tape recording introduces noise, usually called "hiss", caused by the finite size of the magnetic particles in the tape. There is a direct tradeoff between noise and economics. Signal-to-noise ratio is reduced at higher speeds and with wider tracks, increased at lower speeds and with narrower tracks.

By the late 1960s, disk reproducing equipment became so good that audiophiles soon became aware that some of the noise audible on recordings was not surface noise or deficiencies in their equipment, but reproduced tape hiss. A few companies starting making "direct to disk" specialty recordings, made by feeding microphone signals directly to a disk cutter (after amplification and mixing). These recordings never became popular, but they dramatically demonstrated the magnitude and importance of the tape hiss problem.

In the 1970s, advances in solid-state electronics were making the design and marketing of more sophisticated analog circuitry economically feasible. This led to a number of attempts to reduce tape hiss through the use of various forms of volume compression and expansion, the most notable and commercially successful being several systems developed by Dolby Laboratories. These systems divided the frequency spectrum into multiple bands and applied volume compression/expansion independently to each band. The Dolby systems were very successful at increasing the effective dynamic range and signal-to-noise ratio of analog audio recording; to all intents and purposes, audible tape hiss could be eliminated.

In the 1980s, digital recording methods were introduced, and analog tape recording was gradually displaced.

Magnetic Tape Video Storage

Magnetic Tape Data Storage

half-inch reel tape

Magnetic tape was first used to record data in 1951 on the Mauchly-Eckert Univac. The recording medium was a thin band of solid steel. Recording density was 128 characters per inch at a linear speed of 100 ips, yielding a data rate of 12800 characters per second.

IBM computers of the late 1950s used oxide-coated tape similar to that used in audio recording, and IBM's technology soon became the de facto industry standard. Magnetic tape was half an inch wide and wound on removable reels 10.5 inches in diameter. Different lengths were available with 2400 feet and 4800 feet being common.

IBM's drives were mechanically sophisticated floor-standing drives that used vacuum columns to buffer long loops of tape. Powerful reel motors activated the reels as necessary to maintain the length of the tape loop in each column within set limits. The two reels thus spun in rapid, uneven and unsynchronized bursts of motion which were eye-catching and dramatic. Stock shots of vacuum-column tape drives came to symbolize "the computer" in movies and television.

LINCtape (and its derivative, DECtape) were variations on this "round tape." They were essentially a personal storage medium. They featured a fixed formatting track which, unlike standard tape, made it feasible to read and rewrite blocks repeatedly in place. LINCtapes and DECtapes had similar capacity and data transfer rate to the diskettes that displaced them, but their "seek times" were on the order of thirty seconds to a minute.

Most modern magnetic tape systems use reels that are reels are much smaller and are fixed inside a cartridge to protect the tape and facilitate handling. Cartridge formats include QIC, DAT, and Exabyte.

cartridge tapes in drives

Tape is read and written on a tape drive (or "deck") which winds the tape from one reel to the other causing it to move past a read/write head. Early tape had seven parallel tracks of data along the length of the tape allowing six bit characters plus parity written across the tape. A typical recording density was 556 characters per inch. The tape had reflective marks near its end which signaled beginning of tape (BOT) and end of tape (EOT) to the hardware.

Data is typically written to tape in blocks with inter-block gaps between them. Each block is typically written in a single operation with the tape running continuously during the write. There is a complex tradeoff between block size, the size of the data buffer in the record/playback deck, the percentage of tape lost on inter-block gaps, and read/write throughput.

Tape has quite a long data latency for random accesses since the deck must wind an average of 1/3 the tape length to move from one arbitrary data block to another. Tape remains a viable alternative to disk due to its higher bit density and cost per bit. Tape has historically offered enough advantage in these two areas above disk stroage to make it a viable product. The recent vigorous innovation in disk storage density and price, coupled with less-vigorous innovation in tape storage, has reduced the viability of tape storage products.

Most tape systems attempt to alleviate the intrinsic long latency using eitheer indexing, whereby a separate lookup table is maintained which gives the physical tape location for a given data block number, or marking, whereby a tape mark that can be detected while winding the tape at high speed.

Most tape drives now include some kind of data compression. There are several algorithms which provide similar results: LZ (Most), IDRC (Exabyte), ALDC (IBM, QIC) and DLZ1 (DLT). The actual compression algorithms used are not the most effective known today, and better results can usually be obtained by turning off the compression built into the device and using a software compression program instead.

see also: cut a tape, flap, Group code recording, spool, macrotape, microtape, Non Return to Zero Inverted, Phase encoded, Tape Drive, Error Correction, Helical scan, print-through