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APPENDIX 5: Glossary

Magnetic Tape Storage and Handling

 

 

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Magnetic Tape Storage and Handling

A Guide for Libraries and Archives

Dr. John W.C. Van Bogart
National Media Laboratory
June 1995

Table of Contents | Glossary



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Access storage: Storage conditions at or near room ambient conditions that allow tape collections to be readily accessed for immediate playback.

AES: Abbreviation for Audio Engineering Society.

Analog recording: A recording in which continuous magnetic signals are written to the tape that are representations of the voltage signals coming from the recording microphone or the video camera.

Analog-to-digital: The process in which a continuous analog signal is quantized and converted to a series of binary integers.

ANSI: Abbreviation for American National Standards Institute.

Archival storage: Storage conditions specifically designed to extend or maximize the lifetime of stored media. Generally involves the use of temperatures and humidities lower that access storage conditions. Temperatures and humidities are also tightly controlled within a narrow range, and access by personnel is limited.

Backing: See substrate.

Binary number: A number that can be represented using only two numeric symbols - 0 and 1. A number in base 2.

 

Decimal Number Binary Equivalent
0 0
1 1
2 10
4 100
12 1100
100 1100100
1995 11111001011

Binary numbers are used by computers because they can easily be represented and stored by device hardware that utilizes switches, magnetic fields, or charge polarities that are normally in one of two states. The on or off, north or south, or positive or negative states can easily represent the 1s and 0s of a binary number, respectively.

Binder: The polymer used to bind magnetic particles together and adhere them to the tape substrate. Generally, a polyester or polyether polyurethane based system. See polymer.

Bit: A single numeric character. Each bit of a binary number can either be 0 or 1. An n-bit number is composed of exactly n numeric characters. An n-bit binary number can have 2n distinct values. For example, an 8-bit binary number has 28 = 256 distinct values, namely all the numbers between 00000000 (0 in decimal) and 11111111 (255 in decimal), inclusive. 8-bit quantization would discretely sample a signal and assign each sampling a value between 0 and 255, permitting 256 possible values.

Blocking: The sticking together or adhesion of successive windings in a tape pack. Blocking can result from (1) deterioration of the binder, (2) storage of tape reels at high temperatures, and/or (3) excessive tape pack stresses.

Cinching

: The wrinkling, or folding over, of tape on itself in a loose tape pack. Normally occurs when a loose tape pack is stopped suddenly, causing outer tape layers to slip past inner layers, which in turn causes a buckling of tape in the region of slip. Results in large dropouts or high error rates.

 

Coercivity: The level of demagnetizing force that would need to be applied to a tape or magnetic particle to reduce the remanent magnetization to zero. A demagnetizing field of a level in excess of the coercivity must be applied to a magnetic particle in order to coerce it to change the direction of its magnetization. Coercivity is the property of a tape that indicates its resistance to demagnetization and determines the maximum signal frequency that can be recorded by a tape. Hc is the common abbreviation for coercivity.

Cohesive force: The force that holds a material together. The force that holds a material to itself.

Cohesiveness: See cohesive force.

Curvature error: A change in track shape that results in a bowed or S-shaped track. This becomes a problem if the playback head is not able to follow the track closely enough to capture the information.

dB: See decibel.

Decibel: The unit of measure used to indicate relative changes in signal intensity or sound volume. The actual expression for calculating the difference in decibels between signal A and signal B is:

decibel (dB) = 20·logbase10 (signal A amplitude/signal B amplitude)

+6 dB represents a doubling of the signal or a 100% increase
+5 dB represents a 78% increase
+4 dB represents a 58% increase
+3 dB represents a 41% increase
+2 dB represents a 26% increase
+1 dB represents a 12% increase
+0 dB represents no change-signals are equal
-1 dB represents a 11% decrease
-2 dB represents a 21% decrease
-3 dB represents a 29% decrease
-4 dB represents a 37% decrease
-5 dB represents a 44% decrease
-6 dB represents a halving of the signal or a 50% decrease

Digital recording: A recording in which binary numbers are written to the tape that represent quantized versions of the voltage signals from the recording microphone or the video camera. On playback, the numbers are read and processed by a digital-to-analog converter to produce an analog output signal.

Digital-to-analog: The process in which a series of discrete binary integers is converted to a continuous analog signal.

Dropout: Brief signal loss caused by a tape head clog, defect in the tape, debris, or other feature that causes an increase in the head-to-tape spacing. A dropout can also be caused by missing magnetic material. A video dropout generally appears as a white spot or streak on the video monitor. When several video dropouts occur per frame, the TV monitor will appear snowy. The frequent appearance of dropouts on playback is an indication that the tape or recorder is contaminated with debris and/or that the tape binder is deteriorating.

Flange pack: A condition where the tape pack is wound up against one of the flanges of the tape reel.

Format: The arrangement of information tracks on a tape as prescribed by a standard. The two most common categories of recording formats are longitudinal and helical scan.

Head clog: Debris trapped in the playback head of a video recorder. Clogging of the playback head with debris causes dropouts.

Helical scan recording: The recording format in which a slow moving tape is helically wrapped 180° around a rapidly rotating drum with a small embedded record head. The tape is positioned at a slight angle to the equatorial plane of the drum. This results in a recording format in which recorded tracks run diagonally across the tape from one edge to the other. Recorded tracks are parallel to each other but are at an angle to the edge of the tape.

Hydrolysis: The chemical process in which scission of a chemical bond occurs via reaction with water. The polyester chemical bonds in tape binder polymers are subject to hydrolysis, producing alcohol and acid end groups. Hydrolysis is a reversible reaction, meaning that the alcohol and acid groups can react with each other to produce a polyester bond and water as a by-product. In practice, however, a severely degraded tape binder layer will never fully reconstruct back to its original integrity when placed in a very low-humidity environment.

Hygroscopic: The tendency of a material to absorb water. An effect related to changes in moisture content or relative humidity. The hygroscopic expansion coefficient of a tape refers to its change in length as it takes up water upon an increase in the ambient relative humidity.

Longitudinal recording: A recording format in which a slow or fast moving tape is passed by a stationary recording head. The recorded tracks are parallel to the edge of the tape and run the full length of the tape.

Lubricant: A component added to the magnetic layer of a tape to decrease the friction between the head and the tape.

Magnetic particles: The magnetic particles incorporated in the binder to form the magnetic layer on a magnetic tape. Iron oxide, chromium dioxide, barium ferrite, and metal particulate are various examples of magnetic pigment used in commercial tapes. The term pigment is a carry over of terminology from paint and coating technology - the magnetic coating on a tape is analogous to a coat of paint in which the magnetic particle is the paint pigment.

Magnetic pigment: See magnetic particles.

Magnetic remanence: The strength of the magnetic field that remains in a tape or magnetic particle after it is (1) exposed to a strong, external magnetic field and (2) the external field is removed. The property of a tape that determines its ability to record and store a magnetic signal. Mr is the common abbreviation for magnetic remanence. Magnetic remanence, Mr, and magnetic retentivity, Br, both refer to the ability of the tape to retain a magnetic field; however the latter is expressed in units of magnetic flux density.

Magnetic retentivity: See magnetic remanence.

Mistracking: The phenomenon that occurs when the path followed by the read head of the recorder does not correspond to the location of the recorded track on the magnetic tape. Mistracking can occur in both longitudinal and helical scan recording systems. The read head must capture a given percentage of the track in order to produce a playback signal. If the head is too far off the track, recorded information will not be played back.

NARA: The abbreviation for National Archives and Records Administration.

Pack slip: A lateral slip of selected tape windings causing high or low spots (when viewed with tape reel laying flat on one side) in an otherwise smooth tape pack. Pack slip can cause subsequent edge damage when the tape is played, as it will unwind unevenly and may make contact with the tape reel flange.

PET: Abbreviation for polyethylene terephthalate. The polymeric substrate material used for most magnetic tapes.

Polymer: A long organic molecule made up of small, repeating units (literally, many mers). Analogous to a freight train, where each individual unit is represented by a freight car. At very high magnification, a chunk of polymer would resemble a bowl of cooked spaghetti. Plastic materials are polymers. The strength and toughness of plastics is due, in part, to the length of its polymer molecules. If the chains (links in the freight train) are broken by hydrolysis, the shorter chains will impart less strength to the plastic. If enough polymer chains are broken, the plastic will become weak, powdery, or gooey. See binder.

Popped strand: A strand of tape protruding from the edge of a wound tape pack. Print through: The condition where low frequency signals on one tape winding imprint themselves on the immediately adjacent tape windings. It is most noticeable on audio tapes where a ghost of the recording can be heard slightly before the playback of the actual recording.

Quantization: A process in which a continuous signal is converted to a series of points at discrete levels. The quantized version of a ramp, a continuum of levels, would be a staircase, where only certain distinct levels are allowed.

Refreshing: This term can refer to periodic retensioning of tape, or the rerecording of recorded information onto the same tape (or different tape) to refresh the magnetic signal. In the audio/video tape community, refreshing generally refers to retensioning of the tape, but it can also refer to the copying of one tape to another. See transcription.

Relative humidity (RH): The amount of water in the air relative to the maximum amount of water that the air can hold at a given temperature.

Restoration: The process where a tape degraded by age is temporarily or permanently restored to a playable condition. The tape backing procedure is an example of a tape restoration procedure.

Retensioning: The process where a tape is unspooled onto a take-up reel and then rewound at a controlled tension and speed. In performing this procedure, tape pack stresses are redistributed and, thus, the tape is retensioned. This has sometimes been referred to as refreshing (or exercising the tape).

RH: The abbreviation for relative humidity.

Room ambient conditions: The temperature, humidity, and air quality of the surrounding conditions. Those conditions generally found in a library, resource, studio, or office facility with a controlled environment (heating and air conditioning), which should range between 66 to 78° F (19 To 26° C) and 30 to 70% relative humidity year round. Analogous to room temperature conditions, except that this term only refers to the temperature of the room.

Scission: The process in which a chemical bond in a molecule is broken either by reaction with another molecule, such as water, or by the absorption of a high energy photon.

Signal-to-noise ratio: The ratio of the recorded signal level to the tape noise level normally expressed in decibels. Commonly abbreviated as S/N.

SMPTE: Abbreviation for the Society of Motion Pictures and Television Engineers.

Stick slip: The process in which (1) the tape sticks to the recording head because of high friction; (2) the tape tension builds because the tape is not moving at the head; (3) the tape tension reaches a critical level, causing the tape to release from and briefly slip past the read head at high speed; (4) the tape slows to normal speed and once again sticks to the recording head; (5) this process is repeated indefinitely. Characterized by jittery movement of the tape in the transport and/or audible squealing of the tape.

Sticky shed: The gummy deposits left on tape path guides and heads after a sticky tape has been played. The phenomenon whereby a tape binder has deteriorated to such a degree that it lacks sufficient cohesive strength so that the magnetic coating sheds on playback. The shedding of particles by the tape as a result of binder deterioration that causes dropouts on VHS tapes.

Sticky tape: Tape characterized by a soft, gummy, or tacky tape surface. Tape that has experienced a significant level of hydrolysis so that the magnetic coating is softer than normal. Tape characterized by resinous or oily deposits on the surface of the magnetic tape.

Stress: Force per unit area, such as pounds per square inch (psi). A tape wound on a reel with high tension results in a tape pack with a high interwinding stress. See tension.

Substrate: Backing film layer that supports the magnetic layer in a magnetic tape. PET is currently the most commonly used tape substrate.

Tape baking: A process in which a magnetic tape is placed at an elevated temperature for a brief time in order to firm up the tape binder. This procedure is recommended as a temporary cure for the sticky shed or sticky tape syndrome. The tape baking procedure is discussed in the reference, "Sticky Shed Syndrome - Tips on Saving Your Damaged Master Tapes," Mix, May 1990, p. 148.

Tape noise: A magnetic signal on the tape resulting from the finite size and nonuniform distribution of magnetic particles in the magnetic layer of the tape. Tape noise is inherent in any magnetic tape but can be reduced by using smaller pigment sizes in tape formulations. The iron oxide pigments found in less expensive tapes have the largest tape noise level. Ranked in size: iron oxide > chromium dioxide > metal particulate > barium ferrite. Therefore, ranked in order of tape noise: iron oxide > chromium dioxide > metal particulate > barium ferrite.

Tape pack: The structure formed by and comprised solely of tape wound on a hub or spindle; a tape reel consists of a tape pack, the metal, plastic, or glass hub, and flanges. Tape transport: The mechanics used to guide and move the tape through the recording system and past the read and write heads of the recorder. The tape transport consists of the tape guide pins, capstan, rollers, tension controllers, etc.

Tension: Force, or force per tape width. The force on a tape as it is transported through a recorder. A tape wound on a reel with high tension results in a tape pack with a high interwinding stress. See stress.

Thermal: An effect related to changes in temperature. The thermal expansion coefficient of a tape refers to its change in length upon a change in the ambient temperature.

Track angle: The angle that the track of a helical scan recording makes to the edge of the tape. This should correspond with the scan angle of the helical recorder - the angle that the tape makes to the equatorial plane of the rotating drum head. If the track angle and scan angle do not correspond, mistracking will occur.

Transcription: The process of copying all of the information on one tape to another tape of the same or different format. The term refreshing is commonly used by some archivists and librarians to refer to the process of copying information from one tape to a newer tape of the same format (e.g., VHS to VHS). When the information is copied to a different format (e.g., BetaMax to VHS), the terms reformatting and converting have been used.

Trapezoidal error: A change in the angle of a recorded helical scan track. Can result in mistracking.

Vinegar syndrome: Characteristic of the decomposition of acetate based magnetic tape where acetic acid is a substantial by-product that gives the tape a vinegar-like odor. After the onset of the vinegar syndrome, acetate tape backings degrade at an accelerated rate - the hydrolysis of the acetate is catalyzed further by the presence of acetic acid by product.

 


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