The following is a glossary of terms developed to assist members of the BAVC Video Preservation Round Table working group who met in Minneapolis, Minnesota during 1995-1996 to interpret and explore the topic
Analysis and Evaluation Procedures: 1) What is the chemical and physical structure of videotape; 2) What are the causes and characteristics of the common methods of deterioration; and 3) General issues of permanence and durability of videotape.
This document is an attempt to begin a standardization of the terminology used to identify the characteristics of deterioration seen in magnetic videotape. This lexicon could be of use to: video engineers, video catalogers, T.V. and video editors, preservationists, curators, artists, producers, as well as other professionals who work with videotape.
Terminology was derived from scholarly and scientific literature, and from pre-existing glossaries dealing with videotape issues. The following books, brochures, and articles were consulted:
Boyle, Deirdre. Video Preservation: Securing the Future of the Past. New York, New York: Media Alliance, 1993. 41-46.
Eilers, Delos A., Senior Technical Service Specialist, 3M Audio/Video Technology Division. "Audio Magnetic Tape Preservation and Restoration," paper presented at Audio Engineering Society Convention Workshop, March 26, 1992, Vienna, Austria. 7-8.
Meyer, Ronald H., VHS Video Engineer, 3M. Sights and Sounds: St. Paul, MN: 3M Audio/Video Technology Division, 1989. 8-12.
3M Company "Videocassette Tape Physical Damage," Retentivity (date unknown). Magnetic Audio/Video Products Division/3M. 2.
Van Bogart, Dr. John W.C. Magnetic Tape Storage and Handling: A Guide for Libraries and Archives. Washington, DC: The Commission on Preservation and Access and St. Paul, MN: National Media Laboratory, 1995. 30-34.
Perhaps this glossary, which should be considered a working document, can help take us a step towards the implementation of a system of centralized information which utilizes the terms that describe what we see (or don't see) when viewing a tape on our video monitor.©1996 Rebecca Bachman, Walker Art Center Archives
The sticking together or adhesion of successive windings in a tape pack. Blocking can result from deterioration of the binder, storage of tape reels at high temperatures, and/or excessive tape pack stresses.
Wrinkles or creases pressed into the tape by the capstan/pinch roller assembly.
Produces pastel, washed out color.
Produces heavy, saturated colors.
Colors appear to be moving on screen. In color areas of picture, usually most noticeable in highly saturated reds.
Interlayer slippage or magnetic tape in roll form, resulting in buckling of some strands of tape. The tape will in many cases fold over itself causing permanent vertical creases in the tape. Also, if not fixed, will cause increased dropouts. See windowing.
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.
A synchronizing signal on the edge of the videotape which provides a reference for tracking control and tape speed. Control tracks which are heavy dropouts or which are improperly recorded may cause tracking errors or picture jumps.
A tape deformity which may cause horizontal or vertical lines in the playback picture. See wrinkle.
An undesired signal interfering with the desired signal. Can result in several types of picture distortion, mistracking and/or noisy picture.
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.
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.
A wave which has been reflected at one or more points in the transmission medium. Echoes may be leading or lagging the primary signal and appear in the picture monitor as reflections or double images commonly called "ghosts."
Usually occurs on the outside one-sixteenth inch of the video tape. If the tape is sufficiently deformed it will not make proper tape contact with the playback heads. An upper curl (audio edge) crease may affect sound quality. A lower curl (control track) may result in poor picture quality.
Physical distortion of the top or bottom edge of the magnetic tape, usually caused by pack problems such as popped strands or stepping. Affects audio and control track sometimes preventing playback.
A horizontal displacement of upper portion of a picture. Also called skewing.
Very short rapid variations in tape speed which may result in a jumpy or jittery picture.
Tape that has folded over resulting in the oxide surface facing away from the heads.
Degradation caused by tape copying.
A form of low frequency interference, appearing as a narrow horizontal bar moving vertically through the picture.
The accumulation of debris on one or more heads usually causing poor picture clarity during playback. Clogging of the playback head with debris causes dropouts.
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.
This is the portion of the video signal which contains the black and white information. Luminance indicates the amount of light intensity in a picture which is perceived by the eye as brightness.
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, record information will not be played back.
Any unwanted signal present in the total signal.
A lateral slip of select 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.
Any distortion of the magnetic tape which prevents proper head-to-tape contact and is therefore detrimental to the tape playback. These distortions can include edge damage, wrinkles, cinches, and tape stretch.
A loss of magnetization and thus a degradation of recorded information caused by repeated playing of a recorded tape.
A strand of tape protruding from the edge of a wound tape pack.
The condition where low frequency signals on one tape winding imprint themselves on the immediately adjacent tape windings. It is most noticeable on audio recordings where a ghost of the recording can be heard slightly before the playback of the actual recording. Print through is worse heading outward through a tape pack.
A lack of vertical synchronization which causes the video picture to move upward or downward.
Gouging of the magnetic layer or base as the tape passes through a machine. Video tape scratches will cause a loss of head-to-tape contact and appear as a solid line on your screen.
A condition in which the oxide, which forms the recording surface of a videotape, has begun to separate from the base. Loose oxide may clog video heads causing a loss of picture.
Expressed in decibels (dBs), this term describes a ratio or difference of wanted audible or visual information (signal) versus unwanted information experienced by distorted sounds and pictures (noise). Comparatively high decibel numbers mean better sound or visual images.
A bending of picture at top or bottom of television screen caused by the changing of the video track angles on the tape from the time of recording to the time of playback. This can occur as a result of poor tension regulation by the VCR or by ambient conditions which affect the tape.
White flashes appearing in the video image caused by random noise and/or loss of magnetic particles.
Can be caused by a build-up of debris on a guide or head. Sometimes a cleaning of the offending surface will eliminate the squeal. Squeal is also caused by the tape having poor lubrication or losing its lubrication with age. A solution is to overcoat a tape with a lubricant solution, which will eliminate the squeal so a copy can be made.
Unsmooth packing, with transversally mispositioned sections.
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.
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.
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.
A variation in the synchronizing signals. When time base errors are large enough, they may cause skewing or flagging distortion of the video picture.
Evidenced by picture breakup or loss of video in segments of the picture.
A change in the angle of a recorded helical scan track. Can result in mistracking.
An indication of the amount of noise in a black and white picture.
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.
Interlayer slippage or magnetic tape in roll form, resulting in buckling of some strands of tape. The tape will in many cases fold over itself causing permanent vertical creases in the tape. Also, if not fixed, will cause increased dropouts. See cinching.
A physical deformity of the videotape. Any creases or wrinkle in the videotape may produce dropouts or loss of picture information upon playback. See creasing.