Preservation

Archival Formats: Glossary of Terms

Terms Used in the Preservation of Audio, Video and Motion Picture Film

Main Table of Contents for Audio, Video, and Motion Picture Film:

Access Copy: Also called distribution or reference copy. An access copy should provide easy access or review of the content of a less accessible preservation or intermediate copy. The access copy is often relatively low resolution and made available online. An access copy should be a widely supported format that is easy to playback. See Additional Description and NARA's Internal Specifications.

Acetate Film: Also called "safety film" because it was created as a non-flammable alternative to nitrate film stock. Includes cellulose diacetate, cellulose butyrate, cellulose propionate, and cellulose triacetate. Cellulose triacetate has been standard since the 1950s and is usually referred to as simply "acetate". All acetate film is susceptible to vinegar syndrome. Acetate base film was replaced by polyester.

Acetate Magnetic Tape Base: Cellulose acetate was used as a tape substrate (base) in many early magnetic tapes from the 1940s until the mid 1960s. Acetate is susceptible to vinegar syndrome. Acetate magnetic tape bases were replaced by polyester.

Authored DVD: An authored DVD uses a specific file and folder structure to organize audio, video, navigation and backup information. You will see files with various extensions on an authored DVD. These include: .VOB, .IFO and .BUP.

Bit Rate: The amount of sample data that is collected per unit of time. This is usually expressed as ‘bits per second’ (bps) or ‘Megabits per second’ (Mbps), and is the result of sampling rate multiplied by the number of data bits per sample (or bit depth), plus any additional data such as tracking information.

Bit Depth: The number of data bits used for each individual sample. The amount of bits determines the number of discrete levels possible to place a sample within. 8-10 bit is common for video; 16-24 bit is common for audio.

CD-DA: Compact Disc Digital Audio is an audio-only format on CD. Discs are playable on standard compact disc players and software players on a computer.

CD/DVD/BD-R: R indicates that the CD (Compact Disc), DVD (Digital Versatile Disc), or BD (Blu-Ray Disc) is recordable. A few different format variations exist for recordable discs such as +R and -R. If marked with only an R, the disc is write once, meaning that data cannot be erased once written. There are also formats capable of erasing and re-writing data, indicated by RW for re-writable or RE for recordable erasable. Aside from some prerecorded groove and recording parameter data, recordable discs are nearly blank slates capable of being formatted for digital audio, video, or computer data. An exception is the pre-formatted CD-DA recordable disc.

CD/DVD/BD-ROM: ROM is an abbreviation for Read Only Memory. It is a standard for formatting discs to store any type of computer readable data without regard to a specified structure. They are readable only (hence the name) unless discs are writable (RW) or erasable (RE).

Checksum: A small set of data computed through an algorithm with the intent of detecting errors in data files or blocks introduced through storage or transfer. The checksum data accompanies or is otherwise associated with the data files or blocks, and is used to help ensure data integrity.

Color Fade: The result of chemical instability in color print stocks that leads to magenta prints. In these cases, the yellow and cyan layers have faded, leaving magenta as the only prominent color.

Composite Video: Composite video combines all video and synchronization information into one signal. It was developed as a practical way of broadcasting video. It also allowed video to be transferred via a single wire, and was later adopted as a way to record video onto tape. Beginning with Betacam, professional videotape formats successively replaced recording composite video and instead recorded video as separate components. The advantage being increased quality. Consumer videotape formats continued using composite because of the great advantage of directly recording analog broadcasts and the low cost of manufacturing machines. Later as digital broadcasting and data recording proliferated, composite video gave way to component. Below is a list of some of the more common composite videotape formats:

  • VHS
  • S-VHS
  • VHS-C
  • Betamax
  • Video 2000
  • Video8
  • Hi8
  • U-matic 3/4"
  • 1/4" CVC
  • 1/2" EIAJ
  • 1" Type A, B, and C
  • 2" Quadruplex videotape
  • 2" Helical Scan (IVC)

Compression: Use of a method to reduce the size of digital files or streams of data, also referred to as data reduction. Compression is used to either save data storage space or better enable movement over networks or transmission lines. There are many different techniques to compress data, but all fall into one of two overall categories:

  • Lossless: Any data removed can be reconstructed.
  • Lossy: Some or all of the data removed is discarded and gone forever.

Data CD/DVD: See CD/DVD/BD-ROM.

Density: When referring to film, the density is the measure of the blackness of the image. Density is affected by both exposure and processing time.

Diacetate: An early form of acetate film stock, diacetate is easily identifiable because it smells like camphor, or moth balls. Diacetate was used for non-theatrical and amateur films in the 1920s and 1930s. It was replaced by cellulose triacetate and other cellulose acetate film stocks.

Emulsion: The photosensitive coating on film that contains the images and other information.

Film Base: The plastic layer that provides the support needed to carry the images, sound, and any other information. Since the 1890s, motion picture film bases have changed from nitrate to acetate to polyester. Audio and video tape bases have included paper (audio only), acetate, PVC and polyester.

Film Leader: Clear or opaque film material located at the beginning and sometimes the end of a motion picture film. The leader protects the film and will often have identifying information written on it.

Gauge: A characteristic of film that refers to the width, perforations, and other physical features. Gauges range from 7.5mm to 105mm, although 8mm, 16mm and 35mm are most common.

Grooved Recordings: The surface of the media is altered by creating grooves that when read by a stylus, or needle, reproduce sound. Grooved recordings may include cylinders, discs, and belts. Dependent on the type of grooved recording encountered, specific playback equipment and styli may be needed to playback correctly.

Inches per Second: Refers to the length of tape travelling past a read or write head during playback or recording. Inches per second is a valid metric for all magnetic tape and machines (both audio and video), but is normally indicated only for variable speed machines such as open reel audio. Abbreviations include ips, in/s, and in/sec. Higher speeds facilitate a broader range of frequencies able to be recorded on a tape, which generally means more fidelity.

Instantaneous Recording: Also referred to as acetates or lacquers. These discs could be cut and played back instantaneously, unlike other types of discs that needed processing before playback. Became common in the 1930s for both the broadcast and commercial industries and gained popularity up until the introduction of magnetic tape. A lacquer coating is affixed to the base which is usually aluminum, although you may encounter glass or cardboard bases as well.

Intermediate Copy: Also called reproduction master. An intermediate copy is a good quality copy of the original recording. It is produced for making lower resolution derivative files, improving access, and decreasing the use of preservation copies. See Additional Description and NARA's Internal Specifications from the U.S. National Archives and Records Administration.

LaserDisc: A home video format introduced in 1978. The format was the first commercially available optical disc format. It had a number of technical advantages over home video tape systems such as VHS and Betamax, but could not record without expensive semi-professional machines, and had max 120 minutes of recorded material. Once DVD was introduced in 1995, the LaserDisc format started to become an obsolete format.

Magnetic Media: Media in various formats that use magnetic particles to store information. When particles are read by magnetic audio or video heads, the sound and/or images that have been previously recorded will be reproduced. Some examples include: ¼-inch open reel audio, wire recordings, 1-inch video and VHS tapes. Magnetic media may contain analog or digital information dependent on the format.

Magnetic Stripe: A magnetic soundtrack adhered to the side of motion picture film.

Metadata: The National Information Standards Organization (NISO) defines Metadata as “…structured information that describes, explains, locates, or otherwise makes it easier to retrieve, use, or manage an information resource. Metadata is often called data about data or information about information.”

Negative: A film element in which light and color values are reversed. In color film, a negative has colors complementary to real life. Films were usually shot on a negative and then printed to create a positive element. Black and white films appear gray or purplish, and color films have an overall orange hue.

Nitrate Film: The earliest film base. Nitrate film is highly flammable when stored in improper conditions and was replaced by acetate stocks by 1951.

NTSC: Initialism for National Television Standards Committee. In 1941, the U.S. committee developed a standard for encoding analog monochrome (black and white) video and audio television signals. Basically, the standard specifies line count, image refresh frequency, synchronization, modulation schemes, and composite signal encoding. The NTSC standard was augmented in 1953 to enable color television as well as monochrome. This is a map showing where SECAM, NTSC, and PAL systems are used.

Optical Discs: Optical discs are characterized by a substrate laminated with either a stamped reflective layer or a separate recording layer and reflective layer. The discs are read by emitting a focused laser light on the recorded information and sensing the reflection back.

Perforations: The holes in the film that allow for mechanical transport. Also called sprocket holes. The number, arrangement, and placement of perforations are different depending on the film gauge.

PAL: Acronym for Phase Alternating Line. A European standard for encoding analog color and monochrome (black and white) video and audio television signals. The standard, fixed in the early 1960s, took into account the weaknesses related to NTSC's ability to maintain color fidelity. It improved color stability and increased line resolution, but decreased image refresh frequency. Basically, the standard specifies line count, image refresh frequency, synchronization, modulation schemes, and composite signal encoding. This is a map showing where SECAM, NTSC, and PAL systems are used.

Photochemical Preservation: Preservation of a film by photographic means. Printing a new copy on new film stock and then developing and fixing the image as done using traditional photographic processes.

Playback Equipment: Equipment used to listen to and/or view the recorded signal of audio and video. For older formats, playback equipment may be obsolete and professional services may be needed to provide maintenance and repair.

Polyester Film: Also referred to by Kodak's brand name ESTAR, polyester film stocks were introduced in the 1950s and are used today for creating preservation elements. Polyester film stock is durable and is not susceptible to vinegar syndrome.

Polyester Magnetic Tape Base: A chemically stable substrate (base) for audio and video magnetic tape. Polyester bases are durable and not susceptible to vinegar syndrome; sticky shed may still occur, but this will depend on the binder composition. Became the norm for manufacturing by the 1970s.

Positive/Print: Motion picture positives/prints are the equivalent of a still photo print. When shining light through the film the image will appear as it would in real life if projected onto a movie screen. A positive or print is usually made from a negative.

Preservation Copy: A preservation copy is a high quality duplicate of the original record. If the original record deteriorates beyond use the preservation copy should be able to take its place. A preservation copy should last for years or decades - at least long enough to plan for making subsequent copies once these become outdated. See Additional Description and NARA's Internal Specifications from the U.S. National Archives and Records Administration.

Reformatting: A process in which a copy or derivative is created from the original media. This may be done for preservation or access purposes.

Resolution: The measure of how well audio, video, or film can faithfully portray images or sound. Picture cell (pixel) density and bit depth are the units of measure for individual images. Sampling rate and bit depth are the units of measure for moving images and sound.

Reversal Film: A film stock that has no negative, reversal film uses a chemical process so that the film shot in the camera is processed to a positive image. The most famous example would be Kodak's Kodachrome (known primarily as a home movie format), although other manufacturers also produced reversal film. Reversal film was produced in black and white and color formats.

RPM: An abbreviation for Revolutions Per Minute. RPM is a valid metric for all spinning discs or cylinders (both audio and video). It is normally indicated for the various types of grooved discs including Phonographs or Gramophones (often 78 RPM), long play discs (33 1/3 RPM), and singles (45 RPM).

Sampling Rate: The frequency at which information from the original recording is sampled or collected from a continuous signal. The rate is given in hertz (Hz) which equals cycles per second. One common example is the audio CD which has a sampling rate of 44,100 Hz, or 44.1 kHz.

SECAM: Acronym for Séquentiel couleur à mémoire, French for Sequential Color with Memory. A European developed standard for encoding analog color and monochrome (black and white) video and audio television signals. The standard, fixed in the mid 1960s, reduced vertical color resolution to more closely align with human visual color acuity. There are other significant technical differences from NTSC and PAL. One of the weaknesses of SECAM later turned out to be its difficulty to be accurately edited/mixed in studios. The standard specifies line count, image refresh frequency, synchronization, modulation schemes, and composite signal encoding. This is a map showing where SECAM, NTSC, and PAL systems are used.

Soundtrack: The sound that accompanies a film. Soundtracks can be separate (usually production or printing elements) or directly on the film (in the case of release prints and some camera originals). When on the film, a soundtrack can be optical, magnetic, or digital. Older films usually have optical soundtracks, which is a visual representation of the sound waves. Film prints that showed in commercial theaters from the 1990s onward likely have both optical and digital soundtracks.

Spoking: Condition of magnetic tape and motion picture film where excessive pressure caused by shrinkage or too much winding tension eventually causes deformation. It is identifiable by the pack of tape or film showing radial lines emanating out from the hub of the spool like spokes.

Sticky Shed Syndrome (SSS): Sticky shed is a problem unique to magnetic tape; it is caused by the chemical breakdown of the magnetic tape binder and/or backing layer of the tape. As the binder/backing absorb moisture from the surroundings, they become sticky and often shed brown residue to equipment. During playback the tape may squeal and bind, which can damage both tapes and equipment. This process is also referred to as binder hydrolysis.

Track Layout: Refers to the arrangement of recorded information on magnetic tapes. Tracks are either linear (running the length of the tape), or segmented either diagonally or perpendicularly across the tape. There are many variations within each technique, and some formats use a combination of techniques.

Unique Identifier (UID): An identification marking, tag, database entry or file name which guarantees that the same identification is not used elsewhere. UID is immensely important in archives, libraries, or anywhere else where an item must be located unambiguously.

Vinegar Syndrome (film): A deterioration process that affects acetate film base. Vinegar syndrome is identifiable by the vinegar smell that becomes evident as the acetate base breaks down and acetic acid is off-gassed. Films with vinegar syndrome will deteriorate physically, shrinking and losing flexibility. In final stages, films with vinegar syndrome can become fused and resemble a hockey puck.

Vinegar Syndrome (magnetic media): Characteristic of the breakdown of the acetate tape binding and/or backing layer, resulting in the smell of vinegar (acetic acid). The tape may become brittle and difficult to handle.

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