Archival Formats: Glossary of Terms
Terms Used in the Preservation of Audio, Video and Motion Picture Film
Access Copy: Also called distribution or reference copy. An access copy provides easy access or review of content compared to the less accessible preservation or intermediate copy. The access copy is usually low resolution and made available online. An access copy should be a widely supported format that is easy to play back. See the Distribution Copy page on NARA’s Products and Services website for more information about access copies.
Acetate Film: Also called "safety film" because it was created as a non-flammable alternative to nitrate film stock. Types have included 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 largely replaced by polyester for use in duplication and printing in the late 20th century, but is still used today as the base for most in-camera stocks.
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.
Analog: Common examples of analog media include some video tapes like VHS, some audio tapes like cassettes, and grooved media like vinyl records. In the context of “analog vs digital” when referring to dynamic media, an analog signal is encoded in a medium that is infinitely variable (i.e. a continuous signal), as opposed to the data carried in a digital signal (i.e. a discrete signal). A media format that carries an analog signal is said to be an analog format.
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.
Base (film): The plastic layer to which the emulsion and/or magnetized coating is applied. Since the 1890s, motion picture film bases have changed from nitrate to acetate to polyester.
Base (magnetic media): The layer that provides the support needed to carry the binder and magnetic coating. Audio and video tape bases have included paper (audio only), acetate, PVC and polyester.
Bit Rate: The amount of sample data in a digital signal that is collected or streamed per unit of time. Usually expressed as ‘bits per second’ (bps) or ‘Megabits per second’ (Mbps), a dynamic media file’s bit rate is the product of multiplying the sample rate by the bit depth of the samples
Bit Depth: The number of data bits used for each individual sample. The amount of bits determines the number of discrete levels within which it is possible to place a sample. 8-10 bit is common for video and 16-24 bit is common for audio.
Brittleness: The result of a critical loss of plasticizer in a film, usually accompanied by visible warping. Characterized by the increased fragility of a film, and in severe cases by the inability to wind through a film without it fragmenting and crumbling.
Buckle: A warp pattern that is the result of a loss of moisture from the edges of the film, so that the edges of the film shrink relative to the center. Buckle pattern makes it appear that the center of the film “flattens out” into sections rather than maintaining a smooth curve while being wound through.
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).
Channeling: See crazing
Checksum: A relatively small code that is derived from a much larger code by a checksum algorithm for the purpose of verifying that the larger code (usually the data which constitutes a file) remains unchanged during transfer or long-term storage. Any change or corruption of the file can be discerned by comparing the checksum produced prior to a file’s transfer or storage with a second checksum produced later by the same algorithm; if the second checksum does not match the first, then it can be surmised that the file has changed during the intermediate time.
Color Fading: The result of chemical instability in the dye layers of color films. As one or more of the dye layers fade, the others become more prominent, causing a color shift towards the more stable dye. Infamously, the rapid fading of cyan dye in many print and negative stocks from the latter half of the 20th century produces a magenta appearance. Some reversal stocks are also subject to color fade, often producing a purplish or cool appearance.
Composite Video: Composite video combines all video and audio 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, allowing for superior 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 video. Below is a list of some of the more common composite videotape formats:
- Video 2000
- U-matic 3/4"
- 1/4" CVC
- 1/2" EIAJ
- 1" Type A, B, and C
- 2" Quadruplex videotape
- 2" Helical Scan (IVC)
Compression (digital): A method used to reduce the size of digital files or streams of data. Compression is used to 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.
Conservation: Actions taken to ensure the stability and physical survival of moving image and sound recordings. These include appropriate environmental conditions, rehousing, repair, etc. to prevent damage, decay, or loss. This definition may be different for other specialties.
Core: A small cylindrical hub with no flanges, around which film can be wound as an alternative to a film reel.
Data CD/DVD: See CD/DVD/BD-ROM.
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.
Digital: In the context of “analog vs digital” when referring to dynamic media, a digital signal contains data which has been encoded as a series of bits (i.e. a discrete signal), rather than a continuous or infinitely variable signal which characterizes analog media. A media format that carries a digital signal is said to be a digital format.
Digital Preservation: Duplication of a deteriorating piece of media by digital means, which entails using a digitization device to sample the visual and audio components of the media and a computer to record the data into files. This is done for the purpose of creating a more stable digital copy of the original source. The term might also be used to refer to the policies, strategies, and actions such as reformatting and migrating of digital files or content to ensure access over time.
Edge Markings: Markings recorded along the edge of a film often outside the perforations, and usually recorded at the factory where the film stock was produced. Edge markings can contain significant information related to a film stock’s production that may or may not be human-readable: manufacturer, brand, date of manufacture, country of manufacture, manufacturing facility and production line, footage count, etc.
Element: A general term used to refer to any film object created during the production lifecycle of a film, defined by qualities such as whether it is picture or sound, negative or positive, etc.
Emulsion: The photosensitive coating on film that contains the images, optical soundtrack, and other information. Modern color film stocks contain multiple dye layers which, in combination, create full-color images, while black and white emulsions contain silver halide crystals which create images through varying density across the surface of the film.
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.
Generation (film): Refers to the order of duplication of film elements. A copy of a copy of an original negative is said to be a “third generation” element. Multiple film elements can be part of the same generation, e.g. if the same camera negative is used to produce two second-generation intermediate positives.
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. Depending 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 traveling 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 high quality copy of the original element. It is used for making derivative files, improving access, and decreasing the use of preservation copies. See the Reproduction Master page on NARA’s Products and Services website for more information about intermediate copies.
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, depending 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: Any film element in which light and color values are reversed from how the recorded content looks in reality. In color film, a negative has colors complementary to real life. Films are typically shot on negative stock and then printed to create a positive, or print, element. Printing a negative to a non-reversal film stock will always produce a positive. Black and white negatives appear gray or purplish, and color negatives have an overall orange hue due to color masking built into the stock which is advantageous to accurate printing.
Nitrate Film: The earliest film base. Nitrate film is highly flammable when stored in improper conditions and was replaced by acetate stocks by 1951 in the United States.
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. For locations where NTSC systems are used, please see this map.
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: Also called sprocket holes, perforations are the holes in the film that allow for mechanical transport. The number, shape, arrangement, and placement of perforations are different depending on the film gauge and stock type.
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, formalized 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. In essence, the PAL standard specifies line count, image refresh frequency, synchronization, modulation schemes, and composite signal encoding. For locations where PAL systems are used, please see this map.
Photochemical Preservation: Duplication of a deteriorating film by photographic means, which entails developing and fixing the image using traditional photochemical processes, for the purpose of creating a more stable film element.
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 as well in new film production. 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: Any film element which depicts the recorded content as it appears in reality, i.e. without its polarity being inverted (as in a negative). Printing a positive to a non-reversal stock will always produce a negative.
Preservation: Actions taken to reproduce moving image and sound recordings to create a new preservation copy that remains as close to the original intent as possible. This may be a new analog copy or a digital copy that captures the inherent qualities of the source object. This definition may be different for other specialties.
Preservation Copy: A preservation copy is the most original copy and/or 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 the Preservation Master page on NARA’s Products and Services website for more information about preservation copies.
Print: A positive film which has been produced for viewing purposes. As the name suggests, a print film will have been printed from a previous element, so while a camera-original reversal film is viewable, for example, it cannot be called a print. The term “print” may be used as shorthand to refer to a positive copy of a film meant for circulation and projection.
Projection Reel: An object onto which film can be wound for use in a projection system. Reels feature flanges which prevent the film from unspooling. Reels are not ideal for long-term storage and should be replaced by cores when not needed for projection.
Reformatting: A process in which a copy or derivative is created from the original media. This may be done for preservation or access purposes.
Relative Humidity (RH): The amount of water vapor present in the air, expressed as a percentage of the amount needed for saturation at the same temperature. If RH is 100% at a given temperature, any additional moisture added will result in condensation.
Resolution: The measure of how well audio, video, or film can faithfully depict images or sound. For digital still images and the individual images in video, picture cell (pixel) density and bit depth are the units of measure. Pixel density is measured spatially by the number of sample points across and down an image. A common image spatial resolution is 1920x1080 used for HD videos, meaning the image is 1,920 pixels wide and 1,080 pixels tall. Each of those sample points has a number of channels, and a bit depth. The number of channels ranges from one for monochrome to multiple for color. In addition, video is a sequence of still images, and that temporal resolution is measured basically in images per second. Common sampling rates include 24, 29.97, and 60 images per second. For sound, sampling rate and bit depth are the units of measure. A common audio sampling rate is 44.1kHz used for audio CDs, meaning a single second of the audio consists of 44,100 individual samples, and the bit depth for audio CDs is 16 bits per sample. Note that there are other complicating factors, such as the use of lossy compression in both images and sound.
Restoration: The actions taken to produce a version of a piece of media that is as close to the ideal original as possible. In the context of film for example, these actions may include repair, dirt removal, scratch removal, stabilization, color correction, reconstruction of lost content, and reformatting. The related terms “preservation” and “conservation” concern the actions taken to assure that a piece of media will continue to exist in its current form, whereas restoration implies intervention to alter the media to a different state, ideally one which is faithful to its original form. Restoration can simply be understood as an effort to reverse degradation. For further reading, please see Is Restoration the Same as Preservation?
Reversal Film: Reversal film is a type of film that, when processed, appears to be the same polarity (i.e. positive or negative) as whatever it was used to capture. When used in a camera, reversal stock always develops into a positive; when used in film printing, reversal stock develops into a direct copy of whatever other element it was exposed to, either positive or negative. A telltale sign of whether a film stock is reversal is the presence of dense black perf areas, which would normally look clear on a non-reversal stock. The most notable reversal stock brand is Kodachrome, produced by Kodak primarily for amateur motion picture photography.
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. For locations where SECAM systems are used, please see this map.
Shrinkage: The difference between the present dimensions of a film and the dimensions of that film when it was produced, given as a percentage of the original dimensions. Shrinkage of cellulose nitrate and cellulose acetate film bases is inevitable and is caused by chemical off-gassing. Accelerated shrinkage of a film can be a symptom of vinegar syndrome.
Soundtrack: The sound that accompanies a film. Soundtracks can be recorded onto a different film element than the corresponding image content (usually production or printing elements) or recorded directly on the same film alongside the image content (in the case of release prints and some camera originals). Film elements with image and sound are known as composite films. 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. Warped film and tape may exhibit spoking when wound up. When examining a roll of film or tape from the side of the pack, spoking is identifiable by the presence of 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 bases. 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 shrink, become brittle, and be difficult to handle and play.
Warp: A physical imperfection in a piece of film or tape in which the media no longer looks “flat” but instead looks “wavy” or “curled.” Warp is caused by chemical off-gassing that is inconsistent across the whole piece of media, resulting in inconsistent shrinkage of different regions of the media and creating various warp patterns. See the definitions for Buckle and Spoking for more information.