control track
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A control track is a track that runs along an outside edge of a standard
analog Analog or analogue may refer to: Computing and electronics * Analog signal, in which information is encoded in a continuous variable ** Analog device, an apparatus that operates on analog signals *** Analog electronics, circuits which use analog ...
videotape Videotape is magnetic tape used for storing video and usually sound in addition. Information stored can be in the form of either an analog or digital signal. Videotape is used in both video tape recorders (VTRs) and, more commonly, videocassett ...
(including VHS). The control track encodes a series of
pulse In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...
s, each pulse corresponding to the beginning of each
frame A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (con ...
. This allows the video tape player to synchronize its
scan Scan may refer to: Acronyms * Schedules for Clinical Assessment in Neuropsychiatry (SCAN), a psychiatric diagnostic tool developed by WHO * Shared Check Authorization Network (SCAN), a database of bad check writers and collection agency for bad ...
speed and tape speed to the speed of the recording. Thus, the recorded control track defines the speed of playback (e.g. SP, LP, EP, etc.), and it is also what drives the relative counter clock that most
VCR A videocassette recorder (VCR) or video recorder is an electromechanical device that records analog audio and analog video from broadcast television or other source on a removable, magnetic tape videocassette, and can play back the recording. ...
s have.


Significance and use

The control track is used to fine-tune the tape speed during playback, so that the high speed rotating heads remained exactly on their helical tracks rather than somewhere between two adjacent tracks (known as "
tracking Tracking may refer to: Science and technology Computing * Tracking, in computer graphics, in match moving (insertion of graphics into footage) * Tracking, composing music with music tracker software * Eye tracking, measuring the position of t ...
"). Since good tracking depends on precise distances between the rotating drum and the fixed control/audio head reading the linear tracks, which usually varies by a couple of micrometers between machines due to manufacturing tolerances, most VCRs offer tracking adjustment, either manual or automatic, to correct such mismatches. The control track is also used to hold ''index marks'', which were normally written at the beginning of each recording session, and can be found using the VCR's ''index search'' function: this will fast-wind forward or backward to the ''n''th specified index mark, and resume playback from there. At times, higher-end VCRs provided functions for the user to manually add and remove these marks — so that, for example, they coincide with the actual start of the
television program Television, sometimes shortened to TV, is a telecommunication medium for transmitting moving images and sound. The term can refer to a television set, or the medium of television transmission. Television is a mass medium for advertising, e ...
— but this feature later became hard to find. By the late 1990s, some high-end VCRs offered more sophisticated indexing. For example, Panasonic's Tape Library system assigned an ID number to each cassette, and logged recording information (channel, date, time and optional program title entered by the user) both on the cassette and in the VCR's memory for up to 900 recordings (600 with titles).Panasonic Video Cassette Recorder NV-HS960 Series Operating Instructions
VQT8880, Matsushita Electric Industrial Co., Ltd.


Control track damage

A gap in the control track signal of a videotape usually causes a flaw during playback, as the player may lose
tracking Tracking may refer to: Science and technology Computing * Tracking, in computer graphics, in match moving (insertion of graphics into footage) * Tracking, composing music with music tracker software * Eye tracking, measuring the position of t ...
as soon as the track drops out. This usually leads to a gap in video and audio (since there's no information as to what speed the media is recorded at), the VCR's counter will freeze, and upon re-acquiring the control track, the video sync will be momentarily lost. Problems with control track
signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
s are almost always caused by a gap in
recording A record, recording or records may refer to: An item or collection of data Computing * Record (computer science), a data structure ** Record, or row (database), a set of fields in a database related to one entity ** Boot sector or boot record, ...
, e.g. a recording was resumed ''beyond'' (rather than ''at'') the point where recording was previously stopped. A control track
error An error (from the Latin ''error'', meaning "wandering") is an action which is inaccurate or incorrect. In some usages, an error is synonymous with a mistake. The etymology derives from the Latin term 'errare', meaning 'to stray'. In statistics ...
can also be caused by a " dropout" - a defect in the
magnetic Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...
particles on a tape - but this is very uncommon, as the control track is immune to all but the most severe tape damage. A discontinuous control track was (and still is) especially problematic for analog videotape editing. Each gap in the control track, no matter how brief, destroys synchronization and continuity, which can make a segment of a recorded video difficult or impossible to use for editing. The acronym ''LOCT'' (pronounced ''locked''), used by many video professionals, can mean "Loss Of Control Track" and/or "Loss Of Continuous
Timecode A timecode (alternatively, time code) is a sequence of numeric codes generated at regular intervals by a timing synchronization system. Timecode is used in video production, show control and other applications which require temporal coordination ...
." Though either type of "loss" can pose similar problems, and both issues are sometimes directly related, control track and timecode are two different things (see
timecode A timecode (alternatively, time code) is a sequence of numeric codes generated at regular intervals by a timing synchronization system. Timecode is used in video production, show control and other applications which require temporal coordination ...
). Control track problems cannot be "repaired." You can duplicate the tape to restore control track continuity, but this will not fix visual and audible "
glitch A glitch is a short-lived fault in a system, such as a transient fault that corrects itself, making it difficult to troubleshoot. The term is particularly common in the computing and electronics industries, in circuit bending, as well as among ...
es" caused by gaps in the original control track, and the video itself will suffer quality degradation due to "
generation loss Generation loss is the loss of quality between subsequent copies or transcodes of data. Anything that reduces the quality of the representation when copying, and would cause further reduction in quality on making a copy of the copy, can be consid ...
." For this reason, many video professionals would "pre-stripe" every analog videotape to be used in the field or studio beforehand to help ensure good control track throughout. This was done by pre-recording
black Black is a color which results from the absence or complete absorption of visible light. It is an achromatic color, without hue, like white and grey. It is often used symbolically or figuratively to represent darkness. Black and white have o ...
to the entire videotape. Pre-striping also facilitated spot-checking for dropouts on the video portion of the tape (usually seen as brief white horizontal "dashes") before use.


Gallery

Image:Bcn-scanner-head.jpg,
Type B videotape 1–inch type B VTR (designated Type B by SMPTE) is a reel-to-reel analog recording video tape format developed by the Bosch Fernseh division of Bosch in Germany in 1976. The magnetic tape format became the broadcasting standard in continen ...
video Scanner Head, note CTR head built into the side of the scanner, the black circle on the side of the scanner. Image:Ampex-quad-video-head.jpg,
Quadruplex videotape 2-inch quadruplex videotape (also called 2″ quad video tape or quadraplex) was the first practical and commercially successful analog recording video tape format. It was developed and released for the broadcast television industry in 1956 by A ...
Ampex AVR-2 Video Head, note CTR head, the white square on the lower part of the drum.


See also

*
CTL timecode Control track longitudinal timecode, or CTL timecode, developed by JVC in the early 1990s, is a unique technique for embedding, or ''striping'', reference SMPTE timecode onto a videotape. Similar to the way VITC timecode is embedded in the vertic ...


References

{{Reflist Television technology VHS Synchronization