Control track
Encyclopedia
A control track is a track that runs along an outside edge of a standard analog
videotape
(including VHS
). The control track encodes a series of pulse
s, each pulse corresponding to the beginning of each frame. This allows the video tape player to synchronize its scan
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 VCRs have.
A gap in the control track signal of a videotape usually causes a flaw during playback, as the player may lose tracking 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
s are almost always caused by a gap in recording
, e.g. a recording was resumed beyond (rather than at) the point where recording was previously stopped. A control track error
can also be caused by a "dropout
" - a defect in the magnetic 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 , used by many video professionals, can mean "Loss Of Control Track" and/or "Loss Of Continuous Timecode". 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).
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
es" caused by gaps in the original control track, and the video itself will suffer quality degradation due to "generation loss
".
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
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.
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
videotape
Videotape
A videotape is a recording of images and sounds on to magnetic tape as opposed to film stock or random access digital media. Videotapes are also used for storing scientific or medical data, such as the data produced by an electrocardiogram...
(including VHS
VHS
The Video Home System is a consumer-level analog recording videocassette standard developed by Victor Company of Japan ....
). The control track encodes a series of pulse
Pulse (signal processing)
In signal processing, the term pulse has the following meanings:#A rapid, transient change in the amplitude of a signal from a baseline value to a higher or lower value, followed by a rapid return to the baseline value....
s, each pulse corresponding to the beginning of each frame. This allows the video tape player to synchronize its scan
Scan
Scan may refer to:Acronyms:*Schedules for Clinical Assessment in Neuropsychiatry , a psychiatric diagnostic tool developed by WHO*Shared Check Authorization Network , a database of bad check writers and collection agency for bad checks...
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 VCRs have.
A gap in the control track signal of a videotape usually causes a flaw during playback, as the player may lose tracking 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
Signal (electrical engineering)
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
s are almost always caused by a gap in recording
Recording
Recording is the process of capturing data or translating information to a recording format stored on some storage medium, which is often referred to as a record or, if an auditory medium, a recording....
, e.g. a recording was resumed beyond (rather than at) the point where recording was previously stopped. A control track error
Error
The word error entails different meanings and usages relative to how it is conceptually applied. The concrete meaning of the Latin word "error" is "wandering" or "straying". Unlike an illusion, an error or a mistake can sometimes be dispelled through knowledge...
can also be caused by a "dropout
Dropout (electronics)
Dropout within the realm of electronics and electrical engineering, has a number of uses.It is the dropping away of a flake of magnetic material from magnetic tape, leading to loss of signal, or a failure to properly read a binary character from data storage...
" - a defect in the magnetic 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 , used by many video professionals, can mean "Loss Of Control Track" and/or "Loss Of Continuous Timecode". 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).
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
Glitch
A glitch is a short-lived fault in a system. It is often used to describe a transient fault that corrects itself, and is therefore difficult to troubleshoot...
es" caused by gaps in the original control track, and the video itself will suffer quality degradation due to "generation loss
Generation loss
Generation loss refers to 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 considered a form of generation loss...
".
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
Black is the color of objects that do not emit or reflect light in any part of the visible spectrum; they absorb all such frequencies of light...
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.