Timebase correction
Encyclopedia
Time base correction is a technique to reduce or eliminate errors caused by mechanical instability present in analog recording
s on mechanical media. Without time base correction, a signal from a videotape recorder (VTR) or videocassette recorder
(VCR) cannot be mixed with other, more time stable devices found in television studio
s and post-production
facilities. Most broadcast quality
VCRs have simple time base correctors built in though external time base correctors ("TBC"s) are often used.
Time base correction counteracts errors by buffering the video signal and releasing it at a steady rate. "TBC"s also allow a variable delay in the video stream. By adjusting the rate and delay using a waveform monitor and a vectorscope, the corrected signal can now match the timing of the other devices in the system. If all of the devices in a system are adjusted so their signals meet the video switcher at the same time and at the same rate, the signals can be mixed together. A single master clock or "sync generator" provides the reference for all of the devices' clocks.
is encoded relative to the phase of a high frequency color sub-carrier, making the displayed colors extremely sensitive to time base errors. and, importantly, allow the combination and switching of two or more video signals. Unlike audio, video signals cannot be successfully mixed if they are out of time with each other.
ing. Genlocking allows sources that are not themselves genlock-capable to be used with production switchers and A/B roll editing equipment. Stand-alone broadcast model time base correctors typically will genlock the signal to an external sync reference, and also allow the brightness, contrast, chrominance, and color phase ("tint" or "hue") to be adjusted.
Some TBCs featured a Drop Out Compensation (DOC) circuit that enabled videotape flaws caused by oxide drop-out to be temporarily corrected. The DOC circuit required dedicated cabling between the videotape player and the TBC in which irregularities were detected in portions of the video image. Previously captured and stored lines of video would then be superimposed over the flawed video lines.
A variant of the timebase corrector is the frame synchronizer which allows devices that cannot be "steered" by an advanced sync signal to also be time base corrected and/or timed into a system. Satellites, microwave transmitters and other broadcast signals as well as consumer VTRs cannot be sent an advance sync signal. A frame synchronizer stores at least a full frame of video. If the buffer over or under fills, the Frame Sync will hold the last good frame of video until another full frame's worth of video is received. Usually this is undetectable to viewers.
Analog recording
Analog recording is a technique used for the recording of analog signals which among many possibilities include audio frequency, analog audio and analog video information for later playback.Analog recording methods store signals as a continual wave in or on the media...
s on mechanical media. Without time base correction, a signal from a videotape recorder (VTR) or videocassette recorder
Videocassette recorder
The videocassette recorder , is a type of electro-mechanical device that uses removable videocassettes that contain magnetic tape for recording analog audio and analog video from broadcast television so that the images and sound can be played back at a more convenient time...
(VCR) cannot be mixed with other, more time stable devices found in television studio
Television studio
A television studio is an installation in which a video productions take place, either for the recording of live television to video tape, or for the acquisition of raw footage for post-production. The design of a studio is similar to, and derived from, movie studios, with a few amendments for the...
s and post-production
Post-production
Post-production is part of filmmaking and the video production process. It occurs in the making of motion pictures, television programs, radio programs, advertising, audio recordings, photography, and digital art...
facilities. Most broadcast quality
Broadcast quality
Broadcast Quality is an term stemming from quad videotape to denote the quality achieved by professional video cameras and time base correctors used for broadcast television, usually in standard definition...
VCRs have simple time base correctors built in though external time base correctors ("TBC"s) are often used.
Time base correction counteracts errors by buffering the video signal and releasing it at a steady rate. "TBC"s also allow a variable delay in the video stream. By adjusting the rate and delay using a waveform monitor and a vectorscope, the corrected signal can now match the timing of the other devices in the system. If all of the devices in a system are adjusted so their signals meet the video switcher at the same time and at the same rate, the signals can be mixed together. A single master clock or "sync generator" provides the reference for all of the devices' clocks.
Video correction
As far back as 1956, professional reel-to-reel audio tape recorders relying on mechanical stability alone had no audible pitch distortion, and no need for timebase correction. However, the higher sensitivity of video recordings meant that even the best mechanical solutions still resulted in detectable distortion of the video signals. A video signal consists of picture information but also sync and subcarrier signals which allow the image to be framed up square on the monitor, reproduce colors accuratelyIn particular, the color information in NTSCNTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
is encoded relative to the phase of a high frequency color sub-carrier, making the displayed colors extremely sensitive to time base errors. and, importantly, allow the combination and switching of two or more video signals. Unlike audio, video signals cannot be successfully mixed if they are out of time with each other.
Methods
Implicit in the idea of time base correction is that there must be some target time base that the corrector is aiming for. There are two time bases commonly used. The first method is to make the frames, fields and lines come out smoothly and uniformly, at the rates specified by the standards using an oscillator for time reference. The alternative to this method is to align the frames, fields, and lines with some external signal, a procedure called genlockGenlock
Genlock is a common technique where the video output of one source, or a specific reference signal from a signal generator, is used to synchronize other television picture sources together. The aim in video and digital audio applications is to ensure the coincidence of signals in time at a...
ing. Genlocking allows sources that are not themselves genlock-capable to be used with production switchers and A/B roll editing equipment. Stand-alone broadcast model time base correctors typically will genlock the signal to an external sync reference, and also allow the brightness, contrast, chrominance, and color phase ("tint" or "hue") to be adjusted.
Some TBCs featured a Drop Out Compensation (DOC) circuit that enabled videotape flaws caused by oxide drop-out to be temporarily corrected. The DOC circuit required dedicated cabling between the videotape player and the TBC in which irregularities were detected in portions of the video image. Previously captured and stored lines of video would then be superimposed over the flawed video lines.
A variant of the timebase corrector is the frame synchronizer which allows devices that cannot be "steered" by an advanced sync signal to also be time base corrected and/or timed into a system. Satellites, microwave transmitters and other broadcast signals as well as consumer VTRs cannot be sent an advance sync signal. A frame synchronizer stores at least a full frame of video. If the buffer over or under fills, the Frame Sync will hold the last good frame of video until another full frame's worth of video is received. Usually this is undetectable to viewers.