HOME

TheInfoList



OR:

Time base correction (TBC) is a technique to reduce or eliminate errors caused by mechanical instability present in
analog recording Analog recording is a technique used for the recording of analog signals which, among many possibilities, allows analog audio for later playback. Analog audio recording began with mechanical systems such as the phonautograph and phonograph. ...
s on mechanical media. Without time base correction, a signal from a
videotape recorder A video tape recorder (VTR) is a tape recorder designed to record and playback video and audio signal, audio material from magnetic tape. The early VTRs were reel-to-reel, open-reel devices that record on individual reels of 2-inch-wide (5.08& ...
(VTR) or
videocassette recorder 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 Videotape is magnetic tape ...
(VCR) cannot be mixed with other, more time-stable devices found in
television studio A television studio, also called a television production studio, is an installation room in which video productions take place, either for the production of live television and its recording onto video tape or other media such as SSDs, or for t ...
s and
post-production Post-production is part of the process of filmmaking, video production, audio production, and photography. Post-production includes all stages of production occurring after principal photography or recording individual program segments. The ...
facilities. Most
broadcast quality Broadcast quality is a term stemming from quad videotape to denote the quality achieved by professional video cameras and time base correctors (TBC) used for broadcast television, usually in standard definition. As the standards for commercial tel ...
VCRs have simple time base correctors built in though external TBCs are often used. Some high-end domestic analog video recorders and camcorders also include a TBC circuit, which typically can be switched off if required. Time base correction counteracts errors by buffering the video signal as it comes off the videotape at an unsteady rate, and releasing it at a steady rate. TBCs also allow a variable delay in the
video stream Video on demand (VOD) is a media distribution system that allows users to access videos without a traditional video playback device and the constraints of a typical static broadcasting schedule. In the 20th century, broadcasting in the form of o ...
. By adjusting the rate and delay using a waveform monitor and a
vectorscope A vectorscope is a special type of oscilloscope used in both audio and video applications. Whereas an oscilloscope or waveform monitor normally displays a plot of signal vs. time, a vectorscope displays an X-Y plot of two signals, which can r ...
, 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. 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 recorder Reel-to-reel audio tape recording, also called open-reel recording, is magnetic tape audio recording in which the recording tape is spooled between reels. To prepare for use, the ''supply reel'' (or ''feed reel'') containing the tape is plac ...
s relying on mechanical stability alone were stable enough that pitch distortion could be below audible level without time base correction. However, the higher sensitivity of video recordings meant that even the best mechanical solutions still resulted in detectable distortion of the video signals and difficulty locking to downstream devices. 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
NTSC The first American standard for analog television broadcast was developed by National Television System Committee (NTSC)National Television System Committee (1951–1953), Report and Reports of Panel No. 11, 11-A, 12–19, with Some supplement ...
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.


Types

Physically there are only 4 types, dedicated IC, add-in cards for prosumer/professorial VTR/VCRs, desktop standalone units & dedicated rack mount units. In the broadcast world, 1U Rack-mount type time base correctors were common & intended to be easily slid out of a rack on rails to be serviced as these units were meant for 24/7 workloads and typically contained little to no SMD hardware only operator serviceable through-hole electronics making these units a substantially heavy device.


Software Time Base Correction

A modern 5th and final type of TBC being achieved in the 2010s is software defined, packaged inside the open sourc
VHS-Decode

CVBS-Decode
projects which evolved from th
LD-Decode
project this also introduced the .TBC file format a lossless 4fsc digital format not dissembler to D-2 & D-3 videotape storing uncompressed
composite Composite or compositing may refer to: Materials * Composite material, a material that is made from several different substances ** Metal matrix composite, composed of metal and other parts ** Cermet, a composite of ceramic and metallic materials ...
/
s-video S-Video (also known as separate video, Y/C, and erroneously Super-Video ) is an analog video signal format that carries standard-definition video, typically at 525 lines or 625 lines. It encodes video luma and chrominance on two separate channe ...
(combined or Luminance/Chrominance separated) analog video signals in 16-bit unsigned grey scale values these can be opened inside ld-analyse for frame by frame analysis or encoded to a
lossless Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statistic ...
.mkv video file via
FFMPEG FFmpeg is a free and open-source software project consisting of a suite of libraries and programs for handling video, audio, and other multimedia files and streams. At its core is the command-line ffmpeg tool itself, designed for processing of vid ...
. 1135x625 For PAL (17727262 hz) (25i) (4.2GB/min 70MB/s 560mbps) 910x525 For NTSC (14318181 hz) (29.97i) (3.4GB/min 56.66MB/s 453mbps)


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
genlock Genlock (generator locking) 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 picture sources together. The aim in video applications is to ensure the ...
ing.To handle a randomly varying analog signal, a stable "pilot tone" is recorded along with the signal. Upon playback, the pilot tone is extracted with filters and frequency variations in the pilot tone are used to generate an error signal that drives the time base correction circuits. A notch filter is then used to suppress the pilot tone from the recovered signal. 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 time base corrector is the which allows devices that cannot be "steered" by an advanced sync signal to also be time base corrected or timed into a system. Satellites, microwave transmitters and other broadcast signals as well as consumer VTRs cannot be sent an advance sync signal. The synchronizer accomplishes this by (first digitizing in an analog scenario and) writing the incoming digital video into a frame buffer memory using the timing of the sync information contained in that video signal. A frame synchronizer stores at least a full frame of video. Simultaneously the digital video is being read back out of the buffer by an independent timing system that is
genlock Genlock (generator locking) 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 picture sources together. The aim in video applications is to ensure the ...
ed to the house timing reference. If the buffer over or underfills, 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.


See also

*
Drop-out compensator A drop-out compensator is an error concealment device that was commonly used in the analog video era to hide brief RF signal "drop-outs" on videotape playback caused by imperfections in or damage to the tape's magnetic coating. Most compensators wo ...
*
Frame synchronization (video) In video, frame synchronization is the process of synchronizing display pixel scanning to a synchronization source. When several systems are connected, a synchronization signal is fed from the synchronization source to the other systems in the ne ...
*
Video router A video router, also known as a video matrix switch or SDI router, is an electronic switch designed to route video signals from multiple input sources such as cameras, VT/DDR, computers and DVD players, to one or more display devices, such as ...
*
Vision mixer A vision mixer is a device used to select between several different live video sources and, in some cases, compositing live video sources together to create visual effects. In most of the world, both the equipment and its operator are called ...


Notes


References


A digital synchronizer for a video-tape recorder
Bucciarelli, F.V.; Proceedings of the IEEE, Volume 61, Issue 4, April 1973 Page(s):506 - 507

Sony, Sony BVT-800, Professional Timebase Corrector

FOR.A FA-410, Professional Timebase Corrector {{refend Film and video technology Television technology Television terminology Videotape