What is a TBC? - QUESTRONIX

What is a TBC* and why do I need one?
*Timebase Corrector / Time Base Corrector

When a tape is recorded or played in a VCR it is subjected to a range of complex mechanical forces which cause instability in the video signal timing. The most dynamic and troublesome of these is caused by the video heads which protrude ever so slightly (this is called tip penetration) from the spinning head drum. As the head strikes the tape and begins its helical path down the tape, it stretches the tape a tiny amount and the tape springs back.

Because of this stretching and springing back of the tape some lines will be shorter and some lines longer than they should be. This gross time distortion (severe timebase or sync jitter) of the first lines of video usually reduces as the head proceeds further down the tape. All lines are affected, but the first 10% to 20% at the top of the picture are the worst and cause the effect known as flagging or flag-waving which can be seen on some older TV's and any video monitors with slow horizontal synchronisation (long time constant).

It is important to realise that ALL analogue video tape recorders (VTR's) and video cassette recorders (VCR's) do it, bar none! And they all do it slightly differently, so the errors which are recorded will always be a bit different to the errors caused during playback, especially in a different machine. A first generation tape, an original master, will exhibit two layers of timebase error upon playback, even in the machine that recorded it because the mechanical system is affected by so many variables (temperature, friction, humidity, gyroscopic effects, etc.) that that is the best you can ever hope for. Other sources of time distortion are wow and flutter which may be caused by varying tape tension, dragging brakes, sticky guides, the video cassette mechanism, power supply variations, etc, etc.

Every time you copy a copy, the timebase errors are compounded further, eventually making it impossible for a VCR to synchronise with. The signal becomes unrecordable, but a TV or video monitor may still show a recognisable picture because:- 1. Basically, the inertia of an electron beam and it's controlling circuitry is virtually zero compared to a mechanical servo system thus allowing the electron beam to follow the unstable video AND 2. Your eyes are easily deceived. Professional video monitors have operational modes such as underscan, pulse cross and slow sync which can expose timebase errors, but domestic TV's are designed to hide them.

To correct timebase errors when making copies or transferring to a DVD recorder or PC you need a timebase corrector. There are two main types of TBC - external and internal. The external TBC usually has at least two inputs, one for composite video and one for S-Video (Y/C). Composite video fed to the TBC is most often converted into two streams of digital data (one for the Y or luminance portion and one for the C or Chroma portion, the same as S-Video) and stored in memory (the frame store). The original unstable sync is only used to synchronise this process and is stripped off.

A very stable crystal-locked pulse generator is then used to read the data back out of memory. This causes all horizontal lines of video to be restored to the same length (or duration). They are then converted back to analogue video and provided with new, very stable sync signals. At any time, the digital memory effectively contains a whole frame (1 x frame = 2 x fields of 312.5 lines = 625 lines for PAL) which are proceeding through it, line-by-line in a first-in, first-out fashion.

Because of this digital process, the video coming from a TBC exhibits none of the time jitter that is seen at the output of a vcr and, at least as far as the sync is concerned, is a first generation signal which is easily recorded. Internal TBC's are pretty much the same, but usually only store a few lines of video because they have control of the vcr's servos and can control gross mechanical errors directly. All digital Camcorders are inherently TBC's thus having ultra-stable output. The digital tape could be regarded as a digital memory with a huge, non-volatile storage capacity!

External TBC's have some interesting features. For instance, they always output a continuous, stable video signal - even if there is no input or if the input is 'garbage' such as random noise from a receiver with no RF signal being input or perhaps playback of a tape with a huge crease and major dropouts. Some TBC's allow you to select whether you will have colour-bars, a black screen or a freeze-frame upon loss of input - you will see this last effect on TV during live motor racing when the "Race-Cam" link is lost.

A few up-market and all professional TBC's will provide an extra input to allow another (highly stable) video signal to synchronise or 'gen-lock' the output of the TBC. This is useful when you need to mix two video signals together as they have to be solidly locked together and perfectly aligned time-wise. Most TBC's have controls for black level (brightness), video gain (contrast), chroma level (colour saturation), and enhancement / filtering (sharpness).

Internal TBC's in domestic vcr's usually DON'T have video signal adjustments or genlock capability and usually they don't output any signal when stopped, except for whatever signal may be being fed to the active input (known as e-e or electronics to electronics mode). Note that the e-e mode of a vcr with internal TBC does not usually output a stable signal and (in most if not all cases) cannot be used to timebase correct the signal being fed through it unfortunately.

Not so long ago in a domestic analogue editing situation it was not unusual to find that an original miniature camcorder tape was copied to a full-sized VHS or SVHS tape and that this tape was then copied piece-by-piece to another tape during the editing process and that this edited master was used to make the final copies - all without a TBC in sight! No wonder those old tapes look so bad compared to today's digital! It is important to realise - and this really can't be understated - COMPOUNDED TIMEBASE ERRORS CANNOT BE REMOVED, only the current errors due to the playback process can be removed. A TBC cannot extract a perfect video signal from several compounded layers of time-distortion. A grotty tape can be made recordable, but the copy will still look grotty (though stable) because EVERY analogue generation has to be TBC'd to maintain the stability of the video.

Modern video mixers such as the Focus Enhancements (previously Videonics) MX-Pro and Edirol V-4 have digital TBC's built-in to allow digital effects and A/B roll editing. If you have one of these it should be used whenever transferring or copying analogue video to ensure that the final copies are 100% stable and of the highest visual quality. This is especially important when transferring your old Betamax, VHS, SVHS, 8mm and Hi8 tapes to DVD recorders or for video capture into a PC. There is a wide range of high-performance TBCs available such as the CTB-100, CTB-100G, CTB-530, and standards converters such as the CDM640 and CDM-640A that also perform timebase correction. Firewire converters such as the DV2001 and Edirol VMC-1 also perform timebase correction and eliminate the need for an internal video capture card in your firewire-equipped PC.