A time-base corrector, or TBC, is one of the most important pieces of equipment to use when you convert to AVI files or convert tape to DVD. It's also one of the key pieces of video hardware that most DIY home transfer setups are missing. What does a TBC do, and why should you almost always use it when transferring video tapes to DVD?
Tape players, whether they are VCRs or camcorders, rely on mechanical, moving parts to play back a video tape. These parts include rollers, pulleys, tensioners, a helical head that spins, and more.
A tape, which is made of mylar (plastic), passes through all of these parts very closely – especially the helical head. This intense pressure causes the tape to stretch slightly. Luckily, all of these parts are geared to work with an imperfect tape, and allow for a certain amount of "slop." But these parts also can apply a range of pressures on the tape, which it in turn applies to the head. As a result, the tape and resulting signal can become distorted.
Add to this the fact that the tape speed can change over both the short term and the long term. Using different tape brands can cause alignment and tape registration changes. The whole process, moreover, is affected by mechanical wear in the deck, as well as oxide buildup and magnetization on the tape heads.
All of the variables can cause not only quality problems when converting analog video to digital, but timing errors. When a tape plays back on a VCR, each single group of horizontal scan lines comprise a field, and two fields comprise a single frame of video (29.97 frames per second of video). The timing between fields and frames should be exact.
But if these are issues caused by the aforementioned conditions, this precise timing doesn't happen. And if the timing varies too much, serious playback problems can occur. For example, vertical edges in the picture might no longer be straight. Colors can shift, which is caused by field and frame timing problems. Frames may be dropped altogether, which causes audio and video sync problems. If the tape itself has physical damage, these signal problems can be further compounded. When you transfer video to DVD using a analog to digital recorder or converter, these digital devices can go haywire!
The solution is to run the video through a TBC. When a video signal enters a full-frame, external TBC, the information is digitized and stored in a buffer, one field at a time, until an entire frame is present. The timing information is deleted.
Then the TBC replaces every horizontal line timing with its own. The timing is determined by the length of the "retrace" area of the signal. The unit then takes each finished field and performs the same operation with the vertical "retrace" portion, which is where the vertical timing resides.
Finally, the TBC passes the correctly-timed signal through an internal digital to analog converter and outputs it to the DVD recorder or other device. This is how well-done VCR to DVD dubbing is accomplished.
Without a TBC, you wouldn't be able to capture from different tape sources with any level of success. It would be a particular mess when transferring from video tape to computer! Many factors can affect how cleanly a tape transfers. This is why professional video transfer companies rely on time-base correctors – and another reason why it's an excellent idea to hire company that provides DVD and video services to preserve your videotapes.