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.