Time base error
Also called: TBE, line jitter, wobble, timing instability
Time base error is the line to line timing jitter of analog tape that wobbles verticals and drifts color. Learn the cause and how to build it in After Effects.
WHAT CAUSES IT
Time base error is any variation in the timing of the scanlines coming off a tape compared with the stable clock a display or broadcast chain expects. In a helical scan machine, the speed of the tape past the drum, the drum rotation, tape stretch, and mechanical friction all vary slightly, so each line arrives a little early or a little late. The ideal line period is 63.56 microseconds on NTSC, derived from the 15.734 kHz line rate, and a consumer deck can wander by a fraction of a microsecond line to line and by several microseconds over a field. Because the television locks to the horizontal sync pulse of each line, a short line pulls the picture left and a long line pushes it right, so vertical edges ripple. The color subcarrier is recovered from the burst on every line, so timing jitter also destabilizes chroma phase, shifting hue. Broadcast chains used a time base corrector, which writes lines into memory and reads them out against a stable reference to remove the error.
WHAT IT LOOKS LIKE
Vertical edges that should be straight wobble gently from side to side, as if reflected in water, with a period of a few lines up to the whole frame. A door frame bows left near the top and right near the bottom. Fast jitter shows as a ragged edge, where adjacent lines are offset by a pixel or two. Slow errors bend the whole image into an arch or an S curve. Hue may also wander slightly because the chroma phase is tied to the same unstable timing, so skin tones drift between pink and green within a shot.
WHERE YOU SEE IT
Any uncorrected consumer tape playback, old camcorder footage, VHS cut straight into an edit without a time base corrector, and video from worn decks. It is common in found-footage films, in music videos from the 1980s and 1990s, and in test footage that shows tape instability. Games and consoles with unstable clocks can also show it, and security recorders in time lapse mode wobble noticeably because of their varying tape speeds.
RECREATE IT
STATIC DRIFT
CONTROLS DRIFT, ARCH WARP, DRIFT HOLD
DRIFT supplies the line to line horizontal displacement. ARCH WARP adds the slow bow across the frame that comes from a speed variation over a field. Use DRIFT HOLD to keep the wobble steady rather than random. Start both at low values: a time base error should be noticeable on vertical edges but should not tear the picture.
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TERMINAL FADE
CONTROLS WOW, FLUTTER
WOW gives the slow drift in speed and FLUTTER gives the faster one. Use WOW first for a gentle sway, then add FLUTTER at a lower level for fine jitter. If you also want the audio to match, treat the sound separately in your audio editor, since these controls act on the picture.
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HOW TO MAKE IT IN AFTER EFFECTS
1. Pre-compose the footage and apply STATIC DRIFT. 2. Set DRIFT to a low value and look at a vertical edge in the shot, such as a door frame or a building. The edge should shiver slightly. 3. Add ARCH WARP slowly until the frame carries a faint bow at the top or bottom. 4. Use DRIFT HOLD to stabilize the pattern so it does not jump to a new random position every frame. 5. For tape speed variation, apply TERMINAL FADE and raise WOW first and FLUTTER second. 6. Keyframe DRIFT up for a second when the tape would be under stress, such as during a camera movement. 7. Compare against a static shot and a moving shot to make sure the wobble reads on both. 8. If the footage has text or a lower third, put it above the effect so it does not wobble unless you want it to. 9. Render a test and watch at full size, since subtle drift is invisible in a thumbnail.
QUESTIONS
- What is a time base corrector?
- A time base corrector stores each incoming line in memory and reads it out against a stable clock, so the timing jitter from the tape is removed. Broadcast facilities used it for any tape source. Consumer decks often had no such device.
- Why do vertical lines wobble in old video?
- Each scanline starts at a slightly different time, so the line is displaced horizontally on the screen. Lines stack vertically, so a straight vertical edge turns into a wavy line. The wobble comes from tape speed and drum variation. Fast jitter makes a fine ragged edge, while slower variation bends whole objects into arcs, so the wobble you see depends on the speed of the timing error.
- How is time base error different from horizontal tearing?
- Time base error is small, continuous timing variation, a ripple of a few pixels. Horizontal tearing is a larger break in sync where blocks of lines are shifted far from their place. Many real tapes show both at once, with a continuous ripple and occasional larger breaks, so combine them rather than choosing one.
- Can software correct it?
- Digital stabilizers and line-level timing correction can reduce it, and good capture hardware with built-in correction removes most of it. The cleanest approach is to capture through a device that includes a time base corrector. Software cannot always tell a true edge from a shifted line, so aggressive correction can soften detail. Hardware correction before capture is more accurate.