Why does the top of a VHS picture bend?

The tape and head are still settling when each sweep begins, so the first lines are timed slightly wrong. A television without a time base corrector follows those errors, drawing the top lines out of place before it settles. The same timing error exists on every frame, which is why the curve looks identical from one frame to the next.

Is flagging a sign of a bad tape?

It can be. Stretched, worn, or poorly stored tape increases the skew error. A deck with a poorly adjusted tape path also causes it. A small amount is normal on consumer VHS. Cleaning the heads and tape path on the playback deck often reduces it, but a stretched tape will keep some of the bend.

How is flagging different from head switching noise?

Flagging bends the lines at the top of the frame, while head switching noise disturbs the lines at the bottom. They have different causes, one being tape skew at the start of a sweep and the other the handover between heads.

How do I fix flagging in a capture?

Use a time base corrector or a capture device that includes line correction, since it re-times each line against a stable clock. Cropping a few lines off the top removes the worst of it when correction is not available. Recording the flagged lines as a clean crop is usually cheaper than trying to unbend them in post.

Flagging: what it is and how to make it in After Effects

Flagging is the bending of the top lines of a video picture like a flag in the wind. Learn why tape and sync cause it and how to dial it with STATIC DRIFT.

WHAT CAUSES IT

Flagging is a horizontal bending of the lines at the top of the picture, which sway sideways like a flag, caused by timing errors at the start of each field. In a helical scan machine, a head begins a sweep as it first touches the tape, and for the first moments the tape tension and the position of the tape on the drum are still settling. The tape's skew, the amount by which it is tilted or stretched at the entrance to the drum, changes the length of the track, so the first lines are read earlier or later than the nominal timing. Consumer decks have no time base corrector, so these timing shifts reach the television directly. The horizontal oscillator in the display uses a phase-locked loop with a finite response time, so it follows the early timing errors for a few lines and then settles. The result is a curve at the top of the frame that decays towards straight within about ten to twenty lines. A worn tape, a stretched tape, or a misadjusted tape path makes the effect larger.

WHAT IT LOOKS LIKE

The top ten to fifty lines of the picture lean to one side, then curve back to straight below, so a vertical edge near the top bends like a fishing rod. The shape is a smooth curve, sometimes with a little ripple, and it is the same at the top of every frame. Titles and logos placed near the top corners are the most affected. The bend may change between shots, or breathe in and out as the tape tension changes. The remainder of the frame is stable, and this is what separates flagging from general wobble.

WHERE YOU SEE IT

VHS and other helical scan tape played on consumer decks and passed straight to a television or a capture card, particularly with worn tapes or when paused or in special playback modes. It is visible in DVD transfers of old home video and in documentary use of rental tapes. For editors it is a quick signal of tape origin: a small curve at the top of an otherwise stable shot says that this was played on a deck with no correction.

RECREATE IT

STATIC DRIFT

CONTROLS FLAGGING, DRIFT HOLD, STATIC COMPOSITE

FLAGGING is the control for the bend at the top of the frame. Raise it until a vertical edge near the top curves, and stop before the curve reaches the middle of the picture. DRIFT HOLD can keep the bend constant. Use STATIC COMPOSITE to blend it back. For a tape under stress, animate FLAGGING slowly over a few seconds rather than flickering it.

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HOW TO MAKE IT IN AFTER EFFECTS

1. Pre-compose the footage and apply STATIC DRIFT. 2. Choose a frame with a vertical edge near the top, such as a doorway, a window, or a pole, so you can see the curve. 3. Raise FLAGGING from its minimum until the edge bends at the top. 4. Check that the bend fades out over the first part of the frame, leaving the rest straight. 5. Use DRIFT HOLD if the bend is wandering more than you want. 6. Keep the amount modest for a clean tape, and stronger for a worn one. 7. Keyframe FLAGGING over the shot so it breathes slowly, since tape tension changes over time. 8. Combine with a small HEAD SWITCH at the bottom for the full set of tape-edge artifacts. 9. If your footage has a title near the top, place the title in a layer above the effect so it stays steady, or leave it below if you want it to bend with the picture.

QUESTIONS

Why does the top of a VHS picture bend?: The tape and head are still settling when each sweep begins, so the first lines are timed slightly wrong. A television without a time base corrector follows those errors, drawing the top lines out of place before it settles. The same timing error exists on every frame, which is why the curve looks identical from one frame to the next.
Is flagging a sign of a bad tape?: It can be. Stretched, worn, or poorly stored tape increases the skew error. A deck with a poorly adjusted tape path also causes it. A small amount is normal on consumer VHS. Cleaning the heads and tape path on the playback deck often reduces it, but a stretched tape will keep some of the bend.
How is flagging different from head switching noise?: Flagging bends the lines at the top of the frame, while head switching noise disturbs the lines at the bottom. They have different causes, one being tape skew at the start of a sweep and the other the handover between heads.
How do I fix flagging in a capture?: Use a time base corrector or a capture device that includes line correction, since it re-times each line against a stable clock. Cropping a few lines off the top removes the worst of it when correction is not available. Recording the flagged lines as a clean crop is usually cheaper than trying to unbend them in post.

RELATED EFFECTS

Head switching noise Head switching noise is the ragged strip at the bottom of VHS frames. Learn what causes it and how to build it with STATIC DRIFT in After Effects.
Time base error 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.
Horizontal tearing Horizontal tearing is the sideways shearing of bands of lines when analog sync fails. See what breaks in the signal and how to build it with STATIC DRIFT.

LOOKS THAT USE IT

VHS rental tape look The VHS rental tape look: a worn, dubbed cassette with tracking lines, soft color and dropouts, rebuilt in After Effects for trailers, title cards and horror.
1987 camcorder look The 1987 camcorder look: soft Video8 and VHS-C picture, smeared color, date stamp and wobbling timebase, built in After Effects with a repeatable recipe.
90s found footage look The 90s found footage look: Hi8 and VHS-C handheld video with date stamps, auto-iris pumping and dropout, recreated in After Effects for horror.
analog horror look The analog horror look: broadcast and VHS signal failure as dread, with static, tracking loss and color collapse, built in After Effects.