VHS tracking error
Also called: tracking lines, tracking noise, mistracking
VHS tracking error explained: why the heads lose the track, what the noise band looks like, and how to recreate it in After Effects.
WHAT CAUSES IT
A VHS tracking error happens when the playback heads stop following the slanted tracks that the recording heads wrote. On an NTSC deck the drum spins at 1800 rpm, and each head sweeps a thin diagonal stripe across the tape. The transport keeps the heads on those stripes by comparing the 30 Hz control track pulses against the drum phase. If the tape has stretched, the transport is worn, the tape was recorded on a different deck, or the tracking knob is off center, a head slides partly onto the neighboring stripe. Adjacent stripes were recorded with opposite azimuth angles of six degrees, so the head reads the neighbor poorly, but not at zero. The FM luma carrier loses level, the demodulator outputs noise, and the horizontal sync pulses lose their clean edges. The picture shows a band of noise exactly where the head has strayed. In bad cases the line timing breaks and the monitor loses horizontal lock for those lines. The damage forms a band because the head is only misaligned for part of its sweep, and the band sits still or crawls depending on whether the drift is steady or the tape speed is wavering.
WHAT IT LOOKS LIKE
A horizontal band of gray and white speckle, usually one tenth to one third of the frame high, sitting in the lower half of the picture. Inside the band the image is replaced by noise or smeared into streaks, the colors drop out to monochrome, and the lines at the band edges bend sideways by a few percent of the frame width. When the tracking is only slightly off, you see a thin line of snow near the bottom with a small wobble above it. When it is badly off, several bands stack and slide upward, the whole picture goes soft and washed, and the sync tears so the frame shudders left and right. Pausing the tape freezes the band in place, which is a useful reference when you are matching it.
WHERE YOU SEE IT
Rental tapes played on a deck that was not the one that recorded them, home recordings of television, camcorder tapes aged past their rated life, and worn copies from rental stores. In film and video work it signals found footage, horror framed as a recovered tape, retro music videos, and the glitch of a VHS being scrubbed or paused. Tracking controls also appear on the deck's on-screen display, which is why a tracking bar became shorthand for the era.
RECREATE IT
STATIC DRIFT
CONTROLS STATIC BREAK, DRIFT, STATIC CHAOS
Start with STATIC BREAK low so only a thin band forms, then raise DRIFT until the lines at the band edge bend sideways. Keep STATIC CHAOS low at first, because high chaos stops looking like one track fault and turns into general damage. Keep the band in one place for a few seconds rather than animating wildly; real mistracking is steady.
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TERMINAL FADE
CONTROLS TRACKING COLLAPSE, DROPOUT
Use TRACKING COLLAPSE when the tape should look worn rather than just misadjusted, and add a small amount of DROPOUT for flecks inside the band. Raise TRACKING COLLAPSE slowly over the shot. Avoid maxing both controls together, which reads as total failure rather than a mistracking moment.
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HOW TO MAKE IT IN AFTER EFFECTS
1. Place your footage in a composition at its native frame rate, and pre-compose it so the effect works on a flattened layer. 2. Apply STATIC DRIFT to the pre-comp. 3. Set STATIC CHAOS low and raise STATIC BREAK until a single band of disturbance appears in the lower half of the frame. 4. Add DRIFT slowly until the lines at the edges of the band lean sideways. 5. Place a keyframe on STATIC BREAK at the start of the shot at a low value, a second keyframe about a second later at the peak, and a third letting it fall back, so the fault arrives and recedes the way a tracking knob moves. 6. For a worn tape, apply TERMINAL FADE above it and bring up TRACKING COLLAPSE and a little DROPOUT. 7. Scrub a few frames forward and back and check the band is not flickering from frame to frame; if it is, lower STATIC CHAOS. 8. Render a short test at full resolution, since fine noise reads differently at half resolution preview.
QUESTIONS
- Why does a VHS tape show tracking lines?
- The playback heads are not sitting on the same helical tracks that recorded the video. Tape stretch, a worn transport, or a tape recorded on another deck shifts the head position, so it reads noise from the neighboring track. The deck's tracking control moves the head phase relative to the control track to correct it.
- Can tracking errors be fixed after the fact?
- If you still have the tape, adjust the tracking knob or try another deck and capture again. Once the noise is baked into a digital capture the underlying picture is gone, though a temporal filter can soften it. Recapturing from the tape is always better than repairing a bad capture.
- What is the difference between tracking error and head switching noise?
- Head switching noise is a normal narrow disturbance at the very bottom of the frame where one head hands over to the other. Tracking error is a fault that can appear anywhere, is much taller, and moves when the tracking adjustment changes.
- Do DVDs or digital files have tracking lines?
- No. Tracking lines come from helical scan tape playback. A digital transfer can record them if the tape was mistracking at capture, but a file with those lines was made from a tape rather than generating them. Authentic tracking lines in a digital file always trace back to a tape source, so a clean disc should never show them.