Head switching noise
Also called: head switch, bottom-of-frame jitter, head switching point
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.
WHAT CAUSES IT
Head switching noise is the disturbance at the bottom of a VHS picture where the video drum changes from one head to the other. A standard VHS drum carries two heads on opposite sides, and each head reads one field per half revolution, so on an NTSC deck the handover occurs 59.94 times a second. The switch is timed to land a few lines ahead of the vertical blanking interval, roughly six to seven lines before the vertical sync, so that the unavoidable glitch is hidden in the overscan and blanking region of a television. Because the heads have a small time offset against each other, the lines right at the switch are read with a different horizontal timing than the lines above them. The monitor's horizontal oscillator follows that change, so the last lines of the picture lean or jump sideways while it resettles. The effect is strongest when the drum phase is slightly misadjusted or the tape is mechanically worn. On a CRT it was normally cropped, which is why it looks wrong only on flat panels and in full-frame captures.
WHAT IT LOOKS LIKE
A thin strip along the bottom edge of the frame, typically between a few lines and about ten lines tall, where the image is smeared sideways. The top of the strip is clean, then each line within it is displaced a little further left or right, and the bottom pixels can fold back with a hook shape. Brightness at the edge may flicker, and the strip often carries a little extra snow. In a capture of a whole frame you see the hook shape, the edge of the picture blurring, and sometimes a dark or colored band beneath it. The strip stays pinned to the bottom of the frame and does not move with the picture content.
WHERE YOU SEE IT
Any helical scan VHS playback shown in full frame: DVD transfers of old television, online uploads of rental tapes, and camcorder captures from unmasked capture cards. It is a small detail editors use to sell a lo-fi tape source without any large effect. The same feature appears in Betamax and 8mm formats, though VHS made it most famous. Any film that shows a tape in full frame, such as a prop television in a horror film, tends to include it.
RECREATE IT
STATIC DRIFT
CONTROLS HEAD SWITCH, DRIFT, STATIC COMPOSITE
HEAD SWITCH is the main control. Start at a low value so only the last few lines bend, and increase it until the hook shape reads clearly. Leave DRIFT low, because wide drift will bend the whole frame. Use STATIC COMPOSITE to balance how much of the effect shows against the original footage, and keep it subtle if the footage is meant to read as a clean tape.
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HOW TO MAKE IT IN AFTER EFFECTS
1. Pre-compose your footage if you have already stacked other treatments on it, so HEAD SWITCH acts on the final picture. 2. Apply STATIC DRIFT. 3. Set every control at its minimum, then raise HEAD SWITCH until you see the bottom lines kink sideways. 4. Check the bottom edge at 100 percent zoom, because it is easy to push the strip too tall. 5. If you want the strip to look like a mild adjustment error, leave HEAD SWITCH constant and add a very slow keyframe so the hook breathes over a few seconds. 6. If the shot is meant to be cropped for a television, you can leave it a little stronger, then scale the layer slightly so the edge hides under the frame. 7. Add other tape faults, such as tracking error or chroma bleed, in separate steps and keep this one low so it stays recognizable as a small detail. 8. Preview with a title-safe crop on, to see what a viewer on a television would have seen.
QUESTIONS
- Why is head switching noise only at the bottom of the frame?
- The drum is timed so the head handover lands a few lines before the vertical blanking interval. A CRT television hid that region in overscan. The bottom is where it ends up, because the handover must occur near the end of each field.
- Is head switching noise a sign of a bad tape?
- Not necessarily. Every VHS deck produces some of it. It only becomes strong when the drum phase is off, the heads are worn, or the capture shows the full frame instead of the cropped portion a television displayed. A deck with a drifting drum phase or dirty heads moves the strip higher into the picture, which is the real warning sign.
- Can I crop it out?
- Yes. Most of it lives in the last ten lines or so, so a small crop at the bottom of the frame removes it. Some capture software and time base correctors also blank those lines. Cropping is the quickest fix for archival work, but if you want to keep the full frame, a time base corrector hides the strip more gracefully.
- Does S-VHS or digital video have head switching noise?
- S-VHS tape machines do, because they use the same helical scan layout. Digital video files do not generate it; if you see it in a file, it was captured from a tape. Betamax and 8mm helical formats have a similar handover point, though the exact line count differs between formats.