RF interference
Also called: herringbone, radio frequency interference, TV interference, zebra stripes
RF interference is the diagonal herringbone and moving bars that appear when stray radio signals mix into a TV picture. See how it works and how to recreate it.
WHAT CAUSES IT
RF interference is the pattern that appears on a picture when an unwanted radio frequency signal enters the receiver and beats against the video carrier or the color subcarrier. A radio transmitter near the television's tuning range, a hair dryer motor, a fluorescent ballast, or a faulty connector can all inject energy into the tuner or cable. When the interfering frequency differs slightly from a frequency in the picture signal, the two mix and produce a beat at the difference frequency. Because the beat is not locked to the line rate of 15.734 kHz, its phase shifts a little on each successive line, so the pattern appears as diagonal stripes instead of vertical bars. A related example is the 920 kHz beat between the 4.5 MHz sound carrier and the 3.58 MHz color subcarrier, which shows as a fine dot texture in saturated areas. Impulse sources, such as a car ignition, produce short bursts that appear as horizontal streaks of dashes in a few lines.
WHAT IT LOOKS LIKE
Diagonal bands of alternating light and dark, often called herringbone because it looks like the zigzag of a fabric weave, running across the whole picture. The bands are fine and tight when the interfering frequency is high and broad and slow when it is low. They drift, roll, or pulse as the interference changes strength. A strong signal can make the picture lose color and bend at the sides, and in the worst case sync is overwhelmed and the picture tears. Impulse interference appears as short horizontal dashes scattered over a few lines, with a rhythm that matches the source.
WHERE YOU SEE IT
Over-the-air analog television in cities, near amateur or CB radio operators, cheap cable setups, and older receivers with poor shielding. It is common in documentary footage about antenna-era television, in horror and sci-fi set dressing for broadcast signals, and in music videos that need a hacked-signal look. In found-footage scenes it often accompanies the sound of a buzz or a voice breaking in, as if a rogue transmission were crossing the picture.
RECREATE IT
PHANTOM CORRUPT
CONTROLS CORRUPT WARP, PHANTOM NOISE, CORRUPT COMPOSITE
Start with CORRUPT WARP to introduce the wavy diagonal structure, then use PHANTOM NOISE to add the fine texture of the interference. Judge the result by eye against a still of a real herringbone pattern, because the controls do not map directly to a physical frequency. Keep CORRUPT COMPOSITE moderate so the pattern sits on top of the picture.
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STATIC DRIFT
CONTROLS ARCH WARP, DRIFT, STATIC CHAOS
Add ARCH WARP and DRIFT for the side-bending that appears in strong interference, and use STATIC CHAOS sparingly for short impulse-style disruptions. A herringbone is regular, so avoid heavy randomness on the base pattern. Raise the values together during a transmission-breaking moment, then bring them back to a quieter level.
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HOW TO MAKE IT IN AFTER EFFECTS
1. Pre-compose the footage and apply PHANTOM CORRUPT. 2. Raise CORRUPT WARP slowly until diagonal ripples cross the frame. 3. Add PHANTOM NOISE for the fine texture inside the bands. 4. Use CORRUPT COMPOSITE to blend the effect back so the picture remains readable. 5. Apply STATIC DRIFT above it and add ARCH WARP and a little DRIFT so the edges of the picture sway with the strongest bands. 6. Keyframe all three controls together: a quiet level, a rise over a second or two as the interference arrives, a hold at the peak, and a slow fall. 7. If you are adding a sound design with a buzz, align the peak of the keyframes to the loudest part of the sound. 8. For an impulse burst, raise STATIC CHAOS for two or three frames only. 9. Check at full size and at playback speed; the diagonal movement is a motion effect.
QUESTIONS
- Why does interference make diagonal stripes?
- The interfering signal is not synchronized with the line rate, so its phase shifts slightly from one line to the next. The shift builds a diagonal pattern across stacked lines, which looks like slanted stripes or a herringbone. A faster interfering signal produces tighter diagonals, and one that drifts in frequency makes the stripes crawl across the screen.
- What does the 920 kHz beat mean?
- In NTSC, the sound carrier sits at 4.5 MHz and the color subcarrier at 3.58 MHz. If they mix in a nonlinear stage, they leave a difference of about 920 kHz that appears as a fine dot pattern. Careful receiver design removes it.
- How do I remove interference from old television?
- Shorten the cable, add shielding or a filter, move the receiver away from the source, or switch off the nearby device. A tuned trap filter can remove a single steady interfering frequency without hurting the picture much. Interference that gets through a good antenna cable usually comes from the receiver itself or from a nearby motor, so test with the suspect device unplugged.
- Is RF interference the same as snow?
- No. Snow is random noise from a weak signal. Interference is a structured pattern caused by a specific outside signal, which is why it forms bars and herringbone instead of an even speckle. Both can appear together on a weak channel, and editors often layer the two to sell a poorly received broadcast.