Luma noise
Also called: video snow, analog noise, tape hiss, video noise
Luma noise is the fine, streaky grain in the brightness channel of analog video. Learn where it comes from and how to dial it in with TERMINAL FADE.
WHAT CAUSES IT
Luma noise is random variation in the brightness channel of an analog video signal, introduced by every stage that carries the signal. In a tape machine, the head reads a weak magnetic signal that needs a lot of amplification, and the amplifier and the tape itself both contribute thermal noise and magnetic grain. The luma is stored as a frequency modulated carrier, and when the demodulator turns the frequency back into brightness, noise at higher frequencies is converted into a bigger amplitude than low frequency noise, so the noise spectrum rises toward the top of the band. Decks apply a pre-emphasis and a de-emphasis stage to keep this under control, but a consumer VHS recording still has a luma signal to noise ratio only around 40 to 45 dB. Because the noise is generated along the line, it is correlated in the horizontal direction and independent between lines, so it looks like short streaks of varying brightness. Weak antenna signals show the same noise as snow, since the receiver amplifies the weak carrier together with its own thermal noise.
WHAT IT LOOKS LIKE
A fine, animated grain over the whole picture, with a distinctive horizontal grain: each speck is a short dash a few pixels wide and one line tall, not a round dot. The noise is most visible in flat, dark gray areas such as skies and walls and almost disappears on busy detail. Brightness fluctuates randomly from frame to frame, giving a crawling, shimmering texture. At low levels it reads as warm tape hiss. At high levels the picture looks as though it is viewed through falling static, and with a very weak signal the noise overwhelms the picture entirely.
WHERE YOU SEE IT
Any analog tape or broadcast source: VHS and camcorder footage, especially at long play speeds, over-the-air television with a poor antenna, early webcams, and security cameras in low light. It is the base layer of almost every lo-fi video look, since a clean picture with added tape-style noise instantly reads as old. Editors use it under other glitch effects to hold a shot together, and at heavy levels as a dead-channel texture.
RECREATE IT
TERMINAL FADE
CONTROLS GENERATIONS, DROPOUT
Raise GENERATIONS to build up the tape-style degradation, noise included, then add a little DROPOUT for bursts. Keep the level modest, because noise is the base of the look and should sit behind the picture. If you want to judge the result honestly, evaluate it on a dark flat area, where the horizontal grain shows best.
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PHANTOM CORRUPT
CONTROLS PHANTOM NOISE, CORRUPT COMPOSITE
PHANTOM NOISE adds extra noise on top of the tape treatment, and CORRUPT COMPOSITE lets you decide how much of it reaches the final picture. Use it for a stronger weak-signal look. Do not combine both noise sources at high settings, because the result turns into a uniform gray fog instead of streaky grain.
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HOW TO MAKE IT IN AFTER EFFECTS
1. Pre-compose the footage and apply TERMINAL FADE. 2. Raise GENERATIONS gradually and zoom to 100 percent on a flat, dark area of the picture, where noise is easiest to judge. 3. Watch several frames in sequence; the grain should change every frame and read as short horizontal dashes. 4. Add a little DROPOUT if you want occasional bright flecks, but keep it low. 5. For a weaker signal, add PHANTOM CORRUPT and bring up PHANTOM NOISE a small amount at a time. 6. Use CORRUPT COMPOSITE to control how much of the effect reaches the final image. 7. For a signal that gets worse over the shot, keyframe GENERATIONS upward across the shot instead of adding more effects. 8. Be careful with output compression: heavy noise is expensive to encode, so check the delivered file for blocking and raise the bitrate if necessary. 9. If you need the noise to hold up on delivery, render a test of a few seconds first.
QUESTIONS
- What is the difference between luma noise and film grain?
- Luma noise is electronic and magnetic noise in an analog video brightness signal and looks like short horizontal streaks. Film grain comes from the silver halide crystals or dye clouds in emulsion and looks like roundish clumps that are softer and more organic.
- Why does noise look worse in dark areas?
- Noise adds a roughly constant amount of variation to the signal, while dark areas carry less signal. The same noise is therefore a bigger fraction of the picture there, and flat areas have no detail to hide it. This is why noise reduction and film emulation both pay special attention to shadows and flat skies.
- Is video snow the same as luma noise?
- Snow is luma noise at a high level. When a receiver has no usable carrier, the automatic gain control turns the gain up until receiver noise fills the screen, producing the familiar black and white static. Snow therefore marks a missing signal, while ordinary luma noise is present on every analog picture to some degree.
- Can noise be removed after capture?
- Temporal noise reduction averages frames and removes much of it on still shots, but it blurs motion if pushed too far. Capturing from a clean source, with a good deck and a short connection, keeps the noise low to begin with.