Rolling Bands, a Green Cast and a 109 IRE Shirt
A production house shoots a 12-minute campus documentary at 1080/25p with a 1/50 s shutter. Two 50 Hz LED panels and a large window light the interviews. The editor finds horizontal rolling bands on three interview clips and a green cast on another. Deliverables are a 1080/50i broadcast master with EBU bars and 1 kHz tone, and a web file. The client also wants a green-screen segment with a student against a chroma key. The camera records 8-bit 4:2:0 Long GOP at 50 Mb/s. The graphics logo is pure RGB 0,255,0, and the student's white shirt reads 109 IRE on the waveform. The three banded clips were shot at 1/1000 s to hold the bright window; the rest were shot at 1/50 s.
1. Diagnose the rolling bands and the green cast. Which problems can post fix, and which require a reshoot? Justify.
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The rolling bands are temporal aliasing, not a sensor fault. At 1/50 s each frame integrates about 20 ms, two full cycles of 100 Hz ripple from 50 Hz mains, so flicker largely cancels. At 1/1000 s each frame integrates only 1 ms, sampling a different part of the 50 Hz/100 Hz waveform on every frame, so brightness varies across the frame and rolls. Dimmed LEDs with low-frequency PWM make it worse. Post deflicker can reduce the bands, but on a moving face it smears detail and rarely removes them completely. If these three clips are key interviews, reshoot at 1/50 s with the LED dimming disabled or the fixtures on DC, and test 10 seconds before rolling.
The green cast is likely a white-balance or mixed-light error: 3200 K panels against 5600 K daylight. If the green channel is not clipped, post can correct it with a white-balance or offset adjustment and check on a parade. If the green channel is clipped, the detail is gone and no grade will recover it. Check the waveform first; if green is clipped on skin, reshoot.
2. Explain how to deliver a broadcast-legal master. What is wrong with 109 IRE and pure RGB 0,255,0, and what will you change?
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Broadcast legal white is 100 IRE (235 in 8-bit video), and black is 0 IRE (16). A shirt at 109 IRE is over white: highlight detail is clipped and the signal is illegal. The approximate correction is log2(1.09) ≈ 0.12 stop, but because the highlight is already clipped, reducing exposure in post cannot restore detail. The practical fix is to re-grade the clip down, or better, re-expose at capture with zebras at 95–100 IRE and a waveform, using ND or a slight stop-down and then relighting.
Pure RGB 0,255,0 is a computer-display green, outside the broadcast gamut and unsuitable for a key. Use a legal, slightly desaturated green such as RGB 0,180,0 and check it on a vector scope. Keep all graphics inside 16–235 and chroma within legal bars; use a broadcast-safe colour preset and a legaliser at the end of the grade, but do not rely on the legaliser to repair clipped whites. Lay EBU bars and 1 kHz tone at the head of the master and check the full programme on a waveform and vector scope.
3. The chroma-key shot must key cleanly from 8-bit 4:2:0 material. What are the limitations, and what is your shooting and post plan?
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In 4:2:0, chroma is sampled at half resolution horizontally and vertically, so key edges around hair and shoulders are coarser and more prone to jagged or soft mattes. 8-bit also gives limited chroma precision, so uneven green and spill become visible after keying. If the camera offers 4:2:2, use it for this shot even at a higher bitrate; otherwise treat 4:2:0 as a constraint to design around.
Shooting plan: use a clean, evenly lit green screen with no folds, kept within about 1/3 stop across the area; place the student at least 1.5 m from the screen to reduce spill; light the subject separately with a key, fill and backlight; avoid green clothing and shiny surfaces; lock the camera and keep the subject's movement controlled. Expose the green so it is bright but not clipped, and check the green channel on the waveform.
Post plan: use a dedicated keyer with edge refinement and spill suppression, garbage-matte the area, and soften the edge only as much as needed. Do not rely on a single chroma-key filter. If the shot is critical, rent a 4:2:2 recorder for the day; that is cheaper than a failed key.