Vertical roll
Also called: vertical hold loss, rolling picture, frame roll, V-hold
Vertical roll is the picture scrolling up or down when a display loses vertical sync. See how it works and how to recreate it with STATIC DRIFT.
WHAT CAUSES IT
Vertical roll is the continuous scrolling of the picture up or down the screen that happens when a display loses lock on the vertical synchronization signal. The vertical sync is a train of wide pulses lasting about three lines in the vertical blanking interval, repeating once per field, at 59.94 Hz on NTSC. The display's vertical oscillator runs free at a nearly matching rate and is pulled into step by those pulses. If the sync pulses are weak, corrupted, absent, or arrive at a rate the oscillator cannot follow, such as when a tape source is paused or a signal passes through a processor that strips sync, the oscillator drifts against the incoming fields. The picture then starts each field at a different point and the frame boundary, with its black bar of blanking, travels slowly across the screen. The vertical hold control on old sets adjusted the free-running frequency to pull the oscillator back into lock.
WHAT IT LOOKS LIKE
The whole picture slides up or down, and a thick black horizontal bar, the vertical blanking interval, passes through the frame as the image wraps around. The bar may be a clean black stripe or have a ragged edge with some sync pulses visible as a white line. At slow rates the picture moves a few lines per frame and seems to float, and at fast rates it flickers and blurs from bottom to top. Sometimes the roll stops and starts, jumps one frame position, then settles. The picture content remains intact, only its vertical position changes.
WHERE YOU SEE IT
Old CRT televisions with a vertical hold knob, camera feeds into a monitor with a different frame rate, bad switchers, and film shot of a television screen with an out-of-sync camera. A rolling picture is a standard moment in horror films and sci-fi sets to indicate a malfunctioning broadcast, as well as in music videos and transitions that mimic a CRT turning on or flipping.
RECREATE IT
STATIC DRIFT
CONTROLS DRIFT ROLL, DRIFT HOLD, STATIC COMPOSITE
DRIFT ROLL provides the vertical scrolling and DRIFT HOLD lets you steady or release it, similar to turning the vertical hold knob. Start with a slow roll so the picture crawls a few lines per frame. Use STATIC COMPOSITE to mix the result. Avoid very fast rolls, which turn into a blur and lose the sense of a locked picture slipping.
PAY WHAT YOU CAN $29 SUGGESTED NAME YOUR PRICE
HOW TO MAKE IT IN AFTER EFFECTS
1. Pre-compose the footage and apply STATIC DRIFT. 2. Raise DRIFT ROLL gradually until the image scrolls vertically. Choose a slow speed first. 3. Watch the frame wrap as the picture scrolls; the seam where the bottom meets the top is what sells the effect, so check it at full size. 4. Use DRIFT HOLD to settle the picture, and keyframe it to simulate someone adjusting the hold knob: start rolling, ease toward locked, then lose it again. 5. Hold the final state for a few frames so the audience sees the picture steady. 6. Add a small amount of STATIC BREAK for a worse signal if you need it. 7. If the roll is a transition, place the effect on a pre-comp that covers the cut, and keyframe the roll to start before and end after the edit point. 8. Check that the image wraps cleanly with no visible gaps at the top and bottom.
QUESTIONS
- What causes a picture to roll vertically?
- The display has lost lock on the vertical sync pulses. Without them its oscillator runs at its own rate, so each field starts at a different position and the frame appears to scroll. The cause can be a bad cable, a stripped sync signal, or a tape paused mid-frame, and in each case the display simply stops receiving a reliable once-per-field marker.
- How was vertical roll fixed on old TVs?
- By turning the vertical hold knob, which adjusted the free-running frequency of the oscillator until it fell into step with the incoming sync. Later televisions locked automatically and the control was removed. On some sets the hold control also had to be reset after a channel change, because each new signal arrived with slightly different timing and needed a fresh lock.
- Why does a TV filmed by a camera roll?
- The camera's shutter is not in step with the display's refresh, so the camera captures the screen at a different part of each field, creating dark bars that move up or down. This is a separate effect from a sync loss in the signal itself.
- Is vertical roll the same as horizontal tearing?
- No. Roll moves the whole picture vertically because of vertical sync loss. Tearing displaces horizontal bands sideways because of horizontal sync problems. They often appear together when sync degrades, but they are separate faults: roll is vertical sync, tearing is horizontal sync, and each has its own control in the hardware and in the plugin.