Computer monitor scrolling effect on television displays

The computer monitor scrolling effect on television displays refers to the visible flicker, distortion, or rolling bands that appear when a computer screen or monitor is filmed or broadcast on television. This phenomenon, often called the rolling or scrolling bar effect, results from the mismatch between the refresh rate of the computer monitor and the frame rate of the television or video camera capturing the image.
This optical interference produces a visual effect resembling a slow-moving band or flickering wave across the screen, and understanding it requires knowledge of how display and recording technologies synchronise image generation and capture.

Basic Principle of the Effect

Computer monitors and television cameras generate and display images through repetitive scanning cycles. However, the timing of these cycles—the rate at which images are refreshed or recorded—differs between the two systems.

• A computer monitor updates the display several times per second according to its refresh rate (measured in hertz, Hz).
• A television camera or video recorder captures images at a certain frame rate (measured in frames per second, fps).

If these two rates are not perfectly synchronised, the camera captures incomplete or varying portions of the monitor’s image during each frame exposure. As a result, viewers see bright or dark horizontal or diagonal bars slowly moving or “scrolling” across the screen.

Technical Explanation

1. Refresh Rate of Monitors:
   • The refresh rate indicates how many times per second a monitor redraws the image.
   • Common refresh rates for computer displays include 60 Hz, 75 Hz, 120 Hz, and 144 Hz.
   • Older CRT (Cathode Ray Tube) monitors use electron beam scanning lines from top to bottom, while modern LCD/LED displays refresh all pixels electronically but still adhere to fixed frequency cycles.
2. Frame Rate of Cameras and TVs:
   • Television and video cameras record at standard frame rates such as 25 fps (PAL system) or 30 fps (NTSC system).
   • Each frame corresponds to a fraction of a second of exposure.
3. Interference Pattern Formation:
   • When the camera’s frame rate and the monitor’s refresh rate are not synchronised, parts of the image are captured mid-refresh.
   • The difference between the two frequencies produces a beat frequency, which appears as a moving dark or bright band.
   • The speed and direction of the band depend on the difference between the two rates.

For example, if a monitor refreshes at 60 Hz and the camera captures at 30 fps, the mismatch creates a visible interference pattern because the camera captures only half of the monitor’s refresh cycle at each exposure.

Types of Scrolling Effects

1. Horizontal Scrolling Bars: Occur when the refresh and frame rates differ slightly, causing the light intensity of captured frames to vary along horizontal lines.
2. Vertical Banding: Seen when filming modern LCDs or LED walls with certain shutter speeds; vertical bands of light and dark appear due to non-uniform backlight scanning.
3. Diagonal or Moving Waves: Result from rolling shutter effects in cameras, where different parts of the image sensor record light at slightly different times.
4. Flickering: Appears when the refresh rate is significantly out of phase with the camera frame rate, producing rapid brightness fluctuations.

Factors Influencing the Effect

• Display Technology: CRTs exhibit stronger scrolling effects because of line-by-line electron scanning, while LCDs and OLEDs show subtler interference due to global refresh.
• Refresh Rate Difference: The greater the difference between the display’s refresh rate and the camera’s frame rate, the more noticeable the effect.
• Camera Settings: Shutter speed, exposure duration, and frame synchronisation significantly affect visibility. Shorter shutter speeds tend to make the banding more prominent.
• Lighting and Brightness: Higher contrast or brightness levels accentuate the rolling bands.
• Power Supply Frequency: In some cases, the mains power frequency (50 Hz in Europe and India, 60 Hz in the US) influences the display refresh and can worsen the mismatch with camera timing.

Example Scenario

When a computer monitor operating at 60 Hz is filmed by a television camera recording at 25 fps, the refresh cycles and frame captures fall out of phase. The camera captures portions of the monitor’s refresh process at different brightness levels in each frame. This produces a moving light band that seems to roll from the top to the bottom of the screen every few seconds.
The same phenomenon is often seen during live news broadcasts, where a television camera pans across a newsroom displaying computer monitors or LED walls.

Methods to Reduce or Eliminate the Effect

1. Synchronising Refresh and Frame Rates:
   • Adjust the monitor’s refresh rate to match the camera’s frame rate (e.g., 50 Hz for PAL regions, 60 Hz for NTSC).
   • Many professional setups use genlock (generator locking) to synchronise timing signals between devices.
2. Camera Shutter Adjustment:
   • Change the camera’s shutter speed to correspond with the monitor’s refresh frequency (e.g., 1/50 or 1/60 second).
   • This helps capture full cycles of the monitor’s light output per frame.
3. Use of Flicker-Free Monitors:
   • Modern monitors designed for broadcast environments operate with flicker-free backlighting and global refresh synchronisation.
4. Software Correction:
   • Video editing software can apply de-flicker filters to minimise visible scrolling in recorded footage.
5. Lighting Control:
   • Avoid fluorescent or strobe lighting that may introduce additional flicker at power line frequencies.

Modern Context and Technology

With the shift from CRT to LCD, LED, and OLED displays, the scrolling effect has become less pronounced but not entirely eliminated. Modern digital cameras with global shutters, adaptive exposure, and synchronisation options significantly reduce interference.
In professional broadcasting, devices such as frame synchronisers and scan converters ensure that signals from computer displays are adjusted to match television standards before transmission.
Furthermore, HDMI capture systems and screen mirroring technologies bypass optical filming altogether, allowing direct digital transmission of display content, thereby completely avoiding the scrolling effect.

Significance and Understanding

While the computer monitor scrolling effect on television displays may appear as a minor visual imperfection, it illustrates the fundamental principles of synchronisation between electronic systems. It demonstrates how differences in frequency, timing, and image processing can produce perceptible interference patterns.
The phenomenon remains relevant in fields such as broadcast engineering, cinematography, and display design, where precise synchronisation is essential for high-quality visual output.