Description
This guide outlines the visual artifact of scan lines, which are static lines in the LED wall visible when captured with a recording camera.
General problem
Scan lines are visible lines on the recorded image, and are a physical artifact of the relationship between the refresh rate of the LED wall and the refresh rate of the sensor recording the wall. There are no Health alerts that will be present when troubleshooting scan lines, as it is a user configuration / compatibility issue. Proper testing and monitoring of recorded images is essential to making sure there are no scan line artifacts in the recorded image.
Example of scan lines visible in a recorded image by a global sensor camera.
Example of scan lines visible in a recorded image by a rolling shutter camera.
Background
LED walls operate at a significantly higher refresh rate than motion picture recording cameras. This difference enables HELIOS to have advanced features like Camera Mode and GhostFrame, specifically for use in In-Camera Visual Effects (ICVFX). However, precise synchronization of these refresh cycles is essential to achieve the desired effect and to have a properly aligned image by the recording camera.
When the LED wall's refresh rate and the camera sensor's capture time are misaligned, scan line artifacts can occur. These artifacts are influenced by the interaction between the LED's Pulse Width Modulation (PWM) cycle and the camera sensor's charge, capture, and readout process. Additionally, the nature of these scan lines differs depending on whether the camera uses a rolling shutter or a global shutter.
Troubleshooting
Before starting to troubleshoot, it is important to check the refresh rate of the LED tiles. This refresh rate can be viewed on the Camera Mode page, underneath the Sub-frame slice configurator. This refresh rate should be as high as possible, in order to get the cleanest capture for the imaging camera. Generally, a refresh rate of > 7680Hz is recommended. If your refresh rate is somewhere in the range of 3840Hz, that means there is less range for the camera sensor to get a clean capture of the LED wall. If you are seeing scan lines with the naked eye, please contact support@megapixelvr.com with your system log files.
Recommended Troubleshooting Steps
1. Ensure External Sync is active on HELIOS and camera - Check that external sync on HELIOS is activated, and that there is a matching sync signal with the same rate as the content. Make sure that the camera recording frame rate is set to exactly the same rate, and receiving the same sync signal from the same Genlock clock.
2. Adjust the Sync Delay - (Input > Nanosync) Start with microsecond (us) adjustments and then move to smaller units as necessary. The goal here is to change the phase rate of the LED wall, so that it is more in-phase with the camera sensor rate. This can be tricky with rolling shutter camera sensors, as they have a longer capture window with a rolling readout from the sensor (as compared to a global sensor). This may require changing the shutter speed of the camera to get a cleaner capture.
3. (Global sensor cameras only) Make adjustments in camera mode - With global sensor cameras, you can take advantage of their instant sensor readout to synchronize their refresh rate with the LED refresh rate, and make adjustments using sub-frames in camera mode. You can also add black subframe slices to help improve image quality. For example, in the screenshot below, the charge time of the sensor was determined to be occurring during slices 4 and 5. Turning those to black resolved the scan line issue because the LEDs are not emitting light during that time, and having them go to black gives the camera enough time to get a clean capture of the wall.
4. If scan lines are still visible, adjust the shutter angle/shutter speed of the camera - The user should make the shutter angle smaller / shutter speed faster to get a quicker camera capture.
NOTE: If adjusting the Sync Delay in a multi-camera set up, keep in mind that the HELIOS is moving time the same for all cameras. If scan lines are still appearing in other camera views, use the sync generator to offset these individually.
If you continue to experience issues, please reach out to support@megapixelvr.com and include your system's log files for further assistance.
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