INTRODUCTION: The aim of this project was to develop an embedded System which allows for the synchronization of multiple recording devices. In our case the recording devices consist of multiple cameras and a data recorder. As the cameras are spread over a wide area and the data recorder is attached to a skier, it is essential that all devices are synchronized wirelessly and that the system is robust enough for the outdoor environment. The method which is commonly used for synchronization records a jump at the beginning of the run, to have a peak signal recorded on the different devices (camera, force plate, insole pressure measurements or accelerometers). The synchronization is then recovered manually with low accuracy (about one Video frame: 0.02 s). Other available multi-camera systems are either synchronized by using cables to connect all cameras to the same trigger signal, or the unsynchronized video streams are aligned in post processing (Pourcelot, 2000). METHOD: Each of the cameras and the data recorder is attached to one of our synchronization boxes. These boxes contain a GPS receiver which provides a one-pulse-per-second Signal (PPS) which is globally synchronized. All boxes also contain a radio transceiver. After receiving a radio Signal from a designated master box, all boxes wait for the next PPS pulse from their local GPS receiver and then generate a trigger signal which is resynchronized with the PPS signal each second when a new PPS pulse is received. The system has been used to synchronize 6 cameras and multiple force plates embedded in the skier's shoes. Each camera is connected to a laptop, which records the frames with dedicated Software and the data recorder for the force plate receives the same trigger signal in order to later synchronize its recording with the camera frames. RESULTS: The PPS signal is aligned to within 15 ns around the world. Our system introduces an additional misalignment of up to 25 ns which means that our system provides synchronization which is up to 6 orders of magnitude better then the commonly used method. DISCUSSION: The system allows for fast deployments. Each camera has its own battery pack and synchronization box, saving more than 300 m of power and trigger cable. The accuracy provided is excellent and allows for good 3D reconstruction from the camera system as well as perfect matching between the video feeds and the force plate data. CONCLUSION: Our system is a new tool for high accuracy synchronization of distributed devices (possibly moving) in an outdoor environment. It can be used with any kind of device requiring synchronized recordings.
© Copyright 2010 Book of Abstracts. 5th International Congress on Science and Skiing, Dec. 14 - 19, 2010, St. Christoph am Arlberg. Veröffentlicht von University of Salzburg, Interfakultärer Fachbereich Sport- und Bewegungswissenschaft/USI. Alle Rechte vorbehalten.