Joshua Cohen is a Principal at Fat Pencil Studio
feature image by Flickr user Sean MacEntee
A couple months ago I wrote about space junk, and how GPS satellites have become super important to the functioning of our society. One big reason for this is their role in setting network time. A small number of extremely accurate atomic clocks generate timing signals that are synchronized via radio and satellite systems with computer servers that are equipped with a Universal Time (UTC) receiver. These servers can then be used to synchronize the clocks of other computers around the world using Network Time Protocol (NTP).
The primary time and frequency standard for the United States is a cesium fountain atomic clock developed at the NIST laboratories in Boulder, Colorado. Image Credit: NIST/Geoffrey Wheeler
When a security camera captures video footage it is typically saved to a digital video recorder (DVR) device. Often this consists of a hard drive located near the camera, but there are newer devices (IP cameras) that transit video footage over the internet to a remote cloud storage location. All IP cameras, and most newer DVR devices get timing information from a highly accurate network time server. That means the time stamps shown on the video is accurate to within a second of the actual time the event occurred. Some older DVRs do not connect to a network time server and instead rely on an internal clock for timing info. These clocks are less accurate and can drift from actual time, or be set incorrectly by users.
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Legacy security systems must be connected to a power source and send video footage to a DVR system. Image credit: Wikipedia user Hustvedt
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Modern cameras can operate on battery power and be placed anywhere within range of a wifi router. Image credit: The Verge / Arlo
I was curious about the accuracy of time stamps of video collected on buses and light rail vehicles, so I called up a data analyst at TriMet. He handles requests for security video from TriMet vehicles and light rail stations. I learned that all TriMet stationary cameras as well as bus cameras get their timestamp from network time servers. His experience is that the timing of events seen on these cameras correlate very closely with the timing of events in 911 transcripts and police dispatch data.
TriMet does have a few cameras in older light rail vehicles that do not sync with network time. The clocks on these DVRs are set by maintenance staff, and it's been his experience that they are not always set correctly or may drift over time. When video is needed from one of these devices, he follows a process of comparing network time with the DVR time at the time video is downloaded to find a time offset.
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MAX Train Security Camera
Security camera captures police action leading up to arrest in civil rights case.
Unfortunately, most of the video footage that we review for court cases is not so well documented. Many businesses, including gas stations, convenience stores, and night clubs use older DVR systems that either cannot synchronize with network time, or are not configured property. Videos captured on cell phones might not have any time stamp. And so we look for events that we can see and/or hear in the video to get clues about the actual time, and to synchronize clips. This approach takes time, but is more reliable than assuming that time stamp information that may appear on the video clips is correct. For more on this, check out this case study:
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Police Protest
How we used video footage to investigate the circumstances of an arrest and then explain the story to a jury