Recently, I had two encounters with dash camera memory cards.
Both were supposed to contain a video recorded immediately before a road accident.
However, in both cases, there was none.
After extensive data recovery attempts on both cards, I am reasonably sure there was no video,
neither readable normally nor available for recovery.
Thinking back, I recall these situations are quite common.
I had many cases like this in recovery, and
in quite a few of these cases, the video was just not there, never recorded.
A dash camera, like most cameras, uses some form of buffering.
The camera does not flush data to the card immediately.
MP4 format, most commonly used in dash cameras,
stores several separate sets of data in each file, called atoms.
The file holds actual video frames in one part and various reference tables in another.
This layout implies buffering if it is to work in real-time.
Buffering is fine as long as there is time to flush buffered data to permanent storage
(memory card in this case) before shutdown.
It does not work if the shutdown is abrupt because the most recent data gets lost.
Exactly how much is lost depends on the aggressiveness of the buffering.
More aggressive buffering is more efficient and allows less capable CPUs to still handle the task.
At the same time, more aggressive buffering means more data lost in an unexpected shutdown.
In a road accident a dash camera may fail in many different ways.
The camera may lose power, or the card may get disconnected from the camera by the jolt.
While a battery typically mitigates the loss of power (even a small battery does the job),
other failures still cause data loss.
This is why a dash cam is not that reliable in case of an accident.
Aircraft black boxes, for comparison, are specifically designed to continue working throughout
the accident as it unfolds and retain the data after that.
Dash cameras are typically not designed for that.
Filed under: File carving.
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