User guide

SiraView User Guide
9
Discontinuities in Video
Multiplexed video is common in CCTV systems because many cameras need to be recorded
simultaneously. The recording system will switch between cameras and store some frames from
each in turn. In addition, the system may be set up so that a particular channel gets recorded at a
higher frame rate than others.
Some systems are alarm-based, which means they only record video when an alarm is triggered –
perhaps by someone opening a door, or triggering a PIR sensor.
Because of these reasons, it is possible to get video with discontinuities on one or more channels. So
channel 1 may have a fairly constant 10 frames per second of video, but channel 2 only records one
frame per minute and channel 3 only records when someone walks through the front door of a
building.
When playing back this video, channel 1 will update
at a constant rate, channel 2 will update very
slowly (with frames getting up to a minute behind
channel 1) and channel 3 will come and go as and
when it’s available.
To indicate that some frames may be out of date,
SiraView stamps any frames that are older than 1
minute with an overlay showing their age.
You can see at a glance which frames are current,
and which is not. Of course the time stamps under
the frames always give you an accurate indication
of the time, but the overlay highlights the fact that
the frame you are looking at is old.
In fact the same mechanism is used when you’re
rapidly navigating through the video using the
positioning controls – frames that are more than a
minute older than the current time (because you’ve
moved forward quicker than SiraView can update
all the frames) get flagged. When you stop moving
the positioning bar you’ll notice that as the frames
get updated, the overlay disappears.
In addition, when you navigate backwards through
the video too rapidly for SiraView, the frames are
flagged as being from the future – again to indicate
that you should wait for SiraView to catch up
before using the frame.