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Recording
While a proportion of surveillance systems are continuously monitored, these
are in the minority. The majority of systems are not actively monitored and,
even when they are, in most cases, the images are still recorded.
The underlying principle to be considered in relation to the recording of images is
that they may ultimately be required as evidence in criminal proceedings. Despite
sometimes poor quality recordings, many spectacular successes have recently
been achieved in the detection and conviction of serious offenders, thanks to the
existence of recording material.
Analogue Recording
The preferred medium for recording and storing activity has been, and still is, VHS
videotape, usually recorded on a time-lapse video cassette recorder (VCR).These
machines are very similar to domestic video recorders but have extended record
and playback facilities, time and date encoding and alarm inputs.
The extended recording facility is achieved by reducing the frequency of fields
that are recorded, thereby extending the recording period of the tape.The number
of fields is further reduced when the VCR is recording multiple cameras from, for
example, a multiplexer.
VHS VCRs record around 300 TV lines horizontal resolution in monochrome and
240 TV lines in colour.The requirement to record higher resolution images has led
to an increase in the use of Super-VHS VCRs which record about 400 TV lines.
These are recommended where it is intended to record vehicle registration
numbers or other fine points of detail.
VCRs have many moving parts which require regular service to maintain
performance. For this reason, it is recommended that VCRs are serviced once a
year.
Digital Recording
Increasing demand for higher quality, more flexibility and lower maintenance has
encouraged manufacturers to develop recording equipment which more closely
matches the needs of surveillance systems.
This new technology is based on digital processing and storage and, in most
cases, produces higher quality images than analogue systems. Greatly increased
storage capacity also allows higher picture refresh rates and selectable resolution
modes. Camera inputs are multiplexed, stored on a hard disk and, periodically
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The Sony Guide to CCTV
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Issue 3