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Surveillance - glossary of terms

Compression:

A number of methods to reduce the size of digital information.

Conditional refresh:

Technique used in slow and fast scan where only small screen changes are transmitted. Up to a certain percentage of the on-screen picture can be updated before a full picture is required.

Duplex:

(Multiplexer) - a multiplexer with two frame stores, allowing it to show multi-screen pictures while performing time multiplex recording.

Ethernet:

A standard for networking information technology equipment. Information is sent using data packets and Ethernet defines the sending, receiving and error correction of these packets, along with the protocols to avoid collisions of the data whilst on the network. The pieces of equipment, or nodes, on the network are addressed using the TCP/IP protocol.

Field:

One half of a frame, consisting of either the odd or the even numbered line, 50 fields are transmitted every second.

Frame:

One complete TV picture made up of approximately 625 lines. 25 frames are transmitted every second.

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