3.4: Semi-adaptive Dictionary

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Page ID: 50167

Paul Penfield, Jr.
Massachusetts Institute of Technology via MIT OpenCourseWare

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The static-dictionary approach requires one dictionary, defined in advance, that applies to all messages. If a new dictionary could be defined for each message, the compression could be greater because the particular sequences of symbols found in the message could be made into dictionary entries.

Doing so would have several drawbacks, however. First, the new dictionary would have to be transmitted along with the encoded message, resulting in increased overhead. Second, the message would have to be analyzed to discover the best set of dictionary entries, and therefore the entire message would have to be available for analysis before any part of it could be encoded (that is, this technique has large latency). Third, the computer calculating the dictionary would need to have enough memory to store the entire message.

These disadvantages have limited the use of semi-adaptive dictionary compression schemes.