Pulse or digital communications – Repeaters – Testing
Patent
1988-06-13
1990-03-20
Griffin, Robert L.
Pulse or digital communications
Repeaters
Testing
375122, 341 67, H03M 740
Patent
active
049107518
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of reversibly compressing a sequence x(k) of information-carrying symbols which can assume Q discrete values, k being an index which assumes integral number values, the method being that the sequence x(k) of information-carrying symbols is processed in means such as to form a residue w(k), and that the residue w(k) is encoded to form a compressed sequence n(k) of information-carrying symbols which contain the same information as the original sequence x(k).
The invention also relates to an apparatus for carrying out the method.
In practically all connections where digital information is handled, there is the interest of representing the information with as few characters as possible, in most cases with binary digits. This is the case, for example, in storing information in different memories, when it is desired to store as much information as possible in a given memory space. Another case is in the transmission of such as digitally coded speech and image information, when it is desired to transmit as much information as possible with as few bits as possible in order to decrease the transmission rate requirements.
BACKGROUND ART
A known method of compressing digital information is to use a compression technique which is called "predictive coding", and has been described, e.g. in P. Elias: Predictive Coding, Part I and II, IRE Trans. Info. Theory, Vol IT-1, March 1955, pages 16-33, and US-Pat. No. 2,605,361, C. C. Cutler, 1952. This technique includes two operations comprising: information as the original sequence of digital signals,
DISCLOSURE OF THE INVENTION
The sequence of residues in operation 1 is formed by predicting the value of following digital signals in the sequence with the aid of the previous digital signals in the sequence. The difference between the actual digital signal and the predicted signal constitutes the residue. The object of achieving the sequence of residues is to reduce the redundancy in the original sequence of digital signals. In the second operation the residues are encoded, the encoding suitably taking place with code words of varying length, so that residues which occur often are encoded with shorter code words than the residues occuring more seldom.
The object of the present invention is to provide a method and apparatus, with which improved reversible data compression can be achieved.
The object is achieved by a method described in the introduction, characterized by a module Q being carried out on the residues before encoding.
The object is also achieved by an apparatus for carrying out the method, and is characterized by means arranged before the encoder for carrying out a module Q operation.
A higher compression degree can be achieved with this method and apparatus than what is possible with merely reversible predictive coding. The compression degree which can be achieved is determined by the entropy of the sequence w(k) of digital signals after the module Q operation has been executed. The first order entropy, which corresponds to an encoder which encodes w(k) separately for successive values of k is equal to: ##EQU1## where P.sub.w (j) is the relative frequency of w(k)=j, where j=0, 1, . . . , Q-1. The entropy will be lower, and thus the compression degree higher for w(k) that for w(k). This is a consequence of several values of w(k) resulting in the same value of w(k).
For example, if the case is assumed where the next digital signal, i.e. x(k), is predicted to have the same value as the previous digital signal, i.e. x(k-1), the residue thus being w(k)=x(k)-x(k-1), and that Q=256, the residue w(k) will assume the integral number values in the interval (-255, 255). The reduction modulo 256 signifies that w(k) is limited to the interval (0, 255), e.g. by w(k)=-17 and w(k)=239, both giving w(k)=239.
If w.sub.1 (k)=j.sub.1 and w.sub.2 (k)=j.sub.2 both are allowed to result in w(k)=j, the entropy for w(k) will include two terms: (j.sub.2)],
Since compression degree for the sequence of digital signals which has b
REFERENCES:
patent: 3569955 (1971-03-01), Maniere
patent: 4532638 (1985-07-01), Lagger et al.
P. Elias, "Predictive Coding", Parts I and II, IRE Trans. Info. Theory, vol. IT-1, Mar. 1955, pp. 16-33.
Patent Abstract of Japan, vol. 8, No. 66, (E-234), abstract of JP 58-215822, published Dec. 15, 1983.
Griffin Robert L.
Huseman Marianne
Telefonaktiebolaget L M Ericsson
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