Coded data generation or conversion – Digital code to digital code converters – To or from code based on probability
Reexamination Certificate
2000-04-07
2001-12-25
Young, Brian (Department: 2819)
Coded data generation or conversion
Digital code to digital code converters
To or from code based on probability
C341S051000
Reexamination Certificate
active
06333705
ABSTRACT:
The present invention relates to a method and device for coding digital signals, to a method and device for decoding these same digital signals, and to systems using them.
The digital signals in question can be images, video signals, sound signals or data. Here the invention will be described here more particularly in its application to digital signals representing images.
In the field of digital signal coding, it is sometimes advantageous to use several coding techniques for coding the same set of data. In such case, by virtue of a selection method, based for example on an optimum transmission rate allocation criterion, the best coding technique is chosen and applied locally.
For the decoder to be able to correctly decode the coded signals which it receives, an item of information referred to as a “flag” is, conventionally, generally transmitted, making it possible to know which coding technique has been used on each signal portion.
Thus, for example, in accordance with the prior art, a digital image can be divided into blocks, and, for coding each block, it is possible to have a choice between coding by vector quantisation and coding by discrete cosine transform (DCT). Each block is coded by the most appropriate method within the terms of a criterion fixed a priori, and a flag is transmitted with the image signal, in order to indicate, for each block, whether vector quantisation or DCT has been used.
The flag is for example a word able to take as many different values as there are different coding techniques used for coding the blocks.
This way of proceeding according to the prior art notably has the drawback of requiring the transmission of a large number of flags, namely as many flags as there are coded blocks, and therefore reducing the degree of compression of the signals, which consequently limits the transmission rate.
The aim of the present invention is to remedy the aforementioned drawbacks, by eliminating the need to transmit, routinely and explicitly, additional information indicating the coding technique used, in order to arrive at a better compromise between the compression and distortion rates of the signals.
To this end, the present invention proposes a method of coding a digital signal comprising steps of:
dividing the signal into a plurality of subsignals,
choosing a coding technique to code the subsignals among a plurality of coding techniques, so as to produce a stream of coded subsignals,
checking if the decoding of coded subsignals by at least two different decoding techniques associated to the coding techniques is acceptable, and
if at least the decoding of one coded subsignal by at least two different techniques is acceptable, inserting an item of information in the stream of coded subsignals making it possible to select, during decoding, a decoding technique associated with the coding technique used for coding said subsignal.
Thus the coding method takes place in two main phases, including a coding phase proper and a decoding simulation phase, during which there is added, not routinely but only in certain cases, an item of information relating to the coding method used, which makes it possible to reduce the quantity of information produced during coding and therefore to improve the compression rate of the signal.
As a variant, the decoding simulation phase could be omitted, which would make it possible to make the coding and decoding faster, at the risk of obtaining a decoding of lower quality.
According to a particular characteristic, the digital signal is divided into a plurality of frequency sub-bands, themselves divided into blocks constituting the aforementioned subsignals, these subsignals corresponding respectively to the aforementioned plurality of frequency sub-bands.
According to a particular characteristic, in the checking step, a likelihood criterion is used for checking if the decoding of coded subsignals by at least two different decoding techniques associated to the coding techniques is acceptable.
In a particular embodiment, the likelihood criterion consists of:
comparing the value of the distortion between a decoded digital subsignal and adjacent subsignals with a predetermined threshold, and
deciding that the decoded signal is likely if the value of the distortion is less than or equal to this threshold, and
deciding that the decoded subsignal is not likely if the value of the distortion is strictly greater than this threshold.
Such a likelihood criterion makes it possible effectively and simply to eliminate the inappropriate decoding techniques, which result in very high distortion values, reflecting the fact that the decoding technique tested does not correspond to the coding technique used at the start.
In a particular embodiment, in which the digital signal is an image signal and each subsignal is a block of coefficients, the value of the distortion is calculated from the root mean square error between two vectors of coefficients situated along the boundary of one or more blocks.
In this case, a threshold making it possible to decide on the likelihood of the decoded signal can be chosen for example as a function of the nature of the images.
For the same purpose as before, the present invention also proposes a digital signal coding apparatus, comprising:
dividing means for dividing the signal into a plurality of subsignals,
choosing means for choosing a coding technique to code subsignals among a plurality of coding techniques, so as to produce a stream of coded subsignals,
checking means for checking if the decoding of coded subsignals by at least two different decoding techniques associated to the coding techniques is acceptable, and
if at least the decoding of one coded subsignal by at least two different techniques is acceptable, inserting means for inserting an item of information in the stream of coded subsignals making if possible to select, during decoding, a decoding technique associated with a coding technique used for coding said subsignal.
According to a particular characteristic, the dividing means for dividing the digital signal include means for dividing the digital signal into a plurality of frequency sub-bands, themselves divided into blocks constituting said subsignals corresponding respectively to said plurality of frequency sub-bands.
According to a particular characteristic, said checking means checks if the decoding of coded subsignals by at least two different decoding techniques associated to the coding techniques is acceptable by using a likelihood criterion.
According to a particular characteristic, the likelihood criterion consists of:
comparing the value of the distortion between a decoded digital signal and adjacent subsignals with a predetermined threshold, and
deciding that the decoded subsignal is likely if the value of the distortion is less than or equal to said threshold, or
deciding that the decoded subsignal is not likely if the value of the distortion is strictly greater than said threshold.
In a particular embodiment, in which the digital signal is an image and each subsignal is a block of coefficients, the value of the distortion is calculated from the root mean square error between two vectors of coefficients located along the boundary of one or more blocks.
The present invention also proposes a method of decoding a coded digital signal, this coded signal resulting from an original digital signal decomposed into a plurality of subsignals, each subsignal being coded by means of a coding technique among a plurality of coding techniques, the method comprising the steps of:
applying a plurality of decoding techniques to at least one coded subsignal, and
if at least two decoding techniques are acceptable for at least one coded subsignal, reading in the stream of coded subsignals an item of information making it possible to select a decoding technique among the decoding techniques.
According to a particular characteristic, the decoding method further comprises a checking step for checking if at least two decoding techniques are acceptable for at least one coded subsignal, and wherein in the check
Amonou Isabelle
Henry Felix
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Young Brian
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