Method and device for arithmetic encoding or arithmetic decoding

A method for arithmetic decoding of a current spectral coefficient of a current frame using preceding spectral coefficients of the current frame and/or a previous frame, said preceding spectral coefficients being already decoded, wherein said preceding spectral coefficients and said current spectral coefficients, are comprised in one or more quantised spectra resulting from quantising time-frequency-transformed video, audio or speech signal sample values, said method comprising the steps:

• performing a first quantising in which the absolute values of the preceding spectral coefficients are quantised according a first quantisation scheme;
• determining a variance of the absolute values of the preceding spectral coefficients quantised according to the first quantisation scheme, comprising determining a sum of the absolute values of the preceding spectral coefficients quantised according the first quantisation scheme and comparing the determined sum with at least one threshold;
• using the determined variance for selecting one of at least two different non-linear second quantisation schemes;
• performing a second quantising in which the absolute values of the preceding spectral coefficients quantised according the first quantisation scheme are further quantised according to the selected non-linear second quantisation scheme;
• determining a context class out of at least two different context classes, wherein the context class is determined based on an output of the second quantising;
• determining the probability density function using the determined context class and a mapping from the at least two different context classes to at least two different probability density functions, wherein the mapping is realised using a look-up table or a hash table;
• and
• arithmetic decoding the current spectral coefficient based on the determined probability density function.

A device for arithmetic decoding of a current spectral coefficient of a current frame using preceding spectral coefficients of the current frame and/or a previous frame, said preceding spectral coefficients being already decoded, wherein said preceding spectral coefficients and said current spectral coefficients, are comprised in one or more quantised spectra resulting from quantising time-frequency-transformed video, audio or speech signal sample values, said device comprising:

• processing means adapted for:
• performing a first quantising in which the absolute values of the preceding spectral coefficients are quantised according a first quantisation scheme;
• determining a variance of the absolute values of the preceding spectral coefficients quantised according to the first quantisation scheme, comprising determining a sum of the absolute values of the preceding spectral coefficients quantised according the first quantisation scheme and comparing the determined sum with at least one threshold;
• using the determined variance for selecting one of at least two different non-linear second quantisation schemes;
• and
• performing a second quantising in which the absolute values of the preceding spectral coefficients quantised according the first quantisation scheme are further quantised according to the selected non-linear second quantisation scheme, and
• first means for determining a context class out of at least two different context classes, said first means being adapted for determining the context class based on an output of the second quantizing;
• second means for determining the probability density function, said second means being adapted for determining the probability density function using the determined context class and a mapping from the at least two different context classes to at least two different probability density functions, wherein the mapping is realised using a look-up table or a hash table;
• and
• an arithmetic decoder for arithmetic decoding the current spectral coefficient based on the determined probability density function.