Audio encoder and audio decoder using coding contexts with different frequency resolutions and transform lengths

1. An audio encoding apparatus for encoding segments of coefficients, the segments of coefficients representing different time or frequency resolutions of a sampled audio signal, the audio encoding apparatus comprising

a provider for providing the segments of coefficients from an audio stream, the coefficients forming a spectral representation of an audio signal represented by the audio stream at a spectral resolution varying among the segments;

a processor for deriving an entropy coding context for a currently encoded coefficient of a current segment based on a previously encoded coefficient of a previous segment, the previously encoded coefficient representing a different time or frequency resolution than the currently encoded coefficient; and

an entropy encoder for entropy encoding the current coefficient based on the entropy coding context to acquire an encoded audio stream,

wherein the entropy encoder is adapted for encoding the current coefficient in units of a tuple of spectral coefficients and for predicting a range of the tuple based on the entropy coding context,

wherein the entropy encoder is adapted for dividing the tuple by a predetermined factor as often as necessitated to fit a result of the division in a predetermined range and for encoding a number of divisions necessitated, a division remainder and the result of the division when the tuple does not lie in the predicted range, and for encoding a division remainder and the result of the division otherwise, and

wherein the entropy encoder is adapted for encoding the result of the division or the tuple using a group index, the group index referring to a group of one or more codewords for which a probability distribution is based on the entropy coding context, and, based on a uniform probability distribution, an element index in case the group comprises more than one codeword, the element index referring to a codeword within the group, and for encoding the number of divisions by a number of escape symbols, an escape symbol being a specific group index only used for indicating a division, and for encoding the remainders of the divisions based on a uniform probability distribution using an arithmetic coding rule.

2. An audio decoding apparatus for decoding an encoded audio stream to acquire segments of coefficients representing different time or frequency resolutions of a sampled audio signal, comprising

a processor for deriving an entropy coding context for a currently decoded coefficient of a current segment based on a previously decoded coefficient of a previous segment, the previously decoded coefficient representing a different time or frequency resolution than the currently decoded coefficient; and

an entropy decoder for entropy decoding the current coefficient based on the entropy coding context and the encoded audio stream,

wherein the processor is adapted for deriving the entropy coding context based on the previous coefficient, the coefficients forming a spectral representation of an audio signal represented by the audio stream at a spectral resolution varying among the segments,

wherein the processor is adapted for deriving the entropy coding context for the current coefficient, based on neighbouring spectral coefficients previously decoded in one or more of the previous segment and the present segment, and

wherein the entropy decoder is adapted for decoding a group index from the encoded audio stream based on a probability distribution derived from the entropy coding context, wherein the group index represents a group of one or more codewords, and for, based on a uniform probability distribution, decoding an element index from the encoded audio stream if the group index indicates a group comprising more than one codeword, and for deriving a tuple of spectral coefficients of the current segment based on the group index and the element index, thereby acquiring the spectral domain representation in tuples of spectral coefficients.