Method and apparatus for decoding a compressed HOA representation, and method and apparatus for encoding a compressed HOA representation

A method for encoding frames of an input Higher Order Ambisonics (HOA) signal having a given number of coefficient sequences, where each coefficient sequence has an index, comprising

• determining a set of indices of active coefficient sequences (IC,ACT(k)) to be included in a truncated HOA representation;
• computing the truncated HOA representation (CT(k)) having a reduced number of non-zero coefficient sequences;
• estimating from the input HOA signal a first set of candidate directions (MDIR(k));
• dividing the input HOA signal into a plurality of frequency subbands (f1, . . . , fF), wherein coefficient sequences ({tilde over (C)}(k−1, k, f1), . . . , {tilde over (C)}(k−1,f1) of the frequency subbands are obtained;
• estimating for each of the frequency subbands a second set of directions (MDIR(k,f1), . . . , MDIR(k,fF)), wherein each element of the second set of directions is a tuple of indices with a first and a second index, the second index being an index of an active direction for a current frequency subband and the first index being a trajectory index of the active direction, wherein each active direction is also included in the first set of candidate directions (MDIR(k)) of the input HOA signal;
• for each of the frequency subbands, computing directional subband signals ({tilde over (X)}(k−1, k, f1), . . . , {tilde over (X)}(k−1, k, fF)) from the coefficient sequences ({tilde over (C)}(k−1, k, f1), . . . , {tilde over (C)}(k−1, k, fF)) of the frequency subband according to the second set of directions (MDIR(k,f1), . . . , MDIR(k,fF)) of the respective frequency subband;
• for each of the frequency subbands, calculating a prediction matrix (A(k,f1), . . . , A(k,fF)) adapted for predicting the directional subband signals ({tilde over (X)}(k−1, k,f1), . . . , {tilde over (X)}(k−1, k, fF)) from the coefficient sequences ({tilde over (C)}(k−1, k, f1), . . . , {tilde over (C)}(k−1, k, fF)) of the frequency subband using the set of indices of active coefficient sequences (IC,ACT(k)) of the respective frequency subband;
• and
• encoding the first set of candidate directions (MDIR(k)), the second set of directions (MDIR(k,f1), . . . , MDIR(k,fF)), the prediction matrices (A(k,f1), . . . , A(k,fF)) and the truncated HOA representation (CT(k)), wherein the truncated HOA representation (CT(k)) is perceptually encoded in a perceptual encoder.

An apparatus for decoding a Higher Order Ambisonics (HOA) signal, comprising

• an Extraction module configured to extract from the compressed HOA representation a plurality of truncated HOA coefficient sequences ({circumflex over (z)}1(k), . . . , {circumflex over (z)}1(k)), an assignment vector (νAMB,ASSIGN (k)) indicating or containing sequence indices of said truncated HOA coefficient sequences, subband related direction information (MDIR(k+1,f1), . . . , MDIR(k+1,fF)), a plurality of prediction matrices (A(k+1,f1), . . . , A(k+1,fF), and gain control side information (e1(k), β1(k), . . . , eI(k), βI(k)), the Extraction module comprising a Perceptual Decoder configured to perceptually decode the encoded truncated HOA coefficient sequences ({hacek over (z)}1(k), . . . , {hacek over (z)}I(k)) to obtain the truncated HOA coefficient sequences ({circumflex over (z)}1(k), . . . , {circumflex over (z)}I(k));
• a Reconstruction module configured to reconstruct a truncated HOA representation (ĈT(k)) from the plurality of truncated HOA coefficient sequences ({circumflex over (z)}1(k), . . . , {circumflex over (z)}I(k)), the gain control side information (e1(k), β1(k), . . . , eI(k), βI(k)) and the assignment vector (νAMB,ASSIGN (k));
• an Analysis Filter bank module configured to decompose the reconstructed truncated HOA representation (ĈT(k)) into frequency subband representations ({tilde over (Ĉ)}T (k,f1), . . . , {tilde over (Ĉ)}T(k, fF)) for a plurality of F frequency subbands;
• at least one Directional Subband Synthesis module configured to synthesize for each of the frequency subband representations a predicted directional HOA representation ({tilde over (Ĉ)}D (k,f1), . . . , {tilde over (Ĉ)}D(k, fF)) from the respective frequency subband representation ({tilde over (Ĉ)}T (k,f1), . . . , {tilde over (Ĉ)}T(k, fF)) of the reconstructed truncated HOA representation, the subband related direction information MDIR(k+1,f1), . . . , MDIR(k+1,fF)) and the prediction matrices (A(k+1,f1), . . . , A(k+1,fF);
• at least one Subband Composition module configured to compose for each of the F frequency subbands a decoded subband HOA representation ({tilde over (Ĉ)}(k,f1), . . . , {tilde over (Ĉ)}(k,fF)) with coefficient sequences ({tilde over (ĉ)}n(k,fj), n=1, . . . , 0) that are either obtained from coefficient sequences of the truncated HOA representation ({tilde over (Ĉ)}T(k,fj)) if the coefficient sequence has an index n that is included in the assignment vector (νAMB,ASSIGN (k), or otherwise obtained from coefficient sequences of the predicted directional HOA component ({tilde over (Ĉ)}D(k,fj)) provided by one of the Directional Subband Synthesis module;
• and
• a Synthesis Filter bank module configured to synthesize the decoded subband HOA representations ({tilde over (Ĉ)}(k,f1), . . . , {tilde over (Ĉ)}(k, fF)) to obtain the decoded HOA representation (Ĉ(k)).

An apparatus for encoding frames of an input Higher Order Ambisonics (HOA) signal having a given number of coefficient sequences, where each coefficient sequence has an index, comprising

• a computation and determining module configured to compute a truncated HOA representation (CT(k)) having a reduced number of non-zero coefficient sequences, and further configured to determine a set of indices of active coefficient sequences (IC,ACT(k)) included in the truncated HOA representation;
• an Analysis Filter bank module configured to divide the input HOA signal into a plurality of frequency subbands (f1, . . . , fF), wherein coefficient sequences ({tilde over (C)}(k−1, k, f1), . . . , {tilde over (C)}(k−1, k, fF) of the frequency subbands are obtained;
• a Direction Estimation module configured to estimate from the input HOA signal a first set of candidate directions (MDIR(k)), and further configured to estimate for each of the frequency subbands a second set of directions MDIR(k,f1), . . . , MDIR(k,fF)), wherein each element of the second set of directions is a tuple of indices with a first and a second index, the second index being an index of an active direction for a current frequency subband and the first index being a trajectory index of the active direction, wherein each active direction is also included in the first set of candidate directions (MDIR(k)) of the input HOA signal;
• at least one Directional Subband Computation module configured to compute, for each of the frequency subbands, directional subband signals ({tilde over (X)}(k−1, k,f1), . . . , {tilde over (X)}(k−1,k,fF)) from the coefficient sequences ({tilde over (C)}(k−1, k,f1), . . . , {tilde over (C)}(k−1, k, fF)) of the frequency subband according to the second set of directions (MDIR(k,f1), . . . , MDIR(k,fF)) of the respective frequency subband;
• at least one Directional Subband Prediction module configured to calculate, for each of the frequency subbands, a prediction matrix (A(k,f1), . . . , A(k,fF)) adapted for predicting the directional subband signals ({tilde over (X)} (k−1, k,f1), . . . , {tilde over (X)}(k−1,k, fF)) from the coefficient sequences ({tilde over (C)}(k−1, k,f1), . . . , {tilde over (C)}(k−1, k, fF)) of the frequency subband using the set of indices of active coefficient sequences (IC,ACT(k)) of the respective frequency subband;
• and
• encoding module configured to encode the first set of candidate directions (MDIR(k)), the second set of directions MDIR(k,f1), . . . , MDIR(k,fF)), the prediction matrices (A(k,f1), . . . , A(k,fF) and the truncated HOA representation (CT(k)), wherein the encoding module comprises a Perceptual Encoder configured to encode the gain controlled truncated HOA representation (CT(k)).