Patent ID: 8046217

Claim:
An audio decoder which decodes a bitstream and reproduces two audio signals, the bitstream including (i) first coded data indicating a downmix signal obtained by downmixing the two audio signals, (ii) second coded data indicating a gain ratio D between the two audio signals, and (iii) third coded data indicating a phase difference θ between the two audio signals, wherein the third coded data is a value of cos θ associated with the phase difference θ, wherein said audio decoder comprises: a decoding unit operable to decode the first coded data into the downmix signal; a transformation unit operable to transform the downmix signal into a frequency domain signal, the downmix signal being generated by said decoding unit; a determination unit operable to determine, using the value of cos θ that is the third coded data, two phase rotators, a first phase rotator of the two phase rotators forming a phase rotation angle α, and a second phase rotator of the two phase rotators forming a phase rotation angle β, the phase rotation angle α and the phase rotation angle β being obtained by diagonally dividing a contained angle formed by two adjacent sides of a parallelogram where a length ratio between the two adjacent sides is equal to the gain ratio D indicated by the second coded data, and the contained angle being equal to the phase difference θ indicated by the third coded data; a separation unit operable to separate the frequency domain signal into two separation signals using the two phase rotators and the gain ratio D indicated by the second coded data; and an inverse transformation unit operable to inversely transform the two separation signals, respectively, into time domain signals so as to reproduce the two audio signals, wherein said determination unit is operable to determine, as the two phase rotators, either two complex numbers e −jα and e jβ or two conjugate complex numbers e jα and e −jβ of the two complex numbers e −jα and e jβ , wherein the bitstream further includes fourth coded data representing phase polarity information S indicating which of the two audio signals has a phase that is ahead of a phase of another of the two audio signals, wherein said separation unit is operable to generate the two separation signals by respectively multiplying, with the frequency domain signal transformed from the downmix signal by said transformation unit, either the two complex numbers or the two conjugate complex numbers associated with the phase polarity information S indicated as the fourth coded data, and wherein said inverse transformation unit accurately reproduces an advancement or a delay of the phase of each of the two audio signals by inversely transforming the two separation signals generated by multiplying, with the frequency domain signal transformed from the downmix signal by said transformation unit, either the two complex numbers or the two conjugate complex numbers associated with the phase polarity information S indicated as the fourth coded data.