Audio decoder and decoding method using efficient downmixing

A tangible computer-readable storage medium storing decoding instructions that when executed by one or more processors of a processing system cause carrying out a method of decoding audio data that includes encoded blocks of N.n channels of audio data to form decoded audio data that includes M.m channels of decoded audio, M≧1, n being the number of low frequency effects channels in the encoded audio data, and m being the number of low frequency effects channels in the decoded audio data, the method comprising:

• accepting the audio data that includes blocks of N.n channels of encoded audio data encoded by an encoding method, the encoding method including transforming N.n channels of digital audio data, and forming and packing frequency domain exponent and mantissa data;
• and
• decoding the accepted audio data, the decoding including:
• unpacking and decoding the frequency domain exponent and mantissa data;
• determining transform coefficients from the unpacked and decoded frequency domain exponent and mantissa data;
• inverse transforming the frequency domain data and applying further processing to determine sampled audio data;
• and
• time-domain downmixing at least some blocks of the determined sampled audio data according to downmixing data for the case M<N,
• wherein the method includes identifying one or more non-contributing channels of the N.n input channels, a non-contributing channel being a channel that does not contribute to the M.m channels, and
• wherein the method does not carry out inverse transforming the frequency domain data and the applying further processing on the one or more identified non-contributing channels.

An apparatus comprising:

• a processing system that includes one or more processors and a tangible computer-readable storage medium,
• wherein the tangible computer-readable storage medium stores decoding instructions that when executed by at least one of the processors cause carrying out a method of decoding audio data that includes encoded blocks of N.n channels of audio data to form decoded audio data that includes M.m channels of decoded audio, M≧1, n being the number of low frequency effects channels in the encoded audio data, and m being the number of low frequency effects channels in the decoded audio data, the method comprising:
• accepting the audio data that includes blocks of N.n channels of encoded audio data encoded by an encoding method, the encoding method including transforming N.n channels of digital audio data, and forming and packing frequency domain exponent and mantissa data;
• and
• decoding the accepted audio data, the decoding including:
• unpacking and decoding the frequency domain exponent and mantissa data;
• determining transform coefficients from the unpacked and decoded frequency domain exponent and mantissa data;
• inverse transforming the frequency domain data and applying further processing to determine sampled audio data;
• and
• time-domain downmixing at least some blocks of the determined sampled audio data according to downmixing data for the case M<N,
• wherein the method includes identifying one or more non-contributing channels of the N.n input channels, a non-contributing channel being a channel that does not contribute to the M.m channels, and
• wherein the method does not carry out inverse transforming the frequency domain data and the applying further processing on the one or more identified non-contributing channels.