[Feature Article] Spatial audio coding is a term recently coined to describe low bit-rate audio coding that enables the transmission of multichannel spatial audio at fairly low bit rates. In a typical spatial audio coder (see Fig. 1) the audio signal is downmixed to a relatively small number of basic channels (usually one or two), and transmitted along with additional information that enables the reconstruction of the missing channels. Such “parametric” coding includes the calculation of interchannel level, time/phase, and correlation, which are the key parameters of multichannel audio signals containing information about the spatial characteristics of the overall scene. The bit rates employed can be very low compared with traditional multichannel audio codecs, with spatial side chain bit rates of between 32 and 48 kbit/s, in addition to whatever bit rate is employed for the primary channels. The primary channel is often coded using a legacy system for compatibility of the downmixed audio with existing decoders.
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