SEMAC: Spherical Energy Matching Ambisonics Codec for Efficient Higher-Order Ambisonics Compression

Overview & Synopsis

Higher-order Ambisonics (HOA) is an attractive format for immersive spatial audio, but its high channel count and directional properties present significant audio coding challenges. The widely deployed Opus audio codec supports HOA through Channel Mapping Families 2 and 3 (RFC 8486), which code and transmit every Ambisonics channel, so the bitrate grows quadratically with the spherical harmonic order. We introduce SEMAC, which instead represents an Ambisonics signal by a quantized full-order directional energy envelope together with fewer CELT-coded transport channels. CELT separates a per-band energy envelope from the signal shape and restores it after quantization, and SEMAC generalizes this principle to directional signals. Since the reduced transport spans only a lower-dimensional subspace, the decoder re-imposes the envelope observed at the encoder input, restoring the directional energy distribution at full order. A MUSHRA test on fifth-order material compares SEMAC against Opus CMF~3 in matched configurations at 512 and 768 kbit/s. SEMAC with 16 of 36 transport channels is rated above full-order CMF~3 at both rates, and at equal transport channel count above order truncation. The proposed SEMAC strategy of coding fewer transport channels with the directional envelope is found perceptually preferable to coding all HOA channels at low precision.