A first-order DirAC-based parametric Ambisonic coder for immersive communications

Abstract: Directional Audio Coding (DirAC) is a proven method for parametrically representing a 3D audio scene in B-format and is capable of reproducing it on arbitrary loudspeaker layouts. Although such a method seems well suited for low bitrate Ambisonic transmission, little work has been done on the feasibility of building a real system upon it. In this paper, we present a DirAC-based coding for Higher-Order Ambisonics (HOA), developed as part of a standardisation effort to extend the 3GPP EVS codec to immersive communications. Starting from the first-order DirAC model, we show how to reduce algorithmic delay, the bitrate required for the parameters and complexity by bringing the full synthesis in the spherical harmonic domain. The evaluation of the proposed technique for coding 3rd order Ambisonics at bitrates from 32 to 128 kbps shows the relevance of the parametric approach compared with existing solutions.

Accepted at ICASSP 2025 Paper

For this demo:

• The 3D audio scenes were artificially created by panning in 3rd Higher Order Ambisonics (HOA3) speech items from the VCTK corpus, and adding ambience recordings in First Order Ambisonics (FOA) from Ambisonic sound library.
• Mutli-mono EVS was in Constant Bit-Rate, Super-Wideband modus when available to code FOA.
• OPUS was used in Variable Bit-Rate modus (VBR) with Channel Mapping Family (CMF) 3 to code FOA.
• Unlike multi-mono EVS and OPUS, the proposed FO-DirAC-based coding at bitrates demonstrated can transmit and synthesize HOA3.
• FOA and HOA3 signals were binauralized for static headphones playback using the Magnitude Least-Squares (MagLS) binaural rendering and the Sadie II KU100 HRTFs.

Conditions of Use.

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4 static speakers

Original	FO-DirAC 13.2kbps	FO-DirAC 32 kbps	FO-DirAC 64 kbps	FO-DirAC 128 kbps
	n.a.	multi-mono EVS 4x8=32kbps	multi-mono EVS 4x16.4=65.6 kbps	multi-mono EVS 4x32=128kbps
	n.a.	OPUS 32kbps VBR	OPUS 64kbps VBR	OPUS 128kbps VBR

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4 static speakers with outdoor ambience

Original	FO-DirAC 13.2kbps	FO-DirAC 32 kbps	FO-DirAC 64 kbps	FO-DirAC 128 kbps
	n.a.	multi-mono EVS 4x8=32kbps	multi-mono EVS 4x16.4=65.6 kbps	multi-mono EVS 4x32=128kbps
	n.a.	OPUS 32kbps VBR	OPUS 64kbps VBR	OPUS 128kbps VBR

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2 rotating speakers

Original	FO-DirAC 13.2kbps	FO-DirAC 32 kbps	FO-DirAC 64 kbps	FO-DirAC 128 kbps
	n.a.	multi-mono EVS 4x8=32kbps	multi-mono EVS 4x16.4=65.6 kbps	multi-mono EVS 4x32=128kbps
	n.a.	OPUS 32kbps VBR	OPUS 64kbps VBR	OPUS 128kbps VBR

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2 rotating speakers with indoor ambience

Original	FO-DirAC 13.2kbps	FO-DirAC 32 kbps	FO-DirAC 64 kbps	FO-DirAC 128 kbps
	n.a.	multi-mono EVS 4x8=32kbps	multi-mono EVS 4x16.4=65.6 kbps	multi-mono EVS 4x32=128kbps
	n.a.	OPUS 32kbps VBR	OPUS 64kbps VBR	OPUS 128kbps VBR

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Conditions of Use:

• Fraunhofer IIS generated this sound material based on material that is freely available on The University of Edinburgh for the VCTK corpus provided under Creative Commons License, and on The Ambisonic Sound Library for the ambience recordings provided under Creative Commons Attribution 4.0 International, and on The York University for the SADIE II HRTFs provided under Apache License v2.0.
• The content has been processed using generally accepted rules of technology as well as scientific care, but not actual attainment of any expected feature.
• With the exception of willful intent or gross negligence, Fraunhofer IIS shall not be liable that Open Source software or other third party software is free from any error or claim or its fitness for a particular purpose, even if included within the Sound Material.
• The Sound Material shall only be used for testing and appreciating speech synthesis techniques and shall not be copied, publicly transmitted, distributed, lent and modified for any other reason.
• No representation or warranties are made or implied regarding the accuracy, non-infringement, fitness for a particular purpose of Sound Material.
• Unless the conditions shown above are agreed by the recipient, no permission is granted for any use including copy, public transmission, distribution, lend, and modification.
• Copyright and Permission notice shall be duplicated whenever Sound Material are copied, distributed or publicly transmitted.
• The Sound material cannot be distributed with charge. ^[return]