Ambisonics

BBC R&D Ambisonics Intro

Principles

Gerzon, Barton, Fellgett - main theoretical basis
hierarchical approach to directional sound recording
formats
- A-format: microphone recording
- B-format: studio equipment and processing
- C-format: transmission
- D-format: decoding and reproduction
- UHJ - good for mono/stereo compatibility

Ambisonics aims to offer a complete hierarchical approach to directional sound pickup, storage or transmission and reproduction, which is equally applicable to mono, stereo, horizontal surround-sound, or full ‘periphonic’ reproduction including height information.

The more channels used the greater number of dimensions can be reproduced during playback.

A format known as UHJ (‘Universal HJ’, ‘HJ’ simply being the letters denoting two earlier surround sound systems), described originally by Gaskell of the BBC Research Department (Gaskell, 1979), is also used for encoding multichannel ambisonic information into two or three channels whilst retaining good mono and stereo compatibility for ‘non-surround’ listeners.

Remember that ambisonics is not limited to any specific speaker configuration. In our room we have 7.1, but it could use quad, 5.1, or more speakers.

Differences from stereo/quad

Quad
- is fixed to four speakers
- Uses phantom images between two speakers
- worse with off center listening
- common “hole in the middle” problem
Ambisonics
- encodes from all directions, decodes to your loudspeaker setup

Sources

Ambisonic microphone: Zoom H3-VR, Soundfield etc.
Virtual source

Signal formats - A format

B format

The B-format consists of four signals that between them represent the pressure and velocity components of the sound field in any direction, as shown in Figure 4.15. It can be seen that there is a similarity with the sum and difference format of two channel stereo, described earlier, since the B-format is made up of three orthogonal figure-eight components (X, Y and Z), and an omni component (W).

See if you can pick out the three figure of 8 polar patterns and the one omni pattern.

W, X, and Y provide the horizontal spatial information and Z provides the height information.

X - forward-facing figure-eight (equivalent to M in MS stereo), Y is equivalent to a sideways-facing figure-eight (equivalent to S in MS stereo).

The zoom recorder can record directly to B-format with either FuMa or AmbiX or in A-Format. If we record in FuMa an encoding process is not necessary.

A to B Conversion

X = 0.5((LF – LB) + (RF – RB))
Y = 0.5((LF – RB) – (RF – LB))
Z = 0.5((LF – LB) + (RB – RF))
W = 0.5(LF + LB + RF + RB)

C format - aka UHJ

D format - reproduction

Speaker numbers
- Four speakers give adequate surround sound
- six provide better immunity against the drawing of transient and sibilant signals towards a particular speaker
- eight may be used for full periphony with height.

Higher order Ambisonics

Current Developments

Open Source Output format - Opus 1.3
Corporate interest
- Google - Spatial Audio RPC
- Youtube - 180 or 360 degree videos with spatial audio
- Occulus audio
- Microphones - Sennheiser Ambeo VR Mic, Zoom H3-VR
- BBC Research on ambisonics

Gaming

Unity, Unreal
Resonance Audio
Steam Audio
Battlefield 1 & Battlefield V - Dev Talks video