Ambisonics
Ambisonics is a full-sphere surround sound format created by a group of English researchers — among them Michael A. Gerzon, Peter Barnes Fellgett, and John Stuart Wright — under support of the National Research Development Corporation of the United Kingdom. In addition to the horizontal plane, the format incorporates sound sources above and below the listener. The term is used as both a generic name and formerly as a trademark.
Unlike some other multichannel surround formats, its transmission channels do not carry speaker signals. Instead, they contain a speaker-independent representation of a sound field called B-format, which is then decoded to the listener's speaker setup. This extra step allows the producer to think in terms of source directions rather than loudspeaker positions, and offers the listener a considerable degree of flexibility as to the layout and number of speakers used for playback.
Ambisonics was developed in the UK in the 1970s under the auspices of the British National Research Development Corporation.
Despite its solid technical foundation and many advantages, ambisonics had not until recently been a commercial success, and survived only in niche applications and among recording enthusiasts.
With the widespread availability of powerful digital signal processing and the successful market introduction of home theatre surround sound systems since the 1990s, interest in ambisonics among recording engineers, sound designers, composers, media companies, broadcasters and researchers has returned and continues to increase.
In particular, it has proved an effective way to present spatial audio in Virtual Reality applications, as the B-Format scene can be rotated to match the user's head orientation, and then be decoded as binaural stereo.
Introduction
Ambisonics can be understood as a three-dimensional extension of M/S stereo, adding additional difference channels for height and depth. The resulting signal set is called B-format. Its component channels are labelled for the sound pressure, for the front-minus-back sound pressure gradient, for left-minus-right and for up-minus-down.The signal corresponds to an omnidirectional microphone, whereas are the components that would be picked up by figure-of-eight capsules oriented along the three spatial axes.
Panning a source
A simple Ambisonic panner takes a source signal and two parameters, the horizontal angle and the elevation angle. It positions the source at the desired angle by distributing the signal over the Ambisonic components with different gains:Being omnidirectional, the channel always gets the same constant input signal, regardless of the angles. So that it has more-or-less the same average energy as the other channels, W is attenuated by about 3 dB. The terms for actually produce the polar patterns of figure-of-eight microphones. We take their value at and, and multiply the result with the input signal. The result is that the input ends up in all components exactly as loud as the corresponding microphone would have picked it up.
Virtual microphones
The B-format components can be combined to derive virtual microphones with any first-order polar pattern pointing in any direction. Several such microphones with different parameters can be derived at the same time, to create coincident stereo pairs or surround arrays.| Pattern | |
| Figure-of-eight | |
| Hyper- and Supercardioids | |
| Cardioid | |
| Wide cardioids | |
| Omnidirectional |
A horizontal virtual microphone at horizontal angle with pattern is given by
This virtual mic is free-field normalised, which means it has a constant gain of one for on-axis sounds. The illustration on the left shows some examples created with this formula.
Virtual microphones can be manipulated in post-production: desired sounds can be picked out, unwanted ones suppressed, and the balance between direct and reverberant sound can be fine-tuned during mixing.
Decoding
A basic Ambisonic decoder is very similar to a set of virtual microphones. For perfectly regular layouts, a simplified decoder can be generated by pointing a virtual cardioid microphone in the direction of each speaker. Here is a square:The signs of the and components are the important part, the rest are gain factors. The component is discarded, because it is not possible to reproduce height cues with just four loudspeakers in one plane.
In practice, a real Ambisonic decoder requires a number of psycho-acoustic optimisations to work properly.
Currently, the All-Round Ambisonic Decoder can be regarded as the standard solution for loudspeaker-based playback, and Magnitude Least Squares or binaural decoding, as implemented for instance in the IEM and SPARTA Ambisonic production tools.
Frequency-dependent decoding can also be used to produce binaural stereo; this is particularly relevant in Virtual Reality applications.
Higher-order ambisonics
The spatial resolution of first-order ambisonics as described above is quite low. In practice, that translates to slightly blurry sources, but also to a comparably small usable listening area or sweet spot. The resolution can be increased and the sweet spot enlarged by adding groups of more selective directional components to the B-format. These no longer correspond to conventional microphone polar patterns, but rather look like clover leaves. The resulting signal set is then called second-, third-, or collectively, higher-order ambisonics.For a given order, full-sphere systems require signal components, and components are needed for horizontal-only reproduction.
Historically there have been several different format conventions for higher-order ambisonics; for details see Ambisonic data exchange formats.
Comparison to other surround formats
Ambisonics differs from other surround formats in a number of aspects:- It requires only three channels for basic horizontal surround, and four channels for a full-sphere soundfield. Basic full-sphere replay requires a minimum of six loudspeakers.
- The same program material can be decoded for varying numbers of loudspeakers. Moreover, a width-height mix can be played back on horizontal-only, stereo or even mono systems without losing content entirely. This allows producers to embrace with-height production without worrying about loss of information.
- Ambisonics can be scaled to any desired spatial resolution at the cost of additional transmission channels and more speakers for playback. Higher-order material remains downwards compatible and can be played back at lower spatial resolution without requiring a special downmix.
- The core technology of ambisonics is free of patents, and a complete tool chain for production and listening is available as free software for all major operating systems.
- Prone to strong coloration from comb filtering artifacts due to high coherence of neighbouring loudspeaker signals at lower orders
- Unable to deliver the particular spaciousness of spaced omnidirectional microphones preferred by many classical sound engineers and listeners
- Not supported by any major record label or media company. Although a number of Ambisonic UHJ format encoded tracks can be located, if with some difficulty, on services such as Spotify.
- Conceptually difficult for people to grasp, as opposed to the conventional "one channel, one speaker" paradigm.
- More complicated for the consumer to set up, because of the decoding stage.
- Sweet spot which is not found in other forms of surround sound such as VBAP
- Worse localisation for point sources than amplitude panning and counter phase signals blurring imaging
- Much more sensitive to speaker placement than other forms of surround sound that use amplitude panning
Theoretical foundation
Soundfield analysis (encoding)
The B-format signals comprise a truncated spherical harmonic decomposition of the sound field. They correspond to the sound pressure, and the three components of the pressure gradient at a point in space. Together, these approximate the sound field on a sphere around the microphone; formally the first-order truncation of the multipole expansion. is the zero-order information, corresponding to a constant function on the sphere, while are the first-order terms. This first-order truncation is only an approximation of the overall sound field.The higher orders correspond to further terms of the multipole expansion of a function on the sphere in terms of spherical harmonics. In practice, higher orders require more speakers for playback, but increase the spatial resolution and enlarge the area where the sound field is reproduced perfectly.
The radius of this area for Ambisonic order and frequency is given by
where denotes the speed of sound.
This area becomes smaller than a human head above 600 Hz for first order or 1800 Hz for third-order. Accurate reproduction in a head-sized volume up to 20 kHz would require an order of 32 or more than 1000 loudspeakers.
At those frequencies and listening positions where perfect soundfield reconstruction is no longer possible, ambisonics reproduction has to focus on delivering correct directional cues to allow for good localisation even in the presence of reconstruction errors.