Higher Order Ambisonics (HOA)
Ambisonics is both a spatialization algorithm and a recording technique. It aims to recreate the acoustic field, by using a specific encoding/decoding technique that confers to this algorithm its unique flexibility.
It is based on decomposing the acoustic field in several dimensions, forming a stream that is directly decoded to the loudspeaker setup, or that can be recorded and later decoded to any other speaker system.
Higher Order Ambisonics simply refers to a more precise version of first order Ambisonics. It allows a more accurate source localization, and works over larger audience areas.
To understand the working principle of HOA, check the Spatialization Guide to Ambisonics
Speaker Layout Requirements
HOA was designed to work on 2D and 3D surround speaker systems. Try to use speakers that are as regularly spaced as possible across the entire setup, for a maximum homogeneity. However, HOLOPHONIX is using a decoding method that allows non homogeneous speakers (i.e. the spacing between the loudspeaker doen't need to be constant).
This algorithm requires the loudspeaker layout to include frontal and surround speakers. The center of the coordinates system needs to be inside the speaker layout.
Check for error messages in the Logs if an invalid speaker layout is used.
Choosing the HOA Order
When creating an HOA bus, the Order will be automatically computed depending on the loudspeaker quantity. You can still edit manyally the HOA Order.
Always try to match the chosen HOA order with the quantity of loudspeakers. An HOA stream requires at least as many speakers as the number of HOA components for the given order. You can refer to the table below.
It is not always necessary to choose the maximal order, as in practice, a 3rd or 4th order HOA bus generally offers satisfying results.
| HOA Order | 2D | 3D |
|---|---|---|
| Order 1 | 3 | 4 |
| Order 2 | 5 | 9 |
| Order 3 | 7 | 16 |
| Order 4 | 9 | 25 |
| Order 5 | 11 | 36 |
| Order 6 | 13 | 49 |
| Order 7 | 15 | 64 |
When a stream is decoded to a smaller quantity of loudspeakers, its precision will correspond to a lower order.
HOA Parameters
Audience Size Optimization
Ambisonics, by design, uses all the loudspeakers of the setup to spatialize sounds. Its standard decoder is suitable for listeners at the center of the speaker setup, but not for listeners are close to the loudspeakers. These listeners will perceive the sound localization as coming from the closets loudspeakers.
HOLOPHONIX offers multiple optimizations coming from the scientific research, to overcome this challenge. Choose the Audience Size Optimization corresponding to your use case:
- Studio mode will offer a correct spatialization at the sweet stop only
- Live mode offers a spatialization suitable for large audiences
This setting controls the "Decoder Type" setting when using Advanced Parameters.
Blur
Phantom Speaker (3D)
HOA Advanced Parameters
In its basic form, Ambisonics is designed to offer a good sound field reproduction at the center of the speaker system. HOLOPHONIX integrates advanced optimizations to adapt HOA to large audience areas and irregular loudspeaker setups.
- The various HOA decoding Method and Type settings feature strong level differences. Activate the Compensation parameter to limit those level differences when changing those setting.
- Level compensation is not available on dual-band decoders. Change the decoder settings with care.
Decoder Method
The decoding method offers an optimization depending on the loudspeakers layout. By default, HOLOPHONIX uses Energy-Preserving the decoding method, which is suitable for homogeneous and non-homogeneous loudspeaker setups.
Energy-Preserving
- Energy Preserving was designed to keep a constant sound energy when a source is moved across an irregular loudspeaker setup.
All-Rad+
- All-Rad stands for 'All-Round Ambisonics Decoder'. It superimposes two panning methods by first simulating a perfect HOA setup on virtual speakers with Direct Sampling method, and then renders those virtual speakers on the real setup using VBAP. All-Rad+ is a more recent version of All-Rad that was designed to offer level homogeneity characteristics competitive with Energy-Preserving, while still offering the localization precision of All-Rad.
All-Rad 2
- All-Rad 2 is an alternative version of All-Rad, specially crafted for 5.1.4 ITU loudspeaker setups. It offers constant loudness when moving a virtual source across the setup, where other decoders might not achieve this.
Decoder Type
Choose the decoder type according to the proximity of the listeners to the loudspeakers, and the audio content fed to the system.
For venues where the listeners are close to the loudspeakers, In Phase, Basic/In Phase or In Phase/Max Re will often be the best settings, depending on the audio content.
Basic
- The Basic decoder ensures a proper reconstruction of the wavefront at the center of the listening area for low frequencies. With this decoder, all the loudspeakers contribute; they feature level and phase differences.
- recommended use
It is recommended to use Basic decoding only when the listeners are close to the center of the venue.
Max Re
- Max Re was designed based on psychoacoustics criteria to offer better localization than Basic decoding, for frequencies above 700 Hz, at the center of the setup. It concentrates the energy in the direction of the virtual sources. With this decoder, most of the loudspeakers contribute, and feature phase and level differences. However, the level differences are stronger than Basic decoding.
- recommended use
It is recommended to use Max Re only when the listeners are close to the center of the venue.
In-Phase
- In-Phase was created for venues where some listeners are close to the loudspeakers. When computing the loudspeaker feed signals, the in-phase decoder type tries to cancel out of the phase differences, as well as the side and rear lobes that would appear with Basic or Max Re decoding. As a result, only the speakers close to the source are fed with the decoded stream, and they all have a coherent phase.
- recommended use
It is recommended to use In-Phase decoding when the audience features off-centered listeners.
Combined decoders
- It is possible to combine decoding types, each one working on a specific frequency band of the spectrum, with adjustable Cross Over Frequency. The first decoder in the combined name refers to the low frequencies decoder type, and the second to the high-frequencies decoder type.
Cross Over Frequency
Only available for dual-band decoder types, to adjust the crossover frequency between the two decoding bands.
Compensation
Applies a power compensation to balance the level differences between the decoding types and methods. It is not currently available when using dual-band decoders.
Normalization
There is no interest in changing the normalization under normal use; there is no better normalization over another. Changing this setting will have no effect on sound.
In the context of Ambisonics processing, various normalizations of the spherical harmonic functions can be used, and there is (yet) no standard nor consensus in the community. This may lead to compatibility issues between rendering engines. One must ensure that the same convention (normalization) is used for both the encoding stage and the decoding stage.
If HOLOPHONIX is used for both encoding and decoding, then the normalization is automatically adjusted internally by the processor, and you don't have to change about this parameter.
Orientation
The orientation of the B-Format stream can be adjusted with Yaw, Pitch, Roll, to rotate the entire audio scene. See rotation angles.
Blur
This parameter reduces the resolution by lowering the HOA Order (progressively eliminating the high-order components), thus giving a blurring effect to the audio scene.
Phantom Speaker
Only available for 3D HOA, the phantom speakers allows you to create a virtual speaker placed above (Top) or under (Bottom) the speaker setup. When speakers are not evenly distributed, this can solve homogeneity issues.