Higher Order Ambisonics (HOA & HOA2D) Bus

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 verison of first order Ambisonics. It allows a more accurate source localization, and works over larger audience aeras.

tip

For a quick setup, we recommend using the All-Rad+ decoder combined with the In-Phase method. It will provide satisfying results for almost any setup.

note

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, HOA can be efficient on asymmetric layouts, but make sure to choose the decoding settings corresponding to your setup.

Speaker placement requirements

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.

HOLOPHONIX will display an error message in the 'LOG' section if an invalid speaker layout is used. Otherwise, speaker positioning is free.

Choosing 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.

tip

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 Order2D3D
Order 134
Order 259
Order 3716
Order 4925
Order 51136
Order 61349
Order 71564

When a stream is decoded to a smaller quantity of loudspeakers, its precision will correspond to a lower order.

HOA 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.

warning
  • 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#

Choose the decoding method according to your loudspeakers layout. HOLOPHONIX offers many different optimizations to adapt HOA decoding to your layout:

  • Direct Sampling only fits for layouts with regularly spaced loudspeakers,
  • Mode-Matching is suitable for slightly irregular setups,
  • Energy-Preserving, All-Rad and its variants offer the best results on non-homogenous layouts. Those methods are often the best setting.
Recommended Use

Energy-Preserving, All-Rad, All-Rad+ and MVLAD can be used with any valid loudspeaker layout, and are recommended for setups that feature an irregular loudspeaker repartition.

Direct Sampling#

Direct Sampling is the simplest, legacy, decoding method. It is mostly recommended for circular 2D setups that feature regularly spaced loudspeakers. It generally offers poor results on traditional 3D setups.

info

When used on non-uniform layouts, this method will result in a poor level homogeneity.

Mode-Matching#

Mode-Matching is another legacy decoding method, yet more complex than Direct Sampling. It should only be used on 2D and 3D setups that feature regularly spaced loudspeakers, and can provide fair results on slightly irregular setups.

caution

When used on non-uniform layouts, this method will result in a poor level homogeneity, and in some cases it can lead to dangerous levels on individual speakers.

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+#

All-Rad+ is an alternative 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.

MVLAD#

MVLAD ('Most VBAP-Like Ambisonic Decoder') is an alternative decoder that similarly to All-Rad relies on both Ambisonics (with Mode-Matching) and VBAP techniques. It offers results equivalent to All-Rad.

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 crite-ria 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 con-tribute, 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#

info

There is no interest in changing the normalization under normal use; there is no a a normalization over an other. Changing this setting will have no effect on sound.

In the context of Ambisonic 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 worry about this parameter.

Use this parameter only when inputting a pre-encoded HOA stream via a Direct-to-Bus source. In that case, set it up to match the input stream's normalization. Several standard settings are offered for maximum flexibility.

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.