O3A Injector - First Order

O3A Injector - First Order

Host Support

Host TypeSupport


Input4First order B-Format ambisonics (SN3D/ACN O1A or FuMa, switchable)



This plugin is designed to make subtle modifications to four channel first order B-Format while injecting it into an O3A mix. It is intended for use particularly with recordings from first order B-Format microphones.

In general when we refer to B-Format we are referring to 16 channel third order ambisonic (O3A) B-Format, but this plugin is an exception. The first order ambisonic B-Format we are talking about here is less detailed and only uses four channels. The plugin supports two first order input formats, SN3D and FuMa. Please check which you have and set the switch accordingly.

The plugin does not perform conversion from A-Format to B-Format. If your microphone produces ambisonic A-Format, you will need to use software specific to your microphone to convert to B-Format before using this plugin.

After any input format conversion, the plugin has two independent processing stages:

  1. Narrowing
  2. Diffusion

For some material, neither stage is needed as four channel B-Format (in the SN3D/ACN format) maps naturally onto the first order components of the O3A stream. So, it is often fine not to use this plugin at all when injecting first order material into an O3A stream, or to use this plugin with both dials set to zero.

However, there is typically spatial detail missing in the first order material when it is used in a third order context and this missing detail has a subtle effect on how the material sounds, particularly when compared with third order material. Sounds are likely to differ in both "sharpness" and "envelopment". Also, first order material injected directly into a third order stream can sometimes introduce an audible antiphase "ghost" image in the direction opposite the original sound source, which can cause a blurred effect.

Both stages of this plugin can actually be achieved with other plugins (the O3A Order Amplifier and O3A Diffuser from the O3A Manipulators plugin library). This plugin is organised for convenience when working with four channel first order B-Format material.

It is worth noting that the O3A Harpex plugin can be used in place of this one. This uses an active algorithm which estimates original sound directions and uses these to synthesise third order material. For much material, this can produce far sharper imaging.

The plugin is available in the O3A Upmixers plugin library.


Control: Narrowing

Narrowing reduces the amount of spatial information that is used (it applies a gain reduction to the X and Y channels).

This can be useful to remove antiphase images in the opposite direction of sound sources, which are typically heard as a kind of blurring. These are often visible in the O3A Visualiser.

The narrowing control determines how much narrowing is applied. A value of zero leaves the stream unaffected and increasing it results in a less spatially detailed, but hopefully less blurred mix.

Control: Diffusion

The diffusion value controls a set of all-pass filters which are used to smudge the mix gently. This softens transients slightly.

Different all-pass filters are used in different directions, which disrupts an unnatural envelopment or "in-the-head" effect which can sometimes occur.

A zero value for this control leaves the stream unaffected. Higher values change the sense of envelopment, allowing the stream to become more spatially diffuse without changing sound directions.

Control: Format

The input format describes the first order ambisonics that is being fed into this plugin. Setting this incorrectly will give bad results. The options are:

  • "SN3D" (in the ACN channel ordering) is the format used by the O1A and O3A plugins. It is used by some recent first order microphones and is sometimes known as "AmbiX".
  • First order "FuMa" is the same as classic WXYZ B-Format, which dates back to the 1970's. It was used in versions of these plugins prior to version 2.0 and some hardware and software uses it, including a number of 3D microphones.