The short facts
The IEM Plug-in Suite uses the AmbiX convention, that is ACN channel ordering and SN3D normalization. That is the current standard, which most plug-in suites use. Some older suites use the Furse-Malham (FuMa) format, in order to use them with AmbiX, you’ll need to convert their signals to AmbiX and vice versa.
Here’s a short and non-complete list of plug-in suites which also use the AmbiX convetion and can be used directly with the IEM Plug-in Suite:
- Matthias Kronlachner’s ambix plugin suite
- SPARTA / Compass plug-in suites
- SSA-Plugins
- BlueRippleSound plug-ins
- …
AmbiX format
The AmbiX format is the current standard for Ambisonics. It handles two important things: the channel ordering and the normalization. Both are related to the spherical harmonics, the math behind Ambisonics. Let’s take a look!
Channel ordering: ACN
The channel ordering used in the Ambix Format is called ACN which stands for Ambisonic Channel Order. What a greate name! The difference to the FuMa odering is that the channels aren’t ordered alphabetically any more. Here the first oder components for both FuMa and ACN:
FuMa: WXYZ
ACN: WYZX
(W is the omni-directional signal; X, Y, and Z the figure-of-eights in x, y, and z direction)
So why that non-sense? Well, let’s say it’s math… However, in combination with the SN3D normalization it has a neat property, which will we come to in a minute (or second for the fast readers)!
SN3D Normalization
Semi-normalization is used in the AmbiX format. This one makes sure that when you encode a source, the levels of all channels won’t exceed the one in the first (omni, W) channel. In general, this is quite handy, so that clipping doesn’t occur in your DAW.
Note: in a mix, the levels can exceed the one of the omni-channel!
N3D normalization on the other side will boost the signals in each order, which will lead to very high levels let’s say in the 7th order components in channels 50-64. So why would N3D be any good? Well, the math only works in N3D. For a matter of fact, things like beamforming or decoding (which is the same actually), will happen in N3D. But you as a user don’t have to take care of that, the plug-ins will internally apply a conversion if necessary, no worries :-)
Encoding a signal using the IEM Plug-in Suite
Per default, signals will be encoded with SN3D and ACN. You can set the normalization to N3D in most plug-ins, but you shouldn’t, as then the signals won’t be AmbiX compatible any more. Please do that only, if you know what you are doing :-)
When using spherical harmonics, the coefficient (factor/weight) for the omni-channel is usualy 1/sqrt(4*pi) which is something like 0.282094792. In all encoders, the encoding coefficients will be adjusted so that the omni channel will have unity gain.
On the other side, when decoding, e.g. with the **Probe**Decoder the weights are adjusted, so that encoding into one direction and decoding with the same direction will lead to unity again.
The neat property!
Using SN3D and ACN, in other words AmbiX, the first two channels of your Ambisonic signals will hold mid/side signals, which you can simply decode with a mid/side decoder, like the one which comes with Reaper. So a very very simple StereoDecoder would be exactlyt that. Neat, right?