Details on Facebook 360 Ambisonics Mapping from Angelo Farina

EDIT: You can download a JS effect (for Reaper) that does the conversion from ambiX to TBE and another that goes from TBE to 2nd order, 2D, Furse-Malham format here (I’ve included my remapping JS effect too, so you can also go from Furse-Malham to TBE format by converting to that first 🙂 :

WigWare AmbiX, FuMa and TBE Tools

Angelo Farina has published an excellent article detailing how the 9, 2nd order Ambisonic components map to the 8 channels of the Facebook360 TBE format (they decided to pander to the 8-channel limit of pro-tools 🙁 ).  All the details can be found at Angelo’s website:

http://pcfarina.eng.unipr.it/TBE-conversion.htm 

The important bit, for my own notes, is (with added Furse-Malham mapping):

TBE(1) =  0.486968 * Ambix(1)  (FuMa W)
TBE(2) = -0.486968 * Ambix(2)  (FuMa Y)
TBE(3) =  0.486968 * Ambix(4)  (FuMa X)
TBE(4) =  0.344747 * Ambix(3)  
        + 0.445656 * Ambix(7)  (FuMa Z+R)
TBE(5) = -0.630957 * Ambix(9)  (FuMa U)
TBE(6) = -0.630957 * Ambix(5)  (FuMa V)
TBE(7) = -0.630957 * Ambix(6)  (FuMa T)
TBE(8) =  0.630957 * Ambix(8)  (FuMa S)

And, to go from TBE to 2nd order, 2D, Furse-Malham format (as mentioned by Ed, in the comments below):

W =  1.446968601 * TBE(1)
X =  2.047502048 * TBE(3)
Y = -2.047502048 * TBE(2)
U = -1.839587932 * TBE(5)
V = -1.839587932 * TBE(6)

TBE is Facebook360 Two Big Ears format
ambiX is the ambiX format used by YouTube Spatial Media (ACN channel order and SN3D normalisation)
FuMa is the Furse-Malham channel ordering and normalisation scheme.

See https://en.wikipedia.org/wiki/Ambisonic_data_exchange_formats for further details on channel ordering and normalisation schemes.

A polar plot of TBE Channel 4 (the combination of the Z and R channels in FuMa speak) can be seen above (click for higher res image).