Recently I was digging into the art of rendering ambisonics with loudspeakers (that was 8 loudspeakers in different regular arrangements – cube, octagon, bi-rectangle).
During a break in my work I started playing some music from Spotify. Of course I wanted to play it trough all of the loudspeakers, but the simple virtual stereo sources placed in -/+30 degree typical setup wasn’t a thing. I wanted to fill the space around me without losing the left-right information. So I started placing virtual sources in the space, multiplying channels, inverting phases, etc., all of that in ambisonic soundfield that was later rendered to the regular speaker setup. For rendering I used excellent Wigware plugins by Bruce Wiggins from University of Derby, UK, where I had recently occasion to meet him in person and listen to his loudspeaker system, powered with those amazing plugins.
While testing different setups I stopped at one moment, when I placed four sources around me. I started reading about quadrophonics and what was the idea behind playing stereo with four speakers. I found 1970 article on that topic by no other but the father of ambisonics himself – Michael Gerzon.
The most interesting part was that those experiments pushed him towards looking for more realistic surround sound and that resulted in tetrahedral recording and speaker setup and finally the idea of Ambisonics.
In the 1970 article for Hi Fi News Michael Gerzon brings up four speaker setup described by Peter Bouwer.
The sounds fed to the four speakers were as follows: the left-hand speaker was fed with the left stereo channel, the right-hand speaker was fed with the right stereo channel, the ‘front’ speaker was fed with the sum of the two stereo channels, and the rear speaker was fed with the difference ‘between’ the two stereo channels.
I tested this in SAW and it works really good. Here is screen of sources placement in FB360 SAW.
You can get the (3) sum and (4) difference with simple routing scheme in Reaper. Place a stereo track and put your signal or file there. Route it to the next track, where you have Spatialiser plugin. 1/2 channels route to 1/2 channels of Spatialiser track (remember, that it has to have 16 channels if you are working in 3OA). Then route 1/2 channels to 3 channel of Spatialiser track, and then route mono 1 to channel 4 and mono 2 to the same channel but in inverted phase – just tick the button with crossed circle symbol (something like this: ⦲). For me it does the trick.
There are other methods of coping with stereo 2 ambisonics. One of them is placing 5 stereo pairs in +/- 30, +/- 60, +/-90, +/-120, +/-150 degrees positions (proposed by Jack Reynolds), but I like the simplicity of the Bouwer setup method and also the great history behind it!
Let me know what you think about it.
UPDATE (30/09/2019)
On Oculus Connect 6, Oculus Audio Team explain why headlocked music lacks immersion and how they use Quad for better spatialisation. I agree with their concept. Here you can listen and watch this part of OC6: