[LAU] How I built my own IR reverb in ambisonics

[Michael Jarosch]

Hi!

Don't know, if this is actually the academic or scientifically correct 
way to do it, but I got good sounding results - in fact, I managed to 
insert mono-recorded voices into an ambisonic recording with other 
people and it didn't sound that weird…

But I'm always interested in getting things more and more perfect, so I 
hope many of the ambisonics-pros are joining this thread to discuss the 
topic.

This is the software and hardware I used:

  * Røde NT-SF1 1st order Ambisonics microphone
  * Sound Devices MixPre-6 recording device
  * Ubuntu Studio 04.22
  * Carla 2.4.2
  * Røde Soundfield PlugIn (Windows 64bit VST3)
  * Wine
  * Ardour 6.9
  * Audacity 2.4.2
  * LSP-PlugIn Suite (LV2)
  * IEM Ambisonics PlugIn Suite (LinuxVST)

And here is the whole procedure, how I made it…

I made an ambisonics recording with people in a room. After that I took 
a paper bag to create an impulse, about 50cm away from the NT-SF1. (Just 
clapping the hand was too silent in my opinion and I didn't want to use 
balloons because of the plastic waste. Paper bags seem to be an 
relatively environment friendly and cheap alternative to me.) I retried 
as often as it needed to get a good sounding and non distorted record of 
the impulse and its respond.

Getting back to home I copied the A-format of my recordings to my 
computer, fired up Ardour, created a 4-channel track and inserted the 
16-channel-version of carla patchbay into that channel strip. I had to 
deactivate the panner in every channel strip to make Ardour "ambisonics 
compatible", yes, also in the  "Master"-channel which must be blown up 
to 4 channels, too. I imported the tracks that I wanted to be converted.

The only tool in my procedure that wasn't free and open was Rødes 
Soundfield PlugIn, so I had to use some tricks to get manufacturer 
proven A-to-B conversion of my recordings. FalkTX did make carla 
windows-VST-capable, but it's not a bullet prove task… So, I got the 
Soundfield exe-file, installed it with wine and I finally got a dll I 
could drag and drop into carla patchbay's GUI. There it is! Chances are 
Ardour crashes, so I made sure to save after every step. As input 
format, I have to chose "NT-SF1", on output the best choice is to use 
"B-Format (Ambix)", because Ambix is the actual standard and the IEM 
plugins only handles Ambix. Of course, I had to connect Carla's output 
to the input of Soundfield and vice versa.

Time for the A-to-B-convertion. Drag and drop the 4-channel-audio into 
the editor window of Ardour, set "Start" and "End" and use export. Make 
sure to have all 4 channels to be exported into one file. You can use 
both WAV or FLAC. Next step is Audacity. (You don't need to close 
Ardour, sooner or later it will crash… :) )

Instead of stereo files, Audacity handles 4-channel-audio as 4 seperate 
mono files. (But it can be exported to 4-channel-audio, again, if you 
choose "advanced mix options" in the "import/export" division of the 
preferences. You won't need it for the virtual ambisonics IR reverb.) 
But - it is mandatory to keep the audio sample accurate between all the 
tracks! If you want to delete, delete from all of them, simultaneously! 
So, you choose your favorite BANG! out of the inpulse series you 
recorded and delete the rest. Export every mono track as WAV. Use Ambix 
nomenclature: 1st track is W, 2nd is Y, 3rd is Z and 4th is X. Best bet 
is to write the number and the letter in the name of the file. I choose: 
[nameoftheplace]_1_(W).wav and so on…

Now, to actually build the virtual ambisonics IR reverb, fire up Ardour 
and make it ambisonics compatible (deactivate panner). Create a 
4-channel-audio-bus and place a 16-channel Carla patchbay. In the Carla 
patchbay, create 4 "LSP Impulse Responses Mono" - one for each channel. 
(Using LV2 in Carla works pretty stable!) Open the GUI for each "LSP 
Impulse Responses Mono" and use the corresponding WAV in it by clicking 
into the GUI and choose the right file. Inside Carla patchbay it should 
look as follows:

Carla channel 1 output ---> LSP Impulse Responses Mono #1 (with 
[nameoftheplace]_1_(W).wav) ---> Carla channel 1 input
Carla channel 2 output ---> LSP Impulse Responses Mono #2 (with 
[nameoftheplace]_2_(Y).wav) ---> Carla channel 2 input
Carla channel 3 output ---> LSP Impulse Responses Mono #3 (with 
[nameoftheplace]_3_(Z).wav) ---> Carla channel 3 input
Carla channel 4 output ---> LSP Impulse Responses Mono #4 (with 
[nameoftheplace]_4_(X).wav) ---> Carla channel 4 input

Make sure, that every LSP IR Mono instance uses exactly the same values 
of headcut, tailcut and amplification. WIth headcut, you should cut the 
impulse. Now, you can use a mono audio file, place it into the 
ambisonics sphere f.e. with the IEM StereoEncoder, use an auxiliary send 
(don't forget to disable the panner of it, too!) and enjoy the 
reverberation! (Play with Ardour's "strict E/A" to get as much channels 
for each strip you need!) Using an EQ after the virtual reverb is always 
a good idea. LSP IR Mono has one integrated, but my impulses seem to 
have no linear frequency band and needed more than the +/-12 dB the 
plugin offers… (I take IEMs "MultiEQ" for this task - one setting for 
all the channels is another advantage.)

Unfortunately, Carla can't recall LSP IR Mono's WAV-file, so, you have 
to reload it, every time you start your session.

Greets!
I hope, this can be useful for anyone…

Mitsch
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