[CM] ambix ambisonics command line

[Fernando Lopez-Lezcano]

On 6/12/24 09:04, Juan Reyes wrote:
> Hola Nando,
> 
> Thanks a lot for responding.
> 
> Tried looking for '.tetra' files on Fons site but could not found any.
> However, I tried 'theoretical.tetra" and results were not that bad.
> 
> Using four channels sound files with no microphone at all.

No microphone? Hmmm, I don't understand ... how was the four channel 
"A-format" file created?

> FTR, workflow I'm using is combining four channels to get A-Format
> signals in aa Snd instrument as:
> 
> W = FLU+FRD+BLD+BRU
> X = FLU+FRD-BLD-BRU
> Y = FLU-FRD+BLD-BRU
> Z = FLU-FRD-BLD+BRU

These equations already transform capsule signals (FLU, FRD, etc) into a 
B-format four channel Ambisonics signal. If I understand correctly you 
are using these equations, right? (one question is, where are FLU,FRD, 
etc, coming from if not from a microphone?).

If that is what you are doing, you are already getting a WXYZ Ambisonics 
first order signal, no need to further process it. Whether this is 
really "Ambisonics" depends on where FLU/FRD/BRU/BLD come from...

> Once I have these files, I use Fons' 'tetrafile' command to get B-format
> soundfiles with something like:
> 
> 'tetrafile theoretica.tetra a-file.wav b-file.amb'
> 
> Is there a way to get more '.tetra' files ?. Would be great to do more
> experimenting.

The equations you reference above transform four capsule signals into 
first order Ambisonics assuming all capsules are identical, and they are 
co-located (impossible in a real microphone). Unless I misunderstand 
what you are doing, you do not need to use tetrafile, you already have a 
four channel B-format signal.

".tetra" files hold calibration data for individual microphones which 
includes gain compensation for the individual capsules and frequency 
response compensation in the mid and high frequencies that tries to 
correct for the effects of the capsules not being co-located (they 
contain a 4x4 matrix of convolution kernels - filters - that take 
everything into account - I have generated that data for my DIY 3d 
printed microphones from my measurement data).

Each calibration data set corresponds to actual measurements of a 
particular microphone so, technically, it does not make sense to use 
them for any other microphone.

...

Now, artistically you can do whatever you want, of course! Does not 
matter if it makes technical sense or is proper Ambisonics, if you like 
what you hear go for it...

Best,
-- Fernando

See this for more details:
https://ccrma.stanford.edu/~nando/publications/sphear.pdf


> I really appreciate Anders pointing out this method because 'tetrafile'
> command can be embedded in a script to do batch processing.
> 
>    Best of all!,
> 
>    -- Juan
> 
> 
>> Hey Juan,
>> I am assuming you need A-format (capsule signals) to B-format 
>> (Ambisonics) conversion. As Anders wrote, that is heavily dependent on 
>> which Ambisonics microphone was used to record the soundfield.
>>
>> Which one did you use?
>> -- Fernando
>>
>>
> 
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