[linux-audio-dev] Dumb question regarding CMT's ambisonic plugins

fons adriaensen fons.adriaensen at skynet.be
Sat May 28 10:18:08 EDT 2005


Helllo Ico,

> I have been messing with the ambisonic plugins provided in the CMT ladspa 
> collection. I am namely looking into a simple implementation of B-format 
> 1ch->4ch encoder. The problem is that I am failing to figure out how to use 
> this on a simple mono (non-encoded) sound input in order to control its 
> diffusion over 4 channels.

I'm not sure what you mean by diffusion. For a mono source, you basically
have two options:

1. Just send it to the B-format W channel, and not use X,Y,Z

  Thechnically that's not a 'legal' B format, but what you get is that
  the sound will come equally loud out of all speakers.

2. Use a B-format panner to put it into a particular direction.
   Remember that ambisonics encodes source direction, not position,
   i.e. it does not include any notion of distance. This is normally
   encoded by changing the direct / reflections / reverb ratios.

Regarding the CMT set of ambisonics plugins, I remember having tested
some of them at least a year ago and the conclusion was they have some
problems.

> When running it via Ardour and sending it to 4 
> separate busses via this plugin, even though the x, y, and z coordinates do 
> affect the output of the 4 channels, they do not conform to the expected 
> 3,4,2,1 output (I presume these correspond to 4 speaker outputs, but I am 
> also having doubts about this as well, so your help in understanding this 
> better would be most appreciated).

Seems like you need two things:

1. a B-format panner for ardour, so you can mix a number of mono inputs,
  giving each of them a 'direction' and maybe (via additional outputs)
  controlling their distance, and

2. a B-format to 4 speaker decoder following the master mixer strip.

I have these things in house, and I'm using them all the time, but they
are not in a state suitable for release. They will be released some time,
and if you need them badly, I could change some priorities...

OTOH, I remember you've been exploring SuperCollider recently, and that
has everything you need on board. Have a look at the PanB2, PanB and
DecodeB2 Ugens. 

-- 
FA



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