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Effect processing of ambisonic signals prior to or after decoding



Figure 1. Analysis using a matrix-based decoder.



Figure 1. Analysis using a matrix-based decoder.


I am gradually exploring how to work on B-format files. Here’s some screenshots of patches I’ve whipped up to study the effect of processing the sound files before or after the decoding. If it is possible to do processing prior to decoding, it has a number of advantages. I’ll only need to process four channels, rather than the probably larger number of channels determined by the loudspeaker setup. Also the processed sound will be agnostic of speaker setup, making the processed files more flexible.

Figure 1 use the ICST ambidecode~ object. This is a matrix-based and hence linear decoder. Not surprisingly we see that there are no phase differences when comparing signals that were processed prior to and after decoding.

The second screenshot use the Harpex plugin instead. This is not a linear decoder:

A physical approach which reconstructs the recorded sound field is possible, but considering the volume of space occupied by even a single listener’s head, it can only work for frequencies below 800 Hz in the best case. So a psychoacoustical approach must be taken instead, where the loudspeaker feeds are designed to produce a sound field which, while different from the original soundfield, reconstructs the perceived auditory scene over a larger region of space. The design of a decoder therefore requires not only a model of the wave propagation from loudspeaker to listener, but also a psychoacoustical model.

Harpex is a new B-format processing algorithm that takes into account both the non-linear, parametric nature of spatial hearing and our uncanny ability to resolve simultaneous sounds.

As can be seen in Figure 2 there are phase differences between the signals resulting from a pre- or post-decoding processing. Now the next interesting question will be: What sounds the best, processing before or after the decoding?

Over the coming weeks and months I hope that I’ll gradually be able to do similar studies of other common (and less common) effects, in order to build up a versatile toolset of FX processes for ambisonic sound design and mixing.


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