On Tue, Jun 4, 2019 at 2:48 AM Sean Devonport <rsdevonport1...@gmail.com> wrote:
> Thanks so much Aaron! I have seen the ambisonic toolkit around but haven't > played with it too much. Just to be clear, there are two similarly named systems, Ambisonic Toolkit (ATK) by Jo Anderson http://www.ambisonictoolkit.net and Ambisonic Decoder Toolbox (ADT) by me https://bitbucket.org/ambidecodertoolbox/adt Jo's ATK is excellent and has a lot of unique facilities for transforming sound fields. The binaural decoders are the best I've heard for first-order program material. I will look into it and compare the values you > get, to the values I've got. If I run into issues I will definitely contact > you. The speakers are relatively equidistant but I definitely think there > are discrepancies that need to be accounted for. Channeling Nando here -- as a practical matter, you can rarely put the speakers where you'd like to, and even then, they never end up exactly where you've planned. > As well as this, they are > relatively small arrays, so I will need to look into near field > compensation a bit more. I was under the impression the NFC was applied at > the encoding stage only. > The near field effect arises from the fact that when close to a point source the pressure gradient and higher derivatives arise from two components: the direction of propagation (as with plane waves), and the 1/r spreading of the energy over the expanding wavefront. This is the source of the familiar proximity effect in directional microphones. NFC is needed in both encoding and decoding. Both the standard "panning" (encoding) equations and decoder design techniques deal only with the angular part of the Fourier-Bessel decomposition of the soundfield (the spherical harmonics) and ignore the radial part (the spherical Bessel functions), which implies plane waves from sources at infinity. When panning mono point sources into the soundfield, you need to include a forward NFC filter. A correctly designed and calibrated microphone array will capture this naturally. When decoding you need to include inverse NFC filters to compensate for the fact that the loudspeakers are (more or less) point sources. A practical problem is that forward NFC filters have infinite gain at DC. Jerome Daniel's solution is to have a reference decoding distance for Ambisonic program material, say 1 meter -- In the encoder, you encode and then decode for speakers at 1 meter (which produces finite gains at DC). Then in the decoder, the NFC filters correct for the ratio of the reference and actual distance to each loudspeaker. More details in [1] J. Daniel, "Spatial Sound Encoding Including Near Field Effect: Introducing Distance Coding Filters and a Viable, New Ambisonic Format ," *Preprints 23rd AES International Conference, Copenhagen*, 2003. [2] J. Daniel and S. Moreau, "Further Study of Sound Field Coding with Higher Order Ambisonics," *Preprints from 116th AES Convention, Berlin*, no. 6017, 2004. [3] A. J. Heller and E. M. Benjamin, "Design and implementation of filters for Ambisonic decoders," 1st International Faust Conference (IFC-18), Mainz, 2018. Aaron (hel...@ai.sri.com) Menlo Park, CA US > Thanks again Fernando. Such great resources to go from. > > Reading that paper, the AmbiHome system, I notice it speaks about the two > different sets of decoding coefficients for a dual band decoding with > crossover point at 700Hz roughly, in order to maximise the velocity or > energy for phase and loudness respectively. I've seen this used in the > SPARTA plugins aswell. I was wondering if you've implemented this style of > decoding within your Grail system at CCRMA? Or if you have kept the > decoding for one entire band before applying the speaker and room tuning? > > Also, do you normally need to tune the speakers before every performance? > Do you find the quality fluctuates often? > > Thanks again for the feedback everyone! > > -- > Sean Devonport > -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20190609/4931f06d/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit account or options, view archives and so on.
