HOA Tutorial Exercise 06

Extension

Previous: Exercise_05_HOA_b_format_file_playback

Next: Exercise_07_HOA_beamforming

After learning how to play back HOA soundfiles, the examples below demonstarte how to use the classes

• HOATransMirror - mirror the sound field along x,y or z.
• HOATransRotateAz - rotate the soundfield around the z axis.
• HOATransRotateXYZ - rotate the soundfield around the xyz axis.

in order to manipulate the HOA B-format. For the details of the mirroring and rotation transformations please refer to: [1] M. Kronlachner, “Spatial Transformations for the Alteration of Ambisonic Recordings,” Graz University Of Technology, Austria, 2014 p.46-47.

The x,y,z (roll, pitch, yaw) rotator of the HOA scene uses the for the matrix definition from [2] with implentation according to [1] for details about the corrections in the implementation see underlying Faust code by Pierre Lecomte: https://github.com/sekisushai/ambitools/blob/master/Faust/src/hoa_rotator.dsp[2] J. Ivanic and K. Ruedenberg, “Rotation matrices for real spherical harmonics. Direct determination by recursion,” J. Phys. Chem., vol. 100, no. 15, pp. 6342–6347, 1996.

In case you have not done so yet: let's start the sound server, which we configure first to get more audio buses and to have 36 output channels.s.quit; // quit the server first if it is still running with another configuration. ( ServerOptions.devices; s = Server.local; Server.local = Server.default; o = Server.local.options; // Get the local server's options o.numInputBusChannels = 2; // Set Input to number of Inputs o.numOutputBusChannels = 36; // lets start after chan 36 so as not to see the mic input o.numAudioBusChannels = 2048; o.blockSize = 512; o.numWireBufs = 1024 * 16; o.memSize = 2.pow(18); o.sampleRate = 48000; s.makeWindow; s.boot; )

Start the oscilloscope showing all the channels for the selected order of the b-format.( ~order = 3; ~hoaNumChannels = (~order+1).pow(2); s.scope(2); // let's use only two channels as we will monitor it with headphones )

We will load a very short sound file that we loop so that we can observe the soundfield transformations:~hoaSoundFile = Buffer.read(s,HOA.soundsDir++"/eigen_ACN_N3D_CIRMMT_Foyer_short.wav"); ~hoaSoundFile.numChannels;

Let's make a node that can play back the 3rd order HOA sound file.( ~hoaSignal = NodeProxy.new(s, \audio, ~hoaNumChannels); ~hoaSignal.source = { HPF.ar(PlayBuf.ar( ~hoaSoundFile.numChannels, ~hoaSoundFile, loop:1 ), 100) }; ~hoaSignal.fadeTime = 1; )

Here we use HOATransMirror to simply mirror the sound field along the x, y, and z axis.( ~hoaTransformer = NodeProxy.new(s, \audio, ~hoaNumChannels); ~hoaTransformer.source = {|front_back =1, left_right = 0, up_down = 0| var in; in = \in.ar(0!~hoaNumChannels); HOATransMirror.ar(~order, in, front_back, left_right, up_down) }; ~hoaTransformer.fadeTime = 1; )

Then we make the decoder as before.HOADecLebedev26.loadHrirFilters(s); HOADecLebedev26.hrirFilters.size; ( ~decoderOrder = 2; // let's reduce the order ~decoderNumChannels = (~decoderOrder+1).pow(2); ~decoder = NodeProxy.new(s, \audio, 26); ~decoder.source = {var in; in = \in.ar(0!~decoderNumChannels); HOADecLebedev26.ar(~decoderOrder, in, hrir_Filters:1) }; ~decoder.fadeTime = 1; ~decoder.play(0, 26); )

And we connect the NodeProxies to hear something~hoaSignal <>> ~hoaTransformer <>> ~decoder;

Now we can flip the sound field:~hoaTransformer.set(\front_back, 0); ~hoaTransformer.set(\front_back, 1); ~hoaTransformer.set(\left_right, 0); ~hoaTransformer.set(\left_right, 1); ~hoaTransformer.set(\up_down, 0); ~hoaTransformer.set(\up_down, 1);

Here we have an example with HOATransRotateAz where we rotate the sound scene around the z-axis.~hoaTransformer.source = { var in; in = \in.ar(0!~hoaNumChannels); HOATransRotateAz.ar(~order, in, MouseX.kr(-pi, pi)) };

Here we have an example with HOATransRotateXYZ where we rotate the sound scene around all three axis.( ~hoaTransformer.source = { var in; in = \in.ar(0!~hoaNumChannels); HOATransRotateXYZ.ar(~order, in, LFTri.ar(0.02, 0, pi), // pitch LFTri.ar(0.02, 0, pi), // roll LFTri.ar(0.02, 0, pi) // yaw ) }; )

Previous: Exercise_05_HOA_b_format_file_playback

Next: Exercise_07_HOA_beamforming