Encoding FOA | SuperCollider 3.13.0 Help

Authoring

Most users approaching Ambisonics are usually presented with two avenues to author an Ambisonic soundfield: capture a natural soundfield directly with a Soundfield microphone, ¹ or author a planewave from a monophonic signal.² SuperCollider's inbuilt PanB provides the latter solution.

The ATK provides a much wider palate of authoring tools via FoaEncode. These include:

planewave: aka basic directional encoding
omnidirectional: a soundfield from everywhere
virtual loudspeaker array: transcoding standard formats
pseudoinverse microphone array: encoding from discrete microphones or signals

In combination with the ATK's imaging tools, sound images can then be compositionally controlled as required.

The examples below are intended to briefly illustrate some of the first order encoding options made available in the Ambisonic Toolkit.

NOTE: See FoaEncode, FoaEncoderMatrix and FoaEncoderKernel for more details regarding FOA encoding.

Choose a decoder

As the Ambisonic technique is a hierarchal system, numerous options for playback are possible. These include two channel stereo, two channel binaural, pantophonic and full 3D periphonic. With the examples below, we'll take advantage of this by first choosing a suitable decoder with with to audition.

Choose a decoder suitable for your system, as illustrated here. You'll end up definining ~decoder and ~renderDecode .

NOTE: If you choose a kernel decoder, FoaDecoderKernel, be sure to free the kernel after use.

Mono

We have many choices to make when encoding a mono source. These examples include following an encoder with a transformer.

Omni & Push

Encoded as an omnidirectional soundfield (no space!), PinkNoise is used as the example sound source.

In a well aligned, dampend studio environment, this usually sounds "in the head". FoaPush is used to "push" the omnidirectional soundfield so that it becomes a planewave (infinite distance, in an anechoic environment) arriving from some direction.

The soundfield is controlled by MouseX and MouseY, where MouseX specifies the incident azimuth angle (pi to -pi; left to right of display) and MouseY the FoaPush angle (0 to pi/2; bottom to top of display). With the mouse at the bottom of the display, the soundfield remains omnidirectional. Placed at the top of the display, the soundfield becomes directional, and varying left/right position will vary the incident azimuth of the resulting planewave.

If you haven't already choosen a ~decoder and defined ~renderDecode, do so now.

NOTE: A matrix type encoder, see FoaEncoderMatrix: *newOmni for further details.

// ------------------------------------------------------------
// omni encoder
// mono pink noise source

// define encoder matrix
~encoder = FoaEncoderMatrix.newOmni

// inspect
~encoder.kind
~encoder.numChannels
~encoder.dirChannels

(
{
    var sig;                            // audio signal
    var angle, azim;                    // angle and azimuth control

// display encoder and decoder
    "Ambisonic encoding via % encoder".format(~encoder.kind).postln;
    "Ambisonic decoding via % decoder".format(~decoder.kind).postln;

// angle ---> top           = push to plane wave
    //            bottom        = omni-directional
    angle = MouseY.kr(pi/2, 0);

// azimuth -> hard left     = back
    //            centre        = centre
    //            hard right    = back
    azim = MouseX.kr(pi, -pi);

// ------------------------------------------------------------
    // test sig
    sig = PinkNoise.ar;                         // mono pink noise

// ------------------------------------------------------------
    // encode
    sig = FoaEncode.ar(sig, ~encoder);

// ------------------------------------------------------------
    // transform
    sig = FoaTransform.ar(sig, 'push', angle, azim);

// ------------------------------------------------------------
    // decode (via ~renderDecode)
    ~renderDecode.value(sig, ~decoder)

}.scope;
)
// ------------------------------------------------------------

Frequency spreading & Push

Encoded as a frequency spread soundfield, PinkNoise is used as the example sound source. This sounds as spread across the soundfield, with the various frequency components appearing in various places. FoaPush is used to "push" the omnidirectional soundfield so that it becomes a planewave (infinite distance, in an anechoic environment) arriving from some direction.

The soundfield is controlled by MouseX and MouseY, where MouseX specifies the incident azimuth angle (pi to -pi; left to right of display) and MouseY the FoaPush angle (0 to pi/2; bottom to top of display). With the mouse at the bottom of the display, the soundfield remains spread. Placed at the top of the display, the soundfield becomes directional, and varying left/right position will vary the incident azimuth of the resulting planewave.

If you haven't already choosen a ~decoder and defined ~renderDecode, do so now.

NOTE: A kernel type encoder, see FoaEncoderKernel: *newSpread for further details.

WARNING: Kernel encoders require special care. Allow the kernel time to load before attempting to use. Additionally, the kernel buffer should be freed through the use of FoaEncoderKernel: -free after use.

// ------------------------------------------------------------
// frequency spreading encoder
// mono pink noise source

// define encoder kernel
~encoder = FoaEncoderKernel.newSpread