Ambisonic Zoomer

If you are not interested in the theory you can read about how to use AmbisonicZoomer here.

Theory and Implementation Details

The AmbisonicZoomer class allows for a kind of acoustic zoom to the front of the sound field. It was implemented for use with the zoom function with 360-videos in VLC Media Player to place emphasis on sounds in the view direction.

It samples the sound field to the front with a virtual cardioid microphone with order matching the order of the input signal. This mono virtual microphone signal is then re-encoded back to a signal at the front of the sound field and blended with the full sound field based on the zoom value set in AmbisonicZoomer::SetZoom.

A zoom value of 0 will perform no transformation to the ambisonic signal while a zoom of 1 will lead to an extreme zoom, which results in mono signal encoded to the front of the sound field.

Another method for achieving a similar effect would be to decode the Ambisonics signal to a set of points regularly spaced on the sphere, applying a directional weight to these points, then re-encoding the points to Ambisonics. However, this requires a transformation from and to the Ambisonics domain over a large number of points. This makes it more CPU intensive than the simple method used here, and so it is not implemented in libspatialaudio.

AmbisonicZoomer

Configuration

Before calling any other functions the object must first be configured by calling Configure() with the appropriate values. If the values are supported then the it will return true and the object can now be used.

The configuration parameters are:

• nOrder: The ambisonic order from 1 to 3.
• b3D: A bool to indicate if the signal is to be decoded is 2D (azimuth only) or 3D (azimuth and elevation).
• nBlockSize: The maximum block size Process() is expected to handle.
• sampleRate: The sample rate of the signal expected by Process().

Setting the Zoom Amount

The amount of zoom can be set from 0 to 1 using 'SetZoom()'. A value of 0 corresponds to no zoom and the signal will be returned unchanged after processing. A value of 1 means that the maximum amount of zooming will be applied and almost all of the signal will be from the re-encoded forward-facing virtual microphone.

Zooming a Signal

A B-format signal can have zooming applied using the Process() function. The input signal is replaced by the processed signal.

The inputs are:

• pBFSrcDst: A pointer to the source B-format signal.
• nSamples: The number of samples to process.

Code Example

This example shows how to apply zooming to an Ambisonics signal.

#include "Ambisonics.h"

using namespace spaudio;

const unsigned int sampleRate = 48000;
const int nBlockLength = 512;

// Higher ambisonic order means higher spatial resolution and more channels required
const unsigned int nOrder = 1;

std::vector<float> sinewave(nBlockLength);
// Fill the vector with a sine wave
for (int i = 0; i < nBlockLength; ++i)
    sinewave[i] = (float)std::sin((float)M_PI * 2.f * 440.f * (float)i / (float)sampleRate);

// B-format buffer
BFormat myBFormat;
myBFormat.Configure(nOrder, true, nBlockLength);
myBFormat.Reset();

// Encode the signal to Ambisonics
AmbisonicEncoder myEncoder;
myEncoder.Configure(nOrder, true, sampleRate, 0);
PolarPosition<float> position;
position.azimuth = 0;
position.elevation = 0;
position.distance = 1.f;
myEncoder.SetPosition(position);
myEncoder.Reset();
myEncoder.Process(sinewave.data(), nBlockLength, &myBFormat);

// Set up the zoomer processor
AmbisonicZoomer myZoomer;
myZoomer.Configure(nOrder, true, nBlockLength, 0);
myZoomer.SetZoom(0.5f);

// Apply zooming to the Ambisonics signal
myZoomer.Process(&myBFormat, nBlockLength);