Technology

Ambisonics

Ambisonics is a system for capturing, manipulating, and rendering a full sphere sound field, originally developed by Michael Gerzon in the 1970's. The Wikipedia article on ambisonics is a good place to start for a basic understanding. Ambisonics is different than most surround systems in that the channels do not represent speakers but rather are used to represent the sound field via spherical harmonics. We think the math behind this is beautiful (see below), but VVAudio's software takes care of it for you.

Encoding

Encoding is the process of converting the raw microphone signals from a surround microphone, called A-format into a B-format signal. This process was published by Gerzon in his 1975 paper "The Design of Precisely Coincident Microphone Arrays for Stereo and Surround Sound." But when the first commercial tetrahedral microphone was built by SoundField, encoding had to be done with complex analog circuits. Modern digital signal processing allows us to do the job more easily and more accurately. Those strange shapes show the effects of spatial aliasing in a tetrahedral mic.

Starting in 2007, VVAudio worked with Core Sound and Richard Lee, a real ambisonics pioneer, to develop the signal chain for processing the new Core Sound TetraMic. The results were implemented in VVAudio's VVMic for TetraMic and then in VVTetraVST, the first ever commercial implementation of A-format recording. Now with the release of VVEncode, TetraMic encoding can be done natively in ProTools as well.

VVEncode also supports the new Brahma tetrahedral microphone. These mics use a very different approach to calibration and correction which results in a matrix of sixteen convolution-based filters. By making this algorithm available as a plugin for VST and ProTools, VVAudio is continuing its efforts to make ambisonics more accessible.

Decoding

Decoding is the process of converting B-format into speaker feeds. At its most basic level, decoding is just a few sums and differences, much like MS recording but in 3D. Several fine points have been developed over the years including shelf filters to accommodate how we hear at high and low frequencies, near field correction (NFC) to correct for the fact that the speakers are not producing plane waves, and a matrix pseudo-inverse method for calculating the best decode coefficients. VVAudio's soon to be released VVDecode implements all of these methods as described in the classic work "Is My Decoder Ambisonic" by Benjamin, Lee, and Heller.

Parametric Decoding

In order to get better localization, especially when using large numbers of speakers, advanced decoder techniques have been developed that do processing in the frequency domain. Such decoders are called parametric. We will have more to say about parametric decoders soon.

Binaural Decoding

Binaural recording techniques use knowledge about how our ears, head, and body change the sound on the way to our eardrums to deliver a surround sound image using headphones. Binaural has become very popular lately with the rise of virtual reality since it can deliver the same sense of immersion that the visuals strive to achieve. An ambisonic recording can be converted to binaural by rendering several virtual speakers and then processing each with the appropriate HRTF. VVAudio has been working on binaural technologies and the image shows an HRTF from the Listen database.

Ambisonic Processing

By far the most common form of ambisonic processing is a simple rotate, but others like zoom, reverb, and echo, are also possible. Non-ambisonic plugins can be used on ambisonic signals as long as care is taken to process all channels the same way. Sometimes special processing is needed, for example VVAudio's rotate library actually has three different algorithms depending on how fast the soundfield is rotating. The symmetrical nature of ambisonics makes surround specific processes like rotation or spatial EQ practical and well behaved.

Higher Order Ambisonics

VVAudio uses higher order ambisonics (HOA) in simulations to validate the results of advanced first order algorithms.

VVSDK

While only a couple of these technologies are available from VVAudio today in the form of plugins, all of this and more is already in the code library that we use to build our plugins, called VVSDK. This same library has been used by others in applications from VR players for Google Cardboard to acoustic measurement applications in MatLab. Contact us at info@vvaudio.com if you are interested in including VVSDK in your own application.