Technology
Ambisonic microphone Round-up JON THORNTON looks at the growth of ambisonic mic options, and the models worth looking into.
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elative to the history of audio production, Ambisonics is an old technology, dating back to work carried out by Michael Gerzon and others in the 1970s. It’s had something of a renaissance over the last five years, though, as the potential of capturing audio in three dimensions and being able to decode that in different ways has been turbocharged by developments in DSP power and the rise of Virtual and Augmented Reality applications. Put simply, Ambisonics captures a 360o, spherical soundfield, using multiple microphone capsules. It’s perhaps easiest thought of as an extension of MS stereo recording, where a figure-8 microphone’s output is added or subtracted from a central cardioid microphone to give stereo positioning. Replace the cardioid ‘M’ microphone with an omni, and adding a further two figure-8 microphones oriented front/rear and up/down and you end up with four audio components. These four components, known as ‘B Format’ are named W (omni), X (front/rear), Y (left/right) and Z (up/down) and can be used in several ways. For example, by combining them it’s possible to create the pickup of a virtual microphone, in both pattern and angle. And 40 / April/May 2021
because you can use the components to derive multiple virtual microphones, creating one oriented to each position of a 5.1 speaker arrangement allows decoding to this format, or any other. This decoding can then be binaurally rendered for headphone playback, and because it’s virtual, can be constantly rotated by head tracking information — hence the appeal for VR applications. In practice though, the microphone arrangement described above is somewhat impractical, as it’s physically impossible to locate all of those capsules coincidentally. Instead, most current ambisonic microphones employ an array of four cardioid or sub-cardioid capsules arranged in a tetrahedral fashion, such that each capsule is located on each of the four faces of a tetrahedron. This arrangement is known as ‘A Format’ and allows the corresponding ‘B Format’ components to be derived by some relatively simple matrixing. Of course, this doesn’t get around the issue of non-coincidence, but simply minimises and distributes any phase issues equally. As a result, the A-to-B format conversion process requires some complex filtering, the nature of which is somewhat dependent on the exact spacing of the capsules, so varies from manufacturer to
manufacturer. Some address this by providing hardware or software convertors, others by supplying filter coefficients to plug into OEM software. Finally, everything described so far is known as first-order ambisonics (FOA), using only four components. One criticism of FOA is that it doesn’t provide particularly good spatial resolution — in the same way that a coincident stereo pair, relying only on level differences between the two channels, may not deliver as good an image as a spaced pair. What this means in practice is a blurring of the location of sources. Higher orders (more components to the B format, so more resolution) do exist — second-order has a total of nine components, and third-order 16 components. But there’s a trade-off here: more components means more capsules, which means more noise from summing as well as more aggressive filtering requirements, all of which have an impact on overall fidelity. And higher-order microphones can’t simply use traditional polar patterns and matrixing to generate these components. Instead, they rely on larger multi-capsule arrays married to DSP. Here’s a round-up of both some longestablished and new contenders in the field.
