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VR Audio — Differences between A Format and B Format

By Claudio Santos

A Format and B Format. What is the difference between them after all? Since things can get pretty confusing, especially with such non-descriptive nomenclature, we thought we’d offer a quick reminder of what each is in the spatial audio world.

A Format and B Format are two analog audio standards that are part of the ambisonics workflow.

A Format is the raw recording of the four individual cardioid capsules in ambisonics microphones. Since each microphone has different capsules at slightly different distances, the A Format is somewhat specific to the microphone model.

B Format is the standardized format derived from the A Format. The first channel carries the amplitude information of the signal, while the other channels determine the directionality through phase relationships between each other. Once you get your sound into B Format you can use a variety of ambisonic tools to mix and alter it.

It’s worth remembering that the B Format also has a few variations on the standard itself; the most important to understand are Channel Order and Normalization standards.

Ambisonics in B Format consists of four channels of audio — one channel carries the amplitude signal while the others represent the directionality in a sphere through phase relationships. Since this can only be achieved by the combination between the channels, it is important that:

– The channels follow a known order
– The relative level between the amplitude channel and the others must be known in order to properly combine them together

Each of these characteristics has a few variations, with the most notable ones being

– Channel Order
– Furse-Malham standard
– ACN standard

– Normalization (level)
– MaxN standard
-SN3D standard

The combination of these variations result in two different B Format standards:
– Furse-Malham – Older standard that is still supported by a variety of plug-ins and other ambisonic processing tools
– AmbiX – Modern standard that has been widely adopted by distribution platforms such as YouTube

Regardless of the format you will deliver your ambisonics file in, it is vital to keep track of the standards you are using in your chain and make the necessary conversions when appropriate. Otherwise rotations and mirrors will end up in the wrong direction and the whole soundsphere will break down into a mess.

Claudio Santos is a sound editor and spatial audio mixer at Silver Sound. Slightly too interested in technology and workflow hacks, he spends most of his waking hours tweaking, fiddling and tinkering away on his computer.

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