Quiz: Ambisonics – Holophonix Learning Platform

1. The phases of Ambisonics

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Ambisonics is achieved

in two steps: encoding and decoding.
by calculating the path difference of the acoustic wave emitted by a source for each loudspeaker.
using the HRTF linked to the position of the source in space.
by reproducing the acoustic field received at the center of the array.

2. Encoding

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Encoding

is equivalent to digitization (digital conversion) of an analog signal.
produces a stream with as many channels as there are sources.
of a source, is the set of signals specific to each component that subdivides the space: the spherical harmonics. It produces a multichannel stream for each source.

3. The encoding order

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The encoding order N

A.quad (2N+1)qquad B.quad (N+1)^2

describes the precision with which the space will be sampled: the spatial sampling order.
determines the minimum number of loudspeakers required to reproduce the acoustic field at this order (of precision).
means that in 2 dimensions there will be “A” channels in the Ambisonics stream.
means that at least “A” loudspeakers will be needed to reproduce the acoustic field in 2 dimensions.
means that in 2 dimensions there will be “B” channels in the Ambisonics stream.
means that in 3 dimensions there will be “B” channels in the Ambisonics stream.
determines the maximum number of loudspeakers required to reproduce the acoustic field at this order (of precision).
specifies the organization of channels in an Ambisonics stream.
means that at least “B” loudspeakers will be needed to reproduce the acoustic field in 2 dimensions.

4. Ambisonics recording

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Using an Ambisonics microphone at order N=1

requires that the signals from the capsules (A-format) be matrixed to produce a 4-channel Ambisonics stream WXYZ (B-format).
allows a very precise sampling of the space and therefore of the acoustic field.
can be improved by increasing the sampling order.
means the recording can be reproduced with 6 loudspeakers in 3 dimensions.

5. Spatial sampling

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Spatial sampling used in audio

is equivalent to using coincident microphones with complementary directivity patterns to cover the space in 3 dimensions.
allows describing the radiation of a source located at another position from a single point in space.

6. Identification of an Ambisonics stream

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An Ambisonics stream

is determined by the encoding order.
is determined by the sampling frequency (e.g. 48 kHz or 96 kHz)
must specify its normalization (e.g. SN3D or N3D)
is determined by the number of loudspeakers used.
is determined by the organization of its channels (the ACN WYZX ordering at order 1).

7. Decoding

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An Ambisonics stream at order N

A.quad (2N+1)qquad B.quad (N+1)^2

will preserve its encoding accuracy regardless of the number of loudspeakers used.
requires at least “A” loudspeakers in 2 dimensions.
requires only “B” loudspeakers in 3 dimensions.
requires at least “B” loudspeakers in 3 dimensions.
requires only “A” loudspeakers in 2 dimensions.
can be used as a lower-order stream by using only the necessary channels.

8. Decoding order and number of loudspeakers

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begin{array}{rcl} A.quad N_{HP} ge 2 times N_{ordre} + 1 B.quad N_{HP} le 2 times N_{ordre} + 1 C.quad N_{HP} ge (N+1)^2 D.quad N_{HP} le (N+1)^2 end{array}

In 2 dimensions the number of loudspeakers must satisfy condition “B”.
In 2 dimensions the number of loudspeakers must satisfy condition “A”.
In 3 dimensions the number of loudspeakers must satisfy condition “C”.
In 3 dimensions the number of loudspeakers must satisfy condition “D”.

9. Decoding interoperability

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A sound scene is encoded at order N=7.

It is to be decoded in an “Italian-style” theatre on a system of 24 loudspeakers distributed over three horizontal rings at different elevations.

The sound scene will be decoded at order 3 using the first 16 channels of the stream.
The Ambisonics stream contains 15 channels.
The Ambisonics stream contains 64 channels.
The 15 channels of the stream will be used for decoding.
All 64 channels will be used for decoding.

10. Decoding optimization: the type

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Select the correct statements.

The decoding type sets the level ratio between the specific signals of the spherical harmonics in an Ambisonics stream.
The decoding type allows an extension of the sweet-spot zone.

11. Decoding types

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The decoding types are:

inPhase
N3D
basic
“Energy preserving”
maxRe
a maxRe/inPhase combination with a crossover frequency for each.
SN3D

12. Decoding optimization: the method

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The decoding method recommended in the majority of situations is “energy preserving”.

False
True

13. Advantages and disadvantages

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Select the correct statements about Ambisonics.

Source localization will be more precise as the order increases.
Source localization is precise regardless of the decoding order.
Only triplets of loudspeakers are used to reproduce a sound source.
Ambisonics favors an immersive and enveloping diffusion.
All loudspeakers are used to reproduce the acoustic field.

Ambisonics

Quiz: Ambisonics Quiz