"[In first-order Ambisonics],
sound information is encoded into four channels: W, X, Y and Z.
This is called Ambisonic B-format.
The W channel is the non-directional mono component of the signal,
corresponding to the output of an omnidirectional microphone.
The X, Y and Z channels are the directional components in three dimensions.
They correspond to the outputs of three figure-of-eight microphones,
facing forward, to the left, and upward respectively."
first-order Ambisonics 是由一個單聲道和三個方向聲道構成
A-format is the four signals from the capsules – left-front/back and
right-front/back. To get B-format:
X = 0.5 ((LF – LB) + (RF – RB))
Y = 0.5 ((LF – RB) – (RF – LB))
Z = 0.5 ((LF – LB) + (RB – RF))
W = 0.5 (LF + LB + RF + RB)
The 3 basic patterns are: 話筒指向性
- omnidirectional
- figure-8
- cardioid
google 使用的是
Ambisonic audio must use ACN channel ordering and SN3D normalization.
根據通道順序可以分爲
Component ordering
FuMa
SID
ACN
Normalisation
maxN
SN3D
N3D
N2D / SN2D
根據格式
File formats and metadata
AMB
FuMa
.amb Furse-Malham channel ordering and maxN* normalisation.
SUBTYPE_AMBISONIC_B_FORMAT_PCM {00000001-0721-11d3-8644-C8C1CA000000} for integer samples, or
SUBTYPE_AMBISONIC_B_FORMAT_IEEE_FLOAT {00000003-0721-11d3-8644-C8C1CA000000} for floating point.
The definition mandates that the WAVE_EX dwChannelMask must be set to zero.
Furthermore, it recommends that the file should contain a PEAK chunk, containing the value and position of the
highest sample in each channel.
libsndfile
AmbiX
ACN ordering and SN3D normalisation
YouTube 360, Google Cardboard, Google Jump
1AD318C3-00E5-5576-BE2D-0DCA2460BC89.
B-format (FuMa) lists the channels in order: W, X, Y, Z. The W channel also has a -3 dB gain applied.
The ambiX convention lists the channels using the Ambisonics Channel Number (ACN) ordering: W, Y, Z, X.
The channels also use SN3D normalisation (which for first order Ambisonics simply means that the four
channels have a uniform gain normalisation).
ffmpeg -loop 1 -i PS.jpg -i ambiX.wav -map 1:a -map 0:v -c:a copy -channel_layout 4.0 -c:v libx264 -b:v 40000k -bufsize 40000k -shortest PSandAmbiX.mov
現在大概瞭解了 全景聲的構成
需要保存音頻到wav文件中
A WAVE file is often just a RIFF file
with a single "WAVE" chunk which consists of two sub-chunks
-- a "fmt " chunk specifying the data format
and a "data" chunk containing the actual sample data.
Here is the interpretation of these bytes as a WAVE soundfile: