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ambdec.txt
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AmbDec Configuration Files
==========================
AmbDec configuration files were developed by Fons Adriaensen as part of the
AmbDec program <http://kokkinizita.linuxaudio.org/linuxaudio/index.html>.
The file works by specifying a decoder matrix or matrices which transform
ambisonic channels into speaker feeds. Single-band decoders specify a single
matrix that transforms all frequencies, while dual-band decoders specifies two
matrices where one transforms low frequency sounds and the other transforms
high frequency sounds. See docs/ambisonics.txt for more general information
about ambisonics.
Starting with OpenAL Soft 1.18, version 3 of the file format is supported as a
means of specifying custom surround sound speaker layouts. These configuration
files are also used to enable per-speaker distance compensation.
File Format
===========
As of this writing, there is no official documentation of the .ambdec file
format. However, the format as OpenAL Soft sees it is as follows:
The file is plain text. Comments start with a hash/pound character (#). There
may be any amount of whitespace in between the option and parameter values.
Strings are *not* enclosed in quotation marks.
/description <desc:string>
Specifies a text description of the configuration. Ignored by OpenAL Soft.
/version <ver:int>
Declares the format version used by the configuration file. OpenAL Soft
currently only supports version 3.
/dec/chan_mask <mask:hex>
Specifies a hexadecimal mask value of ambisonic input channels used by this
decoder. Counting up from the least significant bit, bit 0 maps to Ambisonic
Channel Number (ACN) 0, bit 1 maps to ACN 1, etc. As an example, a value of 'b'
enables bits 0, 1, and 3 (1011 in binary), which correspond to ACN 0, 1, and 3
(first-order horizontal).
/dec/freq_bands <count:int>
Specifies the number of frequency bands used by the decoder. This must be 1 for
single-band or 2 for dual-band.
/dec/speakers <count:int>
Specifies the number of output speakers to decode to.
/dec/coeff_scale <type:string>
Specifies the scaling used by the decoder coefficients. Currently recognized
types are fuma, sn3d, and n3d, for Furse-Malham (FuMa), semi-normalized (SN3D),
and fully normalized (N3D) scaling, respectively.
/opt/input_scale <name:string>
Specifies the scaling used by the ambisonic input data. As OpenAL Soft renders
the data itself and knows the scaling, this is ignored.
/opt/nfeff_comp <dir:string>
Specifies whether near-field effect compensation is off (not applied at all),
applied on input (faster, less accurate with varying speaker distances) or
output (slower, more accurate with varying speaker distances). Ignored by
OpenAL Soft.
/opt/delay_comp <onoff:bool>
Specifies whether delay compensation is applied for output. This is used to
correct for time variations caused by different speaker distances. As OpenAL
Soft has its own config option for this, this is ignored.
/opt/level_comp <onoff:bool>
Specifies whether gain compensation is applied for output. This is used to
correct for volume variations caused by different speaker distances. As OpenAL
Soft has its own config option for this, this is ignored.
/opt/xover_freq <freq:float>
Specifies the crossover frequency for dual-band decoders. Frequencies less than
this are fed to the low-frequency matrix, and frequencies greater than this are
fed to the high-frequency matrix. Unused for single-band decoders.
/opt/xover_ratio <decibels:float>
Specifies the volume ratio between the frequency bands. Values greater than 0
decrease the low-frequency output by half the specified value and increase the
high-frequency output by half the specified value, while values less than 0
increase the low-frequency output and decrease the high-frequency output to
similar effect. Unused for single-band decoders.
/speakers/{
Begins the output speaker definitions. A speaker is defined using the add_spkr
command, and there must be a matching number of speaker definitions as the
specified speaker count. The definitions are ended with a "/}".
add_spkr <id:string> <dist:float> <azi:float> <elev:float> <connection:string>
Defines an output speaker. The ID is a string identifier for the output speaker
(see Speaker IDs below). The distance is in meters from the center-point of the
physical speaker array. The azimuth is the horizontal angle of the speaker, in
degrees, where 0 is directly front and positive values go left. The elevation
is the vertical angle of the speaker, in degrees, where 0 is directly front and
positive goes upward. The connection string is the JACK port name the speaker
should connect to. Currently, OpenAL Soft uses the ID and distance, and ignores
the rest.
/lfmatrix/{
Begins the low-frequency decoder matrix definition. The definition should
include an order_gain command to specify the base gain for the ambisonic
orders. Each matrix row is defined using the add_row command, and there must be
a matching number of rows as the number of speakers. Additionally the row
definitions are in the same order as the speaker definitions. The definitions
are ended with a "/}". Only valid for dual-band decoders.
/hfmatrix/{
Begins the high-frequency decoder matrix definition. The definition should
include an order_gain command to specify the base gain for the ambisonic
orders. Each matrix row is defined using the add_row command, and there must be
a matching number of rows as the number of speakers, Additionally the row
definitions are in the same order as the speaker definitions. The definitions
are ended with a "/}". Only valid for dual-band decoders.
/matrix/{
Begins the decoder matrix definition. The definition should include an
order_gain command to specify the base gain for the ambisonic orders. Each
matrix row is defined using the add_row command, and there must be a matching
number of rows as the number of speakers. Additionally the row definitions are
in the same order as the speaker definitions. The definitions are ended with a
"/}". Only valid for single-band decoders.
order_gain <gain:float> <gain:float> <gain:float> <gain:float>
Specifies the base gain for the zeroth-, first-, second-, and third-order
coefficients in the given matrix, automatically scaling the related
coefficients. This should be specified at the beginning of the matrix
definition.
add_row <coefficient:float> ...
Specifies a row of coefficients for the matrix. There should be one coefficient
for each enabled bit in the channel mask, and corresponds to the matching ACN
channel.
/end
Marks the end of the configuration file.
Speaker IDs
===========
The AmbDec program uses the speaker ID as a label to display in its config
dialog, but does not otherwise use it for any particular purpose. However,
since OpenAL Soft needs to match a speaker definition to an output channel, the
speaker ID is used to identify what output channel it correspond to. Therefore,
OpenAL Soft requires these channel labels to be recognized:
LF = Front left
RF = Front right
LS = Side left
RS = Side right
LB = Back left
RB = Back right
CE = Front center
CB = Back center
LFT = Top front left
RFT = Top front right
LBT = Top back left
RBT = Top back right
Additionally, configuration files for surround51 will acknowledge back speakers
for side channels, to avoid issues with a configuration expecting 5.1 to use
the side channels when the device is configured for back, or vice-versa.
Furthermore, OpenAL Soft does not require a speaker definition for each output
channel the configuration is used with. So for example a 5.1 configuration may
omit a front center speaker definition, in which case the front center output
channel will not contribute to the ambisonic decode (though OpenAL Soft will
still use it in certain scenarios, such as the AL_EFFECT_DEDICATED_DIALOGUE
effect).
Creating Configuration Files
============================
Configuration files can be created or modified by hand in a text editor. The
AmbDec program also has a GUI for creating and editing them. However, these
methods rely on you having the coefficients to fill in... they won't be
generated for you.
Another option is to use the Ambisonic Decoder Toolbox
<https://bitbucket.org/ambidecodertoolbox/adt.git>. This is a collection of
MATLAB and GNU Octave scripts that can generate AmbDec configuration files from
an array of speaker definitions (labels and positions). If you're familiar with
using MATLAB or GNU Octave, this may be a good option.
There are plans for OpenAL Soft to include a utility to generate coefficients
and make configuration files. However, calculating proper coefficients for
anything other than regular or semi-regular speaker setups is somewhat of a
black art, so may take some time.