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189 lines
8.5 KiB
189 lines
8.5 KiB
AmbDec Configuration Files
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==========================
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AmbDec configuration files were developed by Fons Adriaensen as part of the
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AmbDec program <http://kokkinizita.linuxaudio.org/linuxaudio/index.html>.
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The file works by specifying a decoder matrix or matrices which transform
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ambisonic channels into speaker feeds. Single-band decoders specify a single
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matrix that transforms all frequencies, while dual-band decoders specifies two
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matrices where one transforms low frequency sounds and the other transforms
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high frequency sounds. See docs/ambisonics.txt for more general information
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about ambisonics.
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Starting with OpenAL Soft 1.18, version 3 of the file format is supported as a
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means of specifying custom surround sound speaker layouts. These configuration
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files are also used to enable the high-quality ambisonic decoder.
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File Format
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===========
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As of this writing, there is no official documentation of the .ambdec file
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format. However, the format as OpenAL Soft sees it is as follows:
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The file is plain text. Comments start with a hash/pound character (#). There
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may be any amount of whitespace in between the option and parameter values.
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Strings are *not* enclosed in quotation marks.
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/description <desc:string>
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Specifies a text description of the configuration. Ignored by OpenAL Soft.
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/version <ver:int>
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Declares the format version used by the configuration file. OpenAL Soft
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currently only supports version 3.
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/dec/chan_mask <mask:hex>
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Specifies a hexadecimal mask value of ambisonic input channels used by this
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decoder. Counting up from the least significant bit, bit 0 maps to Ambisonic
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Channel Number (ACN) 0, bit 1 maps to ACN 1, etc. As an example, a value of 'b'
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enables bits 0, 1, and 3 (1011 in binary), which correspond to ACN 0, 1, and 3
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(first-order horizontal).
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/dec/freq_bands <count:int>
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Specifies the number of frequency bands used by the decoder. This must be 1 for
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single-band or 2 for dual-band.
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/dec/speakers <count:int>
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Specifies the number of output speakers to decode to.
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/dec/coeff_scale <type:string>
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Specifies the scaling used by the decoder coefficients. Currently recognized
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types are fuma, sn3d, and n3d, for Furse-Malham (FuMa), semi-normalized (SN3D),
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and fully normalized (N3D) scaling, respectively.
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/opt/input_scale <name:string>
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Specifies the scaling used by the ambisonic input data. As OpenAL Soft renders
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the data itself and knows the scaling, this is ignored.
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/opt/nfeff_comp <dir:string>
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Specifies whether near-field effect compensation is off (not applied at all),
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applied on input (faster, less accurate with varying speaker distances) or
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output (slower, more accurate with varying speaker distances). Ignored by
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OpenAL Soft.
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/opt/delay_comp <onoff:bool>
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Specifies whether delay compensation is applied for output. This is used to
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correct for time variations caused by different speaker distances. As OpenAL
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Soft has its own config option for this, this is ignored.
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/opt/level_comp <onoff:bool>
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Specifies whether gain compensation is applied for output. This is used to
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correct for volume variations caused by different speaker distances. As OpenAL
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Soft has its own config option for this, this is ignored.
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/opt/xover_freq <freq:float>
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Specifies the crossover frequency for dual-band decoders. Frequencies less than
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this are fed to the low-frequency matrix, and frequencies greater than this are
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fed to the high-frequency matrix. Unused for single-band decoders.
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/opt/xover_ratio <decibels:float>
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Specifies the volume ratio between the frequency bands. Values greater than 0
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decrease the low-frequency output by half the specified value and increase the
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high-frequency output by half the specified value, while values less than 0
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increase the low-frequency output and decrease the high-frequency output to
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similar effect. Unused for single-band decoders.
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/speakers/{
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Begins the output speaker definitions. A speaker is defined using the add_spkr
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command, and there must be a matching number of speaker definitions as the
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specified speaker count. The definitions are ended with a "/}".
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add_spkr <id:string> <dist:float> <azi:float> <elev:float> <connection:string>
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Defines an output speaker. The ID is a string identifier for the output speaker
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(see Speaker IDs below). The distance is in meters from the center-point of the
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physical speaker array. The azimuth is the horizontal angle of the speaker, in
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degrees, where 0 is directly front and positive values go left. The elevation
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is the vertical angle of the speaker, in degrees, where 0 is directly front and
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positive goes upward. The connection string is the JACK port name the speaker
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should connect to. Currently, OpenAL Soft uses the ID and distance, and ignores
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the rest.
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/lfmatrix/{
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Begins the low-frequency decoder matrix definition. The definition should
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include an order_gain command to specify the base gain for the ambisonic
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orders. Each matrix row is defined using the add_row command, and there must be
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a matching number of rows as the number of speakers. Additionally the row
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definitions are in the same order as the speaker definitions. The definitions
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are ended with a "/}". Only valid for dual-band decoders.
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/hfmatrix/{
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Begins the high-frequency decoder matrix definition. The definition should
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include an order_gain command to specify the base gain for the ambisonic
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orders. Each matrix row is defined using the add_row command, and there must be
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a matching number of rows as the number of speakers, Additionally the row
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definitions are in the same order as the speaker definitions. The definitions
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are ended with a "/}". Only valid for dual-band decoders.
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/matrix/{
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Begins the decoder matrix definition. The definition should include an
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order_gain command to specify the base gain for the ambisonic orders. Each
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matrix row is defined using the add_row command, and there must be a matching
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number of rows as the number of speakers. Additionally the row definitions are
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in the same order as the speaker definitions. The definitions are ended with a
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"/}". Only valid for single-band decoders.
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order_gain <gain:float> <gain:float> <gain:float> <gain:float>
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Specifies the base gain for the zeroth-, first-, second-, and third-order
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coefficients in the given matrix, automatically scaling the related
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coefficients. This should be specified at the beginning of the matrix
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definition.
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add_row <coefficient:float> ...
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Specifies a row of coefficients for the matrix. There should be one coefficient
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for each enabled bit in the channel mask, and corresponds to the matching ACN
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channel.
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/end
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Marks the end of the configuration file.
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Speaker IDs
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===========
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The AmbDec program uses the speaker ID as a label to display in its config
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dialog, but does not otherwise use it for any particular purpose. However,
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since OpenAL Soft needs to match a speaker definition to an output channel, the
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speaker ID is used to identify what output channel it correspond to. Therefore,
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OpenAL Soft requires these channel labels to be recognized:
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LF = Front left
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RF = Front right
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LS = Side left
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RS = Side right
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LB = Back left
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RB = Back right
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CE = Front center
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CB = Back center
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Additionally, configuration files for surround51 will acknowledge back speakers
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for side channels, and surround51rear will acknowledge side speakers for back
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channels, to avoid issues with a configuration expecting 5.1 to use the side
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channels when the device is configured for back, or vice-versa.
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Furthermore, OpenAL Soft does not require a speaker definition for each output
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channel the configuration is used with. So for example a 5.1 configuration may
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omit a front center speaker definition, in which case the front center output
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channel will not contribute to the ambisonic decode (though OpenAL Soft will
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still use it in certain scenarios, such as the AL_EFFECT_DEDICATED_DIALOGUE
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effect).
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Creating Configuration Files
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============================
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Configuration files can be created or modified by hand in a text editor. The
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AmbDec program also has a GUI for creating and editing them. However, these
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methods rely on you having the coefficients to fill in... they won't be
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generated for you.
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Another option is to use the Ambisonic Decoder Toolbox
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<https://bitbucket.org/ambidecodertoolbox/adt.git>. This is a collection of
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MATLAB and GNU Octave scripts that can generate AmbDec configuration files from
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an array of speaker definitions (labels and positions). If you're familiar with
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using MATLAB or GNU Octave, this may be a good option.
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There are plans for OpenAL Soft to include a utility to generate coefficients
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and make configuration files. However, calculating proper coefficients for
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anything other than regular or semi-regular speaker setups is somewhat of a
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black art, so may take some time.
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