AmbDec Configuration Files ========================== AmbDec configuration files were developed by Fons Adriaensen as part of the AmbDec program . 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 the high-quality ambisonic decoder. 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 Specifies a text description of the configuration. Ignored by OpenAL Soft. /version Declares the format version used by the configuration file. OpenAL Soft currently only supports version 3. /dec/chan_mask 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 Specifies the number of frequency bands used by the decoder. This must be 1 for single-band or 2 for dual-band. /dec/speakers Specifies the number of output speakers to decode to. /dec/coeff_scale 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 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 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 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 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 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-freqyency matrix. Unused for single-band decoders. /opt/xover_ratio 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 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 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 ... 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 Additionally, configuration files for surround51 will acknowledge back speakers for side channels, and surround51rear will acknowledge side speakers for back 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 . 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.