Users Guide to FlightGear sound configuration
Version 0.9.8, October 30, 2005
Author: Erik Hofman <erik at ehofman dot com>

This document is an attempt to describe the configuration of
FlightGear flight simulator's aircraft sound in XML.

Sound Architecture:
------------------
All of the sound configuration files are XML-encoded* property lists.
The root element of each file is always named <PropertyList>. Tags are
almost always found in pairs, with the closing tag having a slash
prefixing the tag name, i.e </PropertyList>. The exception is the tag
representing an aliased property. In this case a slash is prepended to
the closing angle bracket.  (see section Aliasing)

The top level sound configuration file is composed of a <fx>, a
<name>, a <path> sound file and zero or more <volume> and/or <pitch>
definitions.

[ Paths are relative to $FG_ROOT (the root of the installed base package .) ]
[ Absolute paths may be used. Comments are bracketed with <!-- -->.         ]

A limited sound configuration file would look something like this:

<PropertyList>
 <fx>
  <engine>
   <name>engine</name>
   <path>Sounds/wasp.wav</path>
   <mode>looped</mode>
   <condition>
    <property>/engines/engine/running</property>
   </condition>
   <volume>
    <property>/engines/engine/mp-osi</property>
    <factor>0.005</factor>
    <min>0.15</min>
    <max>0.5</max>
    <offset>0.15</offset>
   </volume>
   <pitch>
    <property>/engines/engine/rpm</property>
    <factor>0.0012</factor>
    <min>0.3</min>
    <max>5.0</max>
    <offset>0.3</offset>
   </pitch>
  </engine>
 </fx>
</PropertyList>

This would define an engine sound event handler for a piston engine driven
aeroplane. The sound representing the engine is located in $FG_ROOT/Sounds
and is named wasp.wav. The event is started when the property
/engines/engine/running becomes non zero. 

When that happens, the sound will be played looped (see <mode>) until the
property returns zero again. As you can see the volume is mp-osi dependent,
and the pitch of the sound depends on the engine rpm.

Configuration description:
-------------------------
   
<fx>
        Named FX subtree living under /sim/sound
 
 < ... >
        This is the event separator. The text inside the brackets
        can be anything. Bit it is advised to give it a meaningful name
        like: crank, engine, rumble, gear, squeal, flap, wind or stall
 
        The value can be defined multiple times, thus anything which is
        related may have the same name (grouping them together).
 
   <name>
        This defines the name of the event. This name is used internally
        and, although it can me defined multiple times in the same file,
        should normally have an unique value.
 
        Multiple definitions of the same name will allow multiple sections
        to interfere in the starting and stopping of the sample.

        This method can't be used to control the pitch or volume of the 
        sample, but instead multiple volume or pitch section should be
        included inside the same event.

        The types "raise" and "fall" will stop the playback of the sample
        regardless of any other event. This means that when the type "raise"
        is supplied, sample playback will stop when the event turns false.
        Using the type "fall" will stop playback when the event turns true.

        IMPORTANT:
        If the trigger is used for anything else but stopping the sound
         at a certain event, all sections with the same name *should* have
        exactly the same sections for everything but property and type.

        In the case of just stopping the sample at a certain event, the
        sections for path, volume and pitch may be omitted.

   <path>
        This defined th path to the sound file. The path is relative to the
        FlightGear root directory but could be specified absolute.

   <condition>
        Define a condition that triggers the event.
        For a complete description of the FlightGear conditions,
        please read docs-mini/README.conditions

        An event should define either a condition or a property.
 
   <property>
        Define which property triggers the event, and refers to a node
        in the FlightGear property tree. Action is taken when the property
        is non zero.

        A more sophisticated mechanism to trigger the event is described
        in <condition>
 
   <mode>
        This defines how the sample should be played:
 
        once:           the sample is played once.
                        this is the default.
 
        looped:         the sample plays continuously,
                        until the event turns false.

        in-transit:     the sample plays continuously,
                        while the property is changing its value.

  <type>
        This defines the type os this sample:

        fx:             this is the default type and doesn't need to be defined.

        avionics:       sounds set to this time don't have a position and
                        orientation but are treated as if it's mounted to
                        the aircraft panel. it's up to the user to define
                        if it can always be heard or only when in cockpit
                        view.
   
   <volume> / <pitch>
        Volume or Pitch definition. Currently there may be up to 5
        volume and up to 5 pitch definitions defined within one sound
        event. Normally all offset values are added together and the
        results after property calculations will be multiplied.
        A special condition occurs when the value of factor is negative,
        in which case the offset doesn't get added to the other offset values
        but instead will be used in the multiplication section.
 
     <property>
        Defines which property supplies the value for the calculation.
        Either a <property> or an <internal> should be defined.
        The value is treated as a floating point number.

     <internal>
        Defines which internal variable should be used for the calculation.
        The value is treated as a floating point number.
        The following internals are available at this time:

        dt_play:        the number of seconds since the sound started playing.

        dt_stop:        the number of seconds after the sound has stopped.
 
     <delay-sec>
        Delay after which the sound starts playing. This is useful to let
        a property start two sounds at the same time, where the second is
        delayed until the first stopped playing.

     <type>
        Defines the function that should be used upon the property
        before it is used for calculating the net result:

        lin:            linear handling of the property value.
                        this is the default.
 
        ln:             convert the property value to a natural logarithmic
                        value before scaling it. Anything below 1 will return
                        zero.
 
        log:            convert the property value to a true logarithmic
                        value before scaling it. Anything below 1 will return
                        zero.

        inv:            inverse linear handling (1/x).

        abs:            absolute handling of the value (always positive).

        sqrt:           calculate the square root of the absolute value
                        before scaling it.
 
     <factor>
        Defines the multiplication factor for the property value.
        A special condition is when scale is defined as a negative
        value. In this case the result of |<scale>| * <property) will be
        subtracted from <default>
 
     <offset>
        The initial value for this sound. This value is also used as an
        offset value for calculating the end result.

     <random>
        Add a bit of randomness to the offset. Only used for pitch.
 
     <min>
        Minimum allowed value.
        This is useful if sounds start to sound funny. Anything lower
        will be truncated to this value.
 
     <max>
        Maximum allowed value.
        This is useful if sounds gets to loud. Anything higher will be
        truncated to this value.

   <position>
        Specify the position of the sounds source relative to the
        aircraft center.  The coordinate system used is a left hand
        coordinate system where +Y = left, -Y = right, -Z = down, +Z =
        up, -X = forward, +X = aft.  Distances are in meters.
        The volume calculation due to distance and orientation of the
        sounds source ONLY work on mono samples!

     <x>
        X dimension offset

     <y>
        Y dimension offset

     <z>
        Z dimension offset


   <orientation>
        Specify the orientation of the sounds source.

        The zero vector is default, indicating that a Source is not directional.
        Specifying a non-zero vector will make the Source directional in
        the X,Y,Z direction

     <x>
        X dimension

     <y>
        Y dimension

     <z>
        Z dimension

     <inner-angle>
        The inner edge of the audio cone in degrees (0.0 - 180.0).
        Any sound withing that angle will be played at the current gain.

     <outer-angle>
        The outer edge of the audio cone in degrees (0.0 - 180.0).
        Any sound beyond the outer cone will be played at "outer-gain" volume.

     <outer-gain>
        The gain at the outer edge of the cone.


   <reference-dist>
        Set a reference distance of sound in meters. This is the
        distance where the volume is at its maximum.
        Volume is clamped to this maximum for any distance below.
        Volume is attenuated for any distance above.
        Attenuation depends on reference and maximum distance. See
        OpenAL specification on "AL_INVERSE_DISTANCE_CLAMPED" mode
        for details on exact computation.

   <max-dist>
        Set the maximum audible distance for the sound in meters.
        Sound is cut-off above this distance.


Creating a configuration file:
------------------------------

To make things easy, there is a default value for most entries to allow a
sane configuration when a certain entry is omitted.

Default values are:

type:   lin
factor: 1.0
offset: 0.0 for volume, 1.0 for pitch
min:    0.0
max:    0.0 (don't check)

 
 
Calculations are made the following way (for both pitch and volume):
 
   value = 0;
   offs = 0;

   for (n = 0;  n < max; n++) {
      if (factor < 0)
      {
         value += offset[n] - abs(factor[n]) * function(property[n]);
      }
      else
      {
          value += factor[n] * function(property[n]);
          offs += offset[n];
      }
   }

   volume = offs + value;

where function can be one of: lin, ln, log, inv, abs or sqrt