add audio processor preferences to FAQ

* audio processor preferences were added in change
  https://review.jami.net/c/jami-daemon/+/21976

Change-Id: I56b2c450772aaa01765404d5a0829f89c565f5c4

user/faq.rst

@@ -135,6 +135,8 @@ There are some possible future solutions to this issue, including
 :ref:`swarms <technical-overview:swarms>`, which will allow users to
 set up their own "server" node to receive messages for them.
 
+.. _config-file-location:
+
 Where are the configuration files located?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -666,3 +668,31 @@ encoder=[encoder_name]``, where ``encoder_name`` can be any of
 ``libx264``, ``libvpx``, ``mpeg4``, ``h263``, ``libopus``,
 ``libspeex``, ``g722``, ``pcm_alaw``, or ``pcm_mulaw`` (the FFmpeg
 names for all of Jami's supported encoders).
+
+How can I configure the audio processor?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+An audio processor allows Jami to clean up and process your microphone's audio.
+It can remove echo, reduce noise, and equalize your microphone's volume.
+Additionally, it can detect when you're speaking and send this information to participants in your call.
+The audio processor settings can be set in your ``dring.yml`` file.
+See :ref:`this section to find where this file is located <config-file-location>`.
+
+The relevant preference keys are:
+
+- ``audioProcessor``, which configures which audio processor to use. The valid options are:
+
+  - ``webrtc``: the `WebRTC Audio Processing library <https://www.freedesktop.org/software/pulseaudio/webrtc-audio-processing/>`_
+  - ``speex``: the `Speex DSP library <https://gitlab.xiph.org/xiph/speexdsp>`_
+  - ``null``: disables audio processing (though your system echo canceller may still be used, see below)
+
+- ``echoCancel``, which configures how echo cancelling should be done. The valid options are:
+
+  - ``auto``: try to use your operating system's echo canceller (if it exists), otherwise fall back to the chosen audio processor's echo canceller
+  - ``audioProcessor``: only use the chosen audio processor's echo canceller
+  - ``system``: only use your operating system's echo canceller
+  - ``null``: don't do any echo cancelling
+
+- ``noiseReduce``, ``true``/``false`` to set noise reduction on the audio processor
+- ``automaticGainControl``, ``true``/``false`` to set automatic gain control on the audio processor
+- ``voiceActivityDetection``, ``true``/``false`` to set voice activity detection on the audio processor