Start the audio node.

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-Audio (not written yet)
-=======================
+Audio
+=====
+
+Audio is the most important part of video. It is also the most
+neglected part in most amateur productions; it is easy to care
+about full-HD productions but never remember to give the speaker
+a microphone. Your stream can live with blurry or murky pictures,
+but it cannot live with people not hearing what's being said.
+
+Nageru aims to give the operator meaningful, useful controls for
+processing and mixing audio, with a focus on voice. There are two
+modes for audio processing, namely *simple* and *multichannel*;
+they are selectable from the audio menu.
+
+Be aware that a mix that sounds good on a PA system will not
+necessarily sound good on a stream; PA systems often have rather
+different audio characteristics than a set of home speakers or
+headphones, and there will also frequently be other sounds in the
+room that remove some of the typical “dryness”. However, for simple
+use, reusing such a mix isn't the worst choice you can make.
+
+
+Simple mode
+-----------
+
+**Simple** audio mode is the default, and was the only mode available
+up until Nageru 1.4.0. Despite its name, it contains a powerful
+audio processing chain; however, in many cases, you won't need to
+understand or twiddle any of the knobs availale.
+
+Simple mode allows input from only a single source, and that source
+has to be one of the capture cards. (You choose which one by right-clicking
+on its channel and selecting it as audio source.) The two typical
+cases where this is useful are:
+
+  * When you simply take in audio from one of the cameras,
+    possibly by way of external microphone, or
+  * When you have an external mixer and can embed its output
+    in one of the video inputs.
+
+If you want more than one audio source at a time, or if you want
+to use ALSA inputs, you will need to use multichannel mode; it is
+more complicated, but it is a strict superset of what the simple mode
+can do. (In fact, simple mode constructs a multichannel setup
+behind-the-scenes and then runs the multichannel audio code.)
+
+
+The audio strip
+---------------
+
+.. image:: images/basic-ui.png
+
+The audio strip contains the processing chain for the audio from
+start to end. Note that by default, everything is enabled;
+if you have a premade audio mix that you are confident that you
+want 1:1 into the stream, you can start Nageru with the “--flat-audio”
+flag, that instead starts with everything disabled.
+
+The first step in the pipeline is a **lo-cut** (or equivalently,
+highpass) filter. The exact cutoff frequency is a bit a matter
+of taste (and also depends on the speaker), but the main point
+is that it gets rid of low-frequency hum and a lot of the background
+noise that is not related to the speaker's voice. (If you were
+producing music, you'd probably want it there to make room for
+music *under* it, but the you'd want it higher than the default 120 Hz.)
+
+(TODO: write more)
+
+
+Audio meters
+------------
+
+
+Multichannel mode
+-----------------
+
+
+MIDI control
+------------

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