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<body class="article">
<div id="header">
<h1>upmpdcli Linn Songcast integration</h1>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>The <strong>upmdcli</strong> UPnP front-end to MPD implements support for Linn Songcast with
help from the <strong>sc2mpd</strong> auxiliary process.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_what_is_songcast">What is Songcast</h2>
<div class="sectionbody">
<div class="paragraph"><p>Songcast is a protocol for transporting audio streams across the
network. This is independant from the UPnP framework which mostly deals
with URLs and audio files (usually, but not necessarily, compressed).</p></div>
<div class="paragraph"><p>The streams transported by Songcast are actual real-time audio data, which
can go straight to an audio card for playing.</p></div>
<div class="paragraph"><p>Controlling the streams (connecting, starting, stopping) is done through an
UPnP service named OpenHome <em>Receiver</em>.</p></div>
<div class="paragraph"><p>The typical use of Songcast is to have an audio driver on a Windows or OS X
desktop capture and forward the audio stream to a remote Songcast device.</p></div>
<div class="paragraph"><p>Any application on the desktop will (be compelled to) transparently play to
the remote device, without need to know anything about Songcast.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_implementation_of_songcast_support_in_upmpdcli">Implementation of Songcast support in upmpdcli</h2>
<div class="sectionbody">
<div class="paragraph"><p><strong>upmpdcli</strong> implements the <em>Receiver</em> service, and uses an auxiliary process
(<strong>sc2mpd</strong>) for transporting the audio data. <strong>sc2mpd</strong> is a slight
modification of the sample program which comes with the Linn Songcast
open-source implementation, with the addition of an HTTP interface (based
on <em>libmicrohttpd</em>) to forward the data to <strong>mpd</strong>.</p></div>
<div class="paragraph"><p>Setting up a connection happens as follows:</p></div>
<div class="ulist"><ul>
<li>
<p>
If it finds an executable <strong>sc2mpd</strong> command in the PATH when starting up,
<strong>upmpdcli</strong> advertises a <em>Receiver</em> service.
</p>
</li>
<li>
<p>
The Songcast implementation from the desktop finds out about the
<em>Receiver</em> through the normal UPnP mechanisms and can be instructed to
use it. It then tells the <em>Receiver</em> in <strong>upmpdcli</strong> to start playing.
</p>
</li>
<li>
<p>
<strong>upmdpcli</strong> starts the <strong>sc2mpd</strong> process, which gets ready to receive data
through Songcast, and make it available through HTTP.
</p>
</li>
<li>
<p>
<strong>upmpdcli</strong> instructs <strong>mpd</strong> to play the URL for the <strong>sc2mpd</strong> output.
</p>
</li>
</ul></div>
<div class="paragraph"><p>There is currently a 10-12 S delay between the connection request and the
moment the audio starts playing. This is not normal, but I could not find
the reason for it, so it’s an unavoidable inconvenience at the moment.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_configuration">Configuration</h2>
<div class="sectionbody">
<div class="paragraph"><p>No configuration is necessary by default: if <strong>sc2mpd</strong> is present,
<strong>upmpdcli</strong> will advertise the Songcast capability, and any host with a
Songcast sender installed should be able to use it.</p></div>
<div class="paragraph"><p>However, you can set a number of values in the upmpdcli configuration file
(you <strong>must</strong> set the <strong>upmpdcli</strong> <tt>-c</tt> option for <strong>sc2mpd</strong> to see them):</p></div>
<div class="dlist"><dl>
<dt class="hdlist1">
sclogfilename
</dt>
<dd>
<p>
Name of the file which will receive <strong>sc2mpd</strong> log messages. <tt>stderr</tt> by
default. This <em>can’t be</em> the same file used by <strong>upmpdcli</strong>.
</p>
</dd>
<dt class="hdlist1">
scloglevel
</dt>
<dd>
<p>
Log verbosity.
</p>
</dd>
<dt class="hdlist1">
schttpport
</dt>
<dd>
<p>
HTTP port used by <strong>mpd</strong> to connect to <strong>sc2mpd</strong>. 8888 by default. This must
be an available port on <tt>localhost</tt>, and it will only accept connections from
<tt>localhost</tt>.
</p>
</dd>
<dt class="hdlist1">
sc2mpd
</dt>
<dd>
<p>
Path for the <strong>sc2mpd</strong> executable file (e.g. <tt>/usr/local/bin/sc2mpd</tt>). Only
useful if <strong>sc2mpd</strong> was not installed to a location in the executable $PATH
set for the init scripts. Typically only <tt>/bin</tt> and <tt>/usr/bin</tt> are in
there.
</p>
</dd>
</dl></div>
</div>
</div>
<div class="sect1">
<h2 id="_building_sc2mpd">Building sc2mpd</h2>
<div class="sectionbody">
<div class="paragraph"><p>There are two parts in building <strong>sc2mpd</strong>:</p></div>
<div class="ulist"><ul>
<li>
<p>
Building the Openhome libraries
</p>
</li>
<li>
<p>
Building <strong>scmpd</strong> proper
</p>
</li>
</ul></div>
<div class="paragraph"><p>Building the Openhome libraries is a bit of a black art, and the sc2mpd
source comes with an <strong>ohbuild.sh</strong> script which will try to clone the
Openhome git repositories and build the libs:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>mkdir /my/place/for/openhome
sh ohbuild.sh /my/place/for/openhome</tt></pre>
</div></div>
<div class="paragraph"><p>Miscellaneous error messages will be displayed during the build. Hope for
the best…</p></div>
<div class="paragraph"><p>When this is done configure sc2mpd:</p></div>
<div class="literalblock">
<div class="content">
<pre><tt>sh autogen.sh
./configure --prefix=/usr --with-openhome=/my/place/for/openhome
make
sudo make install</tt></pre>
</div></div>
<div class="paragraph"><p>The build uses static libraries, so you can move the executable to another
machine without needing the openhome directory.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_miscellaneous_remarks">Miscellaneous remarks</h2>
<div class="sectionbody">
<div class="paragraph"><p>Songcast is probably best transported over a wired connection. If you are
doing this over WIFI and experiencing glitches, the wireless is the first
suspect.</p></div>
</div>
</div>
</div>
<div id="footnotes"><hr /></div>
<div id="footer">
<div id="footer-text">
Last updated 2014-11-10 16:15:50 CET
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