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  12 <h1>Frequently Asked Questions</h1>
  13
  14 <ol>
  15   <li><p><b>How does Polypaudio compare with ESOUND/aRts/NAS?</b></p>
  16
  17   <p>Polypaudio is sound daemon similar to ESOUND and NAS, but much more
  18   powerful. aRts is a realtime-synthesizer-cum-sound-server, i.e. it
  19   does much more than Polypaudio. However, I believe that Polypaudio
  20   does what it does much better than any other free sound server.</p>
  21   </li>
  22
  23   <li><p><b>What about ESOUND compatibility?</b></p>
  24   <p>Polypaudio is a drop in replacement for ESOUND. That means: you can
  25   load a esound compatibility module which implements an ESOUND
  26   compatible protocol which allows you to use most of the classic ESOUND
  27   compatible programs (including the command line programs like
  28   <tt>esdcat</tt>).</p>
  29   </li>
  30
  31   <li><p><b>Is Polypaudio a GNOME program?</b></p>
  32   <p>No, Polypaudio has no dependency on GNOME/GTK/GLIB. All it requires
  33   is a UNIX-like operating system and very few dependency
  34   libraries. However, the accompanying GUI tools are writen with
  35   gtkmm, i.e. require both GLIB and GTK.</p></li>
  36
  37   <li><p><b>Can I integrate Polypaudio in my GLIB/GTK/GNOME application?</b></p>
  38   <p>Yes! Polypaudio comes with a GLIB main loop adapter. You can embed
  39   both the client library and the daemon (!) into your GLIB based
  40   application.</p></li>
  41
  42   <li><p><b>Can I integrate Polypaudio in my Qt/KDE application?</b></p>
  43   <p>Yes! Polypaudio uses a main loop abstraction layer that allows you
  44   to integrate Polypaudio in any program that supports main
  45   loops. Unfortunately there is no adapter for Qt publicly available yet.</p></li>
  46
  47   <li><p><b>I want to write a new driver for Polypaudio, are there any docs?</b></p>
  48   <p>Currently, only the client API is documented with doxygen. Read
  49   the source and base your work on a simple module like
  50   <tt>module-pipe-sink</tt>.</p></li>
  51
  52   <li><p><b>What about compatibility with NAS?</b></p>
  53   <p>Is not available (yet?). It is doable, but noone has implemented it yet.</p></li>
  54
  55   <li><p><b>What about compatibility with aRts?</b></p>
  56   <p>Is not available. Since aRts is as synthesizer application you'd have to
  57   reimplement very much code for Polypaudio. It should be easy to
  58   implement limited support for <tt>libartsc</tt> based
  59   applications. Noone has done this yet. It is probably a better idea to
  60   run <tt>arts</tt> on top of Polypaudio (through a polypaudio driver
  61   for aRts, which nobody has written yet). Another solution would be to
  62   embed Polypaudio in the aRts process.</p></li>
  63
  64   <li><p><b>I often hear noises when playing back with Polypaudio, what can I do?</b></p>
  65   <p>There are to possible solutions: run polypaudio with argument
  66 <tt>--high-priority=1</tt> and make yourself member of the group
  67 <tt>realtime</tt>, or increase the fragment sizes of the audio
  68   drivers. The former will allow Polypaudio to activate
  69   <tt>SCHED_FIFO</tt> high priority scheduling (root rights are dropped
  70   immediately after this). Keep in mind that this is a potential security hole!</p></li>
  71
  72    <li><p><b>The <tt>polypaudio</tt>  executable is installed SUID root by default. Why this? Isn't this a potential security hole?</b></p>
  73
  74   <p>Polypaudio activates <tt>SCHED_FIFO</tt> scheduling if the user
  75 passes <tt>--high-priority=1</tt>. This will only succeed when
  76 executed as root, therefore the binary is marked SUID root by
  77 default. Yes, this is a potential security hole. However, polypaudio
  78 tries its best to minimize the security threat: immediately after
  79 startup polypaudio drops all capabilities except
  80 <tt>CAP_SYS_NICE</tt> (At least on systems that support it, like Linux; see <tt>man 7
  81 capabilities</tt> for more information). If the calling user is not a
  82 member of the group <tt>realtime</tt> (which is required to have a GID
  83 < 1000), root rights are dropped immediately. This means, you can
  84 install polypaudio SUID root, but only a subset of your users (the
  85 members of the group <tt>realtime</tt>) may make use of realtime
  86 scheduling. Keep in mind that these users might load their own binary
  87 modules into the polypaudio daemon which may freeze the machine. The
  88 daemon has a minimal protection against CPU hogging (the daemon is
  89 killed after hogging more than 70% CPU for 5 seconds), but this may
  90 be circumvented easily by evildoers.</p></li>
  91
  92   <li><p><b>I want to run polypaudio only when it is needed, how do I do this?</b></p>
  93
  94   <p>Set <tt>autospawn = yes</tt> in <tt>client.conf</tt>. That
  95 configuration file may be found either in <tt>/etc/polypaudio/</tt> or
  96 in <tt>~/.polypaudio/</tt>.</p></li>
  97
  98   <li><p><b>How do I list all polypaudio modules installed?</b></p>
  99
 100    <p><tt>polypaudio --dump-modules</tt></p>
 101
 102    <p>Add <tt>-v</tt> for terse usage instructions.</p>
 103
 104 <li><p><b>How do I use polypaudio over the network?</b></p>
 105
 106 <p>Just set <tt>$POLYP_SERVER</tt> to the host name of the polypaudio
 107 server. For authentication you need the same auth cookies on all sides. For
 108 that copy <tt>~./polypaudio-cookie</tt> to all clients that shall
 109 be allowed to connect.</p>
 110
 111 <p>Alternatively the authorization cookies can be stored in the X11 server.</p></li>
 112
 113 <li><p><b>Is polypaudio capable of providing synchronized audio playback over the network for movie players like <tt>mplayer</tt>?</b></p>
 114
 115 <p>Yes! Unless your network is congested in some way (i.e. transfer latencies vary strongly) it works perfectly. Drop me an email for experimental patches for MPlayer.</p>
 116
 117    <li><p><b>What environment variables does polypaudio care about?</b></p>
 118
 119 <p>The client honors: <tt>POLYP_SINK</tt> (default sink to connect to), <tt>POLYP_SOURCE</tt> (default source to connect to), <tt>POLYP_SERVER</tt> (default server to connect to, like <tt>ESPEAKER</tt>), <tt>POLYP_BINARY</tt> (the binary to start when autospawning a daemon), <tt>POLYP_CLIENTCONFIG</tt> (path to the client configuration file).</p>
 120
 121 <p>The daemon honors: <tt>POLYP_SCRIPT</tt> (default CLI script file run after startup), <tt>POLYP_CONFIG</tt> (default daemon configuration file), <tt>POLYP_DLPATH</tt> (colon separated list of paths where to look for modules)</p></li>
 122
 123
 124 <li><p><b>I saw that SIGUSR2 provokes loading of the module <tt>module-cli-protocol-unix</tt>. But how do I make use of that?</b></p>
 125
 126 <p>A brilliant guy named Lennart Poettering once wrote a nifty tool
 127 for that purpose: <a
 128 href="http://0pointer.de/lennart/projects/bidilink/">bidilink</a>. To
 129 connect to a running polypaudio daemon try using the following commands:</p>
 130
 131 <pre>killall -USR2 polypaudio
 132 bidilink unix-client:/tmp/polypaudio/cli</pre>
 133
 134 <p><i>BTW: Someone should package this great tool for Debian!</i></p>
 135
 136 <p><b>New:</b> There's now a tool <tt>pacmd</tt> that automates sending SIGUSR2 to the daemon and running a bidilink like tool for you.</p>
 137 </li>
 138
 139 <li><p><b>How do the polypaudio libraries decide where to connect to?</b></p>
 140 <p>The following rule applies:</p>
 141 <ol>
 142   <li>If the the application using the library specifies a server to connect to it is used. If the connection fails, the library fails too.</li>
 143   <li>If the environment variable <tt>POLYP_SERVER</tt> is defined the library connects to that server. If the connection fails, the library fails too.</li>
 144   <li>If <tt>$DISPLAY</tt> is set, the library tries to connect to that server and looks for the root window property <tt>POYLP_SERVER</tt> for the host to connect to. If <tt>POLYP_COOKIE</tt> is set it is used as authentication cookie.</li>
 145   <li>If the client configuration file (<tt>~/.polypaudio/client.conf</tt> or <tt>/etc/polypaudio/client.conf</tt>) sets the server address, the library connects to that server. If the connection fails, the library fails too.</li>
 146   <li>The library tries to connect to the default local UNIX socket for polypaudio servers. If the connection fails, it proceeds with the next item.</li>
 147   <li>The library tries to connect to the default local TCP socket for polypaudio servers. If the connection fails, it proceeds with the next item.</li>
 148   <li>If <tt>$DISPLAY</tt> is set, the library tries to connect to the default TCP port of that host. If the connection fails, it proceeds with the next item.</li>
 149   <li>The connection fails.</li>
 150 </ol>
 151 </li>
 152
 153 <li><p><b>Why the heck does libpolyp link against libX11?</b></p>
 154 <p>The Polypaudio client libraries look for some X11 root window
 155 properties for the credentials of the Polypaudio server to access. You
 156 may compile Polypaudio without X11 for disabling this feature.</p></li>
 157
 158 <li><p><b>How can I use Polypaudio as an RTP based N:N multicast
 159 conferencing solution for the LAN?</b></p> <p>After loading all the
 160 necessary audio drivers for recording and playback, just load the RTP
 161 reciever and sender modules with default parameters:</p>
 162
 163 <pre>
 164 load-module module-rtp-send
 165 load-module module-rtp-recv
 166 </pre>
 167
 168 <p>As long as the Polypaudio daemon runs, the microphone data will be
 169 streamed to the network and the data from other hosts is played back
 170 locally. Please note that this may cause quite a lot of traffic. Hence
 171 consider passing <tt>rate=8000 format=ulaw channels=1</tt> to the
 172 sender module to save bandwith while still maintaining good quality
 173 for speech transmission.</p></li>
 174
 175 <li><p><b>What is this RTP/SDP/SAP thing all about?</b></p>
 176
 177 <p>RTP is the <i>Realtime Transfer Protocol</i>. It is a well-known
 178 protocol for transferring audio and video data over IP. SDP is the <i>Session
 179 Description Protocol</i> and can be used to describe RTP sessions. SAP
 180 is the <i>Session Announcement Protocol</i> and can be used to
 181 announce RTP sessions that are described with SDP. (Modern SIP based VoIP phones use RTP/SDP for their sessions, too)</p>
 182
 183 <p>All three protocols are defined in IETF RFCs (RFC3550, RFC3551,
 184 RFC2327, RFC2327). They can be used in both multicast and unicast
 185 fashions. Polypaudio exclusively uses multicast RTP/SDP/SAP containing audio data.</p>
 186
 187 <p>For more information about using these technologies with Polypaudio have a look on the <a href="modules.html#rtp">respective module's documentation</a>.
 188
 189 <li><p><b>How can I use Polypaudio to stream music from my main PC to my LAN with multiple PCs with speakers?</b></p>
 190
 191 <p>On the sender side create an RTP sink:</p>
 192
 193 <pre>
 194 load-module module-null-sink sink_name=rtp
 195 load-module module-rtp-send source=rtp_monitor
 196 set-default-sink rtp
 197 </pre>
 198
 199 <p>This will make <tt>rtp</tt> the default sink, i.e. all applications will write to this virtual RTP device by default.</p>
 200
 201 <p>On the client sides just load the reciever module:</p>
 202 <pre>
 203 load-module module-rtp-recv
 204 </pre>
 205
 206 <p>Now you can play your favourite music on the sender side and all clients will output it simultaneously.</p>
 207
 208
 209 <p>BTW: You can have more than one sender machine set up like this. The audio data will be mixed on the client side.</p></li>
 210
 211 <li><p><b>How can I use Polypaudio to share a single LINE-IN/MIC jack on the entire LAN?</b></p>
 212
 213 <p>On the sender side simply load the RTP sender module:</p>
 214
 215 <pre>
 216 load-module module-rtp-send
 217 </pre>
 218
 219 <p>On the reciever sides, create an RTP source:</p>
 220
 221 <pre>
 222 load-module module-null-sink sink_name=rtp
 223 load-module module-rtp-recv sink=rtp
 224 set-default-source rtp_monitor
 225 </pre>
 226
 227 <p>Now the audio data will be available from the default source <tt>rtp_monitor</tt>.</p></li>
 228
 229 <li><p><b>When sending multicast RTP traffic it is recieved on the entire LAN but not by the sender machine itself!</b></p>
 230
 231 <p>Pass <tt>loop=1</tt> to the sender module!</p></li>
 232
 233 <li><p><b>Can I have more than one multicast RTP group?</b></p>
 234
 235 <p>Yes! Simply use a new multicast group address. Use
 236 the <tt>destination</tt>/<tt>sap_address</tt> arguments of the RTP
 237 modules to select them. Choose your group addresses from the range
 238 <tt>225.0.0.x</tt> to make sure the audio data never leaves the LAN.</p></li>
 239
 240
 241 <li><p><b>Can I use Polypaudio to playback music on two sound cards simultaneously?</b></p>
 242
 243 <p>Yes! Use <a href="modules.html#module-combine"><tt>module-combine</tt></a> for that.</p>
 244
 245 <pre>
 246 load-module module-oss-mmap device="/dev/dsp" sink_name=output0
 247 load-module module-oss-mmap device="/dev/dsp1" sink_name=output1
 248 load-module module-combine sink_name=combined master=output0 slaves=output1
 249 set-sink-default combined
 250 </pre>
 251
 252 <p>This will combine the two sinks <tt>output0</tt> and
 253 <tt>output1</tt> into a new sink <tt>combined</tt>. Every sample
 254 written to the latter will be forwarded to the former two. Polypaudio
 255 will make sure to adjust the sample rate of the slave device in case
 256 it deviates from the master device. You can have more than one slave
 257 sink attached to the combined sink, and hence combine even three and
 258 more sound cards.</p> </li>
 259
 260 <li><p><b>Can I use Polypaudio to combine two stereo soundcards into a virtual surround sound card?</b></p>
 261
 262 <p>Yes! You can use use <a href="modules.html#module-combine"><tt>module-combine</tt></a> for that.</p>
 263
 264 <pre>
 265 load-module module-oss-mmap device="/dev/dsp" sink_name=output0 channel_map=left,right channels=2
 266 load-module module-oss-mmap device="/dev/dsp1" sink_name=output1 channel_map=rear-left,rear-right channels=2
 267 load-module module-combine sink_name=combined master=output0 slaves=output1 channel_map=left,right,rear-left,rear-right channels=4
 268 </pre>
 269
 270 <p>This is mostly identical to the previous example. However, this
 271 time we manually specify the channel mappings for the sinks to make
 272 sure everything is routed correctly.</p>
 273
 274 <p>Please keep in mind that Polypaudio will constantly adjust the
 275 sample rate to compensate for the deviating quartzes of the sound
 276 devices. This is not perfect, however. Deviations in a range of
 277 1/44100s (or 1/48000s depending on the sampling frequency) can not be
 278 compensated. The human ear will decode these deviations as minor
 279 movements (less than 1cm) of the positions of the sound sources
 280 you hear. </p>
 281
 282 </li>
 283
 284
 285
 286 </ol>
 287
 288 <hr/>
 289 <address class="grey">Lennart Poettering &lt;@PACKAGE_BUGREPORT@&gt;, April 2006</address>
 290 <div class="grey"><i>$Id$</i></div>
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