customize.sgml

Intercom 是一个 Unix系统上灵活的语音传输软件。支持标准音频压缩比如GSM, G.711, and G.72x和其他音频编码。Intercom专为高速网络设计来传输高品质的语音

<section id="customize-audio-device">
<title>Audio Devices</title>

<para>If you have more than one sound card, you may wish to specify the audio
device Intercom uses.  There are two variables which control this,
snd_capture_device and snd_play_device.  By default, they are both set to
/dev/dsp but can be modified by the user.  The changes will not take
affect immediately if a call is in progress.</para>

<para>You are free to set snd_capture_device and snd_play_device to
different physical devices.  This might be useful if your Mic and speakers
happen to be on different cards></para>

</section>

<section id="customize-audio-silence">
<title>Handling Silence</title>

<para>Another set of options which you will probably want to configure is
how Intercom handles audio silence.  That is, when you are not speaking,
there is really no reason audio data needs to be sent to your remote party. 
Configuring silence squelching is a bit technical but shouldn't be too hard. 
The option you want to adjust is the silence_thresh variable.  This is an
integer between 0 and 32767 which denotes the minimum sample value below
which is considered silence.  When set to 0, there will be no silence
suppression and all audio data is sent as-is.  Likewise, when set to 32767,
all audio data is squelched and you are essentially muted.  You want to set
this option to a value depending on your mic and surroundings.  Common
values are between 1000 and 7000 but your mileage may vary.  Play with it
until you find the correct value for your setup</para>

<para>The other variable related to silence squelching is silence_time. 
This is a decimal variable which controls how many seconds of silence to
allow before squelching.  By default, this is set to 1.0 (1 second) but some
people like to adjust it.  For example, if you wanted to squelch only if
there is an extended period of silence, you could set it to 30 or so.</para>

</section>

</section>

<section id="network">
<title>Networking Parameters</title>

<para>Intercom provides some options which configure how it communicates
over a network.  Most people probably won't need to adjust these but they
are here in the event that you do.</para>

<section id="customize-network-port">
<title>TCP Port</title>
<para>Intercom uses two sockets in a standard call, a TCP socket for data
communication such as parameter negotiation and a UDP socket for the actual
audio data.  The UDP port used is random and is currently not configurable;
however, the TCP port on which Intercom listens is set through the port
variable.  By default, port is set to 4266 but can be changed through the
set command.  You will need to restart Intercom in order for the setting to
take effect so it will be necessary to save it first.</para>

</section>

<section id="customize-network-lowdelay">
<title>IPTOS_LOWDELAY</title>

<para>Setting IPTOS_LOWDELAY on a socket generally causes packets on that
socket to be sent out before others.  Intercom uses this option as it is a
<emphasis>real-time</emphasis> application where audio delays are not
desired.  This is unlike an application such as a web server where a minor
delay really doesn't matter too much.  By default, Intercom sets this option
on its audio socket.  However, this behavior can be controled, through the
iptos_lowdelay variable.  This is a boolean variable (0 or 1) which when set
to 0, the IPTOS_LOWDELAY option is not applied to the socket.</para>

</section>

<section id="customize-network-udpsize">
<title>UDP packet size</title>

<para>The UDP protocol is packet based.  As such, Intercom sends and
receives audio data in chunks of a certain size.  The packet size Intercom
uses is defendant on the sample rate and compression format used.  It can
vary slightly if encryption is used and can also fluctuate during a call. 
In general, it is desirable to keep packets relatively small to ensure a
minimum audio latency.  We try to be smart about this and it should
rarely be necessary to adjust the sizes.</para>

<para>The UDP packet sizes are controled through two variable.  they are
udp_minsize and udp_maxsize which control the minimum and maximum packet
sizes respectively.  By default, udp_minsize is set to 64 and udp_maxsize is
512.</para>

<para>The udp_minsize variable is most important when audio compression is
used.  If it is too high, audio data will buffer and higher latencies will
result.  However, if it is too low, many packets can be sent which can
cause problems of its own.  The value of 64 is reasonable in most cases.  It
was chosen so as not to buffer up too much data and when using a CODEC such
as GSM at 8k, it will limit packets to a single frame.</para>

<para>The udp_maxsize variable in contrast, is more important when
compression is disabled.  It can also come into play when compression is
used and a high sample rate is employed.  If Intercom is sending packets
which you think are too large, you can decrease this value.  However, don't
bring it too close to udp_minsize.  Intercom needs some flexibility in how
it sizes packets.</para>

</section>

</section>

<section id="customize-misc">
<title>Miscellaneous variables</title>

<para>The following describes some miscellaneous variables which control
Intercom's behavior.  They are straight-forward and don't need a section of
their own.</para>

<note><para>Typing set with no arguments will display a list of valid
variables with a brief description of their function.  It may be more
up-to-date than this document and consulting it is
recommended.</para></note>

<itemizedlist>

<listitem><para>cmd_maxdepth - Controls the depth of command-recursion
permitted before Intercom flags a command-loop and aborts the operation.  A
command-loop can be as simple as setting up an alias a to b and b back to a
or can be much more complicated.  The cmd_maxdepth variable exists to
protect against such occurrences.  The default value is 10 and should be
enough for most anyone.</para></listitem>

<listitem><para>history_limit - This variable controls how many lines to
keep in Intercom's command-history if the readline library is used.  The
default is 100 lines and is usually enough for most
people.</para></listitem>

<listitem><para>reverse_lookup - If set to 1, a reverse DNS lookup is
performed on connected hosts whos results are displayed when a call is
connected and terminated.  If set to 0, this lookup is not done and the IP
address is displayed.  REverse lookups can take time so if connection times
are important, you can disable this; however, the default is enabled ad the
results can be informative.</para></listitem>

<listitem><para>verbose - When set to 1, Intercom becomes a little more
verbose in its output to the console.</para></listitem>

<listitem><para>snd_loopback - When set to 1, audio data which is received
is bounced immediately back to the sender.  It is quite useful for
troubleshooting audio problems.  Local audio is not sent to the remote party
when this option is used.</para></listitem>

</itemizedlist>

</section>

<section id="customize-alias">
<title>Command Aliases</title>

<para>Intercom has a <emphasis>very basic</emphasis> alias system, whereby,
one can define a command which executes another.  This can be most helpful
especially when executing lengthy or repetitive commands.  All
command-aliases are administered by none other than the
<command>alias</command> command.  With <command>alias</command>, you can
list, add, and remove command-aliases.</para>

<para>Do add an alias, the syntax is:</para>
<para><command>alias
<replaceable>&lt;alias&gt;</replaceable>
<replaceable>&lt;action&gt;</replaceable></command></para>
<para>Where <replaceable>&lt;alias&gt;</replaceable> should be substituted for the
alias you would like and <replaceable>&lt;action&gt;</replaceable>for the action. 
For example, to have the <command>c</command> command reference
<command>call</command>, you would enter:</para>
<para><prompt>intercom: </prompt>
<command>alias c call</command></para>

<para>To remove an alias, the syntax is similar.  Enter in <command>alias -u
<replaceable>&lt;alias&gt;</replaceable>&lt;command&gt;</command>.  So, to
remove our c alias above, we would execute:</para>
<para><prompt>intercom: </prompt>
<command>alias -u c</command>
</para>

<section id="customize-alias-examples">
<title>Useful Alias Examples</title>

<para>The following examples demonstrate some useful command-aliases. 
Remember to use proper quoting when entering an alias.</para>

<para>Here is a very simple alias to map the ? command to help.</para>
<para><prompt>intercom: </prompt>
<command>alias '?' help</command></para>

<para>Set up an alias (coffice) to call the office using some special
settings.</para>
<para><prompt>intercom: </prompt>
<command>alias coffice 'call -r 22050 -c g.723-40
office.mycompany.com'</command></para>

<para>Set up an alias whereby the unalias command will remove an
alias.</para>
<para><prompt>intercom: </prompt>
<command>alias unalias 'alias -u'</command></para>

</section>

</section>

</chapter>