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

<!-- $Id: customize.sgml,v 1.3 2002/09/30 05:16:20 shane Exp $ -->

<chapter id="customize">
<title>Customizing Intercom for Your Needs</title>

<para>It is often desirable to change the way a program behaves to better
suit your environment.  Intercom has many options which can be set by the
user.  This chapter takes to step by step through these options and helps
you tailor Intercom to your specific requirements.  We will begin with the
most important options and proceed from there to some options which most
people won't need to change but are provided in the event you do.</para>

<section id="customize-rcfile">
<title>Saving your settings</title>

<para>Before we get into the options Intercom has to offer, you will want to
save your settings once you have things set up.  Intercom reads two files
when it starts, a file called <filename>intercomrc</filename> in SYSCONFDIR
(usually /etc or /usr/local/etc) and <filename>.intercomrc</filename> in the
home directory of the user running intercom.  This file is simply a list of
Intercom commands which get executed on startup.  The rc file may have
comments denoted by the # character at the beginning of the line.</para>

<para>The <command>save</command> command saves all settings (variables,
aliases, and hooks) to the <filename>.intercomrc</filename> file in your
home directory.  Any information previously contained therein is erased. 
<command>save</command> does have a few options which control its behavior,
use <command>help save</command> for a usage summary.  They are fairly
self-explanatory so they will not be discussed here.</para>

</section>

<section id="customize-set">
<title>The set Command</title>

<para>Most of Intercom's customizable features are controled through the
<command>set</command> command.  With <command>set</command>, you can both
display and modify the values of configuration variables.  Typing
<command>set</command> by itself at the command-prompt will display all the
variables available, their values, and a brief description of the variable. 
Some variables will be fairly obvious as to what they do, others undoubtedly
will not.</para>

<para>  To set a variable, you use a construct like this</para>
<para><command>set <replaceable>&lt;variable&gt;</replaceable>
<replaceable>&lt;value&gt;</replaceable></command></para>
<note><para>Replace <replaceable>&lt;variable&gt;</replaceable> and
<replaceable>&lt;value&gt;</replaceable> with the specific variable and value to
set.</para></note>
<para>For example, to set the username to John Smith, you would execute the
following command.</para>
<para><prompt>intercom: </prompt>
<command>set username "John Smith"</command></para>
<para>Notice the quotes around John Smith, this is necessary when setting a
variable to a value which contains spaces.  Failure to use proper quoting
will result in an error.</para>

</section>

<section id="customize-username">
<title>Setting the User Name</title>

<para>The username variable is used during a call to indicate to
the remote party who is calling.  As it is often the deciding factor as to
whether a call is accepted, it is recommended that this be set to an
identifying string which tells people who you are.  Technically, this
variable can be anything you would like provided that it is printable, full
names and email addresses are commonly placed here.  As an example, my
username might be set as follows.</para>
<para><prompt>intercom: </prompt>
<command>set username 'Shane Wegner &lt;[e-mail]&gt;'</command></para>
<para>You might have noticed that the quoting used is a bit different from
the previous example, single quotes are used instead of double quotes.  This
is actually not necessary in this case as double quotes are sufficient but
some special characters do need to be quoted using single quotes.  The rules
are the same as those employed by the Unix shell.</para>
<para>Another example of setting the username variable
is setting it to your Unix login name.  This can be done through variable
substitution.  The following command demonstrates this feature.</para>
<para><prompt> intercom: </prompt>
<command>set username $USER</command></para>
<note><para>This will set the username variable to the value of the
USER environment variable.</para></note>

</section>

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

<para>By default, Intercom is configured for low-bandwidth consumption.  If
you are connecting through a dialup connection, you may not need to modify
these parameters at all.  However, if you have a higher bandwidth
connection, you can achieve higher quality audio by tuning these parameters. 
Feel free to experiment with different settings and use what works best for
you.</para>

<note><para>It is important to note that although you may have your audio
parameters set, a call may not use those parameters.  Generally, the
settings used are those of the caller and not the receiver.  Therefore, be
sure that the caller's client is properly configured before a call is
connected.</para></note>

</section>

<section id="customize-audio-samplerate">
<title>Selecting a Sample Rate</title>

<para>The audio sample rate is quite possibly the single most important
factor in determining audio quality.  The higher the sampling rate, the
better the audio quality.  Of course, the bandwidth requirements are
increased as well.  Technically, Intercom can support any sample rate your
hardware can, however, as the remote caller must also support the sample
rate, it is recommended that you stick to the standard rates.  These are
8000 (8k), 11025 (11k), 16000 (16k), 22050 (22k) and 32000 (32k).</para>

<note><para>Some sound cards do not support the 16k sample rate.  However,
some do and it is often a good one to use if you know that both parties
support it.</para></note>

<para>The sampling rate is controled in one of two ways.  You can either set
the snd_rate variable to your desired sample rate or specify a one-time
sample rate when placing a call.  To set the variable, use a command like
<command>set snd_rate 11025</command> to set the sample rate to 11k.  In
order to place a call with a sample rate other than that in the snd_rate
variable, specify the -r option such as
<command>call -r <replaceable>22050</replaceable>
<replaceable>192.168.1.2</replaceable></command> which will set the rate to 22k
for the duration of the call.</para>

<section id="customize-audio-codec">
<title>Compression Settings</title>

<para>Another setting which plays a large part in audio quality and an even
larger part in bandwidth usage is the compression format (CODEC) selected. 
Certain CODECs give better compression at the expense of audio quality or
CPU utilization.  Alternatively, you can disable compression altogether if
your connection can handle the increased data throughput.</para>

<para>The following table lists the currently supported codecs as well as
the bandwidth they use at several sample rates.  The bandwidth usage is
displayed in killobits per second.  1 killobit equals 1000 bits which is 125
bytes.  Also note that this does not account for Intercom's protocol
overhead which varies depending on other factors.</para>

<table><title>Audio CODEC Bandwidth Usage</title>
<tgroup cols="6" align="left">
<thead>
<row>
<entry>Sample rate</entry><entry>8 K</entry><entry>11 K</entry>
<entry>16 K</entry><entry>22 K</entry><entry>32 K</entry>
</row
<row>
<entry>CODEC</entry><entry>Kbit/sec</entry><entry>Kbit/sec</entry>
<entry>Kbit/sec</entry><entry>Kbit/sec</entry><entry>Kbit/sec</entry>
</row>
</thead>
<tbody>
<row>
<entry>none</entry><entry>128.00</entry><entry>176.40</entry>
<entry>256.00</entry><entry>352.80</entry><entry>512.00</entry>
</row
<row>
<entry>G.711</entry><entry>64.00</entry><entry>88.20</entry>
<entry>128.00</entry><entry>176.40</entry><entry>256.00</entry>
</row
<row>
<entry>G.721</entry><entry>32.00</entry><entry>44.10</entry>
<entry>64.00</entry><entry>88.20</entry><entry>128.00</entry>
</row
<row>
<entry>G.723-40</entry><entry>40.00</entry><entry>55.12</entry>
<entry>80.00</entry><entry>110.25</entry><entry>160.00</entry>
</row
<row>
<entry>G.723-24</entry><entry>24.00</entry><entry>33.08</entry>
<entry>48.00</entry><entry>66.15</entry><entry>96.00</entry>
</row
<row>
<entry>GSM</entry><entry>13.20</entry><entry>18.19</entry>
<entry>26.40</entry><entry>36.38</entry><entry>52.80</entry>
</row
</tbody>
</tgroup>
</table>

</section>

<section id="customize-gain">
<title>Software Gain Control</title>

<para>If you or your remote party is sounding too quiet, the first thing
which should be attempted is to adjust the sound mixer for the capture
source.  Bringing up the input gain can usually help.  Also, some audio
hardware has an analogue mic boost option which you might try enabling. 
However, if all else fails, Intercom does have the ability to amplify or
de-amplify the audio signal (either incoming or outgoing).  The reason you
should use the mixer before trying this is when you amplify a signal, you
also amplify the hiss along with it whereas a hardware gain usually is
better at keeping things clean.</para>

<para>The variables controling gain are input_gain and output_gain
respectively.  The input_gain variable with amplify the signal coming from
your mic before it goes out over the wire.  The output_gain variable
amplifies the incoming signal.  These variables are decimal numbers which
express the gain in decibels.  A value of 0 turns off gain control, a
positive value amplifies the signal by that many decibels and a negative
value will de-amplify the signal.</para>

</section>