Page 328 - Asterisk™: The Future of Telephony

• MP3 files are extremely complex, and require a substantial amount of CPU to
decode. If you have a lot of channels pulling music from the system (for example,
people sometimes like to listen to music through their phone, or a call center may
have several callers on hold), the load on the CPU caused by all of the transcoding
of the stored MP3 files could place too much demand on a machine that is otherwise
suitable to the performance needs of the system.
• Current-generation hard drives hold a lot of data, so there may not be any reason
to worry about cutting down hard drive use. Compressed audio makes sense from
a distribution standpoint (an MP3 is a much smaller download than the equivalent
in .wav format), but once on your system, do we really care how much space they
take up?
• MP3 files don’t usually come with the right sort of licensing. ;-)

Taking all of this into consideration, we recommend that you convert your music sour-
ces into the native format of the various codecs you may be supporting. For example,
if you support μlaw for your internal phones, and G.729 on your VoIP circuits, you will
want to store your music in both formats so that Asterisk will not have to perform
transcoding to play music to calls on those channels.

Free Music
A lot of people do not realize that playing music on hold requires a special license. This
is true even if you play music from CDs that you own, or from the radio. To ensure that
there is no ambiguity, we recommend avoiding the whole matter and using only music
that does not come encumbered with the kind of licenses that the music industry seems
to prefer.
There are many web sites where you can go to get music that is licensed in a manner
that is suitable for music on hold. Two that we have found are http://en.wikipedia.org/
wiki/Wikipedia:Sound/list and http://www.opsound.org/.
Both offer a sizeable collection of music that can be easily downloaded. Note that this
may not all be of professional quality, so listen to all of it before you commit it to your
music-on-hold collection. *

We often use public domain music (or Creative Commons licensed music) on our
systems. Creative Commons music often comes in ogg-Vorbis format (which is con-
ceptually similar to MP3, but not compatible). In order to play .ogg or .mp3 files on our
Asterisk system, we are going to convert them to a format that Asterisk can easily
handle. This requires the following steps:

1. We need to make sure that SoX, the Sound eXchange utility, is installed. If not,
run the following command to install it:

* Seed a search with the term “Creative Commons music” to find more freely usable music.

300 | Chapter 13: Managing Your Asterisk System