[asterisk-users] Re: Where to best start looking for voicemail/moh sound quality problem?

[Paul Davidson]

>
> Date: Mon, 23 Oct 2006 16:48:09 -0400 (EDT)
> From: "Frank Tarczynski" <ftarz at mindspring.com>
> Subject: [asterisk-users] Where to best start looking for
>         voicemail/moh sound quality problem?
> To: asterisk-users at lists.digium.com
> Message-ID:
>         <22138.192.58.204.226.1161636489.squirrel at www.tarczynski.com>
> Content-Type: text/plain;charset=iso-8859-1
>
> I'm running Asterisk 1.2.13 on a Solaris 10 X86 box behind an IPCop
> firewall on a 5Mbps down/512 up cable connection.
>
> I'm having sound quality problems when users call in for voicemail and
> with music on hold.  The sound is choppy and muffled while souding pretty
> good for calls inside the network.
>
> I'd appreciate some pointers as to where to start looking to improve
> things.
>
> I've tried setting QOS paramters for IPCop but I'm sure that had any
> effect.
>
> Frank
>
>
> Frank-

I'm afraid a little more detail may be necessary to help you.  From this, I
have to assume the users are calling in over some sort of network
connection- say SIP or IAX from a provider, and not on the analog, and that
they're making a single stop- at the server hosting Asterisk for MOH and
Voicemail.  If that's not accurate, please let me know.

Given that picture, keep in mind that asymetric bandwidth is always a bad
match for VoIP, and has to be managed very, very carefully.  The simplest
way to think of it is to use a rate limiter to make that 512/512, which will
improve quality, and should still leave you plenty of room for calls,
assuming a reasonable codec compression level.  (not to mention, if your QOS
is set up that way, you get good amount of headroom for downloads and other
traffic)

That being said, both MOH and Voicemail are sort of special- meaning that
they're both using different audio sources, with varying levels of
compression on their own.  If you have high compression (gsm or 729) on your
trunk, and are trying to play back high quality audio (128K mp3 MOH sources,
for instance), the transcoding is bound to produce some 'garble'.  This is
the reason hold music is a bad choice for playback via a cellphone- it
almost always sounds like garbage.  Your voicemail problems- is the quality
of the prompts bad, or the recorded message itself?  Both, and either, have
different audio characteristics, and compression applies differently.

So, to start, try switching your MOH source to an encoding that matches the
trunk- depending on your version of Asterisk and how you've got MOH
configured, this can be done a lot of different ways- check the wiki for
it.  See also if you can match the recording of voicemail to your codec, or
at least find one with a very low translation cost- this may help as well.
For example, if your trunk is using the gsm codec, make sure you're
recording voicemails as gsm, not WAV, wav49, or wav- those can be used as
well, but gsm should be in the mix for best quality.

-pbd
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