The quality of pidgin XMPP voice call is terribly bad.

[crocket]

I use pidgin 2.9.0, farsight2 0.0.27, and libnice 0.1.0 on slackware64 13.37
I built pidgin 2.9.0 against farsight2 and libnice with --enable-vv.

On pidgin, I initiated a voice call with another person on XMPP.
That person also used pidgin.

The voice of the person on the other side stuttered.
When I called myself using two XMPP accounts, I couldn't even
recognize my own voice.
The voice was distorted severely.

I searched the internet, and a web page said it could be a codec
problem, but I don't know if it's the case.

pidgin's voice call is compatible with google talk, but google talk
voice quality is good.

Does anybody know why piding's voice call quality is terribly bad ?