Dr. I Doctor
Dr. I Doctor's Informational Juggernaut
May 1, 2008
Clipped Speech During VoIP Calls
Dear Doctor,
I installed a Voice over IP (VoIP) phone system for our office, initially set up to use Internet VoIP trunks. The reliability and sound quality of Internet phone calls was too poor for our management's taste, so I switched to Plain Old Telephone Service (POTS) analog lines. Alas, to my surprise, we still have significant sound-quality problems. The worst is that the beginning and ending of every spoken sentence seems to be clipped off. But we also hear a distinctive hiss that is both annoying and not audible when I place a call with an analog phone on the same lines. Management is ready to throw out the VoIP system. Help!
Gentle User,
When directly connected to analog POTS lines, a VoIP system should have sound quality very close to that of a traditional PBX. The clipping and hiss symptoms indicate that you may have inadvertently left some sound compression features enabled — features that aren't needed with direct POTS phone trunks.
The most common cause of word clipping is a bandwidth-saving feature called Silence Suppression, which attempts to detect gaps in spoken conversation to reduce the amount of encoded data being sent over the Internet. For analog lines, you should disable this feature.
Hiss is a characteristic of some voice compressor/decompressor (codec) algorithms, which compress encoded audio to further reduce bandwidth requirements. You generally can configure the codec used for specific kinds of calls; for analog calls, the codec should be G.711, resulting in a 64 Kbps uncompressed audio stream (which takes about 80 Kbps in network traffic to transport). On a LAN this bandwidth is negligible. Dr. I Doctor suspects you have inadvertently configured one of the high-compression codecs, such as G.729. Reconfiguring to G.711 should remove all hiss.
Posted by mbeckman at May 1, 2008 4:28 PM
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