VoIP Spear Blog: News & Noise
May 17, 2010

New codecs!

VoIP Spear has always included the ability to calculate MOS using the big three codecs of VoIP: G.711, G.723, and G.729. We've recently added support for GSM, G.722, Speex, and iLBC.

VoIP Spear uses the E-Model of computation for estimating MOS from network statistics like packet loss, latency, and jitter. In order to complete the E-model calculations, we need to know some characteristics about each codec. For example, how much does the codec itself degrade voice quality. Another characteristic would be: how does the codec perform under packet loss conditions.

For G.711, G.723, and G.729, it's very easy to find this information because these codecs have been studied a great deal. This isn't the case for Speex and iLBC. It's hard to find information because they are newer codecs and weren't developed by the ITU. As a result, there haven't been many studies that look at these codecs in the context of the E-Model. Nevertheless, we managed to track down a few papers here and there. We're confident our MOS calculations for Speex and iLBC are accurate.

Calculating MOS from G.722 is a different situation altogether. G.722 is a family of codecs (G.722.1, G.722.2) that is available at several different bit rates. In fact, quality is greatly affected by the bit rate that is used. We didn't want to include all of these bit rates because we want VoIP Spear to be easy to use. Also, some of the other codecs (G.723, iLBC) also provide different options for bit rates so we wanted to be consistent.

In the end, the compromise we arrived at was to assume that G.722 is used at 64kbps. Without any impairments caused by the network, this will provide quality better than even G.711.

We hope you enjoy the new codec choices. If you have any other to suggest, let us know.

Leave a Comment