Another FreeSWITCH First: 48k VoIP

The FreeSWITCH development team has struck again! FreeSWITCH now supports CELT, a new open source audio codec that allows for CD-quality transmission with VoIP. CELT is described as an "ultra-low delay" audio codec that supports both voice and music. 48kHz VoIP can be carried in 48kbps of bandwidth!

Putting this in perspective: CD-quality audio is 44.1kHz. This means CELT supports better than CD-quality audio in a VoIP environment! The only bad news here is that you might need to upgrade your headset so that your microphone is sensitive enough to handle such a wide range. Also, consider bandwidth usage: 48kHz of super high-quality audio requires a mere 48kbps of bandwidth. By comparison, the venerable PCM mu-law codec, G.711u, consumes 64kbps of bandwidth and yields only 8kHz audio quality!

What are some applications of such a high-quality codec with FreeSWITCH?  For one, CD-quality conferencing. Another is radio station backhaul. Have you ever heard a radio talk-show? The host is crystal clear and the callers are sometimes very difficult to hear. This could be remedied by using a higher quality codec for guests and callers. 

New codecs are creating new opportunities for voice and video applications. Another important consideration is the intellectual property involved in many codecs.

The Evolution of "Free Speech"

Anyone who has dealt with the hassles of licensing a proprietary codec, such as the ubiquitous G.729, knows that such patent-encumbered codecs cost more than just money. Developers certainly know the issues: high initial fees and silly licence enforcement schemes. These schemes require resources that could otherwise be applied to advancing the project that uses the codec. 

To get an idea of just how "patent-encumbered" G.729 really is, please see the attached PDF report from the ITU Intellectual Property Rights (IPR) page. By my count, this codec is encumbered by 52 patents from 17 different entities! These entities are part of the G.729 "consortium" and they use Sipro as an agent to handle the business of collecting money from those who use the codec. Beyond that, more patent holders can join the consortium if they demonstrate that their IP is used in the codec. Now look at all the codecs in the ITU's IPR database... That's a lot of encumbered technology.

To combat this trend, a number of technology specialists have devoted themselves to the advancement of "free speech" - that is, unencumbered codec technologies for use in audio and video environments. Many of these codecs are of the highest quality and are as good as, or better, than their proprietary counterparts.  Two well-known codecs are Vorbis (music, general audio) and Theora (video). Another such free and open source codec is called Speex. Speex is optimized for speech and like G.729 it is a "lossy" format meaning that some audio quality is sacrificed in order to minimize bandwidth. Speex can operate at 8kHz, 16kHz, and 32kHz.

The CELT codec represents the best of both worlds: audio quality equal to that of Vorbis while acheiving the low latency needed for VoIP transmissions. We anticipate that more equipment manufacturers and service providers will turn to high-quality, low-cost codecs like CELT. To sum it up, CELT is simply an awesome codec and is a welcome addition to the FreeSWITCH project.

The FreeSWITCH community would like to thank Jean-Marc Valin, the Project Lead for the CELT project, whose work made this exciting addition possible.

FreeSWITCH users will see this codec as mod_celt.

For more information on CELT please visit the project home page.

For more information on other free codecs please visit the Xiph page.