Siren is a family of patented, transform-based, wideband audio codecs developed and licensed by PictureTel Corporation (acquired by Polycom, Inc. in 2001). There are three Siren codecs: Siren 7, Siren 14 and Siren 22.
Siren 7 (or Siren7 or simply Siren) provides 7 kHz audio, bit rates 16, 24, 32 kbit/s and sampling frequency 16 kHz. Siren is derived from PictureTel's PT716plus algorithm. In 1999, ITU-T approved G.722.1 recommendation, which is based on Siren 7 algorithm. It was approved after a four-year selection process involving extensive testing. G.722.1 provides only bit rates 24 and 32 kbit/s and does not support Siren 7's bit rate 16 kbit/s. The algorithm of Siren 7 is identical to its successor, G.722.1, although the data formats are slightly different.
Siren 14 (or Siren14) provides 14 kHz audio, bit rates 24, 32, 48 kbit/s for mono, 48, 64, 96 kbit/s for stereo and sampling frequency 32 kHz. Siren 14 supports stereo and mono audio. It offers 40 millisecond algorithmic delay, using 20 millisecond frame lengths. The mono version of Siren 14 became ITU-T G.722.1C (14 kHz, 24/32/48 kbit/s) in April 2005. The algorithm is based on transform technology, using a Modulated Lapped Transform (MLT).
Siren 22 (or Siren22) provides 22 kHz audio, sampling frequency 48 kHz, bit rates 64, 96, 128 kbit/s stereo and 32, 48, 64 kbit/s mono. Siren 22 offers 40 millisecond algorithmic delay using 20 millisecond frame lengths. In May 2008, ITU-T approved the new G.719 full-band codec which is based on Polycom Siren 22 audio technology and Ericsson’s advanced audio techniques.
Software support 
Siren 7 is commonly used in videoconferencing systems and is also part of Microsoft Office Communicator when using A/V conferencing. Microsoft Office Communications Server uses Siren 7 during audio conferencing. With the default Office Communicator client, point to point audio is by default performed using Microsoft's proprietary codec RTAudio. When a call is promoted into an audio conference (any time 3 or more participants have joined), the codec is switched on the fly to Siren. This is done for performance reasons. Note that even if the conference is reduced to below 3 participants, OCS does not demote the conference to be point-to-point; it remains an A/V conference until the conference is terminated.
In Windows XP and later versions of Windows, the Siren 7 codec is implemented in %systemroot%\system32\SIRENACM.DLL. It is used by MSN Messenger and Live Messenger for sending and receiving voice clips and also as one of the available codecs for the 'Computer Call' feature.
aMSN, an open source Windows Live Messenger clone uses for Siren audio compression and decompression the "libsiren" library, an open source implementation of the codec, written by aMSN developer Youness Alaoui (KaKaRoTo) . The libsiren library has also been copied into libmsn and into the msn-pecan project, which provides plug-in for Pidgin and Adium instant messaging clients.
Usage of Siren 7 and Siren 14 compressions require the licencing of patents from Polycom, in most countries. A royalty free licence for Siren 7 and Siren 14 is available from Polycom if certain fairly basic conditions are met.
Usage of Siren 22 also require the licencing of patents from Polycom.
See also 
- Business Wire (2001-03-26). "PictureTel Announces New Siren Wideband Audio Technology Licensing Program". thefreelibrary.com. Retrieved 2009-09-10.
- Business Wire (2000-07-19). "PictureTel Licenses Audio Technology Suite to Intel". thefreelibrary.com. Retrieved 2009-09-10.
- (2008-08-05) Polycom Enables Acceleration of HD Voice Adoption by Offering Royalty-Free Codec, Retrieved 2009-09-07
- "Polycom Siren/G 722.1 FAQs". Polycom, Inc. Retrieved 2009-09-07.
- Polycom, Inc. (2005-04-12) ITU Approves Polycom Siren14 as New International Standard, Retrieved 2009-09-07
- "Polycom Siren 14/G 722.1C". Polycom, Inc. Retrieved 2009-09-07.
- "ITU Approves Polycom Siren14 as New International Standard". BusinessWire.com. 2005-04-12. Retrieved 2009-09-10.
- Siren 14 information for Prospective Licensees, retrieved 2010-06-08
- "Polycom Siren 22". Polycom, Inc. Retrieved 2009-09-07.
- "G.719: The First ITU-T Standard for Full-Band Audio". Polycom, Inc. 2009-04. Retrieved 2009-09-07.
- "Siren". MultimediaWiki. Retrieved 2009-09-07.
- "MPlayer - Status of codecs support". MultimediaWiki. Retrieved 2009-09-07.
- Microsoft (2001-11). "Media Support in the Microsoft Windows Real-Time Communications Platform". Microsoft. Retrieved 2009-09-07.
- "FreeSWITCH First to Support Polycom's 32khz HD-Audio.". FreeSWITCH. 2008-12-15. Retrieved 2009-09-07.
- "libg722_1 - COPYING". FreeSWITCH. Retrieved 2009-09-07.
- "libg722_1 - README". FreeSWITCH. Retrieved 2009-09-07.
- KaKaRoTo (2008-02-12) MSN Protocol documentation, Pidgin.im mailinglist, Retrieved 2009-09-08
- "msn-pecan 0.0.18 released, now with voice clips support". msn-pecan. 2009-02-16. Retrieved 2009-09-07.
- "msn-pecan". msn-pecan. Retrieved 2009-09-07.
- "Libmsn - is a reusable, open-source, fully documented library for connecting to Microsoft's MSN Messenger service.". Libmsn project at Sourceforge.net. 2009. Retrieved 2009-09-07.
- "SCM Repositories - libmsn - libsiren". Libmsn project at Sourceforge.net. 2009. Retrieved 2009-09-07.
- Xiph.Org Foundation (2009). "CELT - Codec Feature Comparison". Xiph.Org Foundation. Retrieved 2009-09-07.
- Xiph.Org Foundation (2006). "Speex - Codec Quality Comparison". Xiph.Org Foundation. Retrieved 2009-09-07.
- Polycom, Inc. "Siren7/Siren14/G.719 License info". Polycom, Inc. Retrieved 2009-09-07.
- Polycom, Inc. "Polycom Siren 14/G 722.1C FAQs - What are the terms on the free license?". Polycom, Inc. Retrieved 2009-09-07.
- Greg Galitzine (2008-08-06). "Polycom CTO Discusses Siren 7 HD Voice Codec". TMCnet.com. Retrieved 2009-09-07.