Siren is a family of patented, transform-based, wideband audio coding formats and their audio codec implementations, developed and licensed by PictureTel Corporation (acquired by Polycom, Inc. in 2001). There are three Siren codecs: Siren 7, Siren 14, and Siren 22.
Editions
Siren 7 (also written Siren7, or simply Siren) provides 7 kHz audio bandwidth, with bit rates of 16, 24, and 32 kbit/s, and a sampling frequency of 16 kHz. Siren 7 is derived from PictureTel's PT716plus algorithm. In 1999, the ITU-T approved the G.722.1 recommendation, which is based on the Siren 7 algorithm, after a four-year selection process involving extensive testing. G.722.1 provides only bit rates of 24 and 32 kbit/s and does not support Siren 7's 16 kbit/s rate. The algorithm of Siren 7 is identical to its successor G.722.1, although the data formats differ slightly.
Siren 14 (or Siren14) provides 14 kHz audio bandwidth, with bit rates of 24, 32, and 48 kbit/s for mono and 48, 64, and 96 kbit/s for stereo, at a sampling frequency of 32 kHz. Siren 14 supports both stereo and mono audio. It offers a 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 coding technology, using a modulated lapped transform (MLT), a type of discrete cosine transform (DCT) or modified discrete cosine transform (MDCT).
Siren 22 (or Siren22) provides 22 kHz audio bandwidth at a sampling frequency of 48 kHz, with bit rates of 64, 96, and 128 kbit/s for stereo and 32, 48, and 64 kbit/s for mono. Siren 22 offers a 40-millisecond algorithmic delay using 20-millisecond frame lengths. In May 2008, the ITU-T approved the G.719 full-band codec, which is based on Polycom Siren 22 audio technology combined with Ericsson's advanced audio techniques.
Software support
Siren 7 is commonly used in videoconferencing systems and is part of Microsoft Office Communicator for 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 RTAudio codec; when a call is promoted into an audio conference (three or more participants), the codec is switched on the fly to Siren for performance reasons.
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 as one of the available codecs for the "Computer Call" feature.
FreeSWITCH, an open-source communications software, can perform transcoding, conferencing, and bridging of Siren 7/G.722.1 and Siren 14/G.722.1C audio formats. The aMSN open-source Windows Live Messenger clone uses the "libsiren" library, an open-source implementation of the codec written by aMSN developer Youness Alaoui (KaKaRoTo). The libsiren library has also been incorporated into libmsn and the msn-pecan project, which provides a plug-in for the Pidgin and Adium instant messaging clients.
Licensing
Usage of the Siren 7 and Siren 14 audio coding formats requires licensing of patents from Polycom in most countries. A royalty-free license for Siren 7 and Siren 14 is available from Polycom provided certain basic conditions are met. Usage of Siren 22 also requires licensing of patents from Polycom.
See also
- Comparison of audio coding formats
- G.722.1
