Siren (audio format)

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Siren is a lossy compressing audio data format based on a frequency transformation .

Siren comes in three variants that have different widths of frequency reproduction: Siren7, Siren14 and Siren22 with corresponding reproduction of frequencies up to 7, 14 or 22 kHz. Bit rates of 16, 24, 32, 48 or 64 kb / s are possible for monophonic reproduction; twice as much is necessary for stereo. It causes an algorithmic delay ( latency ) of 40 milliseconds.

For the 7 kHz variant there is a free codec reconstructed for the instant messenger aMSN in the form of a program library called "libsiren".

technology

The method divides the signal into overlapping blocks of 20 milliseconds in duration. For this, the quantized root mean square value of the amplitude is explicitly transmitted and the residual signal is described with frequency coefficients from a modified discrete cosine transformation , a variant of the discrete cosine transformation of type 4, which is grouped and quantized into 500 Hz-wide subbands . The values ​​are packed with a kind of Huffman coding .

history

Development of the siren codecs began at PictureTel Corp. for telephony purposes. This developed the audio formats PT724 and PT716, which could transmit audio signals with frequencies of up to 7 kHz ( broadband ) at data rates of 24 and 16 kb / s respectively . This was followed by the further development "PT716plus", which is the direct forerunner of the Siren codecs. With the takeover of the manufacturer, the rights to the technology fell to Polycom in October 2001 . Part of the technology (mainly Siren7 and Siren14) was standardized by the ITU-T in 1999 as G.722.1 and released for conditional free use. The G.719 adopted in 2008 is based on Siren22.

Individual evidence

  1. ↑ Youness Alaoui: Siren. In: MultimediaWiki. October 15, 2006, accessed August 22, 2013 .
  2. ↑ ITU-T Study Group 16 (Ed.): Low-complexity coding at 24 and 32 kbit / s for hands-free operation in systems with low frame loss . ITU-T Recommendation G.722.1. 2nd Edition. Geneva May 14, 2005 (English, itu.int [accessed August 22, 2013]).
  3. ↑ Jeff Rodman: Polycom Siren14 - Information for Prospective Licensees. (PDF; 30 kB) Polycom, February 20, 2009, accessed August 22, 2013 .