RealAudio
RealAudio is an audio file format from RealMedia that is mainly used for audio data streams ( streaming audio ).
RealAudio's compression methods are mostly lossy . Similar to MP3 and Vorbis , RealAudio uses the fact that the human ear does not perceive certain audio information for compression .
Various codecs are used under the term RealAudio, few of which are in-house developments by RealNetworks . Most of them are implementations of existing standards or codecs licensed by other companies. The different codecs identify themselves by a FourCC :
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14_4,lpcJ - RealAudio 1; Optimized for speech, fixed bit rate of 8 kbit / s. The codec used is IS-54 VSELP from TIA .
28_8- RealAudio 2; Optimized for speech, fixed bit rate of 15.2 kbit / s. The codec used is a bit rate reduced version of G.728 LD- CELP from ITU -T.
dnet- RealAudio 3; is also known as Dolby NET; operates at fixed bit rates between 8 and 80 kbit / s. The codec is partially compatible with ATSC / A52 AC-3 .
sipr- RealAudio 4, RealAudio 5; voice codec licensed by Sipro / VoiceAge; also known as ACELP .net; operates at fixed bit rates between 6.5 and 16 kbit / s.
cook- RealAudio G2, RealAudio 8; operates at fixed bit rates between 6 and 96 kbit / s (with multi-channel sound up to 268 kbit / s); In-house development by RealNetworks; named after the main developer Ken Cooke; sometimes the code name Gecko is also used as a designation.
atrc- RealAudio 8; ATRAC 3 codec licensed by Sony ; operates at fixed bit rates between 66 and 352 kbit / s.
raac- RealAudio 10; an MPEG-4 LC- AAC codec licensed from Coding Technologies ; operates at bit rates between 64 and 320 kbit / s.
racp- RealAudio 10; an MPEG-4 HE AAC codec licensed from Coding Technologies; operates at bit rates between 32 and 128 kbit / s.
ralf- RealAudio Lossless Format; Lossless codec, on average a compression of approx. 50–70% of the input signal is achieved (corresponds to approx. 700–1000 kbit / s for CD audio).
whrl- This FourCC was only used for Cook multi-channel audio for a very short time. The format now also bears the FourCC
cook.
For standardized formats IS-54 ( 14_4, lpcJ), G.728 ( 28_8), AC-3 ( dnet) and AAC ( raac, racp) there are free decoder implementations (for example, in FFmpeg ), which means these formats can also be outside the Real Players play become. There cookhas also been a free implementation of the proprietary codec in FFmpeg since December 2005. The information required for this was obtained through reverse engineering .
In addition to RealAudio, RealNetworks also developed and patented the counterpart for video applications, RealVideo .
The file extension of RealAudio is .ra, the MIME type is audio/x-pn-realaudio. The playlist of the Real Audio files is marked with the extension .rm : As a pure text file, it contains the .ra files that are to play or “stream”. RealAudio is also used in RealMedia , mostly together with RealVideo. RealAudio can also be used in Matroska .