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Adaptive Multi-Rate Wideband (AMR-WB) is a patented wideband speech audio coding standard developed based on Adaptive Multi-Rate encoding, using similar methodology as Algebraic Code Excited Linear Prediction (ACELP). AMR-WB provides improved speech quality due to a wider speech bandwidth of 50–7000 Hz compared to narrowband speech coders which in general are optimized for POTS wireline quality of 300–3400 Hz. (AMR-WB ) was developed by Nokia and VoiceAge and it was first specified by 3GPP. AMR-WB is codified as G.722.2, an ITU-T standard speech codec, formally known as ''Wideband coding of speech at around 16 kbit/s using Adaptive Multi-Rate Wideband (AMR-WB)''. G.722.2 AMR-WB is the same codec as the 3GPP AMR-WB. The corresponding 3GPP specifications are TS 26.190 for the speech codec and TS 26.194 for the Voice Activity Detector.〔ITU-T (2003) (ITU-T Recommendation G.722.2 ) Page i. Retrieved on 2009-06-17.〕〔3GPP (3GPP TS 26.190; Transcoding functions; - 3GPP technical specification ) Retrieved on 2009-06-17.〕〔3GPP (3GPP TS 26.194; Voice Activity Detector (VAD); - 3GPP technical specification ) Retrieved on 2009-06-17.〕 The AMR-WB format has the following parameters:〔(Voice Age white paper ) Retrieved on 2012-02-22.〕 *Frequency bands processed: 50-6400 Hz (all modes) plus 6400-7000 Hz (23.85 kbit/s mode only) *Delay frame size: 20 ms *Look ahead: 5ms *AMR-WB codec employs a bandsplitting filter; the one-way delay of this filter is 0.9375 ms 〔3GPP (3GPP TS 26.976 - Performance characterization of the Adaptive Multi-Rate Wideband (AMR-WB) speech codec ; Chapter 25 Transmission Delay ) Retrieved on 2014-04-09.〕 *Complexity: 38 WMOPS, RAM 5.3KWords *Voice activity detection, Discontinuous Transmission, Comfort Noise Generator *Fixed point: Bit-exact C *Floating point: under work. A common file extension for AMR-WB file format is .awb. There also exists another storage format for AMR-WB that is suitable for applications with more advanced demands on the storage format, like random access or synchronization with video. This format is the 3GPP-specified 3GP container format based on ISO base media file format.〔(RFC 4867 - RTP Payload Format and File Storage Format for the Adaptive Multi-Rate (AMR) and Adaptive Multi-Rate Wideband (AMR-WB) Audio Codecs ) Page 35〕 3GP also allows use of AMR-WB bit streams for stereo sound.==AMR modes== AMR-WB operates, like AMR, with nine different bit rates. The lowest bit rate providing excellent speech quality in a clean environment is 12.65 kbit/s. Higher bit rates are useful in background noise conditions and for music. Also lower bit rates of 6.60 and 8.85 kbit/s provide reasonable quality especially if compared to narrow band codecs. The frequencies from 6.4 kHz to 7 kHz are only transmitted in the highest bitrate mode (23.85 kbit/s), while in the rest of the modes the decoder generates sounds for this band by using the lower frequency data (75-6400 Hz), along with random noise in order to simulate this high frequency band.〔Kuo, Sen M., Bob H. Lee, and Wenshun Tian. Real-Time Digital Signal Processing: Fundamentals, Implementations and Applications. John Wiley & Sons, 2013.〕 All modes are sampled at 16 kHz (using 14-bit resolution) and processed at 12.8 kHz. The bit rates are the following: * Mandatory multi-rate configuration * * 6.60 kbit/s (used for circuit switched GSM and UMTS connections; should only be used temporarily during bad radio connections and is not considered wideband speech) * * 8.85 kbit/s (used for circuit switched GSM and UMTS connections; should only be used temporarily during bad radio connections and is not considered wideband speech; provides quality equal to G.722 at 48 kbit/s for clean speech) * * 12.65 kbit/s (main anchor bitrate; used for circuit switched GSM and UMTS connections; offers superior audio quality to AMR at and above this bit rate; provides quality equal to or better than G722 at 56 kbit/s for clean speech) * Higher bitrates for speech in adverse background noise environments, combined speech and music, and multi-party conferencing. * * 14.25 kbit/s * * 15.85 kbit/s * * 18.25 kbit/s * * 19.85 kbit/s * * 23.05 kbit/s (not targeted for full-rate GSM channels) * * 23.85 kbit/s (provides quality equal to G.722 at 64 kbit/s for clean speech; not targeted for full-rate GSM channels) Notes: "The codec mode can be changed every 20 ms in 3G WCDMA channels and every 40 ms in GSM/GERAN channels. (For Tandem Free Operation interoperability with GSM/GERAN, mode change rate is restricted in 3G to 40 ms in AMR-WB encoder.)" 〔3GPP (3GPP TS 26.976 - Performance characterization of the Adaptive Multi-Rate Wideband (AMR-WB) speech codec ; Chapter 4.2 ) Retrieved on 2014-04-10.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Adaptive Multi-Rate Wideband」の詳細全文を読む スポンサード リンク
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