From section 4.3 of the specification:
- "Principles of the extended adaptive multi-rate wideband codec. The AMR-WB+ audio codec contains all the AMR-WB speech codec modes 1-9 and AMR-WB VAD and DTX. AMRWB+ extends the AMR-WB codec by adding TCX, bandwidth extension, and stereo. The AMR-WB+ audio codec processes input frames equal to 2048 samples at an internal sampling frequency Fs . The internal sampling frequency is limited to the range 12800-38400 Hz, see section 8 for more details. The 2048-sample frames are split into two critically sampled equal frequency bands. This results in two superframes of a 1024 samples corresponding to the low frequency (LF) and high frequency (HF) band. Each superframe is divided into four 256-samples frames. Sampling at the internal sampling rate is obtained by using a variable sampling conversion scheme, which re-samples the input signal. The LF and HF signals are then encoded using two different approaches: the LF is encoded and decoded using the "core" encoder/decoder, based on switched ACELP and transform coded excitation (TCX). In ACELP mode, the standard AMR-WB codec is used. The HF signal is encoded with relatively few bits (16 bits/frame) using a bandwidth extension (BWE) method. The basic set of rates are built based on AMR-WB rates in addition to bandwidth extension. . . .
- "ACELP encoding and decoding are similar to standard AMR-WB speech codec. The ACELP coding consists of LTP analysis and synthesis and algebraic codebook excitation. The ACELP coding mode is used in AMR-WB operation within AMR-WB+ codec. In TCX mode the perceptually weighted signal is processed in the transform domain. The Fourier transformed weighted signal is quantised using split multi-rate lattice quantisation (algebraic VQ). Transform is calculated in 1024, 512 or 256 samples windows. The excitation signal is recovered by inverse filtering the quantised weighted signal through the inverse weighting filter (same weighting filter as in AMR-WB)."
- From the VoiceAge AMR-WB+ Web page (consulted April 4, 2007): "Emerging end-to-end digital communication systems enable the use of wideband audio coding in a large and varied collection of applications for mobile environments, including mobile TV/radio, streaming services for music, news and sports and multimedia messaging for person-to-person and business-to-person communications. . . . Extended Adaptive Multi-Rate Wideband (AMR-WB+) addresses mixed speech and audio content, providing superior-quality sound across all content types, even at low and very low bit rates. . . . AMR-WB+ delivers high-quality music, speech-over-music, and speech-between-music, making it perfectly suited to low-bit-rate mixed-content audio applications . . . ."