4 General
26.1713GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecGeneral descriptionSpeech codec speech processing functionsTS
The AMR-WB speech coder consists of the multi-rate speech coder, a source controlled rate scheme including a voice activity detector and a comfort noise generation system, and an error concealment mechanism to combat the effects of transmission errors and lost packets.
The multi-rate speech coder is a single integrated speech codec with nine source rates from 6.60 kbit/s to 23.85 kbit/s, and a low rate background noise encoding mode. The speech coder is capable of switching its bit-rate every 20 ms speech frame upon command.
A reference configuration where the various speech processing functions are identified is given in Figure 1. In this figure, the relevant specifications for each function are also indicated.
In Figure 1, the audio parts including analogue to digital and digital to analogue conversion are included, to show the complete speech path between the audio input/output in the User Equipment (UE) and the digital interface of the network. The detailed specification of the audio parts is not within the scope of this document. These aspects are only considered to the extent that the performance of the audio parts affect the performance of the speech transcoder.
Figure 1: Overview of audio processing functions
1) 8-bit A‑law or μ-law PCM (ITU-T recommendation G.711), 8000 samples/s
2) 14‑bit uniform PCM, 16 000 samples/s
3) Voice Activity Detector (VAD) flag
4) Encoded speech frame, 50 frames/s, number of bits/frame depending on the AMR-WB codec mode
5) Silence Descriptor (SID) frame.
6) TX_TYPE, 3 bits, indicates whether information bits are available and if they are speech or SID information
7) Information bits delivered to the 3G AN
8) Information bits received from the 3G AN
9) RX_TYPE, the type of frame received quantized into three bits
10) Silence Descriptor (SID) flag
11) Time Alignment Flag (TAF), marks the position of the SID frame within the SACCH multiframe