06.713GPPAdaptive Multi-Rate speech processing functionsGeneral descriptionTS
presents a reference configuration where the various speech processing functions are identified. In this figure, the relevant documents for each function are also indicated.
In , 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 Mobile Station (MS) and the digital interface of the PSTN. The detailed specification of the audio parts are contained in GSM 03.50 . These aspects are only considered to the extent that the performance of the audio parts affect the performance of the speech transcoder. Note that the inband signalling is not shown in Figure 1, refer to Figure 2. The mode indications (UL codec mode and DL codec mode) are used to select the mode of the speech encoder and speech decoder in Figure 1.
Figure 2 presents a general block diagram of the overall AMR system for uplink and downlink over the same radio interface. Mobile Station (MS), Base Tranceiver Station (BTS) and Transcoding and Rate Adaptor Unit (TRAU) are shown in the figure. The detailed specification of AMR link adaptation, channel quality estimation and inband signalling is given in GSM 05.09 . The AMR multirate adaptation is based on quality measurements of the radio channel. The measurements are processed to give the UL Quality Indicator and the DL Quality Indicator. The UL Quality Indicator is mapped to an UL Mode Command and the DL Quality Indicator is mapped to a DL Mode Request in the BTS and MS, respectively. The UL Mode Command and the DL Mode Request are sent to the transmitter using the reverse link. UL codec mode and DL Mode Request are sent as inband signals in the uplink radio channel. DL codec mode and UL Mode Command are sent as inband signals in the downlink channel.
Figure 1: Overview of audio processing functions
1) 8‑bit A‑law or -law PCM (ITU-T recommendation G.711), 8 000 samples/s;
2) 13‑bit uniform PCM, 8 000 samples/s;
3) Voice Activity Detector (VAD) flag;
4) Encoded speech frame, 50 frames/s, number of bits/frame depending on the AMR codec mode;
5) SIlence Descriptor (SID) frame (marked SID_FIRST or SID_UPD);
6) TX_TYPE, 2 bits, indicates whether information bits are available and if they are speech or SID information;
7) Information bits delivered to the radio subsystem;
8) Information bits received from the radio subsystem;
9) RX_TYPE, the type of frame received quantized into three bits, (classified by the RSS).
Figure 2: Block diagram of the AMR system.