4 General

26.1933GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecRelease 17Source controlled rate operationSpeech codec speech processing functionsTS

Source Controlled Rate operation (SCR) is a mechanism for the AMR Wideband Speech Codec, which allows to encode the input signal at a lower average rate by taking speech inactivity into account. The SCR scheme may be used for the following purposes:

-to save power in the User Equipment;

-to reduce the overall interference and load in the networks.

SCR in the transmitting path (uplink) shall be in operation in UEs, if commanded so by the network. The UE shall handle SCR in the receiving path (downlink) at any time, regardless, whether SCR in the transmitting path is commanded or not.

4.1 General organisation

The default SCR mechanism described in this document requires the following functions:

-a Voice Activity Detector (VAD) on the transmit (TX) side;

-evaluation of the background acoustic noise on the transmit (TX) side, in order to transmit characteristic parameters to the receive (RX) side;

-generation on the receive (RX) side of a similar noise, called comfort noise, during periods where the transmission is switched off.

The Voice Activity Detector (VAD) is defined in [7] and the comfort noise functions in [6]. Both are based partly on the speech Codec and its internal variables, defined in [4].

In addition to these functions, if the parameters arriving at the RX side are detected to be seriously corrupted by errors, the speech or comfort noise must be generated from substituted data in order to avoid seriously annoying effects for the listener. These functions are defined in [5].

An overall description of the speech processing parts can be found in [1]. An overview of one link SCR operation is shown in Figure 1.

Figure 1: Block diagram of one link SCR operation