This document describes the Source Controlled Rate (SCR) operation of the Adaptive Multi-Rate Wideband speech Codec. The implementation of this SCR operation is mandatory in all UMTS equipment implementing the Adaptive Multi-Rate Wideband speech Codec. The description is structured according to the block diagram in Figure 1. This structure of distributing the various functions between system entities is not mandatory for implementation, as long as the operation on the speech decoder output remains the same. Annex A describes the Discontinuous Transmission (DTX) operation of the Adaptive Multi-Rate Wideband speech Codec in Codec Type FR_AMR-WB for the GSM system.