4 General

26.1923GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecComfort noise aspectsRelease 17Speech codec speech processing functionsTS

A basic problem when using SCR is that the background acoustic noise, which is transmitted together with the speech, would disappear when the transmission is cut, resulting in discontinuities of the background noise. Since the SCR switching can take place rapidly, it has been found that this effect can be very annoying for the listener – especially in a car environment with high background noise levels. In bad cases, the speech may be hardly intelligible.

This document specifies the way to overcome this problem by generating on the receive (RX) side synthetic noise similar to the transmit (TX) side background noise. The comfort noise parameters are estimated on the TX side and transmitted to the RX side at a regular rate when speech is not present. This allows the comfort noise to adapt to the changes of the noise on the TX side.