Appendix III to ITU-T Recommendation G.711 describes a toolbox to provide audio quality enhancements to ITU-T G.711. The toolbox comprises four tools that are algorithms initially developed in the context of ITU-T G.711.1 wideband speech and audio codec.

The four tools aim at enhancing the quality of ITU-T G.711 legacy for both encoder and decoder sides. At the encoder side is a noise shaping tool which is used in combination with a modified ITU‑T G.711 encoder to perceptually shape the coding noise of the PCM encoder and produce a compatible bit stream. At the decoder, the three tools offer an improved audio quality and/or a better robustness against packet losses. The first tool is a noise gate which is used to increase the clearness of the audio signal during quasi-silent periods. The second tool is a postfilter which reduces the PCM quantization noise of legacy ITU-T G.711. The third is a frame erasure concealment algorithm which is used to extrapolate the signal in case of erased frames. The toolbox has been tested with a frame size of 5 ms. The overall complexity of the toolbox is about 4 WMOPS.

All of these tools can be used separately or in combination.

This appendix contains an electronic attachment containing the respective ANSI-C source code.