VoIP Voice and Fax Signal Processing

3.8 Codecs AND OVERLOAD LEVELS

Relations between overload and maximum signal levels are given in the G.191 manual [ITU-T-G.191 STL (2005)]. As explained in the G.711 codec, G.711 accepts 3.17 dBm for μ-law and 3.14 dBm for A-law as an undistorted sine wave. In the case of G.722, it can accept 6 dB more than G.711, up to 9 dBm. In practice, ADC and digital-to-analog converter (DAC) devices come with some of the bit selection options in quantization. When such options are available, instead of ±8159 as full-scale quantization, two times ±8159 can be considered as the main input. Engineers may also shift data by 1 bit to the left to make it achieve 6 dB of more quantization. The better option would be directly getting required full-scale amplitude from ADC/DAC and scaling down to feed it to other modules. Some codecs accept 15-dBm levels as input. The meaning here is they accept signal levels ±32767, i.e., four times (12 dB) more than usual G.711 levels at their input. These levels are also required to be considered in association with other module operations as some of them may consider G.711 levels as the highest levels.