In the detection of tones for fax and modem, three different families of tones are considered.
A summary on the fax and modem tones are given in Table 14.2. For complete specifications, refer to the ITU recommendations [ITU-T-V.8 (2000), ITU-T-V.25 (1996), ITU-T-G.168 (2004)] and RFC2833 [Schulzrinne and Petrack (2000)].
The CNG detection involves passing the samples through a band-pass filter (BPF) centered at 1100 Hz. A typical bandwidth of BPF is between 64 and 100 Hz. An output of BPF is envelope detected and checked for proper power levels, duration, and repetition. Power levels have to be calibrated usually with a μ-law sine wave treating an 8159 sine wave amplitude as +3.17-dBm power.
The accepted power level for CNG is wider in range. Based on power level, several signals could pass through a 1100-Hz centered BPF and appear as CNG detection. To avoid false detections, it is required to monitor out-of-band energy. To take care of this requirement, it is required to pass the input through band elimination or a notch filter centered at 1100 Hz. The power levels observed in the notch filter path act as a reference for out-of-band energy. ...