VoIP Voice and Fax Signal Processing

9.2 WIDEBAND VoIP ADAPTER

VoIP adapter software and hardware details are given in Chapter 2 for narrow-band voice. The main deviations in implementation of wideband voice are marked with a sampling frequency of 8, 16, and 8/16 kHz. The common module details are given in previous chapters. In the wideband mode of operation, it is expected that phones of both narrowband and wideband are used with FXS interfaces. Even if the FXS interface supports wideband, fax machines on an FXS interface make calls in narrowband. The FXO interface is connected to the PSTN central office, and the PSTN is always of narrowband voice. Hence, several mixed operations will occur with wideband and narrowband voice combinations. The favorable part of these combinations is that the FXS interface devices support software configurations to use devices in both narrowband and wideband mode. Several VoIP systems are built with front-end interface devices made to work for both narrowband and wideband mode. Hence, the support of wideband is more of a software change on some existing VoIP systems.

VoIP adapter main functional blocks are shown in Fig. 9.2 for narrowband and wideband combinations. The explanation of modules is given under the assumption that FXS is used with wideband phone, and FXO is for narrow-band PSTN. The SLIC–CODEC device for FXS is of wideband capable with the required programming or configuration. FXO devices are of narrowband. The PCM interface creates communication between FXS/FXO devices ...