In order to play voice messages on a voice modem, the voice data should be presented to the modem in an encoded format. When the modem records a voice message, the recorded voice will be digitized and presented to the application in an encoded format. Unfortunately, there is no standard encoding format that every modem supports. Supported formats vary from chipset to chipset and sometimes even within the same modem chipset. To make things worst, the way in which modems return the list of the supported voice formats is not standard.
To handle this situation Voice C++ uses a default voice-encoding format for every supported chipset. The default format is the following:
Rockwell chipset modems: Rockwell ADPCM, 7200 Hz.
US Robotics modems: 8Khz, 8 bit, mono, PCM encoded WAV files.
Lucent chipset modems: 8Khz, 8 bit, mono, PCM encoded WAV files.
Cirrus logic chipset modems: 11Khz, 8 bit, mono, PCM encoded WAV files.
Conexant HCF chipset modems:11Khz, 8 bit, mono, PCM encoded WAV files.
These formats are supported by most modems within the given chipset and they produce the best available voice quality.