The main issue in digitized voice is the amount of storage required. A good phone-quality voice digitization will produce about 64 Kbits of data for each second of voice. The hard disk will quickly be filled up by digitized voice at this digitization rate.
Speech compression is needed to reduce the digitization data rate.
Reducing the speech data rate further and maintaining good voice quality requires a sophisticated and complicated signal processing algorithm. It also requires a lot of digital signal processing computation power. We call this sophisticated speech data compression capability advanced voice capability. The U-1496 series modems support five voice digitization schemes. However, the advanced CELP scheme is supported only on the plus version and the U-1496 LCD model.