Signal-to-Noise Analysis of a PCM Voice System Based on Analog/Digital Filtering

In the field of communications, an ever-increasing number of system functions is realized in digital form. In this context, the problem of economical analog/digital conversion becomes an important issue. The cost of analog/digital conversion, however, is often dominated by the associated filters required for the prevention of foldover distortion. For this reason, an approach characterized by analog filtering (with relaxed requirements) before A/D conversion, and additional, digital, time-shared filtering after A/D conversion, becomes an attractive solution. In this paper, the signal-to-noise ratio of such a system is analyzed. The analysis includes quantization noise and overload distortion from the analog/digital conversion process, additional noise due to codec imperfections and round-off noise from the digital filters and interpolators. A design example, that was also implemented in hardware, demonstrates that the specifications of the CCITT can be easily met.