We believe that the filtering problem for a digital local exchange is best studied by division into two parts. The quantisation noise filter design entails a trade-off between this filter and the type of coder. The complexity of the bandlimiting filter to achieve the CCITT specification is divided between the analogue prefilter and the second part of the digital filter. Several designs of digital filter are described which may be suitable for large scale integration.

M. Carey

Papers

Filtering for code conversion in digital telephone exchanges