SpanDSP is a library of DSP functions for telephony, in the 8000
sample per second world of E1s, T1s, and higher order PCM channels.
It contains low level functions, such as basic filters. It also
contains higher level functions, such as cadenced supervisory tone
detection, and a complete software FAX machine.  The software has
been designed to avoid intellectual property issues, using mature
techniques where all relevant patents have expired. See the file
DueDiligence for important information about these intellectual
property issues.