freeswitch/libs/spandsp/DueDiligence

Intellectual Property Due Diligence
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Modems and voice coding are heavily patented areas. Implementing these without
serious consideration of IP issues would be foolish. This document describes
the basis on which the software has been implemented.


A check of the intellectual property information at the ITU web site shows a
number of patent claims against the current standards implemented by spandsp.
It is important to realise, however, that some of these patents have long
since expired (group III fax dates back to the 1970s). Also, many are
related to recent additions to the FAX standard, such as colour FAX handling,
which few people ever use.

The V.14 rate adaption standard seems free of patent encumberance.

One patent is listed as relevant to the V.17 standard. It is a patent from
IBM, but the ITU database does not specify its nature. I believe it is
related to the trellis coding used, and I think it has expired. I do not
know for sure. The techniques used in the implementation should be free of
patent encumberance. Most of the implementation is similar to the V.29
modem. The key addition the trellis code processing. The trellis encoding
is trivial. The decoding uses Viterbi techniques, which are quite old.

The V.21 standard dates from the 1950s. The V.23 standard is also very old.
There is no possibility that any patents related to it are still in force.
However, the implementation also needs to be free of patented techniques.
The implementation only uses very mature numerical oscillator and quadrature
correlation techniques, so there should be no patent issues.

Only one patent is listed as relevant to the V.29 standard. This dates from
the 1970s, and must have expired. The modem has been implemented using only
very mature techniques, none of which can be less than 20 years old. There
seem no possibility, therefore, that any patents are still in force related
to the techniques used.

Some aspect of the V.8 standard seems to have patents associated with it,
according to the ITU patent database. I am unclear what these are. V.8 is a
very simple standard. There seems to be nothing innovative about it.

Many patents are listed as relevant to the T.30 standard. However, they all
appear to relate to newer features, such as colour FAX, added in recent years.
The current implementation only covers the original features from the late
1970s, where there appear to be patent issues.

The T.4 standard defines the image compression and decompression techniques
used for group 3 FAXes. The spandsp implementation is based on code derived
from freely available implementations of T.4. These have existed for a number
of years without IP issues. The standard is old enough for any patents to have
expired, anyway.

V.42bis compression uses the LZW algorithm. This is the same algorithm used in
GIF files. Unisys patented this algorithm. However, the Unisys patent has now
expired.