| is a cross-platform Audio I/O library used by Audacity. PortAudio development is led by Phil Burk and Ross Bencina, but some of the Audacity developers are major contributors to PortAudio as well. There are two major versions of PortAudio available, of which Audacity uses the more recent v19.
This is the earlier version of PortAudio, but it has limited support for more modern audio . In particular, it has no support on Linux. The Audacity version of PortAudio has been patched extensively, in particular to get our Mac support to work with a wider range of devices.
PortAudio v19 is the current version of PortAudio and is still being developed. The API is regarded as "stable" by the PortAudio developers, but the degree of completeness in the implementations is variable. We rely on SVN "snapshots" of the codebase. Often the version of PortAudio used in Audacity has patches that have yet to make it back to the PortAudio SVN.
PortAudio v19 is the only way to get ALSA support under Linux. In some cases it's also the only way to get certain external devices to work with Audacity, especially Firewire devices on Mac (though USB devices on Mac can be more problematic on v19 than v18).