Finally I managed to get the DV Audio working on the
Firewire port. The FFMPEG DV Audio decoder was
not present in my install and I couldn't work out how
to add it. So in the end I used libdv to extract the audio
from the DV frame. A frame contains 1920 samples
of audio and the MP2 encoder wants 1152 samples, it
took a while for the penny to drop but I have it all
working now, with nice clean audio and a very small
lag between the video and the audio.
The next task will be to add code to do special effects,
overlays, mixes, testcards even green screens. Once
the video is in memory it is very easy to manipulate.