export_tcaud options for AC3 sound?

Hello,

I'm trying to digitize VHS tapes using my bt878 cards (bttv is v4l2) and 
get them on a DVD, and using different sources e.g. [1], I came to the 
following test combination (hence the 10s):

transcode \
         -x v4l2=resync_margin=1:resync_interval=250,oss \
         -i /dev/video0 \
         -e 48000,16,2 \
         -p /dev/dsp \
         -c 0-00:00:10 \
         -J smartyuv,pv \
         -y ffmpeg,tcaud \
         --export_prof dvd-pal \
         --export_asr 2 \
         -D0 \
         -o tvrecord.m2v \
         -m tvrecord.ac3

mplex -f8 -o tvrecord.mpg tvrecord.m2v tvrecord.ac3

and it works quite well (I'm nevertheless open for improvement 
suggestions), but for 2 things:

1. direct encoding into one file would be nicer but I understand that 
it's not possible but I'd like to be proven wrong, it would spare me 
half of the disk space required.

2. Much more important, the resulting audio file is not AC3 but MP3, I 
guess I'm missing the magic option for tcaud, but I couldn't find any 
documentation, not on the web, no manpage, and tcmodinfo neither.
Any clue on how to fix this?

Thanks, Eric

[1] http://www.transcoding.org/transcode?Tutorials/Authoring_PC_Media_To_DVD