[dvd::rip] tcprobe detects only 2 channels on AC3 6 channels file ?

I'm trying to use the direct passthtough parameter of transcode. I'd like to transcode vob files to avi (ffmepg) with 6 channels AC3 audio. Unsuccessfull.

Transcode seems to always dowmix to stereo. I've investigated and my pb seems to come from tcprobe that does not detect 6 channels but 2.

My original vob files own well 6 channels here is a quick check under ffmpeg and tcdecode on the same files :

ffmpeg -i 004.vob gives :

FFmpeg version SVN-r10193, Copyright (c) 2000-2007 Fabrice Bellard, et al.
configuration: --prefix=/usr/local --enable-shared --enable-gpl --enable-pp --enable-pthreads --enable-libogg --enable-libtheora --enable-libvorbis --cpu=k8 --enable-libmp3lame --enable-liba52 --enable-libfaad --enable-libfaac
libavutil version: 49.5.0
libavcodec version: 51.41.0
libavformat version: 51.12.2
built on Aug 23 2007 07:50:05, gcc: 4.1.1
Input #0, mpeg, from '../vob/002/5elements-004.vob':
Duration: 00:29:35.1, start: 5395.014367, bitrate: 4838 kb/s
Stream #0.0[0x1e0]: Video: mpeg2video, yuv420p, 720x576, 7800 kb/s, 25.00 fps(r)
Stream #0.1[0x81]: Audio: liba52, 48000 Hz, 5:1, 448 kb/s
Stream #0.2[0x83]: Audio: liba52, 48000 Hz, 5:1, 448 kb/s
Stream #0.3[0x1c0]: Audio: mp2, 48000 Hz, stereo, 192 kb/s
Stream #0.4[0x1c2]: Audio: mp2, 48000 Hz, stereo, 192 kb/s
Stream #0.5[0x1c4]: Audio: mp2, 48000 Hz, stereo, 192 kb/s

tcprobe -i .004.vob -H 100 gives :

import frame size: -g 720x576 [720x576]
     aspect ratio: 16:9 (*)
       frame rate: -f 25.000 [25.000] frc=3
                   PTS=5395.5104, frame_time=40 ms, bitrate=7800 kbps
      audio track: -a 0 [0] -e 48000,16,2 [48000,16,2] -n 0x50 [0x2000] (*)
                   PTS=5395.4304, bitrate=192 kbps
                   -D 1 --av_fine_ms 39 (frames & ms) [0] [0]
      audio track: -a 1 [0] -e 48000,16,2 [48000,16,2] -n 0x2000 [0x2000]
                   PTS=5395.3344, bitrate=448 kbps
                   -D 4 --av_fine_ms 15 (frames & ms) [0] [0]
      audio track: -a 0 [0] -e 48000,16,2 [48000,16,2] -n 0x50 [0x2000] (*)
                   PTS=5395.4304, bitrate=192 kbps
                   -D 1 --av_fine_ms 39 (frames & ms) [0] [0]
      audio track: -a 3 [0] -e 48000,16,2 [48000,16,2] -n 0x2000 [0x2000]
                   PTS=5395.3344, bitrate=448 kbps
                   -D 4 --av_fine_ms 15 (frames & ms) [0] [0]
      audio track: -a 0 [0] -e 48000,16,2 [48000,16,2] -n 0x50 [0x2000] (*)
                   PTS=5395.4304, bitrate=192 kbps
                   -D 1 --av_fine_ms 39 (frames & ms) [0] [0]
detected (4) subtitle(s)

Could it comes from a special transcode compilation parameter or simply a 'hidden' option ?

thanks for helping me

regards

Pierre