can't get output of exec ('nice -n 19 ffmpeg -i "file"')

Hi, i have the following php statements; I'm wondering why exec()'s $output remains empty.. Any help is greatly appreciated.. <?php $cmd = 'nice -n 19 ffmpeg -i "'.$sourcePath.'"'; exec ($cmd, $output, $result); return array (

           'cmd' => $cmd,
           'output' => $output,
           'result' => $result

); which when executed returns ["cmd"]=>string(128) "nice -n 19 ffmpeg -i "/data/web/secret/20080929 201651/work/MVI_1993.avi"" ["output"]=>array(0) { } ["result"]=>int(1) } if i run the same "cmd" on the commandline, it nicely returns: FFmpeg version SVN-rUNKNOWN, Copyright (c) 2000-2004 Fabrice Bellard configuration: --enable-gpl --enable-pp --enable-pthreads --enable-vorbis --enable-libogg --enable-a52 --enable-dts --enable-libgsm --enable-mp3lame --enable-faad --enable-dc1394 --disable-debug --enable-shared --prefix=/usr libavutil version: 0d.49.0.0 libavcodec version: 0d.51.11.0 libavformat version: 0d.50.5.0 built on Sep 29 2008 18:43:44, gcc: 4.1.2 20061115 (prerelease) (Debian 4.1.1-21) Input #0, avi, from '/data/web/LIVE_WEBSITES/www/veerman.name/mediaBeez_content/media/upload/20080929 201651/work/MVI_1993.avi': Duration: 00:01:45.6, start: 0.000000, bitrate: 1746 kb/s Stream #0.0: Video: mjpeg, yuvj422p, 320x240, 15.00 fps(r) Stream #0.1: Audio: pcm_u8, 11024 Hz, mono, 88 kb/s Must supply at least one output file