[SOLVED] batch transcoding fails

I've searched everywhere...
I'm trying to batch transcode mts 1080p files to DNxHD

when I do just one file at a time, no probs
when i select multiple files and transcode there is and error made "Failed to create file"
error log shows

Input #0, mpegts, from '/media/VIDEO/00056.MTS':
Duration: 00:00:21.63, start: 0.501778, bitrate: 16308 kb/s
Program 1
Stream #0:0[0x1011]: Video: h264 (High) (HDMV / 0x564D4448), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 50 fps, 50 tbr, 90k tbn, 50 tbc
Stream #0:1[0x1100]: Audio: ac3 (AC-3 / 0x332D4341), 48000 Hz, stereo, s16, 256 kb/s
Stream #0:2[0x1200]: Subtitle: hdmv_pgs_subtitle ([144][0][0][0] / 0x0090)
[NULL @ 0x16629c0] Unable to find a suitable output format for 'copy'
copy: Invalid argument

(it's interesting that only one of the files is mentione in the log -perhaps process didn't get any further that that one)

otherwise...
can any one give some pointers or links to ffmpeg commands that can do the same job, particularly for a whole directory of mts files?
thanks

Forums:

I've found out that if I make a shortcut key for Transcode a dialogue opens up which is different to the right-click or Clip menu Transcode and it will batch-transcode without error.

Now I choose "demux with MKV" (for mts files) and the result is very good after rendering the .mkv clips.

demux is nicely fast too.

edit: the joins are smooth. but I noticed after more careful observation, and encoding at a lower bps (to eliminate my cpu as culprit), that the resulting movie is jumpy -that's multiple mts clips demuxed with mkv, then rendered as project to any format..

so I am nearly stuck for making AVCHD projects.... where is the sticky telling exactly how it's done?