I believe the fact is you don't have to put -ac = 5 or 6 cause ac3 already means 5.1 , i may be wrong but i believe you have to do it without specifying the -ac parameter it would be like this ffmpeg -i 00080.MTS -vcodec libxvid -b 18000k -acodec ac3 -deinterlace -s 1440x1080 output.avi"
I believe the fact is you don't have to put -ac = 5 or 6 cause ac3 already means 5.1 , i may be wrong but i believe you have to do it without specifying the -ac parameter it would be like this ffmpeg -i 00080.MTS -vcodec libxvid -b 18000k -acodec ac3 -deinterlace -s 1440x1080 output.avi"