Using ffmpeg with audio in a video

I am using ffmpeg to convert one video format to another:

"ffmpeg -ac 1 -i vid1.mp4 -sameq vid1.avi"

And I am getting the error:

"encoding 6 channel(s) is not allowed in mp2"

Now I understand what this means - and the transcoding works fine if I switch sound transcoding off (using "-an" option). How do I get around this and still retain the sound?

More information

Input file:

Seems stream 0 codec frame rate differs from container frame rate: 47.95 (5000000/104271) -> 23.98 (24000/1001) Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'vid1.mp4': 开发者_高级运维 Duration: 00:02:03.24, start: 0.000000, bitrate: 8236 kb/s Stream #0.0(und): Video: h264, yuv420p, 1920x816, 23.98 tbr, 23.98 tbn, 47.95 tbc Stream #0.1(und): Audio: aac, 48000 Hz, 5.1, s16 At least one output file must be specified

Try to put -ac 2 after -i vid1.mp4, to set 2 audio channels on the output file.

As a general rule, options are applied to the next specified file.

I ended up transcoding without the sound and it worked.