Sometimes it’s necessary to completely mask out one of the stereo channels of a file and have that channel be output on both sides. For example, if you have a file, stereo.mp3, but want to hear only the left channel of the file through both sides of a headphone, use the following command:

mplayer stereo.mp3 -af pan=1:1:0

or try

mplayer stereo.mp3 -af pan=1:0:1

to hear only the right channel. Then, to convert the file permanently:

mencoder stereo.mp3 -af pan=1:1:0 -oac copy -o mono.mp3

which will create a new file, mono.mp3, that will have only the left channel. If the file has video, try this:

mencoder stereo.wmv -af pan=1:1:0 -oac mp3lame -ovc copy -o mono.wmv

Notice the -oac mp3lame option. The -af options will be ignored with -oac copy.

Also, during the conversion, you may see a bunch of these messages:

Pos: 522.3s  14486f (60%) 2110.43fps Trem:   0min  32mb  A-V:-0.002 [117:127]
71 duplicate frame(s)!
Pos: 522.3s  14487f (60%) 2110.27fps Trem:   0min  32mb  A-V:0.001 [117:127]
30 duplicate frame(s)!
Pos: 522.4s  14488f (60%) 2110.42fps Trem:   0min  32mb  A-V:-0.000 [117:127]
31 duplicate frame(s)!
Pos: 522.4s  14489f (60%) 2110.56fps Trem:   0min  32mb  A-V:-0.001 [117:127]

These messages will not prevent the conversion from completion. Just let the file finish and it should play.

Note

Sometimes a : is used instead of an =. For example pan:1:1:0 instead of pan=1:1:0.