[FFmpeg-user] how to pass output of lavfi to xv?

[Anatoly]

I need to monitor audio levels while capturing audio/video. To achieve
this I tried simple thing first:
# ffmpeg -i Like\ an\ e-bird.mp3 -lavfi showvolume -f xv audiolevel
And I've got:
...
Stream mapping:
  Stream #0:0 (mp3) -> showvolume
  showvolume -> Stream #0:0 (rawvideo)
Could not write header for output file #0 (incorrect codec
parameters ?): Not yet implemented in FFmpeg, patches welcome

At the same time I can play any video on xv device without problem,
like this:
# ffmpeg -i video.avi -f xv video
...
Stream mapping:
  Stream #0:0 -> #0:0 (mpeg4 (native) -> rawvideo (native))
Press [q] to stop, [?] for help

Also, I can encode output of lavfi to file:
# ffmpeg -i Like\ an\ e-bird.mp3 -lavfi showvolume -c:v h264 -an
volmon.mp4