Downloading a youtube video and extract audio

Ever played a game of the STALKER series? I am not all that fond of ego shooters, but I enjoy movies made by the russian director Tarkovsky, and the STALKER triology is based upon his famous movie of the same name. You can find plenty of movie snippets on YouTube.

Anyway, I also came across this little benchmark test (yeah, there is no visible difference between DirectX10 and 11, but that's not the point here), which had a very catchy tune. A few visitors ask where the music was taken from, but no one replied. Now, it can be rather difficult to search for an instrumental piece of music on the net, when all you have got is the music.
Update: Or you may find the tune in another vid, and actually get a reply. The track above is Dream Catchers by A.L.N.P.S.V from the Tunguska Chillout Grooves vol. 1.

So I searched for a way of somehow extracting the audio in a .ogg file. All my attempts (even trying to use some sort of audio capture) failed, until my friend Alexandru Csete told me about this nifty python script which can download youtube videos.

Unpacking it, and executing it from command line

$ ./youtube-dl http://www.youtube.com/watch?v=6QpAMzTCDpg

which returns a .mp4 file. The audio channel can now be extracted with ffmpeg:

$ ffmpeg -i 6QpAMzTCDpg.mp4 -vn -acodec vorbis -aq 50 audio.ogg

The "-vn" option tells ffmpeg to ignore the video part. If the "-acodec vorbis" is not specified, the audio had some aac format, which e.g. my mobile phone is not capable of handling, so this had to be stated explicitly.

I spent some time to figure out the quality setting. First I tried "-ab 64000" setting, but somehow ffmpeg did all the encoding at about 60 kpbs irrespectively of the -ab setting. The default settings were pretty bad, and especially in this example you can easily hear losses. (If you listen to the faint woodwind-like sounds close to 5:30 you hear they are almost gone on lower quality setting.) Using the "-aq 50" option I got a good result.

I checked with the "file" command to see what format the audio was:

$ file audio.ogg
audio.ogg: Ogg data, Vorbis audio, stereo, 44100 Hz, ~0 bps

Apparently the file command cannot recognize the bps correctly.

I also tried to convert to mp3 format, but even if I had installed the "lame" library, ffmpeg refused to find it. I did not pursue this further, since I am actually quite happy with the ogg/vorbis format.