How to improve the sound of a videofile
I have been watching the videos from the lectures with great interest, but the sound recording seems to be a bit problematic. The volume level varied a lot through the recordings, so I have made a quick fix for the first lecture, amplifying the quiet parts and normalizing the volume. You can find my edit at http://julipan.blip.tv/ but please note that I am not a good sound engineer and the result could be a lot better if I knew better what I was doing. Below is an explanation of how I went about in case anyone is interested in the editing other lectures.
The (suggestions for) instructions
- Audacity, levelator or other sound editing software
- the codecs (if you can view the movie you probably have them)
- a command line terminal, preferrably a GNU one...
- the video files (can be downloaded from cyberone.blip.tv by pressing the export button below the streaming movie - choose the mp4 files, not the flv ones)
Step one; extract the audio from the videofile with mplayer:
mplayer -vc null -vo null -ao pcm CyberOne01.mp4
This will create a file containing sound only called audiofile.wav
Step two; edit the audiofile
Import it with a sound editing program like Audacity where you have a graphical view of the audio. Select all the areas with low volume and use the "amplify" filter on them. Select All (Ctrl-A) and use the "normalize"-filter. When you are finished with editing the audio, export it as another wav-file.
Alternative editing suggestion for step two:
Use Gigavox's levelator, a non-free but gratis one-step compressor, normalizer and limiter. Just drop the extracted .wav file from step one into the levelator window and use the output file in step three. A real time-saver ..and sounds great! (Note: only Windows & Macintosh, no Linux version yet) -- hr
Step three; mix together the video track and the new audio track with ffmpeg
ffmpeg -i CyberOne01.mp4 -i audiofile01_normalized.wav -map 0.0 -map 1.0 CyberNormal01.mp4
That's it.. upload the files and share alike ;-)