Post-processing
Post-processing is what you do with your films after you recorded them, if you don't want to watch them and delete them after. For example, you might want to
- remove errors (e.g. due to weak signal) from the stream: see cleaning
- bring the film to a different format, e.g. the more effective XviD (or DivX) to save hard-disk space.
The latter step is called re-encoding (=recoding), if you change the codec (i.e. from MPEG-2 to e.g. XviD). It is called requantisation, if you only change the parameters of the stream (i.e. reduce the MPEG-2 PS bitrate, which is most likely not a good idea as the bitrate of the transmitted signal is much less than the one of DVDs and visible artifacts already do occur at the common bitrate of 2000kbit/s MPEG-2).
As a note on image quality: all the common codecs are lossy, i.e. the quality of the encoded video/audio signal you recieve is not as good as the original one. If you re-encode, you can only create more losses/artifacts. Therefore, in order to minimize any additional loss, I use generous XviD bitrates of e.g. 900 kbit/s, which still use less diskspace than the original MPEG-2 file.
Examples
Properly working solutions can be shared here:
Example 1: vdrsync, lame, ffmpeg
This works with the files as saved with VDR. I use VDRadmin-am to manage and schedule my DVB recordings. Then I reduce the filesizes for the recordings that I want to archive. So requirement: one or a few output files
001.vdr
in your VDR video directory, e.g. in
/videos/Simpsons/2006-05-06.05.29.90.99.rec/
First we divide the PS into its components:
vdrsync 001.vdr 002.vdr 003.vdr 004.vdr (and however many .vdr you have)
This will create two output files, one for video and one for audio. It won't take long, as it just copies raw data. Then we re-encode the MP2 audio stream into a good quality MP3:
lame --mp2input -q 5 -h --vbr-new c0.mpa audio.mp3
Finally, we re-encode the video and just copy the temporary MP3 audio file in there in the same step:
ffmpeg -deinterlace -i e0.mpv -i audio.mp3 -f avi -vcodec mpeg4 -b 900 -g 300 -bf 2 -acodec copy ~/film.avi
(Note that options for a file are left of it, here the -deinterlace for the file e0.mpv.)
This makes a 1.0GB AVI out of a 2.1GB MPEG2 PS, the quality is pretty good. However, I noticed that sometimes the synchronisation of audio-video is slightly off. I can't stand that as I personally look at people's mouths speaking... You can try
ffmpeg -itsoffset 00:00:00.200 -deinterlace -i e0.mpv -i audio.mp3 -f avi -vcodec mpeg4 -b 900 -g 300 -bf 2 -acodec copy ~/film.avi
which would make the video 200 msec later than the audio. But this won't work if the non-synchronisation varies over the film.
Example 2:
Your go: share your insights with us...