Post-processing

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Post-processing is what you do with your films after you recorded them to make the files robuster, handier or smaller. Normally one wouldn't go through the trouble if it's just for a quick watch and delete, but files should be post-processeed if you want to archive them.

Common post-processing steps are

  • remove errors (e.g. due to weak signal) from the stream: see cleaning
  • cut out the adverts, the beginning and the end of the transmission
  • bring the film to a different format, e.g. the more efficient MPEG4 (XviD or DivX) to save hard-disk space or bring more films onto a DVD.

The last 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).

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. 1200 kbit/s, which still use less diskspace than the original MPEG-2 file.

A note on MPEG2 files recorded from DVB: transmission errors are unavoidable... While your player (e.g. gxine) can deal with it, any post-processing has enormous problems with it - either your re-coding program will crash, or, even worse, it might seem okay but then (after hours of encoding, maybe after wasting a DVD to burn it to) for some films some really annoying de-synchronisation might occur. This is why error correction before re-coding is very important.


Examples

Properly working solutions can be shared here:

Example 1: ProjectX, mplex, avidemux

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 4 films fit on a single layer DVD (4.4 GB). So requirement: one or a few output files of VDR like

001.vdr 002.vdr 003.vdr

in your VDR video directory, e.g. in

/videos/vdr/The_Long_Good_Friday/2006-09-12.20.59.90.99.rec

First I use ProjectX for error correction.

me@comp:/videos/vdr/The_Long_Good_Friday/2006-09-12.20.59.90.99.rec$ java -jar /usr/bin/px/ProjectX.jar -demux 001.vdr 002.vdr 003.vdr -out /home/me/videos2convert/ -name longgoodfriday

This will create a video and an audio file (error-free) in the folder specified

/home/me/videos2convert/longgoodfriday.m2v
/home/me/videos2convert/longgoodfriday.mp2

as well as some text file that shows you how many errors you had in the original stream.

Now I use mplex to combine them again (mux them):

mplex -f 8 -o longgoodfriday_muxed.mpeg2 longgoodfriday.m2v longgoodfriday.mp2

At the end I open the file

longgoodfriday_muxed.mpeg2

in avidemux which I use to cut out (by hand) the unnecessary parts at the beginninng and end of film, and the adverts. The GUI is really nice - it takes about 5 minutes to cut out 4 blocks of ads in a 2 hour film. I also use it to encode to MP3 (lame) and MPEG4 (normal 16:9 film with bitrate 1300 kbit/s for action films, 1100 kbit/s for slower films), saving as AVI.

avidemux


Example 2: vdrsync, lame, ffmpeg

Note: this works only in theory - as soon as normal transmission errors occur, vdrsync aborts...

This works with the files as saved with VDR. 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 3:

Your go: share your insights with us...


See also