Friday, October 28, 2011

Converting DVDs for viewing on a tablet, while inlining captions

Previously, I  described how to convert HDTV videos for my EEE Pad Transformer.  Now, I'll go over something a bit more difficult.

My wife and I have some DVDs of Bollywood films that we enjoy watching.  Aaja Nachle, Om Shanti Om, 3 Idiots, Billu, among others.  These films are mostly in Hindi, but there are English subtitles available.  As we don't understand Hindi, we watch the movies with the subtitles.  The Android media viewer that comes with the tablet doesn't have a way to select subtitles from an alternate video stream.

Now, I wanted to make files of these movies that I could watch on the Android tablet.  As noted in the previous article, the resulting files have to be H.264 Baseline profile, and under 2GB in size.

Here's how I did this.  Note that this procedure required no less than 70 GB of free disk space to hold a large intermediate file, as I wanted to avoid artefacts introduced by running through multiple codecs, so I used a lossless intermediate state.

First of all, I used the MythTV option to rip a perfect copy of the DVD.  That gave me a file, say 3IDIOTS.vob.

Next, I used mencoder to inline the captions directly into the video stream:

mencoder -ovc lavc -lavcopts vcodec=ljpeg:aspect=16/9 \
    -vobsubid 0 -oac lavc -lavcopts acodec=flac \
    -o 3idiots 3IDIOTS.vob

The output file, 3idiots, was, as noted, huge.  It consisted of a lossless jpeg video stream, with the subtitle 0 track overlaid on the video stream itself.

Next, the file had to be converted to H.264 Baseline.  In this case, I decided, rather than setting a qmax, that I would set a bitrate.  That way I could be certain ahead of time what the final size of the file would be, though at the cost of increased trancoding time.  To get a fixed bitrate, it is necessary to run ffmpeg in two passes, once to collect statistics, and the second time to generate the file itself.  Here's how this is run:

ffmpeg -pass 1 -i 3idiots -vcodec libx264 -vpre fast \
    -vpre baseline -b 1400 -acodec libfaac -ab 64k \
    -ac 2 -ar 44100 -threads 3 \
    -deinterlace -y junkfile.mp4

ffmpeg -pass 2 -i 3idiots -vcodec libx264 -vpre fast \
    -vpre baseline -b 1400k -acodec libfaac -ab 64k \
    -ac 2 -ar 44100 -threads 3 \
    -deinterlace 3idiots.mp4 

The "junkfile.mp4" file can be deleted.  The H.264 file, 3idiots.mp4, came in at 1.8 GB, and was of quite acceptable quality to view on the tablet.

Converting HDTV videos for viewing on a tablet

I have an Android-based tablet computer, the EEE Pad Transformer.  My MythTV computer can record digital over-the-air broadcasts in high definition now that I have put an HDHomerun on my network.  So, it would be nice to be able to transfer some HDTV programs to the Android computer to watch them there while traveling.  The HDTV shows are 1080i, encoded as mpeg2 video, at a bitrate of close to 16000 kbits/sec.

So, what are our constraints?  The Android computer is not powerful enough to play videos without hardware assist, and that hardware assist is only available when viewing H.264 videos encoded with the baseline profile.  It doesn't work on main profile H.264 videos.  Also, the Micro-SD card that I plug into the tablet must be formatted as VFAT, it isn't recognized when I reformat it to any more modern Linux filesystems, so our files are going to have to be under 2GB in size.  Also, the Android screen is only 1280x800, so there's no point copying a 2560x1080 file there, the machine will have to reduce the resolution, we might as well do it before we copy it to the card.

So, a 1 hour show, recorded on the MythTV box, is about 8 GB and in the wrong format.  We convert it in two steps.  First, cut out any commercials and transcode it at high quality.  For network broadcast television that chops off about 25% of the file size, and you probably didn't want to watch the commercials while sitting on the train/airplane anyway.

Next, it has to be transcoded to H.264 Basline.  This can be done with ffmpeg:

ffmpeg -i PROGRAM.mpg -vcodec libx264 -vpre fast \
     -vpre baseline -s hd720 -qmax 30 -acodec libfaac \
     -ab 128k -ac 2 -threads 4 -ar 44100 -deinterlace \

This takes the HDTV .mpg file from mythtv, "PROGRAM.mpg", and converts it.  We use the libx264 video codec, fast settings, baseline profile, formatted for a high definition 720 line screen.  "qmax" sets a limit on quality loss, I usually use a value between 25 and 30.  We use the FAAC audio codec at 128kbits/sec, deinterlace the result, and write it to "PROGRAM.mp4".

The resulting file, about 45 minutes of air time, is about 600 MB in size.