1. Introduction

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2. Required software

• MPlayer is a multimedia player which supports a very broad range of file formats and media codecs: MPEG1, AVIs (including all Windows codecs, all DivX^? codecs), DVDs, VCDs, SVCDs and so on. I use it for watching all kind of files.
• MEncoder is part of the MPlayer package and is used for encoding video and sound. It is capable of transcoding DVDs.
• transcode is a set of tools that allow to transcode from a broad list of formats to a broad list of other formats including transcoding from DVD to AVI or (S)VCD.
• You know that most DVDs are encrypted. libdvdcss is a library that decrypts DVD contents.
• DVDs contain information about the titles / chapters / angles and audio languages contained in the files. These information are stored in .IFO files. libdvdread is often used to parse these files and extract the informations that are needed.
• lame (Lame Ain't an MP3 Encoder) encodes MP2/WAV audio to the MP3 format.
• vobcopy can be used to copy VOBs from a DVD. Decryption is done on the fly.
• dvd::rip is a graphical frontend for transcode.

I'll present you two methods for DVD transcoding - using MEncoder

and transcode. Nevertheless you need some files for both method.

2.1. A word on codecs

In an ideal world there would be only one codec - the best one. But this world is far from ideal. The result is that there is a wide range of video and audio codecs to chose from. First I'll explain the different types of codecs:

• Native codecs are normal Linux binaries (most often shared objects, libCODECNAME.so). Support for them is trivial.
• Windows codecs are the original or slightly modified Windows dynamic libraries (DLLs, AX and the likes). These codecs do not work natively under Linux - you need a special library to use these codecs. This library is called avifile. Some months ago this was the only way to watch AVIs under Linux.

Today most codecs come with a native version for Linux (like XviD, DivX 4/5 and others).

Here's a list of supported video codecs:

• MPEG4 is an official video compression standard. There is no codec whose name is simply MPEG4. If you talk about MPEG4 you don't talk about a specific codec - you talk about a set of techniques to compress videos. MPEG4 compliant means that a codec produces files that can be played by other MPEG4 compliant codecs.
• DivX ;-) is the "original" hacked Microsoft MPEG4 codec that started all those ripping frenzies. It does not support all MPEG4 features like b-frames^* or global movement compensation (GMC ^*). Some still use it under Windows with the excellent tool Nandub. It is seldom used under Linux because there are codecs that provide better quality. It comes as a Windows dynamic library (divx.dll and some others) and requires avifile.
• DivX 4 and 5 are the official successors. DivX 5 replaces DivX 4. That's why you can't have both codecs installed at the same time (well technically you can, but you shouldn't, for obvious reasons). These are native Linux shared objects called libdivxdecore.so and libdivxencore.so. This is closed source. For Windows there are different versions available and only the commercial version supports all encoding options like b-frames^*, GMC^*

  or quarter PEL^*. The free version can still be used to view all files created with even the commercial version. Unfortunately there's only the free version for Linux which doesn't support all those nice options. DivXNetworks^? are thinking about providing a commercial solution for Linux as well. But don't expect that codec to be available soon.

• XviD (it's DivX read backwards) is an Open Source MPEG4 implementation that is really good considering compression and picture quality. Support for advanced MPEG4 features (b-frames^*, GMC^*) is being worked on or has already been implemented.
• libavcodec or short lavc is another Open Source MPEG4 compliant video codec which is superior in performance and quality to DivX 5 and even to XviD. This codec supports b-frames^*. It is part of the ffmpeg project.
• Of course there are the MPEG1 compliant codecs that you need for VCDs and the MPEG2 compliant codecs for SVCDs or DVDs. At the moment this guide aims at producing AVIs so I won't go into detail about either one of these.
• Most other codecs are either outdated (like Intel Indeo 5) or other cracked version of Microsoft's MPEG4 codec (like AngelPotion). Still other codecs are under heavy development (like VP4) and do not present a workable solution for us here.

This guide tries to focus on two codecs: XviD and lavc. Reasons are that both provide excellent quality, both are fast and you only need a MPEG4 compliant decoder (like DivX 5 or XviD) on a Windows system for playback. Playback under Linux is no problem at all -

MPlayer or Xine do happily play DivX 4/5, XviD^? and lavc encoded movies.

Again a list, this time for audio codecs:

• MP3 is short for MPEG1 layer 3 and is an official audio compression standard. If you talk about MP3 you actually talk about a compression technique, not about a codec in particular. There are more codecs for MP3 than I can name in a full day.
• lame is an acronym for "Lame Ain't an MP3 Encoder" (although it is ;-)). It provides a encoder that produces high quality MP3 files. Both transcode and MEncoder use lame for audio compression.
• AC3 is again an official audio compression standard. Almost all DVDs contain AC3 encoded audio. Today there are decoders for both Windows and Linux that work with AC3 sound inside AVIs. The advantage is that no re-compression is necessary (recompressing always loses quality) and that multi channel sound (Dolby Surround and the likes) will be preserved. The disadvantage is that AC3 sound needs more space than MP3 encoded sound.
• Vorbis is a new Open Source audio compression. It is available for both Windows and Linux. Its advantages are a better quality : size ratio compared to MP3 and multi channel support. Unfortunately you cannot safely include Vorbis audio into AVI files - but I'm working on a tool that can join video and Vorbis audio in an OGG container. As soon as it is ready I will include it in this guide.

I hope this clears things up a bit.

2.2. Common libraries / codecs

I won't go into too much detail in this chapter. There is more documentation on how to install included with each package. Refer to that if you have problems. Also note that MPlayer has rather strong requirements regarding versions of gcc, binutils and other central system components. Refer to the MPlayer installation documentation.

Note: Before downloading software check if your distribution already contains that software. I use Debian Woody (testing) and have found that a lot of software is already available prepackaged.

• libdvdcss - Head over to <a href="http://www.dtek.chalmers.se/groups/dvd/downloads.shtml" target="_blank">the Ogle project page and download libdvdcss-0.0.3.ogle3.tar.gz (note that version numbers may differ as software is always under development). Un-tar-gz it, compile it, and install it in /usr/local
. Please note that there different versions of this library floating around (0.0.3, 1.0.1). MPlayer still suggests that you use 0.0.3.
• libdvdread - Get it from the same web page as libdvdcss. Note that Debian Woody already contains libdvdread. A simple apt-get install libdvdread2 libdvdread2-dev should do the trick.
• XviD codec - Needed if you want to use transcode. Short instructions taken from the MPlayer documentation:
  
  • cvs -z3 -d:pserver:anonymous@cvs.xvid.org:/xvid login When asked for a password simply hit enter.
  • cvs -z3 -d:pserver:anonymous@cvs.xvid.org:/xvid co xvidcore
  • cd xvidcore/build/generic
  • Edit Makefile.linux to fit your needs.
  • make -f Makefile.linux
  • Copy the xvidcore/src/divx4.h file to /usr/local/include/decore.h and to /usr/local/include/encore2.h. Be sure to make a backup of the files you will overwrite!
  • Copy libxvidcore.so and libcore.a to /usr/local/lib/
  For the compilation you might need a special version of nasm which you can get <a href="http://homepage1.nifty.com/herumi/soft/petit/nasmsse2.tgz" target="_blank">here.
• vobcopy - Get it from <a href="http://lin-prog-newbie.sourceforge.net/projects/c/index.html" target="_blank">the homepage.
• avifile - As explained it is needed to access Windows codecs for which there are no native Linux binaries. Head over to the homepage. Before you do that check again if your Linux distribution already provides avifile (Debian Woody does). You still need the Windows codecs from the avifile homepage or directly from <a href="http://www2.mplayerhq.hu/MPlayer/releases/w32codec-0.60.tar.bz2" taget="_blank">MPlayer's homepage.
• DivX 5 - although this guide does not focus on DivX 5 you can get it from http://www.divx.com/. If you have some time to spare than I suggest make some comparisons between DivX 5 and XviD or lavc and see for yourself that it is worse than the two latter codecs.

• -i /space/tng-biggoodbye/vob/004 obviously tells transcode where to find the files. It can be a file, a device or a directory containing the files.
• -w 4357,250,100 sets the video encoding parameters: bit rate, maximum key frame interval and crispness.
• -a 1 selects audio track number 1 (starting with 0).
• -b 128,0,0 sets the audio encoding parameters for lame: bit rate, VBR and quality. Have a look at lame's man page for an explanation of the parameters -V and -q.
• -s 3.311 causes lame to scale the audio and thus normalizing it on the fly.
• -V causes transcode to do image processing in the YUV color space. Without -V images would be converted to the RGB color space. Note that some external filters only work with either color space. YUV processing gives a huge speed boost.
• -f 25 sets the frame rate.
• -B 12,10,8 sets the fast scaling options: the picture will be scaled down to height - 12 * 8 rows and to width - 10 * 8 columns.
• -R 1 is the marker for the first (of two) encoding pass.
• -x vob,null - the video input comes from VOB files and the audio input will be skipped (it isn't needed for the first pass anyway).
• -o /dev/null - We don't need the video either, so just discard it.
• -y xvidcvs,null - Output video using XviD and discard audio.

The second command line is not that different. It just skips all options that would discard output (like -o real-file-name and -y xvidcvs). For a more complete reference have a look at transcode's and lame's man pages.

As a last note you will find that transcode as of version 0.6.0pre6-20020529 contains support for libavcodec via a still experimental export module written by me. You can activate it by using -y ffmpeg4 (remember: libavcodec is part of ffmpeg) instead of your previous codec. dvd::rip does not support it at the moment (read: it is not in the list of selectable codecs).

4.3. Using MEncoder

As mentioned earlier there is no GUI for MEncoder at the moment. Therefore I won't provide screenshots and you have to do everything by hand. This is a process called "three pass encoding".

4.3.1. Encoding the audio

The first step is to encode only the audio. Make sure you know what audio language you want to use - you need either the audio id (which is 128 for the first stream, 129 for the second etc.) or the two-letter country code (like en for English or de for German). You can find out about these codes by running MPlayer in verbose mode:

mplayer -dvd 1 -v. It should print a lot of lines. Search these for lines similar to:
[open] audio stream: 0 audio format: ac3 language: en aid: 128
[open] audio stream: 1 audio format: ac3 language: de aid: 129
[open] audio stream: 2 audio format: ac3 language: es aid: 130
Here I have three audio streams: English, German, Spanish and their IDs.

Now call MEncoder to encode the audio for you:

nice -+19 cat /space/*vob | nice-+19 mencoder -ovc frameno -o frameno.avi -oac mp3lame -lameopts abr:br=128 -alang de -
Here's an explanation for the command line arguments used:

• nice -+19 - Gives MEncoder the lowest process priority so it won't disrupt your normal work.
• -ovc frameno - Output Video Codec is frameno

  which means that no video data is actually written or processed at all.

• -o frameno.avi - Write the output to the file called frameno.avi. The audio must be written to a file with that name as MEncoder will read the audio from exactly this file with the exact name in the next step.
• -oac mp3lame - Output Audio Codec is the lame

  encoder library.

• -lameopts abr:br=128 - options for lame. It tells lame to produce a ABR (average bitrate, it's a variable bitrate setting with the average supplied by the user) with the average bitrate of 128kbit/s.
• -alang de
- use the German sound track. An alternative is using -aid 129.

Wait some time and you'll have your audio. MEncoder even prints some suggestions for the video bitrate:
Recommended video bitrate for 650MB CD: 1845
Recommended video bitrate for 700MB CD: 1992
Recommended video bitrate for 800MB CD: 2287
As you can see these values are rather high - but that's because an Ally McBeal^? episode is only 41minutes long. So I'd go for two episodes per CD and give the video a bitrate of about 1000.

4.3.2. Generating a preview (first pass)

Now's the time to decide about the video codec you're going to use. I'm assuming that AVI is the desired result with one of the different DivX encoders. MEncoder has support for DivX 4 or DivX 5 (same under Windows actually, you have either v4 or v5 running) as well as for XviD or lavc. Although most of you won't have heard of libavcodec or the ffmpeg-project before you should be advised that lavc is far superior to DivX 4 or 5

and at least as good as XviD. So I will only provide examples for lavc as you don't even need special codecs for playback: MPlayer plays them just fine (naturally), and for Windows you simply need DivX 5 (free edition is absolutely sufficient). So here we go.

Let's assemble the command line options one by one:

• -oac copy - MEncoder needs the audio information from the first pass in order to keep audio/video synchronization. You mustn't use -nosound, although I suggested it in earlier versions of this guide. If you want the technical details then have a look at <a href="http://mplayerhq.hu/pipermail/mplayer-users/2002-August/019551.html" target="_blank">the MPlayer mailing list.
• -o /dev/null - The output of the first pass isn't needed either, so just dump it.
• -ovc lavc - chose lavc as the Output Video Codec.
• -lavcopts vcodec=mpeg4:vbitrate=1000:vhq:vqmin=2:vqmax=31:vpass=1 - libavcodec actually supports a handful of different video codecs. With vcodec=mpeg4 we chose the MPEG4 compliant encoder. vbitrate=1000 is the desired bitrate. vhq

  (very high quality) tells the encoder to spend a lot of time optimizing the results. It is worth it. vpass=1 finally tells the encoder that this is the first pass only. Using vqmin and vqmax tells the codec how much it may compress the video at least and at most. Keeping these at their defaults (see man mencoder) restricts the achievable output bitrate a lot so I advice choosing a broader range. Note that I'm no expert on quantizers. If I'm wrong here please drop me a line.

• -vop scale=640:480 - scale the picture down to 640 by 480 pixels. I've read on the mailing list that a new switch has been added that automatically calculates the height from the width or the other way round so that the aspect ratio will be kept. I haven't tried it myself but it should look like this: -vop scale -xy 640 . Note that while it is possible to actually enlarge the picture I strongly advise against it. There are several drawbacks: the encoder needs a lot more bits to achieve the same quality you'd get when encoding a smaller picture and then scaling it up while playing it back, and you might even get distorted pictures (Peter Schuller reported such a case to me). Always scale down.
• If your picture is interlaced you can deinterlace it with -npp lb.
• We only want a preview so we provide the start position and the length: -ss 0:20 specifies 20seconds into the movie as our beginning, and -endpos 0:30 says that we want 30seconds to be processed. This parameter is named badly as it's not the end position but the duration that it specifies.

nice -+19 mencoder -oac copy -o /dev/null -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1000:vhq:vpass=1 -vop scale=640:480 -npp lb -ss 0:20 -endpos 0:30 /space/first.vob

4.3.3. Generating a preview (second pass)

For this step we can keep most of our previous command line. Of course we change the vpass=1 to vpass=2 to tell the encoder that it's the second pass this time. Of course we should save the final output and replace -o /dev/null with something useful, e.g. -o ally-preview.avi.

Here's the command line:
nice -+19 mencoder -oac copy -o ally-preview.avi -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1000:vhq:vpass=2 -vop scale=640:480 -npp lb -ss 0:20 -endpos 0:30 /space/first.vob
After MEncoder finishes take a look at your preview. Adjust parameters if needed. Redo the preview until you're happy with it.

4.3.4. Generating the final AVI (both passes)

The calls for the final options simply leave out -ss and

-endpos. Unfortunately MEncoder can't handle multiple input files on the command line. That's what the cat is for. Just enter:
cat /space/*vob | nice -+19 mencoder -oac copy -o /dev/null -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1000:vhq:vpass=1:vqmin=2:vqmax=31 -vop scale=640:480 -npp lb -
cat /space/*vob | nice -+19 mencoder -oac copy -o ally.avi -ovc lavc -lavcopts vcodec=mpeg4:vbitrate=1000:vhq:vpass=2:vqmin=2:vqmax=31 -vop scale=640:480 -npp lb -
Note that -ss does not seem to work with the cat *vob | mencoder... variant. So for the preview just specify the first vob as the last argument on the command line (just look up).

Now go get a beer and a nice book.

Back to the table of contents

5. Subtitles

On a DVD subtitles are stored as pictures that are shown on top of the movie by your movie player. That way the authors have a wide choice of how their subtitles look like (and it makes subtitles in Asian languages much easier to implement). For us this may or may not be a problem - depending on whether we want to include subtitles directly into the picture of have them as a separate file/stream.

5.1. Converting subtitles to text format

Very often you don't want to be forced to see the subtitles. This is not possible if you include the subtitles in the picture during encoding. You have to extract the subtitles from the DVD into an external file/stream that the user can activate (or not). I will describe the process of converting the DVD subtitles into a text format that is widely used. Text subtitles can be easily scaled by the player (by selecting an appropriate font) and they are really small (most often below 100KB).

For this process you must have transcode and its sources. You need tccat and tcextract from transcode itself and the files in transcode/contrib/subrip from the transcode

sources.

5.1.1. Compiling the tools

Unfortunately no binary package (RPM, deb) that I know of includes subrip so we have to compile and install it ourselves. But this is rather easy.

1. Make sure you have gocr and ispell installed on your system. They come with most modern distributions. But if yours does not contain gocr then head over to <a href="http://jocr.sourceforge.net/" target="_blank">its website.
2. Change into subrip's source directory (cd transcode/contrib/subrip) and invoke make.
3. Copy the three programs srttool, subtitle2pgm

   and pgm2txt to a directory in your PATH.

4. You may have to change the script pgm2txt if your gocr does not support the -p option: at the end there are two lines containing -p ${DBPATH}. Simply remove it (after consultin gocr's manpage).

5.1.2. Extracting the subtitle stream

Here I assume that you've copied your DVD with vobcopy -m meaning that it has been completely mirrored including the .IFO files. If not then you'll have to adjust the sources.

First let's see which subtitles are available. We can use mplayer

for this task:
mplayer -dvd-device /space/st-tng/disc1/ -dvd 1 -vo null -ao null -frames 0 -v 2>&1 | grep sid
This causes mplayer to just print a lot of information about the source and not to play anything at all. It should give you a list of subtitles:
[open] subtitle ( sid ): 0 language: da
[open] subtitle ( sid ): 1 language: de
[open] subtitle ( sid ): 2 language: en
[open] subtitle ( sid ): 3 language: es


[open] subtitle ( sid ): 4 language: fr
[open] subtitle ( sid ): 5 language: it
[open] subtitle ( sid ): 6 language: nl
[open] subtitle ( sid ): 7 language: no
[open] subtitle ( sid ): 8 language: sv
[open] subtitle ( sid ): 9 language: en

Now that we have the sid (subtitle ID) for the language that we want we can fire up the transcode tools and let them extract the raw subtitle stream:
tccat -i /space/st-tng/dic1/ -T 1 -L | tcextract -x ps1 -t vob -a 0x22 > subs-en
The -a 0x21 is the subtitle stream's hexadecimal number: 0x20 + sid. Here I use the English subtitles.

5.1.2. Converting the raw stream

Ok, we have a raw subtitle stream - but what can we do with it? First we have to convert each subtitle entry into a picture. This can be easily done with
subtitle2pgm -o english -c 255,255,0,255 < subs-en
Here's a catch however. With -c you can specify the grey levels used in the conversion. The idea is to make the job for gocr as easy as possible. Therefore you might have to experiment with the parameters - but this is easy, too. I've taken the following samples from my Star Trek - The Next Generation DVD:

• -c 0,255,255,255 - this is obviously wrong.
• -c 255,0,255,255 - this looks good.
• -c 255,255,0,255 - we don't want this one.
• -c 255,255,255,0 - we don't want this either.

As you can see you need a picture that does not contain outlined characters.

subtitle2pgm creates a lot of images - one for each subtitle - and a control file, called english.srtx in my case, that contains the duration for each subtitle. The next step is to let gocr recognize the text:
pgm2txt english
Be warned - gocr will ask you often about charcters that it can't recognize. This is normal. Once you're done you should run ispell

over all the newly created text files:
ispell -d american english*txt
Adjust the languange to your needs, of course.

The last step is to let srttool include the actual text into the .srtx file:
srttool -s -w < english.srtx > english.srt

Voila, you have a working subtitle file. You can watch them with e.g.
mplayer -sub english.srt mymovie.avi

Back to the table of contents

6. Additional tricks

There are some more topics involved in video production.

6.1. Fixing desynched audio

This does not work with AVIs that use VBR/ABR MP3s. This step uses avisync which is part of the transcode package.

avisync is easy to use: just provide an input file (-i name.avi), an output file (-o newname.avi) and the synchronization offset in frames (-n count). From

avisync's help: count>0: audio starts with frame 'count'. count>0: prepend 'count' padding audio frames. An example:
avisync -i ally-not-synched.avi -o ally-synched.avi -n -2

6.2. Splitting AVIs

This does not work with AVIs that use VBR/ABR MP3s. This step uses avisplit which is part of the transcode package. Use it only with AVIs generated by transcode.

avisplit is easy to use: just provide an input file (-i name.avi) and when it should split. You can split after a fixed size (-s size_in_megs), after a number of frames (-f f1-f2) or after a number of seconds (-t s1-s2). An example:
avisplit -i ally-big-file.avi -o ally -s 700
This will produce files named ally-0000.avi, ally-0001.avi etc. that are each 100megs big at most.

6.3. Extracting audio from VOBs/MPEGs/AVIs

MPlayer can be used easily to extract sound. The -vo null options tells MPlayer not to show the video.

• VOB -> AC3: mplayer -vo null -nogui -alang de -dumpaudio -dumpfile lang-de.ac3 *vob
  This will extract the German audio to a file called lang-de.ac3.
• VOB -> WAV: mplayer -vo null -nogui -aid 128 -ao pcm -aofile lang-de.wav *vob
  This will extract audio track no. 128, downmix the AC3 sound to PCM and write the results to lang-de.wav.
• MPG -> MP2: mplayer -vo null -nogui -dumpaudio -dumpfile audio.mp2 myvideo.mpg
  This will extract the audio to a file called audio.mp2.
• AVI -> WAV: mplayer -vo null -nogui -ao pcm -aofile audio.wav myvideo.avi
  This will extract the audio, convert it to PCM and write the resulting .WAV to audio.wav.

I'm not listing all possible options. Generally use -dumpaudio -dumpfile newaudio.extension if you want to extract the audio without processing and -ao pcm -aofile newaudio.wav if you want the WAV itself.

6.4. Using separate audio files

Sometimes you want to play another audio file along with your video. MPlayer makes this rather easy: mplayer -audiofile anotherlang.mp3 myvideo.avi plays video from myvideo.avi

and audio from anotherlang.mp3. A lot of Windows media players support external audio files, too. This way you can provide several language versions as separate audio files. A combination often used is to have the AVI include the English audio track and to provide additional audio tracks in separate files.

6.5. A visual AVI editor: avidemux

If you've done video editing with Windows before you probably know the excellent OpenSource^? editor <a href="http://www.virtualdub.org/" target="_blank">VirtualDub. An OpenSource^? utility for Linux that aims at providing the user with a similar set of features is avidemux. You can use it in order to easily cut specific parts from a movie, to apply filters, to reencode movies and to do a lot more. I suggest you take a look at it even if using the command line is as natural to you as it is for me.

Back to the table of contents

7 Links

7.1. Libraries, codecs

• libdvdread and libdvdcss: <a href="http://www.dtek.chalmers.se/groups/dvd/downloads.shtml" target="_blank">http://www.dtek.chalmers.se/groups/dvd/downloads.shtml
• DivX 5: http://www.divx.com/
• XviD: http://www.xvid.org/
• ffmpeg (the home of libavcodec): http://ffmpeg.sourceforge.net/

7.2. Media players

• MPlayer: http://www.mplayerhq.hu/
• Xine: http://xine.sourceforge.net/
• Ogle: <a href="http://www.dtek.chalmers.se/groups/dvd/index.shtml" target="_blank">http://www.dtek.chalmers.se/groups/dvd/index.shtml

7.3. Media encoders

• MEncoder (part of MPlayer): <a href="http://www.mplayerhq.hu/" target="_blank"> http://www.mplayerhq.hu/
• transcode: <a href="http://www.theorie.physik.uni-goettingen.de/~ostreich/transcode/" target="_blank">http://www.theorie.physik.uni-goettingen.de/~ostreich/transcode/
• dvd::rip (GUI for transcode): <a href="http://www.exit1.org/dvdrip/" target="_blank">http://www.exit1.org/dvdrip/
• some Perl scripts for transcode: <a href="http://www9.informatik.uni-erlangen.de/~Vogelgsang/bp/tctools.html" target="_blank">http://www9.informatik.uni-erlangen.de/~Vogelgsang/bp/tctools.html

7.4. Other documentation

Both MPlayer and transcode come with standard Unix man pages: man mplayer, man mencoder and

man transcode show these pages. Additionally providing -h or --help as the only parameter to one of these programs will give you a listing of command line options (which might not be complete).

• MPlayer's documentation: <a href="http://www.mplayerhq.hu/DOCS/" target="_blank">http://www.mplayerhq.hu/DOCS/
• transcode's documentation and examples: <a href="http://www.theorie.physik.uni-goettingen.de/~ostreich/transcode/examples.html#top" target="_blank">http://www.theorie.physik.uni-goettingen.de/~ostreich/transcode/examples.html#top
• The best place on the 'net for DVD ripping and conversion is doom9. Although it focuses on Windows it still provides a lot of insight on DVDs, codecs in general. Also the forum is quite excellent. If your really interested in video processing then this is the place for you.

7.5. Mailing lists

I strongly suggest that you search/browse the mailing lists for information and subscribe yourself. A lot of information is available there that has not made it into the main documentation or the command line reference.

• MPlayer's user mailing list: <a href="http://mplayerhq.hu/mailman/listinfo/mplayer-users" target="_blank">http://mplayerhq.hu/mailman/listinfo/mplayer-users
• MPlayer's developer mailing list: <a href="http://mplayerhq.hu/mailman/listinfo/mplayer-dev-eng" target="_blank">http://mplayerhq.hu/mailman/listinfo/mplayer-dev-eng
• transcode's user mailing list: <a href="http://www.theorie.physik.uni-goettingen.de/pipermail/transcode-users/" target="_blank">http://www.theorie.physik.uni-goettingen.de/pipermail/transcode-users/
• dvd::rip's mailing list: <a href="http://www.exit1.org/dvdrip/list.cipp" target="_blank">http://www.exit1.org/dvdrip/list.cipp

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8. Glossary

8.1. CBR, VBR and ABR

These three acronyms stand for Constant Bit Rate, Variable Bit Rate and Average Bit Rate.

With CBR the encoder spends the same amount of bits on each frame, regardless of how much it would really need. This has the disadvantage that for example complete black pictures or complete silence in an audio frame would waste a lot of bits while pictures or audio frames with a lot of elements would benefit from these bits.

VBR now takes as much bits as it needs. So black pictures get very few bits and complex pictures get as much as they need. While this provides the best quality the final size can't be predicted at all.

This is were ABR kicks in. You specify an average bit rate you want to achieve and the encoder distributes the bits it has available. Those bits it saves on black pictures it spends on complex pictures. It is a compromise between quality and size predictability.

8.2. Global Motion Compensation (GMC)

Global motion compensation (GMC) helps in zooming (bringing the camera closer) and panning (swiveling the camera), if the objects remain static in type and only change in size and position within an image. Activation of GMC is advisable if nature films or landscape documentation are to be compressed.

8.3. Letterboxing

Perhaps you've noticed that video is sometimes surrounded by black bars prior to encoding. This process is called letterboxing. It can help to maintain the correct aspect ratios for some players. Others use the black bars for displaying subtitles. The disadvantage is that these bars enlarge the picture and therefore require more bandwidth to encode although completely black parts shouldn't need much space - but the transition between the picture and the black bars does need a lot of bits that could be used elsewhere.

8.4. P/I/B-Frames, GOPs

This was taken from doom9's Forum:

This is a quick explanation of an MPEG "GOP", or, "Group Of Pictures".

GOP:

• begins with an "I" frame, followed usually by a number of "P" and "B" frames
• each GOP is independent: all frames needed for predictions are contained within each GOP
• GOPs can be as small as a single I frame, or as large as desired, but usually no more than 15 frames in length for MPEG2. With MPEG4 GOPs can be as large as the maximal keyframe interval (usually 200 - 300 frames). Most codecs permit GOPs of unlimited length.
• The longer the GOP, the more efficient, but less robust the coding

I frame:

• "Intra-coded" frames : average 7:1 reduction.
• like JPEG, every video frame is broken into blocks of 8x8 pixels of Y, R-Y, and B-Y
• blocks are grouped into "macroblocks" of 16x16
• The standards say that macroblocks are grouped horizontally into slices which have similar average block levels although real life codecs don't necessarily follow that rule.
• Again the standards say that multiple slices form a frame, and that these frames are the resulting "I" frames, although codecs don't really follow that either.

P frame:

• P frames are predicted based on prior I or P frames plus the addition of data for changed macroblocks.
• average about 20:1 reduction, or about half the size of I frames

B frame:

• Bidirectionally predicted frames based on appearance and positions of past and future frames macroblocks.
• B frames require less data than P frames, averaging about 50:1 reduction.
• B frames require more decoder buffer memory because 2 frames are compared during the reconstruction process.
• B frames also require manipulation of the coding order: frames moving from the coder to the decoder are NOT in presentation sequence.

For every macroblock in a P frame the encoder decides whether it already knows this block from the preceding frame or whether it's completely new. In the former case it only encodes the differences (INTER mode). In the latter case it encodes the whole macroblock (INTRA mode).

The situation for B frames is like this: "I don't know that block" (INTRA mode), "I know that block from the preceding I or P frame (backward mode)", "this block looks like the block in the following frame (forward mode)" or "this block looks like a mixture of the blocks from the preceding and the following frames (bidirectional mode)".

MPEG1 GOPs are always "IBBPBBPBBPBB" (this is the order in which the frames are displayed, not necessarily the order in which they were encoded/saved). MPEG2 GOPs look like that as well. They may as well contain three B frames. The DivX5^? MPEG4 codec only uses "IBPBPBPBPB" because it's easier to implement and because B frame support for MPEG4 codecs is rather new. You may expect to see more B frames in the future.

8.5. Quarter Pixel (QP)

Quarter pel or quarter pixel affects the precision in filtering of macro blocks. DivX^? 4 works with half pel (1.5, 1.5); 1.25, 1.75, etc. are possible starting with DivX^? 5. Conventional division of an image into macro blocks is refined on the basis of integers - 16x16 or 8x8 - using the supplementary information from what are known as virtual blocks. This allows the motions of objects in images to be reproduced more realistically.

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9. Changelog

TODO list:

• Add chapter about subtitles and letterboxing
• Add chapter about DVD -> (S)VCD transcoding
• Add more documentation about what to clip when you have letterboxed material
• Add information about using DivX5^? despite its inferior quality because a lot of people want to use it :)
• Add chapter about including multiple audio tracks in an AVI

August 23, 2002:

• Changed the mencoder command line from -nosound

  to -oac copy.

• Added a hint about avidemux in the 'tricks' chapter.
• Added -m to vobcopy's command line.
• Finally changed my recommendation from vqmin=1 to vqmin=2 as lavc might produce buggy files with 1.
• Fixed a lot spelling mistakes (thanks to ispell :)).

June 3, 2002:

• First translation of this guide into German by Nicolai Lissner

June 3, 2002:

• Changed mencoder ... /space/*vob to cat /space/*vob | mencoder ... - for the audio processing (thanks to E. Kevin Hall).
• Added a note about region codes as they are a source for cryptic error messages during the ripping process (thanks to Bradley Alexander).
• Added a note about transcode's new export module for libavcodec, export_ffmpeg4.

May 5, 2002:

• Fixed the glossary entry about P/I/B frames and GOPs (thanks to Christoph Lampert)
• Added a subchapter about aspect ratios (thanks to Christoph Lampert)
• Fixed some typos (VBR -> ABR and others, thanks to various readers who reported them)
• Fixed mencoder command lines for multiple vob files (thanks to Christoph Terhechte and Frank Otto)

May 3, 2002:

• Fixed vpass=1 which should be vpass=2 in several places (thanks to Markus Liebl)

May 2, 2002:

• Added explanations for transcode's command line options
• Added warnings about scaling pictures up (thanks to Peter Schuller)
• Added vqmin and vqmax to the recommended options for libavcodec (thanks to Peter Schuller)
• Added some links, fixed some typos

May 1, 2002:

• Added explanations about audio and video codecs
• Fixed a lot of grammatical errors (thanks to Doug Shea)
• Added this changelog ;)
• Made two versions available - one big file or or file per chapter
• Guide available as one tar ball for download
• Provided an explanation and some sample images for interlaced video (thanks to Jason Smith)
• Started a glossary (thanks to Robos, creator of vobcopy)
• Added some links, fixed some errors

April 30, 2002:

• Initial release

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10 About

This guide was written by Moritz Bunkus. I'm a 24 year old student of computer science at the Technical University Braunschweig, Germany. Obviously I have too much spare time to waste :-)

This is the first release of this guide. Please provide feedback - what did you like? What did you dislike? Where should I provide more information? Please also correct mistakes that I have made. And please tell me about programs / tools / scripts / documentation that I've missed. I can be reached via email: moritz@bunkus.org

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