Lair Of The Multimedia Guru

March 2, 2006

The AVI container file format

Some people love it some hate it, but no doubt it is probably the most common container format for videos though its usage is declining as supperior formats like matroska, nut and others gain popularity

Why is/was AVI so successfull

well theres no way to awnser this with certanity, we can only guess

  • allows storing of audio and video encoded with almost any codec without complicated codec specific hacks (ogg is probably the most common example which fundamentally failed here, but mpeg-ps/ts has the same issue, and .mp4 is thanks to the standard comitees also full of codec specific hacks, which ironically wouldnt be needed at all)
  • simple muxing and demuxing (quicktime/.mov/.mp4/mpeg-ps fails here)
  • no known software patents (asf/wma/wmv fails here)

AVI the format vs. reallity or why some developers hate AVI

The biggest problem with AVI at least in my humble oppionion is not some technical limitation of AVI but the fact that many encoders (muxers actually to be precisse) generate incorrect AVI files which violate the AVI spec from MS, that again means that players need to be much more complex to deal with all these variations …
some examples of such variations

  • putting all the audio in a single chunk, yeah try to implement seeking …
  • having an index which doesnt match the chunks, and just to be sure dont set the header flags which indicate that the index must be used
  • putting several variable sized frames into a single chunk
  • or the very common way of storing mp3 in AVI, simply cut the mp3 frames completely randomly and then scream “avi doesnt support vbr mp3” (hi virtualdub and clones)

Common missconceptions about AVI

AVI doesnt support variable bitrate audio
This is simple nonsense, there are 2 ways to store frames (audio/video/…) in AVI, the first is to set sample_size to the size in bytes of a packet and store an integer number of such packets per chunk, the second is to store 1 frame per chunk and set sample_size=0 and thats what has to be done for variable size packets it works with audio as well as video. Now what some encoders actually do is they set sample_size=1 and then chop the audio stream up at random, this surprisingly works with cbr mp3 but not vbr mp3 but its totally wrong even for cbr mp3
AVI doesnt support variable framerate
again this isnt true, storing variable framerate in AVI is not efficient but it is possible and its not a hack and it doesnt stretch the AVI spec, its fully suported, you just set the framerate to the least common multiple of the rates you want to use, and use 0-byte chunks for “skiped” frames
storing variable samples per packet audio in AVI (vorbis)
while this is still possible, this stretches the spec somewhat, sample_size must be 0, now there are 2 ways to store things, 1. the one with large overhead this sets rate/scale to a common timebase (gcd of samples per packet/samplerate) and then adds 0 sized packets to keep the timestamps correct, 2. the one with small overhead, this sets rate/scale to (lcm of samples per packet/samplerate) and packs several packets intoeach chunk which isnt really allowed but well …
AVI has 24byte per chunk overhead, ODML-AVI has 32byte per chunk overhead
well ODML-AVI without the old redundant index has just 16byte per chunk overhead and if you stretch the spec like some encoders successfully do by having chunks and index missmatch then you can get away with 8byte per chunk overhead, and yes you can still seek in this
Filed under: Container Formats — Michael @ 23:53

7 Comments »

  1. How about B-frames, or more appropriately, out of order frames (IBBPBBP)? Is it a misconception that AVI can not handle those?

    Comment by Multimedia Mike — March 3, 2006 @ 5:20

  2. My biggest pain with AVI is inability to seek/play in an incomplete file. The MPlayer tools ‘avi-fix’ and ‘-idx’ barely make up for this, but not as well as you’d like to…

    – ods15

    Comment by ods15 — March 3, 2006 @ 14:49

  3. BTW, I’d hardly say nut is gaining popularity… :) I think 90% of all nut files in the world are probably in my hd…

    Comment by ods15 — March 3, 2006 @ 15:57

  4. Thanks for the write up.
    How do you deal with AviStreamHeader::dwStart parameter? Is it in milliseconds or frames?

    Comment by denis — June 27, 2008 @ 8:40

  5. With what program can read AVI conatainer file ???!! Pls HELP

    Comment by Niki — March 21, 2009 @ 0:18

  6. What program can read AVI conatainer file ???!!

    Comment by Niki — March 21, 2009 @ 0:19

  7. hellow,
    Please advice me whether i will be able to store AVI files from Panasonic 50 DVC pro camera? @ 50mbps ? which editing software will help ?
    thx

    Comment by peekay — June 26, 2010 @ 5:40

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