Compressing a .mov file?

Relationship between file formats for video (and audio)

• Hi I am struggling to understand the relationships between the different file formats for video (and, by association, audio). I understand that there are different ways of compressing video files - one may use codecs for Mpeg 1, Mpeg 2 or Mpeg 4. I believe that mpg 1 is the basis for VCD, mpg 2 is the basis for DVD and Mpg 4 is the basis for DivX or Xvid. Then you get .avi video files and .mov files. And then all the video files (like .mpg or divx) supposedly have audio files - in a variety of formats. So a DivX file "contains" an .mp3 audio file? My questions are: - are the compression algorithms for mpeg 1, mpeg 2 and mpeg 4 standardised? - are there different ways a particular compression standard can be implemented, for example, mpeg 4 can be used in both DivX and Xvid files? Why would there be a difference? - how does an mpeg, .mpg or divx file relate to an .avi file or .mov file? - If you compress a video to, for example, divx format, is there a separate compression for the audio component and that gets "rolled in" to the divx format? - Files with extenstion .mpg; are these mpeg 1? (but then what about the audio?) or "mpeg 1 for video plus some other format for audio)? I've tried to ask questions to indicate my lack of knowledge! I'm really trying to find out how the different compression algorithms and file formats and extensions relate to one another (e.g. DivX is mpeg 4 compression for video AND mp3 compression for video?). Thanks.

• Answer:

  Well, you said you ask the questions to indicate your lack of knowledge, but it tells me that you are almost there. Here are my answers to your questions. First, MPEG stands for the motion picture experts group. They make a variety of video (and occasionally audio) standards. Like you stated mpeg 1 is vcd, mpeg 2 is dvd (and sometimes svcd depending on a few things), and mpeg 4 is the *basis* for what divx started out as, though divx is now something entirely different. Mpeg is always a standard. Mpeg 1 by definition is compressed in the same way all the time, and can play in a variety of players, for example. VCD is a standard within Mpeg 1 that defines what resolution and bitrates may be used with the mpeg 1 standard to be vcd compliant. Mpeg 2 is similar, and is a video compression standard. Any mpeg 2 player should play any properly encoded mpeg 2 file. Within the mpeg standard there is also audio compression. The mpeg standard dictates what parts of the data file are to contain the audio data. The Mp3's you doubtlessly have heard of is a short name for "Mpeg 1, layer 3", which is to say, a way to encode audio into an mpeg 1 file. A vcd, for its audio, uses mpeg 1, layer 2, which is less efficient than layer 3, but requires less processing to decode, which is why it was used in the past. To answer your second question, yes there are different ways a compression standard can be implemented. For example, .asf files were an early implementation of mpeg 4 made by microsoft. Divx originally was created as a hacked version of microsoft's asf format in order to allow it to contain a higher quality audio stream (mpeg 1 layer 3, or mp3 for short) and slightly improved video. Divx, however, as it has several versions (now up to version 5)has evovled into an entirely different way to encode video than mpeg 4. Xvid, similarly, is a video compression that works similarly to divx, but is encoded using a different algorithm. Also, mpeg can be implemented in several ways. Mpeg 2, for instance can be implemented as DVD format video, which dictates using a specific resolution, bitrate, allowable audio formats, the file organization expected on the disc, encryption / copy protection standards, and so on. SVCD is another standard that uses the same video compression algorithm that DVD does, but expects a different file organization, different resolution and bitrate, different audio formats, and so on. The reason for these differences is that different formats (dvd discs versus cds) and target audiences (internet video users vs home consumers) have different expectations, limitations, and requirements. Since cds can hold less, formats that were designed for them have lower resolutions and lower quality overall. Formats such as divx that were designed to run on a computer and be transferred using the internet, can be more computationally demanding to display, in order to accomidate a lower available bitrate. Your next question, how do video standards relate, is best answered by explaining the avi format. AVI stands for Audio/Video Interleaved, and is simply a standard that specifies how a video file is to explain to windows how its audio and video streams are organized. AVI is a non-specific format, that is, any multitude of audio and video compression formats can be combined together, and the file may still be an .avi file. .mov is similar in nature, but those files are usually only playable by apple's quicktime, as .mov is a proprietary format. Mpeg, on the otherhand, is a very specific format, and there are only a small number of available audio and video compression formats that may be used for a file to be a valid mpeg file. You can even rename a .mpg file to .avi and it will still play just fine in most players, since .avi is really a catch all format. So long as you have the right codec (encoder/decoder program) on your computer, your media players should play any avi file encoded with the same codecs. Divx and Xvid are examples of codecs. To answer your next question, yes a different audio format gets rolled into the divx. The divx encoder dictates which audio formats are allowed and how they are to be inserted into the file in order to be a valid divx file. As with any avi file, there may be audio codecs that are customarily used to encode the audio, but so long as both the creator of the video and the person playing the video both have the same audio codec, just about any audio format will work. Mp3 is a common format that is used. For your next question, customarily, .mpg files are mpeg 1 files, however, this is not required. mpeg 2 or mpeg 4 files may be labeled .mpg, but they will not play in players that only play mpeg 1 files. Sometimes mpeg 2 files are labeled .m2v, but more often than not, all mpeg files, no matter what version, have the file extension of either .mpg or .mpeg. Lastly, some video editing programs will allow you to remove the audio from an mpeg file, giving you two files, a file normally with an extension of .mp2 or .mpa and another file with an extension of either .mv1 or .mpv. The dvdrhelp glossary may help with some of this ( http://www.dvdrhelp.com/glossary ) Search strategy: Poke around at Vcdhelp.com and dvdrhelp.com