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Can I get 5.1 surround output using MPEG-4?

Discussion in 'Desktop & Laptop Computers Forum' started by John_N, Jan 20, 2003.

  1. John_N

    John_N
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    Hi

    A quick question. I was wondering if an MPEG-4 coded file would be capable of giving me 5.1 surround? It's a little unclear in the documentation as to whether we have just stereo or the full 5.1 surround?

    Lets assume that my home movie that I have recorded digitally on my machine is already in 5.1 surround format and can be output using the SPDIF output. If I want to compress this to mpeg4, can I still get the 5.1 surround output?

    Any ideas?
    Cheers
    John
     
  2. John_N

    John_N
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    ... Not a single reply... :(

    I've been looking at the mpeg4 spec and it looks like it should be able to support a full surround soundtrack.

    I'm making a custom multimedia jukebox in my study. It currently has some of my CD collection on it and I'm thinking of transferring some of my DVD collection onto it.

    However I would obviously prefer to have the movie 'coded' as an mpeg4 to save HD space.. But this is no use to me if I can't then play it back through my projector and sound system in original 5.1 surround (ie - if the audio is downmixed to stereo for example)...

    Ideas?
     
  3. MAW

    MAW
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    Yes you can. You can't compress the sound track though, AFAIK. As you may know, compressing to mpeg 4 is a long, complicated and involved process. I've got the software, but don't do it often cos I ain't got the time, and the pc is in constant use, mine is the hcpc downstairs and the kids use it for homework & network games as soon as I leave the seat. Processing can take up to 12 hrs or so. You do also lose some resolution, depending on the level of compression you seek. You've checked ou divx.com etc?
     
  4. John_N

    John_N
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    Thanks

    I've got a dedicated server machine that is 24/7 available for number crunching so that isn't an issue for me. I know that it can take several hours though.

    I understood that all the movie (video and sound) lived in the .VOB files. Even if I were to decrypt the .VOBS and put them in my hard drive, they still contain the video and sound.

    Am I correct then that the encoding process basically compressed the video only and can separate the sound track out?

    I've checked DIVX web site and it isn't really clear to me. I guess I'll have to try it out.
     
  5. MAW

    MAW
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    It's not very clear til you start doing it. Even then..... Have you goit broadband? The online help is pretty good, and Gordian knot at least is not troubled by multitasking. You only need the vobsub bit if you want the subtitles, I didn't suss that, couldn't work out what it was for! Never mind....
     
  6. Beastie Boy

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    Hi all. Congratulations on a great forum.

    I thought I would use my first post to try and give some help to others.

    5.1 surround sound is certainly possible with MPEG4. I have encoded many movies using DivX 5 and Gordian Knot. As the output ia an AVI file, almost any audio fomat can be used. I generally use stereo mp3 to save disc space as I don't yet have a surround system.

    The process is quite easy with the feeware that is now available. The best online resource is www.doom9.net but if anyone would like some advice, I would be more than willing to help.

    Cheers, Beastie.
     
  7. John_N

    John_N
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    That sounds promising.
    Do you know if we can specify different levels of compression?

    A normal movie on my multimedia jukebox takes 7GB of drive space. This is being viewed via DVI to the projector (when the DVI cable arrives).

    Obviously any hard drive space saving is a good thing, but also any compression artifacts are going to be magnified on a big screen. Therefore - do you think I could get a reasonable level (even 4gb would be a big improvement) of compression and still have good quality?

    The 5.1 surround soundtrack is good news though. I'm going to have to look at this further.
     
  8. Desmo

    Desmo
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    Jonh, I've Divx'd a few films and, with a 5.1 soundtrack, can get the average film at an extremely good quality (on a 32" TV) down to around 2Gb.
    4Gb is going to be no problem.

    doom9 is THE place to go to find out about all of this.
     
  9. Beastie Boy

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    The highest bitrate that can be specified is 6000kbps (kilo BITS per second). However, depending on the compressibility of the movie, this bitrate will not always be reached.

    DivX has a minimum DRF (detail removing factor?) of 2. This means that the DivX will be compressed roughly twice as much as the source. This means that even at the maximum bitrate, some artifcts may be introduced. However, DivX is a very clever codec. It tries to fool the eye and 'hide' these artifacts in the motion of the film. If you were to freeze frame the movie, the artifacts would be evident, but when the movie is playing they become much less so.

    DivX was designed to compress film into very small files. It can easily fit most movies onto 2 x 700MB CDs (with MP3 sound) and works surprisingly well at 1000- 1500kbps. However, as you are not so limited for storage space, simply crank it up to max and see what you think of the results.

    Bear in mind that DivX files are not resized during playback like 16:9 DVDs, so encode at the resolution that you wish to use for playback ,perhaps no greater than 800 wide though. You can encode at a lower resolution and allow your hardware to supersample the image. I encode between 574 and 672 wide depending on the source. Try to keep the resolution in multiples of 16 for compatibility. If you use Gordian Knot then all this is very easy asnd it also sorts out the aspect ratio for you.

    Cheers, Beastie.
     

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