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Conversion to DVD confusion

Discussion in 'MPEG-1 and MPEG-2 encoding (AVI to DVD)' started by smarskwid, Dec 11, 2007.

  1. smarskwid

    smarskwid Member

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    Okay, I am a resonably intelligent guy, but I just cannot wrap my mind around the whole aspect ratio, video heigth/width on a PC vs television scan lines thing.

    I like to make homebrewed DVD video discs which I can expect to play on all standard set-top DVD players. The source video for my DVDs is in a variety of formats (wmv, avi, xvid, divx...you name it). The source video also varies in aspect ratio and video heigth/width. The heigth/width is all over the place. I convert all the source video to 352 X 240 MPEG-1, 1800 kbps video bitrate, mpeg layer 2 audio 224 kbps bitrate/48,000 khz frequency. It's basically VCD with DVD compliant audio. I never change the aspect ratio when re-encoding. I keep the same as the source file. Most DVD authoring software accepts such an MPEG as compliant (and I believe that it is technically compliant), and all players I have encountered will play the resulting VOB files. The low bitrate is acceptable as far as my playback tastes go, and it allows me to fit a ton of video on a 4.7 GB blank DVD. I am perfectly willing to trade off quality in favor of DVD video discs with 5 hours of video content.

    Here are my concerns. I concede the lossy nature of my chosen MPEG specifications. But what I DON'T want is to have a distorted picture when playing back the DVD. You know what I mean...where things are either fatter or thinner than they should be. I also don't want to have any of the video cropped out. When I re-encode source video to 352 X 240 MPEG (let's say the original source video is 480 X 360)...is the resultant DVD video going to play back on a standard 4:3 television cropped or with distorted proportions?

    I am aready aware of a few things. I understand that the source video itself may have introduced unctonrollable perversion of the picture, and that the television naturally does not display the complete frame of the MPEG. The perimeter gets cut off a bit on your standard 4:3 television.

    I encode my MPEGs with MediaConcept MPEG encoder. I author my DVD with Ulead DVD Workshop, which accepts my MPEG-1 as described above and does not re-encode but simply builds the VIDEO_TS structure.

    One other thing...can someone explain the difference in encoding to 720 X 480 (which would then have to be MPEG-2, I think) instead of 352 X 240? Does it increase the file size of the resulting MPEG and/or add encoding time? At the low bitrate I use, would one look any better than the other when played back on a television?
     
  2. attar

    attar Senior member

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    If you have a DivX that is 640x480 then it has a SAR (size aspect ratio)of 4:3 (640 ÷ 480 = 1.333, same as 4 ÷ 3).
    This file also has square Pixels (the PAR =1 )

    If you multiply the SAR by the PAR you get the DAR (display aspect ratio).

    When converted to MPEG-1, 352 x 480 , the SAR is 1.467 and the PAR is .909.
    Multiply SAR by PAR (1.467 x .909 = 1.33 which is the same ratio as 4:3).
    The PAR changes because this is intended to be viewed on a TV which does not have square pixels (the PC does)

    When converted to DVD format, the SAR is now 1.5 and PAR is .889 which is still 4:3.
    Why - because that's the DVD standard.

    The VCR is set to output the video to a 4:3 TV.
    The information in the video tells it that it is a 4:3 input and the TV displays it full screen.
    If the input was 16:9, the VCR knows that it is going to a 4:3 TV, and puts the image in the letterbox for viewing.

    You can convert AVI to half D1 and it processes faster.

    DVD format is 720x480, which is more pixels than half D1, so the amount of detail in half D1 has to fit in the full frame.
    It cannot add detail that it doesn't have, so the picture is degraded.

    PS the information is my understanding of aspect ratio.This, after agonizing through numerous, simple (God help us) explanations.
     
    Last edited: Dec 11, 2007
  3. MysticE

    MysticE Active member

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    Mpeg layer 2 audio is not DVD compliant for NTSC spec.

    The audio data on a DVD movie can be PCM, DTS, MPEG-1 Audio Layer II (MP2), or Dolby Digital (AC-3) format.
     
  4. smarskwid

    smarskwid Member

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    Agreed; I meant mp2. Sorry.
     
  5. mistycat

    mistycat Active member

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    With MainConcept, this bitrate calculator will give you the proper Aspect Ratio to use: http://www.tomzavodny.cz/program/bitcalc/index_en.php Input your source resolution at #8, select the target resolution (NTSC/PAL), DVD=720 x (480/576) and this will give you a resize resolution to use in MainConcept at crop and scale, make sure to untick keep proportion there (only) and encode as 4.3. To encode as 16.9 (smaller letterboxing), increase height by 1.33.
     

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