Compressing

Discussion in 'Digital Video' started by MLO, Oct 30, 2008.

  1. MLO macrumors member

    Joined:
    Dec 31, 2007
    #1
    Hi,

    I taught myself FCP - what a beast. Could someone dump down the concept behind compressing, I feel like I am in math class, never was a good student of math.

    Especially for getting a HD video after editing down to 10 mb for a website.

    Thanks
     
  2. -DH macrumors 65816

    Joined:
    Nov 28, 2006
    Location:
    Nashville Tennessee
    #2
    I assume you meant "dumb down" the concept. Compressing video usually means transcoding it to another codec, typically with a reduced resolution and sometimes also a reduced frame rate (DVD-Video being the exception). The resulting file will usually have a much lower data rate, depending on the intended method of delivery. For example; a video file intended for web viewing will need to have a much lower data rate than a video file intended for DVD.

    Compressing also means that you're throwing away data. Since standard video is made up of roughly 30 frames per second (NTSC; 25fps for PAL), the resulting native file may be rather large. Using DV as an example, an hour of DV footage requires about 13.3gb and has a sustained data rate requirement of about 3.6mbps. That is too large and demanding for web or even DVD delivery. So you have to compress it according to how it will be viewed.

    Say you've made a 30 minute edit of DV footage (6.65gb file size). If you want to make a DVD-Video, it will have to be converted to MPEG-2 video and AIF or AC3 audio. MPEG-2 only has one full frame (known as the I frame) for every 15 original frames. The 14 frames following the full frame only contain the information that has changed since the preceding frame. All the information in the frame that didn't change is thrown away in the frames following an I frame. That isn't exactly accurate, but rather a brief overview of how it works.

    For video that is intended for web viewing, first you'll need to decide which codec to use. There's Flash, WMV, h.264, DiVX, MPEG-4, MPEG-1, Real Media and others.

    -DH
     

Share This Page