Please explain video bit rate further…

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    • #89346
      AvatarGainer
      Member

      I don’t understand your video on “What is bit rate”. I want it explained more. First I need you to define bit rate. And how compressing comes along it? What’s that ‘two frames containing one color, the colors are treated as one color’? Is IT the compressing? And, what if a video has only 1 Mbps bit rate? And, how does increasing resolution while keeping bit rate constant, make the image quality worse? And how does increasing frame rate while keeping bit rate constant, make image quality worse?
      Please explain those….

    • #213649
      Mike WilhelmMike Wilhelm
      Keymaster

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      Hi Gainer-

      Here's an example:

      Let's say you have a 1 second video clip that has a resolution of 10 x 10, is 200Mbps and there are 30 frames every second. (these are ridiculous numbers but it makes the math easy).

      That tells us that the file is 200Mb and frame of video is approximately 6.67Mb (200Mbps / 30frames).

      Let's say you want a file size that is half that, or 100Mb. Now every frame is 3.3Mb. To get the size down, the encoding software has to start taking shortcuts, merging pixels, etc and the image quality gets worse.

      Now let's say you want have the same image quality but instead it's 20 x 20 resolution. Well, that's four times the pixels! If you don't take any shortcuts by merging pixels, therefore keeping the same image quliaty, it makes sense that your file size would be 4x bigger. After all, each frame has 4x the pixels.

      So if you want to increase the resolution to 20 x 20 but keep the same file size (i.e., bitrate), the encoder has to start cutting corners and degrading image quality (that's compression). Ergo, image quality gets worse.

      There are many ways compression cuts corners. Two examples are when several pixels of similar, but still different, colors are next to each other, encoding software may change them all so they're all the exact same color, hoping your eye cannot see the difference. Another way is if two frames come one after annother and are mostly the same, it might discard the data from the second frame and just copy the first.

      Now, I'm not a software engineer, so a true expert might take issue with this explanation, but I feel I've come pretty dang close. 🙂

    • #213650
      Avatarshendrick
      Inactive

      These Xiph.org videos are a great resource to understand digital video (ep. 1),  and audio (ep. 2).

       

      https://wiki.xiph.org/Videos/A_Digital_Media_Primer_For_Geeks

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