Understanding Frame Rate per sec and megabits per sec

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    • #85506
      Avatarcollie581
      Participant

      Hi I am new to this and I am trying to work out things I don't understand.  As I understand it the megabits per second relate to the quality of the image being recorded and the frame rate per second is the number of individual frames recorded each second.  If the camera is set to 30mbps and the frame rate is 25fps I should get a certain quality of image.  Now if I change the frame rate to 50fps and leave the quality at 30 mbps what happens?  How is it possible for the mbps figure to stay the same?  Surely it would have to produce twice the number of frames in a second and as a result the quality of each frame would drop.  Obviously my understanding of how this works is incorrect but I would be really grateful if some one could put me right as I can see this pretty basic to being able to get the best I can out of my camcorder.  Thanks

    • #212253
      Avatarmcrockett
      Member

      One thing we need to know is this:  Is it shooting progressive or interlaced video.  When it's shooting 25fps, it's obviously progressive.  But when you switch it to shoot 50fps, is it still shooting progressive frames, or are they interlaced?

    • #212254
      Avatarcollie581
      Participant

      Ah!  Now you have introduced another thing I don't understand!  I think it is interlaced as I see an "i" after the 50.  To let you understand better my camcorder is a Canon XA20.  I can set it to AVCHD or MP4.  When set to AVCHD  24mps or less I can change the frame rate to 50i or PF24.  If I set the movie format to MP4 I can't change the frame rate at all.  The option is greyed out on the menu.  I would like to know what setting I should choose to get the best quality from my camera.  Thanks.

    • #212255
      Avatarmcrockett
      Member

           Aha!  Yes, 50i means that it's interlaced at 50 frames per second.  However, is it really 50 frames per second?  The answer is actually no.  Your 25fps are progressive frames, meaning that you are getting 25 complete still frames every second.  Your 50i means that you get 50 interlaced frames per second.  So in a second, your first frame will be the first line of pixels, the third line of pixels, the fifth and so on.  Your second frame will be the even rows of pixels from the previous frame.  Then the third frame will be a new image, but only the odd rows of pixels again.  Then the fourth frame will be the evens again from the the third frame.  So in reality, 50i is still just 25 images, but you get 50 half images, which is why your bit rate doesn't change.

           As for what settings should you choose, it really depends.  If you're uploading to YouTube, you'll want to use progressive.  If you upload an interlaced video to YouTube, it will be re-encoded to progressive, and not in a very clean manner.  It will basically look like fecal matter smothered in animal carcases.  However, if you're going to burn it to a DVD, and you want smoother motion, interlaced may be the way to go.

           Hope this clears it up a bit for you.

       

      Mike

    • #212256
      Avatarcollie581
      Participant

        Mike thank you. That helps a lot. I think I will just keep it at MP4 24mps for the time being.  Once i I have more experience I will try different settings. 

    • #212287
      Avatarazlabaz
      Participant

      I'm in a similar position having just started to understand the consequences of frame rates and video quality. A clear explanation, thank you.

    • #214834
      Avatarsparkster
      Member

      I am quite a newbie too and I found this Videomaker training video very useful in understanding more about frame rates.

      https://www.videomaker.com/courses/17585-camera-controls-settings-frame-rates

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