Tutorial - Splitting a large file using Avidemux

by Musicman August 19, 2012
In this pictorial howto, I show how to split a large video file using Avidemux in Ubuntu Feisty. Screenshots aplenty!


Avidemux is "a free video editor designed for simple cutting, filtering and encoding tasks. It supports many file types, including AVI, DVD compatible MPEG files, MP4 and ASF, using a variety of codecs. Tasks can be automated using projects, job queue and powerful scripting capabilities."

Avidemux is available in the Ubuntu multiverse repository, but is also available as a binary download for Mac, Windows, Gentoo and Mandriva, Debian and as source via svn.

Why would I split my video files?

There are a number of reasons to split your files into several smaller ones.
  • At the moment, engagemedia.org cannot take files that are too large. This is actually a bug that we are working to fix.
  • Engagemedia.org auto transcodes your file into flash for embedded viewing. While we love viewing vids in browser, we don't think that anyone want's to watch 1hr of video in browser. Am willing to be wrong on this point, but I would imagine it would be more desirable to watch shorter things in browser, and those that want to watch the whole thing can download full video for watching in VLC (or other media browser of choice).

How to split your video

1. Start Avidemux:

Start Avidemux

2. Open the file you wish to make smaller. File->open->

Select the file

3. Your loaded file will show you the first frame. i've chosen a film that was sent to us for encoding and uploading called EarthDream. It goes for about an hour:

Loaded file

4. We want to find all the black frames so that we can edit our video in a natural break in the video - we don't want to be cutting people off mid interview! To do this, we go to Tools->Scan for black frames:

Finding black frames

5. You will be asked where to save the output of this process. It's a basic text file which is dead easy to understand:

save to text file

6. This will churn away, depending on the size and content of your video:


7. Ok, now we are getting to the nitty gritty. The output can be seen in the image below. As you can see, it's fairly intuitive. You need to peruse the black frame output and decide how long each part of the film is going to be. I've decided to make them about 20 minutes long each. You can see above that there is a black frame at roughly 10 minutes, but after that it goes (roughly) 23 minutes, 27 minutes, 47 minutes, 54 minutes and the end, 65 minutes. So, I've decided to go 23, 47, end - splitting it into 3 roughly equal bits:

text output

8. This is where we actually cut and save our video up. The tools we use are at the bottom of the screen, here are the main parts you will need highlighted:

The Avidemux tools...

  • On the far left we have the "Frame" box. This tells you which frame you are up to at any point in time. You can also jump to another frame, when the video is stopped, by typing in the frame number and hitting return. As you can see, it also shows hte total number of frames available (97553 in this case).
  • In the middle, we have our Marker tools, imaginatively named A and B. "A" is the first frame of you "selected area". "B" is the last frame of your "selected area". There are two things to note here:
    1. If you set A to be a frame after B, Avidemux will auto correct - the value in A will move to B and the A value will become B. There is a better explanation of this on the avidemux wiki.
    2. Once you have set the Markers, and you save the video file, the frames from A to B-1 will be saved.

    You can't see it in the text output image above, but whenever there is a mid-video black frame, there are at least 3 in a row (eg: frames 35540, 35541 and 35542). We will come back to the significance of this shortly.
  • On the far right we have the positions at which A and B are marked currently.

9. By typing 35541 into the Frame box (I've put a red dot next to it), and hitting the B marker button, we can set what we want to be the first part of our video. We are setting the cut to the middle black frame, because we know that it will not be included in this part of the vid:

10. Now, by choosing File->Save->Save Video, we can save the selection we have just marked out with the A and B markers:

save the video selection

11. You will then be asked to name the file - call it something obvious and sensical: in this case I've chosen the simple EarthDream_part1.avi:

name the file

12. Sometimes, but not everytime, you may be asked about smart copies and Q Factors. These both have to do with the way that the original encoding took place. Click yes to smart copies. The Q Factor can be thought of as a "Quality Factor" - the smaller the number, the bigger the file, and better the quality. The larger the Q Factor, the smaller the file and the worse the quality.

smart copy?Q Factor

13. From here it get's very simple. Once the save has finished, you back down to the Frame box, and put in the number of the next black frame. Then hit the A marker button (yes, that says "A"). You can see the result of hitting the A marker below in the far right hand corner - as we mentioned above in step 8 (note 2), the A frame was after the B frame, so the value of B has been moved to A, and the new value of B is now the value that we want.

next selection

14. As far as editing goes, that's it. Repeat steps 10-12 as often as you need.

15. Ok, now for the final trick - the screenshot/thumbnail to put with your film(s). Avidemux is also a media viewer, so press play and watch until you find something that will best represent your film. When the film is stopped or paused, you can step through the frames using the button's highlighted below to find the frame that is best:

16. Then File->Save->Save jpeg image and it will save that frame as a jpeg. Don't forget to do a new one for each part of your film - to make it easier to distinguish them:

save as jpeg"

Voila! we are ready to upload our videos!