Preparing videos for FutureLearn courses

January 28, 2022

This week we started up our new online course, Motion Capture: The Art of Studying Human Activity, and we are also rerunning Music Moves: Why Does Music Make You Move? for the seventh time. Most of the material for these courses is premade, but we record a new wrap-up video at the end of each week. This makes it possible to answer questions that have been posed during the week and add some new and relevant material.

To simplify making these wrap-up videos, I am this time around recording them with my Samsung Galaxy s21 Ultra and a set of Røde Wireless GO II microphones. Time is limited when making these videos, so I have decided to quickly trim the files with FFmpeg instead of spending time in video editing software.

I have started shooting videos in 4K, not necessarily because I need it right now, but all my equipment supports 4K these days, and it feels more future-proof. However, FutureLearn does not like 4K and is rather picky about the files to be uploaded:

File format: .mp4 / .mov / .m4v
File size: up to 5GB
Codec: H264
Frame rate: 25 fps
Bit rate: min 2 Mbps constant bit rate
Sound: AAC 44khz stereo

So how do you go about creating such files? Well, FFmpeg comes to the rescue again:

ffmpeg -i input.mp4 -vf "scale=1920:1080 , fps=25" -ar 44100 -b:v 8M -minrate 2M input.mp4

The one-liner is relatively self-explanatory. First, I apply a video filter that scales down the video to 1080p and reduces the framerate to 25fps. Then I specify that the audio should be reduced to 44100 Hz. FutureLearn wants a bitrate of 2 Mbps but does not specify a preferred bitrate. I decided to go for 8 Mbps, the suggested bitrate for 1080p uploads to YouTube. I added a minimum bitrate of 2 Mbps at the end, but I don’t think it is necessary since the bitrate used for MP4 files is constant.

All in all, this means that I can do the complete video editing with two simple one-liners, one for trimming the file and the one above for converting to the correct format. That way, I should manage to create two such wrap-up videos each week for the coming weeks.