Testing Blackmagic Web Presenter

Blackmagic Web PresenterWe are rapidly moving towards the start of our new Master’s programme Music, Communication & Technology. This is a unique programme in that it is split between two universities (in Oslo and Trondheim), 500 kilometres apart. We are working on setting up a permanent high-quality, low-latency connection that will be used as the basis for our communication. But in addition to this permanent setup we need solutions for quick and easy communication. We have been (and will be) testing a lot of different software and hardware solutions, and in a series of blog posts I will describe some of the pros and cons of these.

Today I have been testing the Blackmagic Web Presenter. This is a small box with two video inputs (one HDMI and one SDI), and two audio inputs (one XLR and one stereo RCA). The box functions as a very basic video/audio mixer, but the most interesting thing is that it shows up as a normal web camera on the computer (even in Ubuntu, without drivers!). This means that it can be used in most communication platforms, including Skype, Teams, Hangouts, Appear.in, Zoom, etc., and be the centerpiece of slightly more advanced communication.

My main interest in testing it now was to see if I could connect a regular camera (Canon XF105) and a document camera (Lumens DC193) to the device. As you can see in the video below, this worked flawlessly, and I was able to do a quick recording using the built-in video recorder (Cheese) in Ubuntu.

So to the verdict:


  • No-frills setup, even on Ubuntu!
  • Very positive that it scales the video correctly. My camera was running 1080i and the document camera 780p, and the scaling worked flawlessly (you need the same inputs for video transition effects to work, though, but not really a problem for my usage).
  • Hardware encoding makes it easy to connect also to fairly moderate PCs.
  • Nice price tag (~$500).


  • Most people have HDMI devices, but SDI is rare. We have a lot of SDI stuff, so it works fine for our use.
  • No phantom power for the XLR. This is perhaps the biggest problem, I think. You can use a dynamic microphone, but I would have preferred a condenser. Now I ended up connecting a wireless lavalier microphone, with a line-level XLR connection in the receiver. It is also possible to use a mixer, but the whole point of this box is to have a small, portable and easy set up.
  • 720p output is ok for many things we will use it for, but is not particularly future-proof.
  • It has a fan. It makes a little more noise than when my laptop fan kicks in, but is not noticeable if it is moved one meter away.

Not perfect, but for its usage I think it works very nicely. For meetings and teaching where it is necessary to have a little more than just a plain web camera, I think it does it job nicely.

Trim video file using FFMPEG

This is a note to self, and hopefully to others, about how to easily and quickly trim videos without recompression.

Often I end up with long video recordings that I want to split or trim. One a side note sometimes people call this “cropping”, but in my world cropping is to cut out parts of the image, that is, a spatial transformation. Splitting and trimming are temporal transformations.

You can of course both split and trim in most video editing software, but these will typically also recompress the file on export. This reduces the quality of the video, and it also takes a long time. A much better solution is to trim losslessly, and fortunately there is a way to do this with the wonder-tool FFMPEG. Being a command line utility (available on most platforms) it has a ton of different options, and I never remember these. So here it goes, this is what I use (on Ubuntu) to trim out parts of a long video file:

This will cut out the section from about 1h19min to 2h18min losslessly, and will only take a few seconds to run.

Visualisations of a timelapse video

Yesterday, I posted a blog entry on my TimeLapser application, and how it was used to document the working process of the making of the sculpture Hommage til kaffeselskapene by my mother. The final timelapse video looks like this:

Now I have run this timelapse video through my VideoAnalysis application, to see what types of analysis material can come out of such a video.

The average image displays a “summary” of the entire video recording, somehow similar to an “open shutter” in traditional photography. This image allows for seeing what has been moving and what has not been moving throughout the entire sequence.

Average image
Average image 

The motion average image is somehow similar to the average image, but it summarises the motion images through the entire sequence, that is, only the parts of the image that changed.

Motion average image
Motion average image

What I call a motion history image, is the motion average image overlaid only a single frame from the original video. I typically create such motion history images using both the first and last frames of the video, as can be seen below.

Motion history image, based on first video frame
Motion history image, based on first video frame
Motion history image, based on last video frame
Motion history image, based on last video frame

Finally, I have also created both horisontal and vertical motiongrams of the timelapse video. The horisontal motiongram displays the vertical motion, which in this case is how the sculptor moved back and forth when sitting at the table. The edge of the table can be seen as the “stripe” running throughout the image.

Horisontal motiongram, displaying vertical motion
Horisontal motiongram, displaying vertical motion

The vertical motiongram, on the other hand, displays horisontal motion, that is, how the artist moved sideways throughout the process. Here it is very interesting to note the rhythmic swaying pattern, as the sculptor moved back and forth in what seems to be a periodic pattern.

Vertical motiongram, displaying horisontal motion
Vertical motiongram, displaying horisontal motion

I also have some more motion data, which it will be interesting to study in more detail in Matlab.


TimeLapser-screenshotI have recently started moving my development efforts over to GitHub, to keep everything in one place. Now I have also uploaded a small application I developed for a project by my mother, Norwegian sculptor Grete Refsum. She wanted to create a timelapse video of her making a new sculpture, “Hommage til kaffeselskapene”, for her installation piece Tante Vivi, fange nr. 24 127 Ravensbrück.

There are lots of timelapse software available, but none of them that fitted my needs. So I developed a small Max patch called TimeLapser. TimeLapser takes an image from a webcam at a regular interval (1 minute). Each image is saved with the time code as the name of the file, making it easy to use the images for documentation purposes or assembling the images into timelapse videos. The application was originally developed for an art project, but can probably be useful for other timelapse applications as well.

The application will only store separate image files, which can easily be assembled into timelapse movies using for example Quicktime.

Below is a video showing the final timelapse of my mother’s sculpture: