“The black hole”

"The black hole" or: Dealing with the black screen in Zoom

“Good! Much better than I thought at the beginning! Only always this black screen in Zoom.” is what you often hear TUHH teachers say when you ask them how the digital summer semester 2020 went. We asked ourselves:

  • What reasons do students have to start their video or not?
  • Which technical and data protection aspects are relevant from the point of view of the TUHH Computer Centre?
  • How do teachers deal with the “black screen” in Zoom?

Selected sound bites shed light on this topic.


Georg Spieß (1st Chair in ASTA) and Mareike Wendelmuth (University Policy Officer in ASTA):

1. what is the advantage of the screen being switched off for the students?

“In our view, there are three key reasons for students to have their video turned off:

  1. Showing the private premises and thus the private circumstances of one’s life is a strong intrusion into the privacy of the individual student. Because even otherwise we decide very consciously who we give an insight into our private living space. In contrast to most students, teachers have the opportunity to record their events outside their private rooms, for example in the office. Unfortunately, a virtual background is often not possible with the technical conditions of every student. In addition, it is not possible to completely prevent the recording of transmissions.
  2. By switching off the video, it is also easier to follow the event on a mobile phone, e.g. on the train or when technical equipment is not available.
  3. The transmission of the video requires a higher data rate, which is only available to a limited extent. This can make it difficult to follow the event smoothly.

2. what disadvantage does the switched off screen have for the students?

“A lack of facial expressions and gestures reduces communication. This can significantly affect cooperation and the students’ sense of community. This can also lead to the demotivation of the teachers, because they also lack resonance.

3. should teachers ask students to turn on their video?

“Under no circumstances should students be forced to turn on their cameras. However, they may be encouraged to do so and set an example. Sensitive handling and understanding of individual circumstances is to be expected from everyone, especially the teachers.

4. which solution related to turning videos on or off do students prefer?

“From the point of view of mutual consideration, we think it makes sense, especially in small group work, to turn on the cameras to improve mutual communication. In the current situation, many of us lack direct contact with other people. Video communication can at least partially compensate for this. Despite these advantages, the personal freedoms and needs of each individual must always be respected. For larger events, the advantages of mutual video transmission are only partially apparent to us. From our point of view, good and active moderation of the various (participants’) contributions is more important for the quality of the course.

Dr. A. C. Wawrzyn (TUHH Computer Centre):

1. can the zoom network withstand the switching on of all participant videos in courses?

“I am not aware of a load test at TUHH with a maximum of 300 participants, but Zoom somewhat circumvents the problem of too many video streams by only showing a limited number on one page. At smaller resolutions, for example, this is 4×6 and at larger resolutions 7×7, which are arranged next to each other as small tiles. You then have to “scroll” to the other participants that you cannot see at the moment. Halfway up-to-date client hardware can manage this. The Zoom servers can also cope with 100+ participants.”

At the beginning of the semester, students were asked to switch off their video in order to save bandwidth. To what extent does the TU Hamburg have an increased data volume due to switching on the video function?

“The bottleneck at the start of the semester was less the bandwidth of the connection to the DFN and more the load on the VPN servers. The latter were particularly stressed by the high number of teleworkers. It was common to connect to the TUHH via VPN in order to have access to resources and then video conferences were started. As a result, these video streams were also tunneled through the TUHH VPN servers and only then decoupled to the internet. At the beginning of the semester, the RZ reduced the problem for users of the Cisco AnyConnect client by making split-tunnelling settings that decoupled known IP address ranges from Zoom Cloud servers before the VPN tunnel. With other VPN clients (e.g. native under Mac or Linux) the problem persists. Here it helps to terminate the VPN or to reduce the throughput by deactivating the video stream. How the utilisation of the bandwidth to the DFN will behave in the hybrid semester is still open. The lecture recording system Mediasite is expected to stream significantly more to the internet. In addition, there are more staff and students at TUHH again, so an increase can be expected compared to a pure attendance semester, where most of the action takes place on the campus network.”

3.what is the data centre’s preferred solutionfor turning video on or off?

“From a data protection point of view, it would be good in any case if the individual participant starts without his or her own video and chooses to share it (or not). The rest depends on the use case: In a smaller group, videos are incredibly helpful for the flow of conversation because you can raise your hand visually when you want to say something or see the reaction of the other person. That’s when you should turn them on. In large groups, but also especially in lectures, it is actually enough to see the speaker / lecturer. Ultimately, mini-tiles have hardly any added value and there are definitely bottlenecks with the providers (peering between Telekom and DFN was poor in some cases at the beginning of the pandemic and led to packet losses). Here you can save resources and switch off the video.”

Haibo Ruan (Lecturer, Institute of Mathematics):

“Teaching for me is something interactive and sharing. Perhaps especially in the age of digital teaching, such qualities as interactivity and a sense of community in teaching and learning activities are more in demand than ever, otherwise you easily “lose” the learners. I always explain to my students at the beginning of my course that I value such an interactive and shared learning atmosphere very much and I want to design our course together with them so that you can learn really well and have fun with it. However, this can only be achieved if all participants (including the lecturers) have the feeling that they can talk to each other at eye level. I then ask for the sound and picture to be released by all participants. This has always worked well so far, most of them happily join in.”

Christian Kautz (Head of the Didactics of Engineering):

“For me, being able to see the students on the screen was less important last semester than I had originally expected. It was certainly a completely new experience to deal with the same people for weeks without knowing what they look like. But at the same time, I associated a certain voice with the names of some of the students, so they did have a certain identity or reality for me. In the breakout rooms (each with three to five people) I repeatedly asked students to turn on their cameras. Some did, others did not. But since we were working with concrete texts or simulations, screen sharing was more important than seeing faces anyway. As long as some kind of interaction was visible, the need to have a picture of the person was rather secondary for me.”


In summary, it is important in teaching with Zoom,

  • Be aware of and consider the different perspectives of the pros and cons of turning the camera on or off,
  • engage in explicit dialogue with students at the beginning of a course on the topic,
  • Zoom events can be designed in an activating way using small methods so that students can build subject and personal competences – with or without camera release. For example, questions and feedback from students can be collected in chat, small group work can take place in breakout sessions and Zoom surveys can be used for the method of peer instruction.

Thus, teaching in Zoom cannot be a “shouting out words & absorbing them in the black hole”, but a positively designed, virtual teaching/learning process.

We thank all contributors for their sound bites and Joseph Rüffert for his support with this blog post.

Researched: Joseph Rüffert, summarised: Ulrike Bulmann

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