Mixing multiple microphones for hybrid meetings

Behringer MX400 as summing mixer

During the last 1.5 year I have been working mostly from home, as all of my direct colleagues. Initially it took us some time getting used to doing our group meetings online, but by now we know how to make those pleasant, inclusive and efficient. Now that many people are vaccinated, we expect/hope that we’ll soon be able to get back to the university for work. However, there are a few aspects of online meetings that I value and hope to can maintain. The travel time is much less, making it easier to quickly join a meeting that otherwise would be held on the other side of campus. It is rather trivial to have people join from abroad, e.g. previous colleagues that want to keep their connection and contribute to the Donders knowledge and culture. Everyone can share their screen much easier. The chat is used to post background material, links to relevant papers, etc. Consequently, I expect that we will not all of a sudden switch back to in real-life meetings, but rather we will have a (possibly infinite) period in which some people attend in real-life, and others online.

In our MEG meeting and the hackathon we have experimented with different aspects of hybrid meetings and documented our findings. We quickly learned that to ensure lively discussions, real-life and online attendees should be able to hear each very well. Spontaneously talk between live participants is easy, but the online participants should be able to hear everything without extra strain and be able to chime in.

One of the core components to support hybrid meetings in the DCCN meeting rooms is a Meeting Owl, which is a smart 360-camera combined with a directional microphone and a speaker. The experience with that is overall quite OK, but its microphone is simply not so good. When listening to meetings from home, the in real-life attendees are more difficult to hear and understand than the other online attendees. Using my experience of setting up a good audio system at home, I started thinking about and planning some improvements to the audio in our DCCN meeting rooms.

The microphone is a very important component to pick up the sound of all in real-life attendees. The “Oval Office”, one of our most used meeting rooms, is suited for about 20 people around an elongated table. We tested a Superlux ECM999 omnidirectional microphone during a meeting in that room and with online participants: according to the people online it was considerably better than the built-in microphone of the Owl. Hence we decided to go for two of these ECM999 microphones, one at each side of the table, connected to a Behringer audio interface. Using two microphones results in a stereo signal, but we don’t want to use them in stereo. Although Zoom supports stereo sound, it is not enabled by default and therefore prone to user errors. The setup in that room should also be compatible with other online platforms, such as Teams, Skype, Jitsi, and other web-based systems. These do not support stereo at all, since they are built for mono microphone input from a webcam or laptop.

Behringer UMC204HD with two microphone/line input channels

Connecting two microphones to the first two channels of the audio interface results in stereo: the 1st channel is audible as the left channel, the 2nd channel as right. At home where I use this interface, I can mix and pan the channels with specialized software such as LadioCast, or a DAW such as Ableton Live. However, the setup at work should be robust and easy to use for other colleagues that are less computer- and audio-savvy. Hence, I figured that the signals should be mixed to mono using an analog solution. Note that I did consider more sophisticated audio interfaces from Focusrite, MOTU, and Steinberg. These have built-in DSP processing that can be configured for effects and mixing. However, the use of specialized software is something I wanted to avoid, and my online explorations of the reviews and manuals pointed in the direction that the DSPs affect the analog outputs of these interfaces but that the digital audio towards the computer is not mixed but remains as separate channels. That make sense for the normal/intended usage of those interfaces, as you would want to record all channels and do the mixing and post-processing in DAW software.

Back of the Behringer UMC404HD with the 4 inserts on the right.

To implement the analog stereo-to-mono conversion, I used the inserts that are offered on the back of the Behringer UMC404HD or the UMC204HD. These inserts use 1/8 inch TRS jacks, where the tip is the “send”, the ring is the “receive” and the sleeve the shared ground. The multi-channel (aka stereo) signal can be converted to mono by feeding the “send” of channel 1 and 2 to a small line-level mixer, and feeding the mixed or summed signal back to the “receive” of both channel 1 and 2.

Behringer MX400 inside view

Specialized summing mixers can get quite expensive, but any line-level mixer should be able to do the job. I opted for a 4-channel mono Behringer MX400 that I still had lying around. Rather than ordering or making specialized insert cables and a output Y-splitter to get the mixed signal back into the inserts, I modified the MX400 mixer. It has five female jacks (4 mono inputs, 1 mono output), and those happen to be TRS jacks with only the tip and the sleeve connected and the ring not in use.

By connecting the tip of the output (on the right in the photo below) to the ring of the 4 inputs, and by using stereo patch cables between the UMC404HD inserts and the MX400 inputs, the mixed/summed output is presented back on all channels of the audio interface just prior to the ADC. Consequently, multiple microphones can be connected to the UMC404HD, but the computer will receive the same summed/mixed signal on all 4 digital channels. The online meeting software (Zoom, Skype, etc) will use the first channel of the audio device that you select, which now contains the analog sum of all connected microphones.

Behringer MX400 modification
Modification to the MX400 to send its output back to the inserts; now to be used with stereo patch cables

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