This post is part of a series on designing a wireless microphone system for hybrid online meetings, i.e. with some people present in person and others present online. See also the previous and next post in this series.
Pondering about wireless microphones for a classroom or for a larger scale conference/meeting room, I identified some requirements:
Imagine that you would have a number of rechargeable clip-on microphones that all transmit their audio wirelessly to a single base station. The base station could also act as a charging station, i.e. when not in use the microphones would be docked in it. The base station would be connected to the central laptop/computer as if it is a single external microphone. Bluetooth lapel microphones exist, but Bluetooth does not allow connecting a lot of microphones to the same computer. Proprietary radio systems such as used by audio companies like Sennheiser are not DIY friendly. There are easy to use RF modules, but those are more suited for IoT applications and not streaming audio. This actually sounds like an ideal application for a 5G device-to-device network, but components for those are not easily available yet.
I think wifi would have enough bandwidth and would be able to support a large number of clip-on microphones: a dedicated wifi access point has no problems dealing with 50 to 100 connected clients. It can be a dedicated/closed network since there is no reason to have the microphones connected to the internet, except perhaps to receive software updates. Also, other devices such as laptops don’t have to connect to this wifi network, except when a web interface is considered for configuration and audio mixing (see below).
For the clip-on microphones, I am considering using an ESP32 connected to an I2S MEMS microphone and a small (e.g 500 mAh) LiPo battery. These can be housed in a custom 3D printed case with a clip to attach it to the clothing, and a hiddeon connector at the bottom for charging in the docking bay. The ESP32 needs firmware that sets up and maintains the wifi connection, processes the I2S audio, does threshold detection and, when loud enough, transmits the audio over wifi.
For the base or docking station and wifi access point, I am considering a Raspberry Pi Zero W combined with a HifiBerry DAC+ Zero. The line-level output of the HifiBerry would be provided on a standard 3.5 mm female jack, such that a standard TRS or TRRS jack-to-jack cable can be used to connect the base station output to the laptop microphone input. The base station requires software that receives the (UDP?) wifi input streams of all ESP32 modules, normalizes them, and mixes them into a single audio output.
Some additional features I was thinking of for the base station are a volume indicator, e.g., a Neopixel that turns green-orange-red). Furthermore there could be a mute button for every microphone, a solo button (muting all but the one that has been selected), and knobs to adjust the volume level for each channel. These could be implemented using physical buttons placed next to the charging bays in the dock, but also through a web interface.
Usability in a classroom or meeting room by people that have no technical understanding of the system is also crucial. If the base station would have physical mute buttons and/or volume knobs for each of the channels, the clip-on microphone modules must be clearly labeled/numbered. Possibly they could all be 3D printed in a different color and the corresponding charging bay (with the knob/button next to it) in the base station would then have the same color. The individual microphones don’t have to be recognizable if audio mixing or muting is not needed.
Considering that this system might be used at the same time in multiple neighboring classrooms, the wifi signal amplitude should be strong enough to have within classroom reception, but as weak as possible to not interfere between classrooms and with the regular internet wifi.
I think that the base station could be made for about 50-100 euro (hardware costs only, and mainly depending on whether it has buttons and knobs for each channel) and that each clip-on microphone can be made for about 10-15 Euro. For a system comprised of 30 clip-on microphones to accommodate a complete classroom that would amount to €350-550. For a smaller meeting room system with 8 clip-on microphones, it would be around €150.
There is quite some development and testing needed for this. For prototyping I have ordered an Adafruit HUZZAH32 and a I2S microphone breakout board from a local and fast (and also more expensive) supplier and some comparable but cheaper components from Aliexpress. Let’s start with a single microphone, similar to this or this baby monitor. If I can get that to work with a Raspberry Pi, the next step would be to check how well that scales to a larger number of ESP32 microphones.