Ambisonic Microphone
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This project has arisen for the development of the final practice of the subject of Creative Electronics, a subject belonging to the University of Malaga (UMA), School of Telecommunications.
The whole project has been carried out by Rocio Liceras Ramirez y Guillermo Jose Martos Aguilera, students of Electronic Systems Engineering at the University of Málaga
We started from the instructable by DJJules about the Ambialice. Our original idea was to replicate it exactly as it was, until we realized that if we wanted to build a low-cost ambisonic microphone, we would still need an audio interface with at least four channels, which are not exactly cheap. This made the approach somewhat inconsistent, so we began looking for different alternatives to avoid the need for such an interface.
https://www.instructables.com/Ambi-Alice-a-First-Order-Ambisonic-Microphone/
We considered several options, and the most viable one was adapting the design to work with USB interfaces, which can be found at a relatively low price and usually offer one or two inputs. During our search, we found many interfaces that were not compatible with our needs, as we required stereo jack inputs and some form of power supply. We did find one interface that was ideal, as it provided around 5 volts, but its price was high and we would have needed four units, which brought us back to the original problem. For this reason, we decided to discard this option.
After testing several other interfaces, we found some that provided enough voltage for the microphone to be audible, although not comparable to its performance when operating at the ideal voltage.
We attempted to power the microphones externally, but we were unable to achieve this due to the input impedance, although we believe it is quite possible. The necessary schematics will be included in the corresponding section.
As we were running out of time, we decided to proceed without external power and built the microphone in such a way that it could be powered through either a jack or XLR connection. Audio samples with the final result will be included, where it can be observed that the microphone performs ideally with loud sounds, while quieter sounds, such as ambient noise, are affected by an effect similar to a low-pass filter.
Ambi-Alice
As part of our project, in addition to attempting to create the low-cost version, we also replicated the Ambi-Alice microphone.
We didn’t encounter major difficulties during its construction, as it is well documented; however, we did find some links that were no longer available or we looked for cheaper alternatives.
This section is intended to provide the links we used in case someone else needs them.
- XLR conectors
https://a.aliexpress.com/_EJhlEyC
- Colored tape
https://a.aliexpress.com/_ExeMIkg
- Silicone stabilizers
https://a.aliexpress.com/_EwWj0QG
- Microphone capsules
- https://micbooster.com/product/jli-2590a/?v=12470fe406d4
In this last case, we would like to clarify that we chose this website because we are from Spain, and being a UK-based site, we thought we would have fewer customs issues (which we ended up having anyway). Depending on where you are, it might be more convenient to order from a US-based site or from this UK-based one.Nuestra versión
Our Version
As briefly mentioned in the introduction, the main difference between the Ambi-Alice and our microphone lies in the connection cables. In our case, we needed them to be Jack connectors in order to use USB interfaces.
Let’s start from the beginning: our idea was for the microphone capsules to connect to female Jack connectors. Then, the cables connecting to the interface could be of two types: male Jack – male Jack or male Jack – XLR. Therefore, the entire cable setup is designed with this duality in mind.
*See schematic and wiring photos*
As can be seen, no resistors or capacitors are present, since, as mentioned in the introduction, we were unable to power it externally and the voltage provided by the USB interfaces is low, making a voltage divider unnecessary.
Additionally, as observed in the finished microphone, we designed and 3D-printed new parts, such as the microphone handle and the bottom cover.
https://www.mediafire.com/folder/4nohzyjoxkamd/Microfono
*See 3D design photo*
The Jack connectors were purchased at a physical electronics store and are 3.5 mm jacks with a – mm casing.
Finally, the interfaces used were the following:
https://www.tienda.zabalavera.com/articulos/ais-adp-a106-0768
Four of them will be required, in addition to a USB hub. This can be any hub, but to avoid the same mistake we made, it is recommended to get one with the USB ports in a vertical orientation, since otherwise the interfaces may interfere with each other, making it impossible to connect them all.
*See audio interface photos*
We want to reiterate that this version does not achieve the desired quality due to the lack of microphone power. It works perfectly with loud noises; however, for ambient noise, which is one of the main intended uses, the capture is not as expected.
Voicemeeter Potato
Once our microphone was assembled, we encountered a new problem: we have four microphone inputs, but we need them to be synchronized and usable as a single input.
Commercial interfaces solve this problem internally: they allow multiple inputs and outputs to be connected and present them to the system as a single audio device.
The first option we tried was ASIO4ALL, but we ran into several issues:
- Each input appeared with a random name, which changed every time it was connected or disconnected.
- Performance was very slow, as it is not designed for this type of usage.
- Management of multiple inputs was unreliable.
Searching for alternatives, we found Voicemeeter Potato, an advanced virtual audio mixer for Windows.
This mixer offers many features that go beyond our needs, but for our microphone, we are particularly interested in how it allows connecting and grouping multiple microphones.
The attached image shows how the connections should be made: each microphone is connected to a separate analog input (A1, A2, A3, and A4), with all channels configured in mono.
In the image, the devices appear connected but unavailable. When they are properly available, they appear in gray, and the VU meters light up, displaying the signal captured by each microphone.
At this stage, it is very important to place the microphones in the correct order (see the Ambi-Alice Instructable), as this will be the order later recognized by the recording application.
The settings used in the application are as follows:
*See settings image*
Generally, it should not be necessary to modify these parameters. However, if any issues arise, it is recommended to use this configuration as a reference.
Ambisonic Recorder
The application is developed in C++ using Visual Studio together with openFrameworks and other addons.
BRT (Binaural Rendering Toolbox) is an audio processing addon developed by the University of Málaga (UMA) that provides tools for spatial audio rendering, with a particular focus on binaural reproduction. It enables the rendering of audio signals into binaural output using HRTFs, allowing a realistic spatial perception of sound over headphones in real time.
The BRT addon is integrated, which in this case, is responsible for the binauralization of the different audio signals, allowing the audio to be perceived binaurally during real-time monitoring while recording.
https://github.com/GrupoDiana/ofxBRTLibrary
However, the resulting WAV file is exported in B-format (W, X, Y, Z) following the FuMa convention, meaning that the output is not binauralized.
As shown in the user interface image, there are eight control elements. The first two buttons are used to start and stop the audio playback, which corresponds to what is heard through the headphones (if they are being used). The next two buttons control the recording process.
The Playback and Stop Play buttons allow the reproduction of the most recent recording made during the current execution of the program. There is also a button to save the WAV file, and finally, a checkbox that allows the user to switch between A-format and B-format signal visualization.
Finally, any change performed is reflected in the command line, allowing real-time monitoring of the application’s state and actions.
Microphone Power
Essentially, the circuit required to power the microphones is shown in the image. A resistor with a value similar to the output impedance of the microphone capsules would be needed, connected between the positive supply (+V) and the microphone input, followed by a coupling capacitor to separate the signal from the power supply and route it to the output.
We attempted to power the microphones, for example, with 5 V, but consistently obtained very low values and did not have time to investigate this thoroughly. Therefore, it is left to the user to experiment with resistor and voltage values to achieve proper microphone power.
In summary, the basic schematic is simple, but the exact values depend on the capsules used and the available power source.
Audio Files
In this section we want to include some of the recorded audio files. Most of them were recorded using the AmbiÁlice version; however, the file called Torre_del_Mar was recorded with our version. The low-pass effect can be heard at the beginning of the conversations, and loud sounds are reproduced clearly, as near the end of the audio a sound of church bells can be heard.
Acknowledgments
We want to thank our professors, Arcadio, Luis, and Francisco, for helping us and encouraging us to try new things without fear of failure.
We have learned a lot while developing it, both about the ambisonic world and different audio devices, since before this, we didn’t even know how to use an interface.
Finally, we want to thank DJJules for his Ambi-Alice instructable, as without it this project would have been impossible.