A guide for converting a handheld microphone (with PTT) into a USB microphone for use on VATSIM.
I do not have formal education in electrical engineering or electronics. This guide is based on my personal experiences and experimentation. While I strive to provide accurate information, I cannot guarantee the safety or effectiveness of these methods. Please proceed with caution, especially when working with soldering or modifying electrical components.
Constructive feedback and tips for improving this guide are welcome and can be commented below.
The Holmco MH-50 or the Peiker TM110 are relatively easy to find. Both are similar to the microphones used by air traffic controllers (at least in Norway). Since Peiker and Holmco are German brands, a good place to look is ebay.de. Search terms like "Peiker," "Stabmikrofon," or "Holmco" often yield relevant results. Although listings may specify shipping to Germany, many sellers are willing to ship internationally. If you are lucky you might get one in very good condition for 50 EUR or less.
| Holmco MH-50 | Peiker TM110 |
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The diagram shows the basic concept for interfacing the microphone with a computer. To build this adapter box, you will need the following components:
- USB sound card for the audio
- USB gamepad for the PTT
- USB hub to consolidate the connections into a single USB cable
- USB cable to connect to your computer
- 5-pin connector matching the plug on the microphone
Below are some notes about selecting the right components.
The microphone circuit is usually separate from the PTT circuit, meaning that you should be able to record audio without pressing the PTT. The PTT is only for activating "transmission" in the voice application on your PC.
If, however, the PTT is part of the microphone circuit, you will need to open up the mic housing and rewire it. Turning the mic on and off with the PTT (disconnecting it every time you release the PTT) is not ideal because the sound card will take time to adjust its gain each time the mic is reconnected.
Why use a USB sound card? You could connect the microphone to the 3.5mm microphone input on your PC, but depending on the microphone's specs and your computer's sound card, the signal might be too weak. A USB sound card with decent audio quality ensures consistent sound across any computer.
I found these unbranded blue USB sound cards with the C-Media CM108 chip work well. They are cheap (search for CM108 on AliExpress), has a high bias voltage (around 5v) and it's easy to access the components while still being compact. Unfortunately it's not uncommon for these USB sound cards to add a significant level of electrical noise to the audio - so I recommend ordering at least two and pick the best one.
The PTT is simply a momentary on/off button. The easiest way to interface it with a PC is to buy a cheap gamepad, take it apart, and solder the two PTT wires directly to one of the contact areas for the buttons. Voice applications like "Audio for VATSIM" support a gamepad/joystick button for push-to-talk (PTT). Discord also has a "push-to-mute" feature for voice-activated speech. This is the most plug-and-play option.
Even with the housing removed, USB gamepads are quite large. For a more compact option you may consider the "Arduino Beetle." This Arduino is based on the ATMEGA32U4 chip, which can be programmed to function as a USB gamepad (ArduinoJoystickLibrary). Note that you will need some basic knowledge when it comes to Arduino and handling libraries. However, once it's been programmed, simply solder the PTT circuit to one of its analog inputs, and it's plug-and-play!
A third option: the CM108 (similar to the CM119) microcontroller on the USB sound card has some unused GPIO (general-purpose input/output) pins that can interface with the PTT. This is the most compact option since you can eliminate the USB hub and USB gamepad—just the USB sound card is needed. Be aware that this option requires careful soldering, and it's easy to damage the USB sound card if you make a mistake. More about this in the final part of this guide.
| USB Gamepad | Arduino Beetle | CM119 Chip |
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Microphones sold on eBay may come with various connectors, or none at all. In any case you should get a panel connector to mount on the adapter box. I recommend replacing the connector with a 5-pin GX16 connector. Be sure to order both a male connector for the cable and a female "panel type" connector for the enclosure box.
When you cut off the plug and strip off the outer insulation, you'll typically find four cords wrapped in a braided copper shield (see image below).
To prevent electrical noise from entering the microphone cable, ensure the shielding is connected to mic- (ground). I suggest twisting and soldering it to one of the connector pins and making the connection to ground inside the adapter box. Make sure it's properly insulated, for instance by wrapping it in a shrink tube, to avoid contact with other wires (this is crucial!).
If you still encounter electrical noise, especially when touching the microphone, ensure the copper shield is physically connected to the microphone housing on the microphone end of the cable.
The C-Media CM108 and CM119 chips feature unused GPIO pins that can be used for PTT control. This blog post, focusing on HAM radio hardware, inspired me to interface the PTT button directly with the USB sound card, eliminating the need for a gamepad or USB hub. I found that it is possible!
On the CM119 chip, connect one end of the PTT circuit to the REGV pin (pin 37) and the other to the GPIO3 pin (pin 13). Be careful, as short-circuiting adjacent pins may prevent the USB device from working. A useful trick is to use a 30-gauge wire (super thin) and thread it under adjacent pins to hold it in place while soldering.
What I'm trying to illustrate above is a "pull-up circuit": when the PTT is not pressed, the GPIO pin reads "high" (current flows to the pin), and when pressed, it reads "low" (current flows to ground). You can also use a "pull-down circuit". In any case I recommend adding a resistor since the GPIO pin requires very little current to detect it. I used a 4.4 kOhm resistor.
With this option you may actually be able to fit all components inside the microphone's metal sleeve, meaning that you can have a single USB cable running directly from the microphone to your computer.
The PTT can be programmed to emulate a keyboard stroke using the HidPttEmulator. This program works only on Windows, but similar software can be made for Mac and Linux as well.
If you purchase a second-hand microphone, it is likely a dynamic microphone that requires signal amplification. Depending on the microphone's specifications, some may produce sufficient audio levels for the USB sound card, while others may be too faint.
In the latter case, you have two options:
- Replace the internal microphone with one that is suitable for the USB sound card.
- Install an amplifier.
Personally, I have always opted for the first option, although I have been experimenting with the second.
I found a very small board online that features both an amplifier (MAX4466) and an audio compressor (SSM2167), compact enough to fit inside the microphone housing. The amplifier boosts the audio signal, while the compressor reduces the dynamic range, making the sound much clearer and crisper. The power consumption is so low that it can even be powered by the bias output from the CM108 chip. Sounds perfect, right?
Unfortunately, I'm not completely satisfied with these. While they do amplify the audio from the dynamic microphone nicely, they also introduce a loud "pumping" noise in the background. I haven't been able to pinpoint the issue yet, but after reviewing the SSM2167 datasheet, it seems like the problem might be related to the capacitor value on the PCB. If anyone finds a solution - please share it!