@@ -2,4 +2,16 @@ The Raspberry Pi based user terminal contains two custom hardware components: a
...
@@ -2,4 +2,16 @@ The Raspberry Pi based user terminal contains two custom hardware components: a
# Motorola Speaker-Microphone
# Motorola Speaker-Microphone
To match the user experience with real LMR systems, the user terminal provides a Motorola speaker-microphone, the same model that is used by some LMR radios. A simple custom hardware interface was needed to connect the speaker-microphone to the Raspberry Pi.
The Motorola speaker-mic is connected to an external USB audio adapter attached to the Raspberry Pi. We use a USB adapter with volume up and down button inputs repurposed from an old Logitech headset. The speaker-out and microphone-in signals on the Motorola speaker-mic are connected to the corresponding audio ports on the USB audio adapter.
The condenser microphone in the Motorola speaker-microphone requires bias voltage to power the internal amplifier. Fortunately, microphone input ports on most modern USB audio adapters provide bias voltage (typically 3 V).
The PTT button on the speaker-mic is connected in series with the microphone. When the button is pressed, bias voltage from the microphone port is connected to the base of a NPN BJT transistor. The transistor opens and connects the volume-up button input to the ground. The transistor isolates the digital volume up button port from the microphone audio line, preventing digital noise from leaking into the microphone signal.
In Linux, the volume up button is represented as a USB HID device and its state can be read via the corresponding evdev (Linux input) device. If you sound card does not have volume up and down buttons, you can connect the PTT button directly to a GPIO port on the Raspberry Pi. Both variants are equally easy to work with in software.
