Audio Test Microphone

A minimal USB Audio Class 2.0 (UAC2) microphone that streams a generated test signal, useful for verifying the audio device stack end to end.

What it does

• Enumerates as a UAC2 microphone with 1 input channel at 48 kHz, 16-bit.
• An audio task fills a buffer with a continuously incrementing 16-bit ramp counter and writes it to the IN endpoint every 1 ms, simulating data from an I2S source. The counter resets when the streaming interface is closed.
• Handles UAC2 control requests: feature-unit mute/volume, clock-source sample-rate and clock-validity, and input-terminal connector.
• Blinks the on-board LED to indicate USB state (not mounted / mounted / suspended).

USB Descriptors

Configuration

Notable tusb_config.h settings:

#define CFG_TUD_AUDIO                                 1
#define CFG_TUD_AUDIO_FUNC_1_SAMPLE_RATE             48000
#define CFG_TUD_AUDIO_ENABLE_EP_IN                    1
#define CFG_TUD_AUDIO_FUNC_1_N_CHANNELS_TX            1
#define CFG_TUD_AUDIO_FUNC_1_N_BYTES_PER_SAMPLE_TX    2   // 16-bit
#define CFG_TUD_AUDIO_EP_SZ_IN                        TUD_AUDIO_EP_SIZE(TUD_OPT_HIGH_SPEED, CFG_TUD_AUDIO_FUNC_1_SAMPLE_RATE, CFG_TUD_AUDIO_FUNC_1_N_BYTES_PER_SAMPLE_TX, CFG_TUD_AUDIO_FUNC_1_N_CHANNELS_TX)
#define CFG_TUD_AUDIO_FUNC_1_EP_IN_SZ_MAX            CFG_TUD_AUDIO_EP_SZ_IN

Building

CMake:

mkdir build && cd build
cmake -DBOARD=raspberry_pi_pico ..
cmake --build .

Make:

make BOARD=raspberry_pi_pico all

FreeRTOS variant

A FreeRTOS build is in examples/device/audio_test_freertos — identical single-channel UAC2 test microphone, with the device, audio, and LED-blink work split across FreeRTOS tasks.

Try it

The device appears as a single-channel USB microphone. On Linux, list it with arecord -l and capture with arecord (for example arecord -D hw:CARD=MicNode -c 1 -f S16_LE -r 48000 test.wav); the recorded samples form a rising ramp. The included src/plot_audio_samples.py records and plots the signal (requires sounddevice and matplotlib).