Audio Multi-Rate Microphone

A single-channel USB microphone test example that supports multiple sample rates and adapts its descriptors to the negotiated USB bus speed — UAC1 at Full-Speed and UAC2 at High-Speed.

What it does

• Enumerates as a single-channel microphone. The configuration descriptor returned depends on link speed: a UAC1 (Audio 1.0) configuration at Full-Speed and a UAC2 (Audio 2.0) configuration at High-Speed. A device-qualifier and other-speed-configuration descriptor are provided so it works at either speed.
• Supports discrete sample rates of 32 kHz, 48 kHz and 96 kHz. At Full-Speed the rate is selected through the UAC1 endpoint sampling-frequency control; at High-Speed through the UAC2 clock-source frequency range.
• Offers a 16-bit format; the High-Speed UAC2 configuration adds a second format with 24-bit samples carried in 32-bit slots, selected via the streaming interface's alternate setting.
• An audio task generates an incrementing ramp signal sized to the current sample rate and sample width, and writes it to the IN endpoint every 1 ms. The ramp resets when the streaming interface is closed.
• Handles both UAC1 and UAC2 control requests (mute, volume, sample frequency) dispatched on the active audio version.
• 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_MAX_SAMPLE_RATE                96000
#define CFG_TUD_AUDIO_FUNC_1_N_FORMATS                      2
#define CFG_TUD_AUDIO_ENABLE_EP_IN                          1
#define CFG_TUD_AUDIO_FUNC_1_N_CHANNELS_TX                  1
#define CFG_TUD_AUDIO_FUNC_1_FORMAT_1_N_BYTES_PER_SAMPLE_TX 2   // 16-bit in 16-bit slots
#define CFG_TUD_AUDIO_FUNC_1_FORMAT_1_RESOLUTION_RX         16
#define CFG_TUD_AUDIO_FUNC_1_FORMAT_2_N_BYTES_PER_SAMPLE_TX 4   // 24-bit in 32-bit slots (UAC2 only)
#define CFG_TUD_AUDIO_FUNC_1_FORMAT_2_RESOLUTION_RX         24
#define CFG_TUD_AUDIO_FUNC_1_EP_IN_SZ_MAX                   TU_MAX(CFG_TUD_AUDIO10_FUNC_1_FORMAT_1_EP_SZ_IN, TU_MAX(CFG_TUD_AUDIO20_FUNC_1_FORMAT_1_EP_SZ_IN, CFG_TUD_AUDIO20_FUNC_1_FORMAT_2_EP_SZ_IN))

Building

CMake:

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

Make:

make BOARD=raspberry_pi_pico all

Try it

The device appears as a single-channel USB microphone. On Linux, list it with arecord -l and record at a chosen rate with arecord, e.g. arecord -D hw:CARD=MicNode -c 1 -f S16_LE -r 96000 test.wav, trying 32000/48000/96000 to exercise rate switching. On a High-Speed host you can also select the 24-bit format (-f S24_3LE / S32_LE depending on the host). The included src/plot_audio_samples.py records and plots the signal.