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USB Audio asynchronous explicit feedback sample


This sample demonstrates how to implement USB asynchronous audio playback with explicit feedback. It can run on any board with USB and I2S support, but the feedback calculation is currently only implemented for the Nordic nRF5340 IC.

The device running this sample presents itself to the host as a Full-Speed Asynchronous USB Audio 2 class device supporting 48 kHz 16-bit 2-channel (stereo) playback.

Explicit Feedback

Asynchronous USB Audio is used when the actual sample clock is not controlled by USB host. Because the sample clock is independent from USB SOF it is inevitable that 1 ms according to audio sink (I2S) will be either slightly longer or shorter than 1 ms according to audio source (USB host). In the long run, this discrepancy leads to overruns or underruns. By providing explicit feedback to host, the device can tell host how many samples on average it needs every frame. The host achieves the average by sending either nominal or nominal ±1 sample packets every frame.

The dummy feedback implementation, used when there is no target-specific feedback code available, returns a feedback value that results in host sending nominal number of samples every frame. Theoretically it should be possible to obtain the timing information based on I2S and USB interrupts, but currently neither subsystem provides the necessary timestamp information.

Explcit Feedback on nRF5340

The nRF5340 is capable of counting both edges of I2S LRCLK relative to USB SOF with the use of DPPI, TIMER and GPIOTE input. Alternatively, if the GPIOTE input is not available, the DPPI and TIMER peripherals on nRF5340 can be configured to provide relative timing information between I2S FRAMESTART and USB SOF.

This sample in both modes (direct sample counting and indirect I2S buffer output to USB SOF offset) has been tested on nRF5340 DK.

The sample defaults to indirect feedback calculation because direct sample counting requires external connection between I2S LRCLK output pin to GPIOTE input pin (hardcoded to P1.09) on nRF5340 DK. In the indirect mode no extra connections are necessary and the sample can even be used without any I2S device connected where I2S signals can be checked e.g. on logic analyzer.

Building and Running

The code can be found in samples/subsys/usb/uac2_explicit_feedback.

To build and flash the application:

west build -b nrf5340dk/nrf5340/cpuapp samples/subsys/usb/uac2_explicit_feedback
west flash