Controller for USB Audio Gadget

The controller subscribes to Playback/Capture Rate alsa controls defined by the gadget alsa device and starts/stops appropriate playback/capture processes on the gadget side.

Communicating from the Gadget

When no playback/capture runs on the USB host side or the USB cable is disconnected, the respective Playback/Capture Rate controls report 0. When playback/capture is started, the controls report the actual samplerate in Hz. The default control names are the names hard-coded in the kernel audio gadget code, configurable with params -p/--pctl and -c/--cctl.

Playback/Capture Processes on the Gadget Side

The process commands are specified by params -x/--pcmd resp. -y/--ccmd. The controller executes the commands directly without any shell. Every occurence of string {R} is replaced with current samplerate in Hz, as reported by the corresponding alsa control. The default commands run alsaloop to Loopback devices.

Capture

The direction notation is from the gadget view (same as the gaudio alsa notation). Capture means FROM the gadget alsa device (i.e. playback from the USB host view).

Playback

Playback means TO the gadget alsa device (i.e. capture from the USB host view)

Debouncing

USB audio drivers of USB hosts test functionality of the USB device during enumeration. Also pulseaudio tries to open alsa devices. In order to avoid bounced starting/killing the gadget-side processes, the controller implements a debouncer, delaying start of the respective process by a timeout in param -d/--timeout. Value 0 disables the debouncing. With parameter -t/--show-timing the controller measures and reports the time between start and stop events, allowing to set debouncing timeout optimal for the specific usage. The optimal value is slightly larger than the maximum reported stop-start time when plugging the USB cable in. My linux host enumeration bounces take around 25ms, therefore the default value is set to 50 ms. That means that at every playback/capture start on the USB host the first 50ms of data will be lost, but the controller will not run any process on the gadget side during enumeration.

Requirements

If alsaloop is used, the version in alsa-utils 1.2.6 supports Capture/Playback Pitch gadget feedback controls.
All required patches for the audio gadget have not been submitted yet, subject to change soon.

Installation

Installing latest stable Rust with rustup.
cd gadget_ctl
cargo build --release
The binary is compiled to target/release/gadget_ctl

Running

Alsaloop 1.2.6 is not in raspbian yet, therefore exec commands must be specified appropriately:

target/release/gaudio_ctl -vv -t -g UAC2Gadget -p "Playback Rate" -c "Capture Rate" -d 40 -x "/home/pi/work/alsa-utils/alsaloop/alsaloop -vvvv -r {R} -S playshift -f S32_LE -C hw:Loopback,1 -P hw:UAC2Gadget  --latency=10000" -y "/home/pi/work/alsa-utils/alsaloop/alsaloop -vvvv -r {R} -S captshift -f S32_LE -P hw:Loopback,1 -C hw:UAC2Gadget --latency=10000"

pavhofman/gaudio_ctl