The diagram and photo below show an audio loopback dongle for the headset connector that we call the "Dr. Rick O'Rang audio loopback dongle." The Chrome hardware team designed this circuit and plug for functional testing; however it has many other uses too. The Android audio team uses it to measure round-trip audio latency, through the Larsen effect (audio feedback loop) or autocorrelation.
To ensure that the output signal will not overload the microphone input, we cut it down by about 20dB. The resistor loads tell the microphone polarity switch that the audio loopback dongle is a US/CTIA pinout Tip Ring Ring Shield (TRRS) plug.
Using an alternative dongle
The IK Multimedia iRig 2 is an analog interface that can replace our own loopback dongle. To use this interface, follow these steps:
- Plug the iRig 2 into the 3.5 mm headset jack located on your Android device or USB C to 3.5 mm adapter.
- Connect a 1/4 inch mono guitar cable between the input and output jacks of the iRig 2 to form a loopback.
- Set the toggle switch to FX.
- Adjust the gain knob as needed.
This isn't a recommendation or endorsement; your experience might vary. Similar interfaces from other suppliers might also work, but be sure they're analog so as to avoid the added latency from a digital interface.