| ## Audio Latency Measurement |
| |
| Audio output latency as measured by WALT is the time that passes from the moment an application |
| decides to output a tone until it can be detected via the headphone jack. Microphone latency is |
| defined similarly. |
| |
| Low latency audio IO on Android can be achieved via JNI C/C++ code. |
| Documentation and sample code can be found on the |
| [High Performance Audio website](http://googlesamples.github.io/android-audio-high-performance/). |
| |
| |
| ### Reported values |
| |
| We are trying to stick to the following (overlapping) principles |
| 1. Timestamp events as close to hardware as possible. Most events up the stack can be easily timed with software alone. |
| 1. Measure time intervals that are likely to have low variability. |
| |
| ##### Playback |
| |
| In order to avoid warm up latency during audio playback it is |
| [recommended to constantly enqueue buffers containing silence](http://googlesamples.github.io/android-audio-high-performance/guides/audio-output-latency.html#avoid-warm-up-latency). |
| WALT app follows this pattern. |
| |
| The audio data buffers are enqueued in the |
| [player callback](https://github.com/google/walt/blob/v0.1.6/android/WALT/app/src/main/jni/player.c#L107) |
| and the latency reported by WALT app is the time from the |
| [Enqueue() call](https://github.com/google/walt/blob/v0.1.6/android/WALT/app/src/main/jni/player.c#L123) |
| until there is a detectable signal on the wire. Note that this does not include the time between the moment the app decided to output a tone until the Enqueue() call. This is somewhat counterintuitive but this time is deliberately omitted. In case of the WALT app code this time is likely be uniformly distributed between 0 and the length of the buffer (5 ms in case of Nexus 5) and therefore would contribute considerable variance but little interesting information if included in the reported latency. |
| |
| ##### Recording |
| The reported latency is the time from the moment the last frame in a buffer was recorded until the |
| [recorder callback](https://github.com/google/walt/blob/v0.1.6/android/WALT/app/src/main/jni/player.c#L345) |
| receiving that buffer is executed. |
| |
| TODO: Is the round trip latency expected to be Recording latency + Playback latency + one buffer length? |
| |
| ### Sample measurements |
| |
| | Device | OS version | Buffer | Playback [ms] | Recording* [ms] | |
| | :--- | :--- | :--- | ---: | ---: | |
| | Nexus 5 | M4B30Z (6.0.1) | 240 frames @ 48 kHz = 5 ms | 27.6 | 2.5 | |
| | Nexus 5X | NRD91P (7.0) | 192 frames @ 48 kHz = 4 ms | 14.9 | 3.5 | |
| | Nexus 7 | LMY47Q (5.1) | 240 frames @ 48 kHz = 5 ms | 32.1 | 16.3 | |
| | Nexus 9 | MMB29K (6.0.1) | 128 frames @ 48 kHz = 2.6 ms | 9.8 | 1.0 | |
| | Nexus 6P | MHC19I (6.0.1) | 192 frames @ 48 kHz = 4 ms | 15.3 | 1.6 | |
| | Pixel | NDE63P (7.1) | 192 frames @ 48 kHz = 4 ms | 8.9 | 1.7 | |
| | Pixel XL | NDE63H (7.1) | 192 frames @ 48 kHz = 4 ms | 9.1 | 1.6 | |
| |
| \* WALT clock synchronization accuracy is about 1 ms hence the relative error for recording latency can be fairly high. |
| |
| #### Published round trip measurements |
| Superpowered Inc. maintains an open source app for measuring round trip audio latency - |
| [Superpowered Latency App](https://github.com/superpoweredSDK/SuperpoweredLatency). |
| |
| * [Audio round trip measurements published by Android group](https://source.android.com/devices/audio/latency_measurements.html#measurements) |
| * [Audio round trip measurements published by Superpowered Inc.](http://superpowered.com/latency) |
| |
| |
| ### Hardware |
| |
| Audio signal for measuring microphone latency is generated as a square wave using the Teensy tone() |
| function ([currently at 5 kHz](https://github.com/google/walt/blob/v0.1.6/arduino/walt/walt.ino#L310)). |
| The signal is attenuated by a simple circuit similar to the |
| [ChromeOS/Android audio loopback dongle](https://source.android.com/devices/audio/loopback.html). |
| |
| Audio output signal from the phone is detected when audio line voltage crosses a predefined |
| threshold (currently about 65 mV). |
