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diff --git a/docs/AudioLatency.md b/docs/AudioLatency.md new file mode 100644 index 0000000..feac41e --- /dev/null +++ b/docs/AudioLatency.md @@ -0,0 +1,67 @@ +## 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). |