The sample demos how to use OpenSL ES to create a player and recorder in Android Fast Audio Path, and connect them to loopback audio. On most android devices, there is a optimized audio path that is tuned up for low latency purpose. The sample creates player/recorder to work in this highly optimized audio path(sometimes called native audio path, low latency path, or fast audio path). The application is validated against the following configurations:
- Android L AndroidOne
- Android M Nexus 5, Nexus 9 This sample uses the new Android Studio with CMake support, and shows how to use shared stl lib with android studio version 2.2.0, see CMakeLists.txt for details
- Android Studio 2.2+ with NDK bundle.
- Download Android Studio
- Launch Android Studio.
- Open the sample directory.
- Open File/Project Structure...
- Click Download or Select NDK location.
- Click Tools/Android/Sync Project with Gradle Files.
- Click Run/Run 'app'.
App will capture audio from android devices and playback on the same device; the playback on speaker will be captured immediately and played back...! So to verify it, it is recommended to "mute" the playback audio with a earspeaker/earphone/earbug so it does not get looped back. Some device like Nexus 9, once you plug in an external headphone/headspeaker, it stops to use onboard microphone AND speaker anymore -- in this case, you need turn on the microphone coming with your headphone. Another point, when switching between external headphone and internal one, the volume is sometimes very low/muted; recommend to increase the playback volume with volume buttons on the phone/pad after plugging external headphone.
Low Latency Verification
execute "adb shell dumpsys media.audio_flinger". Find a list of the running processes
Name Active Client Type Fmt Chn mask Session fCount S F SRate L dB R dB Server Main buf Aux Buf Flags UndFrmCnt
F 2 no 704 1 00000001 00000003 562 13248 S 1 48000 -inf -inf 000033C0 0xabab8480 0x0 0x600 0
F 6 yes 9345 3 00000001 00000001 576 128 A 1 48000 0 0 0376AA00 0xabab8480 0x0 0x400 256
execute adb shell ps | grep echo
- find the sample app pid
- check with result on step 1.
if there is one "F" in the front of your echo pid, player is on fast audio path
For fast audio capture [it is totally different story], if you do NOT see
com.example.nativeaudio W/AudioRecord﹕ AUDIO_INPUT_FLAG_FAST denied by client
in your logcat output when you are creating audio recorder, you could "assume" you are on the fast path.
If your system image was built with muted ALOGW, you will not be able to see the above warning message.
A couple of knobs in the code for lower latency purpose:
- audio buffer size
- number of audio buffers cached before kicking start player The lower you go with them, the lower latency you get and also the lower budget for audio processing. All audio processing has to be completed in the time period they are captured / played back, plus extra time needed for:
- audio driver
- audio flinger framework,
- bufferqueue callbacks etc Besides those, the irregularity of the buffer queue player/capture callback time is another factor. The callback from openSL may not as regular as you assumed, the more irregularity it is, the more likely have choopy audio. To fight that, more buffering is needed, which defeats the low-latency purpose! The low latency path is highly tuned up so you have better chance to get more regular callbacks. You may experiment with your platform to find the best parameters for lower latency and continuously playback audio experience. The app capture and playback on the same device [most of times the same chip], capture and playback clocks are assumed synchronized naturally [so we are not dealing with it]
- The sample is greatly inspired by native-audio sample
- Don Turner @ Google for the helping of low latency path
- Ian Ni-Lewis @ Google for producer/consumer queue and many others
If you've found an error in these samples, please file an issue.
Copyright 2015 Google, Inc.
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