Low-latency audio playback on Android

For lowest latency on Android as of version 4.2.2, you should do the following, ordered from least to most obvious:

1. Pick a device that supports FEATURE_AUDIO_PRO if possible, or FEATURE_AUDIO_LOW_LATENCY if not. (“Low latency” is 50ms one way; pro is <20ms round trip.)

2. Use OpenSL. The Dalvik GC has a low amortized cost, but when it runs it takes more time than a low-latency audio thread can allow.

3. Process audio in a buffer queue callback. The system runs buffer queue callbacks in a thread that has more favorable scheduling than normal user-mode threads.

4. Make your buffer size a multiple of AudioManager.getProperty(PROPERTY_OUTPUT_FRAMES_PER_BUFFER). Otherwise your callback will occasionally get two calls per timeslice rather than one. Unless your CPU usage is really light, this will probably end up glitching. (On Android M, it is very important to use EXACTLY the system buffer size, due to a bug in the buffer handling code.)

5. Use the sample rate provided by AudioManager.getProperty(PROPERTY_OUTPUT_SAMPLE_RATE). Otherwise your buffers take a detour through the system resampler.

6. Never make a syscall or lock a synchronization object inside the buffer callback. If you must synchronize, use a lock-free structure. For best results, use a completely wait-free structure such as a single-reader single-writer ring buffer. Loads of developers get this wrong and end up with glitches that are unpredictable and hard to debug.

7. Use vector instructions such as NEON, SSE, or whatever the equivalent instruction set is on your target processor.

8. Test and measure your code. Track how long it takes to run–and remember that you need to know the worst-case performance, not the average, because the worst case is what causes the glitches. And be conservative. You already know that if it takes more time to process your audio than it does to play it, you’ll never get low latency. But on Android this is even more important, because the CPU frequency fluctuates so much. You can use perhaps 60-70% of CPU for audio, but keep in mind that this will change as the device gets hotter or cooler, or as the wifi or LTE radios start and stop, and so on.

Low-latency audio is no longer a new feature for Android, but it still requires device-specific changes in the hardware, drivers, kernel, and framework to pull off. This means that there’s a lot of variation in the latency you can expect from different devices, and given how many different price points Android phones sell at, there probably will always be differences. Look for FEATURE_AUDIO_PRO or FEATURE_AUDIO_LOW_LATENCY to identify devices that meet the latency criteria your app requires.