Android Audio Smoothness: Performance Analysis and Optimization Guide

This document targets Android audio developers and focuses on improving audio smoothness by analyzing and mitigating audio stutter and noise issues.

1. Introduction

The goal is to provide a reference for developers to understand the causes of audio glitches such as buffer underruns, pop noises, and background‑thread scheduling delays.

2. Audio Formats

Human hearing ranges from 20 Hz to 20 kHz. Typical PCM parameters include channel count (mono, stereo, 5.1, 7.1), bit depth (16‑bit, 32‑bit), and sample rate (8 kHz–384 kHz). The document lists common configurations and shows how frame size is calculated (e.g., 16 bit × 2 ch = 4 bytes per frame).

3. Common Noise Types

Pop noise (breaks in data continuity, often seen during fast‑forward or variable‑speed playback)

Zero‑padding (inserting zeros when data production lags)

Clipping (signal exceeds encoding range)

Beeps, howling, environmental background noise, and random white noise

4. Audio Pipeline Basics

The Android audio stack consists of AudioTrack/AudioRecord entry points, AAudio, OpenSL ES, FastMixer, Low‑Latency, and ULL paths. AAudio uses shared memory (mmap) to minimise copy overhead and supports three performance modes: NONE, LOW_LATENCY, and POWER_SAVING.

5. Audio and CPU Scheduling

The document explains Linux task states (R, S, D, T, X, Z) and scheduler classes (STOP, DEADLINE, RT, CFS, IDLE). It details how Android assigns audio threads a high‑priority nice value (≈101) and how cgroup migration and timerslack affect background playback.

6. Performance Case Studies

APP playback thread priority too low (nice 120) causing buffer starvation.

Game audio thread using FastMixer with insufficient priority.

NativeThread priority mis‑set, leading to severe stutter.

Blocking audio APIs (e.g., isBluetoothScoOn) on critical threads.

Calling AudioTrack.start() on the render thread causing 580 ms block.

Improper FrameCount settings (too large → silence, too small → stutter).

Excessive audio‑related Binder calls exhausting the system thread pool and triggering device reboot.

7. Joint Optimization Recommendations

Set audio‑related threads to at least nice 104 (preferably 101).

Avoid time‑consuming AudioManager calls in high‑priority threads.

Run playback logic inside a foreground Service to keep timerslack low.

Minimise frequent audio‑state queries.

8. References

Links to Apple Audio Toolbox, Windows Multimedia Class Scheduler Service, and several Chinese technical articles.