Fix Audio Stuttering On Raspberry Pi With BlueALSA

by Sebastian Müller 51 views

Introduction

Experiencing audio stuttering with your Raspberry Pi-based AirPlay bridge after it's been running for a day or more? You're not alone! This article dives deep into troubleshooting this frustrating issue, specifically when using BlueALSA for Bluetooth audio. We'll break down the problem, analyze potential causes, and explore solutions to get your Shairport Sync setup running smoothly. This comprehensive guide aims to provide valuable insights and practical steps for anyone facing similar audio glitches, ensuring a seamless and enjoyable listening experience. Understanding the intricacies of audio streaming over Bluetooth and the potential bottlenecks in a Raspberry Pi environment is key to resolving these issues, so let's get started and explore the common pitfalls and effective strategies for troubleshooting.

The Problem: Audio Stutters After Long Periods of Uptime

Many users building AirPlay bridges with Raspberry Pi and BlueALSA have reported a peculiar issue: audio plays perfectly initially, but after the device has been powered on for an extended period (typically around a day), the audio begins to stutter. This issue seems to be more prevalent on 64-bit operating systems, but it also occurs on 32-bit Raspberry Pi OS Lite. The standard installation involves using the package manager (apt install bluez-alsa-utils) without specific configurations in asoundrc, making it easier to switch Bluetooth output devices. However, this setup sometimes leads to long-term audio stuttering problems. Initially, increasing audio_backend_buffer_desired_length_in_seconds and ALSA's buffer_size can mitigate constant stuttering, but the problem resurfaces after prolonged use.

Symptoms

  • Audio plays smoothly initially but starts stuttering after about a day of uptime.
  • Stuttering correlates with increasing “Resend Reqs” in logs.
  • The issue is exacerbated when starting or stopping audio streams.
  • Using a 64-bit OS seems to make the problem occur sooner and more frequently.

Initial Mitigation Attempts

To address the constant audio stutter, adjustments were made to the buffer settings. Specifically, the audio_backend_buffer_desired_length_in_seconds in the Shairport Sync configuration and the buffer_size in ALSA were increased. While these changes initially alleviated the stuttering, the problem returned after the device operated for an extended period. This suggests that the initial buffer adjustments were only a temporary fix, and the underlying issue persists, requiring a more thorough investigation. These intermittent issues are particularly challenging to diagnose, as they indicate a potential resource constraint or a degradation in performance over time, rather than a consistent configuration problem.

Diagnosing the Issue

To effectively troubleshoot, it’s essential to methodically check potential causes. In this scenario, several factors were considered and investigated to pinpoint the root cause of the audio stuttering: Wi-Fi power management, CPU temperature, RAM usage, CPU usage, and versions of Raspberry Pi OS. Here's a breakdown of the diagnostic steps and findings:

1. System Resource Monitoring

The first step involves monitoring system resources to identify any bottlenecks or performance issues. Key areas to check include:

  • CPU Usage: High CPU utilization can lead to audio processing delays and stuttering. Monitoring CPU load over time can reveal if specific processes or services are consuming excessive resources.
  • RAM Usage: Insufficient memory can cause the system to swap data to disk, which is a slow operation that can disrupt audio playback. Monitoring RAM usage can help determine if the system is running out of memory.
  • CPU Temperature: Overheating can cause CPU throttling, which reduces performance and can result in audio stuttering. Monitoring CPU temperature can help identify if thermal issues are a factor.

2. Wi-Fi Power Management

Wi-Fi power management settings can sometimes interfere with real-time audio streaming. Power-saving features may cause the Wi-Fi adapter to enter a low-power state, leading to connectivity issues and audio disruptions. Disabling Wi-Fi power management can help determine if this is the cause of the stuttering.

3. Background Processes

Background processes can consume system resources and interfere with audio playback. Identifying and disabling unnecessary background processes can help improve audio performance. In this case, a small WSGI Flask server handling Bluetooth device and Wi-Fi selection was running in the background. Although it was mostly idle, it was considered as a potential cause and temporarily disabled for testing.

4. Operating System and Software Versions

The specific versions of the operating system and related software components can impact system performance. Testing with different OS versions (e.g., 32-bit vs. 64-bit) and software versions (e.g., BlueALSA, Shairport Sync) can help identify compatibility issues or bugs that may be causing the audio stuttering.

Findings

Despite thorough checks, no immediate red flags were found in terms of CPU temperature, RAM usage, or CPU usage. Wi-Fi power management was also ruled out as a cause. The issue seemed to occur more often and earlier on a 64-bit OS, which is a significant clue. The background Flask server, though disabled for testing, didn't resolve the problem, indicating that it wasn't the primary cause. The key observation from the logs was the increasing “Resend Reqs,” suggesting potential network or buffering issues over time. This comprehensive approach to diagnosing the issue is crucial for systematically eliminating potential causes and focusing on the most likely culprits.

Analyzing Log Output

Log analysis is crucial for pinpointing the root cause of the audio stuttering. The logs from Shairport Sync provide valuable insights into the audio stream's behavior and potential issues. Here's a breakdown of the key log entries and their implications:

Key Log Sections

  • BlueALSA PCM Initialization: This section shows the process of Shairport Sync connecting to BlueALSA, setting up the audio output stream, and configuring buffers. The log entries related to /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink indicate the specific Bluetooth device being used.

    Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:1309: Getting BlueALSA PCM: PLAYBACK 00:00:00:00:00:00 a2dp
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:1062: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Setting constraints
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:532: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Initializing HW
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:567: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: FIFO buffer size: 1024 frames
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:581: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Selected HW buffer: 45 periods x 1920 bytes == 86400 bytes
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:600: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Initializing SW
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:600: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Initializing SW
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:600: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Initializing SW
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:639: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Prepared
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:600: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Initializing SW
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:639: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Prepared
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:356: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Starting
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:226: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Starting IO loop: 17

  • IO Thread Management: These logs indicate the starting, pausing, and stopping of the audio IO thread, which is responsible for pushing audio data to BlueALSA. Frequent starting and stopping might indicate instability or buffering issues.

    Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:231: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Pausing IO thread
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:392: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Stopping
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:161: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: IO thread cleanup
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:639: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Prepared
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:356: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Starting
Aug 06 18:14:55 pi shairport-sync[517]: D: bluealsa-pcm.c:226: /org/bluealsa/hci0/dev_34_81_F4_7B_1C_2F/a2dpsrc/sink: Starting IO loop: 17

  • Sync Error and Resend Requests: This section is the most telling. The increasing “Resend Reqs” value over time strongly suggests that the system is struggling to maintain a consistent audio stream. This could be due to network congestion, buffer underruns, or other timing issues.

    Aug 06 18:15:05 pi shairport-sync[517]: Sync Error ms | Net Sync PPM | All Sync PPM | Missing | Resend Reqs | Output FPS (r) | Output FPS (c)
Aug 06 18:15:05 pi shairport-sync[517]:         -0.01            0.0            0.0         0             5              N/A              N/A
...
Aug 06 18:36:39 pi shairport-sync[517]:         -1.56            5.7            5.7         0           885         44099.99         44100.00

Key Observations

  1. Increasing Resend Requests: The progressive increase in “Resend Reqs” is a critical indicator. It means that audio packets are not being delivered or processed in a timely manner, leading to stuttering as the system tries to recover missing data.
  2. Frequent IO Thread Pausing and Resuming: The logs show numerous instances of the IO thread being paused and resumed. This might indicate that the audio stream is frequently interrupted, possibly due to buffering issues or synchronization problems between Shairport Sync and BlueALSA.
  3. Sync Errors: The