Ecasound Buffer Overruns and Underruns in JACK

When Ecasound encounters a buffer overrun or underrun while operating as a client under the JACK Audio Connection Kit, the system experiences an "xrun." This article explains how JACK and Ecasound detect these timing errors, the audible and operational consequences during processing, and how the system attempts to recover.

In digital audio systems, a buffer underrun occurs when an application fails to deliver processed audio data to the output buffer before the sound hardware needs it. A buffer overrun occurs when incoming audio data arrives faster than the application can read and clear it from the input buffer. Within the JACK ecosystem, both scenarios are categorized under the single term "xrun."

Because JACK operates as a strict real-time audio server, it enforces fixed-period deadlines on all connected clients. When Ecasound cannot finish executing its signal-processing chains, effects, or disk I/O within the allocated period window, JACK cannot wait. To prevent total playback failure, JACK drops the delayed audio frames or injects silence into the stream to maintain synchronized timing across the rest of the graph.

When this occurs, JACK alerts the Ecasound process through its registered xrun callback. Ecasound immediately logs the event, reporting the occurrence directly to standard error or the console interface. These reports indicate that timing synchronization was lost for that specific processing cycle.

The practical impact of an xrun in Ecasound depends on the operation:

• Audio Output and Monitoring: The listener will hear distinct audible artifacts, such as clicks, pops, or momentary dropouts of silence, caused by the missing samples in the stream.
• Audio Recording: If Ecasound is capturing streams from JACK to disk, an overrun results in lost samples. This leads to gaps or micro-splices in the recorded waveform, compromising the continuity and fidelity of the final audio file.
• Synchronization: Ecasound attempts to resynchronize its internal chain processing clock with JACK's transport. While playback usually continues immediately after the interruption, persistent xruns indicate that the system cannot sustain real-time execution.

Frequent xruns typically stem from the JACK buffer size being set too small for the complexity of Ecasound’s routing, high CPU contention, or inadequate real-time thread scheduling permissions on the host operating system.