Features/GlitchFreeAudio

Glitch-free Audio

Summary

The PulseAudio sound server has been rewritten to use timer-based audio scheduling instead of the traditional interrupt-driven approach. This is the approach that is taken by other systems such as Apples CoreAudio and the Windows Vista audio subsystem and has a number of advantages, not the least in reduced power consumption.

Owner

• Name: Lennart Poettering

Current status

• Targeted release: Fedora 10
• Last updated: 2008-04-09
• Percentage of completion: 50%

Detailed Description

Lennart has written a great explanation of the technical details here .

Benefit to Fedora

Lennarts writeup has a detailed discussion of advantages (and disadvantages) of the glitch-free approach. Some highlights are:

• Less wakeups, reduced power consumption
• Dynamic latency adaption
• Less dependent on audio hardware
• Minimized chance of drop outs

Scope

The feature requires a pretty-much rewritten core of PulseAudio.

Test Plan

• Play sound on the desktop in various ways, e.g. music in rhythmbox, or a video in totem, or some flash video in firefox. Verify that the sound experience is the same as on F9.
• While doing the above, run powertop and observe that pulseaudio is not increasing the power consumption as much as it used to.
• Repeat the above with different sound cards

User Experience

Unfortunately not much of this is directly visible. The location where this is best visible is powertop.

Dependencies

The glitch-free logic will only be enabled on mmap()-capable ALSA devices and where hrtimers are available. The non-glitch-free logic will be preserved as compatibility code with older kernels and limited sound hardware or drivers.

From Lennarts writeup:

• System timers on Unix are not very high precision. On traditional Linux with HZ=100 sleep times for timers are rounded up to multiples of 10ms. Only very recent Linux kernels with hrtimers can provide something better, but only on x86 and x86-64 until now. This makes the whole scheme unusable for low latency setups unless you run the very latest Linux. Also, hrtimers are not (yet) exposed in poll()/select(). It requires major jumping through loops to work around this limitation.

• Generally, this works reliably only on newest ALSA, newest kernel, newest everything. It has pretty steep requirements on software and sometimes even on hardware.

Contingency Plan

If the glitch-free PulseAudio exhibits glitches that cannot be fixed in time for F10, we can just revert to the last 'traditional' PulseAudio release.

Documentation

Lennarts detailed explanation is here: http://0pointer.de/blog/projects/pulse-glitch-free.html

Release Notes

The PulseAudio sound server has been rewritten to use timer-based audio scheduling instead of the traditional interrupt-driven approach.

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