Posted on Fr 18 Juli 2008

PulseAudio FUD

Jeffrey Stedfast

Jeffrey Stedfast seems to have made it his new hobby to bash PulseAudio. In a series of very negative blog postings he flamed my software and hence me in best NotZed-like fashion. Particularly interesting in this case is the fact that he apologized to me privately on IRC for this behaviour shortly after his first posting when he was critizised on #gnome-hackers -- only to continue flaming and bashing in more blog posts shortly after. Flaming is very much part of the Free Software community I guess. A lot of people do it from time to time (including me). But maybe there are better places for this than Planet Gnome. And maybe doing it for days is not particularly nice. And maybe flaming sucks in the first place anyway.

Regardless what I think about Jeffrey and his behaviour on Planet Gnome, let's have a look on his trophies, the five "bugs" he posted:

  1. Not directly related to PulseAudio itself. Also, finding errors in code that is related to esd is not exactly the most difficult thing in the world.
  2. The same theme.
  3. Fixed 3 months ago. It is certainly not my fault that this isn't available in Jeffrey's distro.
  4. A real, valid bug report. Fixed in git a while back, but not available in any released version. May only be triggered under heavy load or with a bad high-latency scheduler.
  5. A valid bug, but not really in PulseAudio. Mostly caused because the ALSA API and PA API don't really match 100%.

OK, Jeffrey found a real bug, but I wouldn't say this is really enough to make all the fuss about. Or is it?

Why PulseAudio?

Jeffrey wrote something about 'solution looking for a problem' when speaking of PulseAudio. While that was certainly not a nice thing to say it however tells me one thing: I apparently didn't manage to communicate well enough why I am doing PulseAudio in the first place. So, why am I doing it then?

  • There's so much more a good audio system needs to provide than just the most basic mixing functionality. Per-application volumes, moving streams between devices during playback, positional event sounds (i.e. click on the left side of the screen, have the sound event come out through the left speakers), secure session-switching support, monitoring of sound playback levels, rescuing playback streams to other audio devices on hot unplug, automatic hotplug configuration, automatic up/downmixing stereo/surround, high-quality resampling, network transparency, sound effects, simultaneous output to multiple sound devices are all features PA provides right now, and what you don't get without it. It also provides the infrastructure for upcoming features like volume-follows-focus, automatic attenuation of music on signal on VoIP stream, UPnP media renderer support, Apple RAOP support, mixing/volume adjustments with dynamic range compression, adaptive volume of event sounds based on the volume of music streams, jack sensing, switching between stereo/surround/spdif during runtime, ...
  • And even for the most basic mixing functionality plain ALSA/dmix is not really everlasting happiness. Due to the way it works all clients are forced to use the same buffering metrics all the time, that means all clients are limited in their wakeup/latency settings. You will burn more CPU than necessary this way, keep the risk of drop-outs unnecessarily high and still not be able to make clients with low-latency requirements happy. 'Glitch-Free' PulseAudio fixes all this. Quite frankly I believe that 'glitch-free' PulseAudio is the single most important killer feature that should be enough to convince everyone why PulseAudio is the right thing to do. Maybe people actually don't know that they want this. But they absolutely do, especially the embedded people -- if used properly it is a must for power-saving during audio playback. It's a pity that how awesome this feature is you cannot directly see from the user interface.[1]
  • PulseAudio provides compatibility with a lot of sound systems/APIs that bare ALSA or bare OSS don't provide.
  • And last but not least, I love breaking Jeffrey's audio. It's just soo much fun, you really have to try it! ;-)

If you want to know more about why I think that PulseAudio is an important part of the modern Linux desktop audio stack, please read my slides from 2007.


Many people (like Jeffrey) wonder why have software mixing at all if you have hardware mixing? The thing is, hardware mixing is a thing of the past, modern soundcards don't do it anymore. Precisely for doing things like mixing in software SIMD CPU extensions like SSE have been invented. Modern sound cards these days are kind of "dumbed" down, high-quality DACs. They don't do mixing anymore, many modern chips don't even do volume control anymore. Remember the days where having a Wavetable chip was a killer feature of a sound card? Those days are gone, today wavetable synthesizing is done almost exlcusively in software -- and that's exactly what happened to hardware mixing too. And it is good that way. In software mixing is is much easier to do fancier stuff like DRC which will increase quality of mixing. And modern CPUs provide all the necessary SIMD command sets to implement this efficiently.

Other people believe that JACK would be a better solution for the problem. This is nonsense. JACK has been designed for a very different purpose. It is optimized for low latency inter-application communication. It requires floating point samples, it knows nothing about channel mappings, it depends on every client to behave correctly. And so on, and so on. It is a sound server for audio production. For desktop applications it is however not well suited. For a desktop saving power is very important, one application misbehaving shouldn't have an effect on other application's playback; converting from/to FP all the time is not going to help battery life either. Please understand that for the purpose of pro audio you can make completely different compromises than you can do on the desktop. For example, while having 'glitch-free' is great for embedded and desktop use, it makes no sense at all for pro audio, and would only have a drawback on performance. So, please stop bringing up JACK again and again. It's just not the right tool for desktop audio, and this opinion is shared by the JACK developers themselves.

Jeffrey thinks that audio mixing is nothing for userspace. Which is basically what OSS4 tries to do: mixing in kernel space. However, the future of PCM audio is floating points. Mixing them in kernel space is problematic because (at least on Linux) FP in kernel space is a no-no. Also, the kernel people made clear more than once that maths/decoding/encoding like this should happen in userspace. Quite honestly, doing the mixing in kernel space is probably one of the primary reasons why I think that OSS4 is a bad idea. The fancier your mixing gets (i.e. including resampling, upmixing, downmixing, DRC, ...) the more difficulties you will have to move such a complex, time-intensive code into the kernel.

Not everytime your audio breaks it is alone PulseAudio's fault. For example, the original flame of Jeffrey's was about the low volume that he experienced when running PA. This is mostly due to the suckish way we initialize the default volumes of ALSA sound cards. Most distributions have simple scripts that initialize ALSA sound card volumes to fixed values like 75% of the available range, without understanding what the range or the controls actually mean. This is actually a very bad thing to do. Integrated USB speakers for example tend export the full amplification range via the mixer controls. 75% for them is incredibly loud. For other hardware (like apparently Jeffrey's) it is too low in volume. How to fix this has been discussed on the ALSA mailing list, but no final solution has been presented yet. Nonetheless, the fact that the volume was too low, is completely unrelated to PulseAudio.

PulseAudio interfaces with lower-level technologies like ALSA on one hand, and with high-level applications on the other hand. Those systems are not perfect. Especially closed-source applications tend to do very evil things with the audio APIs (Flash!) that are very hard to support on virtualized sound systems such as PulseAudio [2]. However, things are getting better. My list of issues I found in ALSA is getting shorter. Many applications have already been fixed.

The reflex "my audio is broken it must be PulseAudio's fault" is certainly easy to come up with, but it certainly is not always right.

Also note that -- like many areas in Free Software -- development of the desktop audio stack on Linux is a bit understaffed. AFAIK there are only two people working on ALSA full-time and only me working on PulseAudio and other userspace audio infrastructure, assisted by a few others who supply code and patches from time to time, some more and some less.

More Breakage to Come

I now tried to explain why the audio experience on systems with PulseAudio might not be as good as some people hoped, but what about the future? To be frank: the next version of PulseAudio (0.9.11) will break even more things. The 'glitch-free' stuff mentioned above uses quite a few features of the underlying ALSA infrastructure that apparently noone has been using before -- and which just don't work properly yet on all drivers. And there are quite a few drivers around, and I only have a very limited set of hardware to test with. Already I know that the some of the most popular drivers (USB and HDA) do not work entirely correctly with 'glitch-free'.

So you ask why I plan to release this code knowing that it will break things? Well, it works on some hardware/drivers properly, and for the others I know work-arounds to get things to work. And 0.9.11 has been delayed for too long already. Also I need testing from a bigger audience. And it is not so much 0.9.11 that is buggy, it is the code it is based on. 'Glitch-free' PA 0.9.11 is going to part of Fedora 10. Fedora has always been more bleeding edge than other other distributions. Picking 0.9.11 just like that for an 'LTS' release might however be a not a good idea.

So, please bear with me when I release 0.9.11. Snapshots have already been available in Rawhide for a while, and hell didn't freeze over.

The Distributions' Role in the Game

Some distributions did a better job adopting PulseAudio than others. On the good side I certainly have to list Mandriva, Debian[3], and Fedora[4]. OTOH Ubuntu didn't exactly do a stellar job. They didn't do their homework. Adopting PA in a distribution is a fair amount of work, given that it interfaces with so many different things at so many different places. The integration with other systems is crucial. The information was all out there, communicated on the wiki, the mailing lists and on the PA IRC channel. But if you join and hang around on neither, then you won't get the memo. To my surprise when Ubuntu adopted PulseAudio they moved into one of their 'LTS' releases rightaway [5]. Which I guess can be called gutsy -- on the background that I work for Red Hat and PulseAudio is not part of RHEL at this time. I get a lot of flak from Ubuntu users, and I am pretty sure the vast amount of it is undeserving and not my fault.

Why Jeffrey's distro of choice (SUSE?) didn't package pavucontrol 0.9.6 although it has been released months ago I don't know. But there's certainly no reason to whine about that to me and bash me for it.

Having said all this -- it's easy to point to other software's faults or other people's failures. So, admitting this, PulseAudio is certainly not bug-free, far from that. It's a relatively complex piece of software (threading, real-time, lock-free, sensitive to timing, ...), and every software has its bugs. In some workloads they might be easier to find than it others. And I am working on fixing those which are found. I won't forget any bug report, but the order and priority I work on them is still mostly up to me I guess, right? There's still a lot of work to do in desktop audio, it will take some time to get things completely right and complete.

Calls for "audio should just work (tm)" are often heard. But if you don't want to stick with a sound system that was state of the art in the 90's for all times, then I fear things *will have* to break from time to time. And Jeffrey, I have no idea what you are actually hacking on. Some people mentioned something with Evolution. If that's true, then quite honestly, "email should just work", too, shouldn't it? Evolution is not exactly famous for it's legendary bug-freeness and stability, or did I miss something? Maybe you should be the one to start with making things "just work", especially since Evolution has been around for much longer already.

Back to Work

Now that I responded to Jeffrey's FUD I think we all can go back to work and end this flamefest! I wish people would actually try to understand things before writing an insulting rant -- without the slightest clue -- but with words like "clusterfuck". I'd like to thank all the people who commented on Jeffrey's blog and basically already said what I wrote here now.

So, and now I am off hacking a bit on PulseAudio a bit more -- or should I say in Jeffrey's words: on my clusterfuck that is an epic fail and that no desktop user needs?


[1] BTW 'glitch-free' is nothing I invented, other OS have been doing something like this for quite a while (Vista, Mac OS). On Linux however, PulseAudio is the first and only implementation (at least to my knowledge).

[2] In fact, Flash 9 can not be made fully working on PulseAudio. This is because the way Flash destructs it's driver backends is racy. Unfixably racy, from external code. Jeffrey complained about Flash instability in his second post. This is unfair to PulseAudio, because I cannot fix this. This is like complaining that X crashes when you use binary-only fglrx.

[3] To Debian's standards at least. Since development of Debian is very distributed the integration of such a system as PulseAudio is much more difficult since in touches so many different packages in the system that are kind of private property by a lot of different maintainers with different views on things.

[4] I maintain the Fedora stuff myself, so I might be a bit biased on this one... ;-)

[5] I guess Ubuntu sees that this was a bit too much too early, too. At least that's how I understood my invitation to UDS in Prague. Since that summit I haven't heard anything from them anymore, though.

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