The state of Linux audio
Detta är en ganska lång post om "the state of Linux audio", men det kan nog vara en ganska så intressant läsning för geeks. Stort tack till Sean McNamara som är involverad i ALSA projektet.
What is the state of audio in Linux?
How does it compare against other operating systems?
Is the Linux audio functionality good or bad?
What could be better?
What is planned for the future?
This sounds like a homework question to me
That said, there are no "right" answers to these questions which encapsulate every opinion about this topic. A comprehensive answer would take many pages -- one could write a book on the different
viewpoints. But here's my personal viewpoint (comments inline).
What is the state of audio in Linux?
How does it compare against other operating systems?
I'm going to answer these two questions at the same time.
Depends on how you define "audio" and "Linux".
[list="a"][*]If you define audio = "the ability to send/capture PCM data to/from a sound card and have it playback/capture at low latency", and Linux = "the Linux kernel", I'd argue that ALSA is the most
featureful, stable, and comprehensive implementation of more audio drivers than any other platform. Not only does ALSA support virtually every consumer audio device on the market; it also supports many high-end pro audio solutions, as well as sound chips for embedded systems. I'd say that Windows supports the same core consumer audio devices out of the box, it probably supports all pro audio devices with third-party drivers, and supports few to no embedded SoC chips.
So in sheer number of chipsets supported out of the box, I think ALSA takes the cake. In addition, fragmentation at the kernel level is fairly low; the OSS/Free implementation in the Linux kernel is quickly fading into bitrot oblivion, and 4-Front Technologies' OSS 4.x is the only real competitor to ALSA's kernel-level support. The fragmentation in the kernel is really between two implementations, where one implementation (ALSA) is what a vast majority of people use, if only because distributions ship it as the default sound stack. If all you're interested in is the kernel, I'd argue that you can envision Linux audio as a thriving, successful, victorious project whose only kernel-level goals going forward are to maintain the current level of excellence they've achieved, by supporting new hardware as it comes down the R&D pipe.
[*]If you define audio more broadly to include userspace support for audio editing, consistent audio playback, and a worry-free user experience where applications "just work", and you define Linux as "a
typical distribution of GNU/Linux which includes many FOSS graphical applications and utilities", the scenario is much uglier. At the user space level, there are tens of different APIs available which offer
mostly-overlapping feature sets; unfortunately, almost every library claims one or more compelling features which distinguish it from the pack.[/list=a]
Due to the strong emphasis on consistency in the large desktop environment efforts, GNOME and KDE, software belonging under the umbrella of these projects often behaves well as ALSA applications. In
KDE 4's case, we have Phonon, which eventually gets around to using ALSA. As Colin said, these projects are quickly moving to also support native PulseAudio.
When an independent software developer (not associated with GNOME or KDE) writes a new application making heavy use of audio, they are almost equally likely to choose among one of a half-dozen leading competitors offering easy to use or featureful audio programming APIs:
FMOD, OpenAL, libao, libpulse, ALSA, JACK, and so on. While many of these advertise support for ALSA as a back-end, such implementations are hit-or-miss as to whether they actually work. Once you start
interposing a plugin library that translates ALSA calls into PulseAudio calls (a necessary step towards desktop integration and API interoperability), the chances of correct functionality diminish.
Basically, the more pass-through layers the audio has to run through before it's sent out your speakers, you have exponentially worse chances of it actually succeeding (and if it does succeed, the sound quality and reliability also diminish the more indirection you have).
In this way, the standard UNIX model of employing many small programs/libraries together in cooperation falls down with audio. It's just very hard to ensure proper interoperability between different audio APIs.
A consequence of this fragmentation, coupled with the fact that ALSA has no "forced" mixing of multiple applications' sound at once, means that the reliability of sound playback on a user's desktop can be extraordinarily poor when they have installed more than one or two audio-playing applications.
If your distribution does not ship any software mixing installed and ready to use out of the box, which has been the case for every distro until ca. 2007, then you will automatically run into problems if you
try to play sound from two applications at once. But even if your distribution has PulseAudio or ALSA's dmix enabled by default, any of the above mentioned libraries may decide at any time that it doesn't
want to play nicely with other applications, and hog your sound card, shutting out any other applications indefinitely. This peculiar limitation is fundamental to ALSA's design and cannot be fixed without breaking backwards compatibility.
PulseAudio is solving many of these problems, including a mandate that any applications using PulseAudio must allow their audio to be mixed with other applications. But PulseAudio still sits on top of a fragile and complicated ALSA userspace, which limits Pulse's ability to do its job. Here are two issues foremost in my mind pertaining to ALSA userland, presented at a high level:
[list="1"][*]Most developers, novice or experienced, are unable to correctly write an ALSA client that cooperates nicely with ALSA plugins such as ALSA<->Pulse. A huge percentage of existing ALSA clients are semantically misaligned with any ALSA userspace backend except the most basic direct hardware access (which, again, lacks any sort of mixing). This is partly the fault of these developers, and partly the fault of the API designers for creating something that is capable of being misused so easily. Application writers should not be given a choice between using a slower, more compatible code path or a faster, less compatible code path; Murphy's Law says that most developers will choose the less compatible path to make their application faster, giving users no choice to fall back to the more compatible path. And of course (Murphy's Law again) most users will need the compatibility
mode to get the application's audio output to work at all. I'm referring to mmap'ed playback for anyone who is familiar with ALSA internals, but this is merely one example of several.
[*]The ALSA plugin interface leaves (too) many things open to the interpretation of the plugin writer. This is mostly a documentation issue: syntax of function prototypes means absolutely nothing if those
implementing the APIs do not know what state the program must be in before and after each function call, what sort of errors must be thrown, and so on. As a result, it is very difficult to write an ALSA
plugin and proclaim, "There; any sane ALSA client will now be able to interface seamlessly with my sound server/library through my ALSA plugin." This is exacerbated by the client-side problems in the
previous point -- there are even fewer sane ALSA clients than there are sane ALSA plugins :P[/list=1]
Is the Linux audio functionality good or bad?
This question is SO subjective that I think I'll let you infer this from the rest of my post, rather than trying to address it directly.
What could be better?
As I said, unification on a core set of components is the most important step going forward. This has to be a conscious effort made between audio stack developers (PulseAudio and ALSA's maintainers) and application authors. On the one hand, the audio stack needs to end its decade-long streak of uniform mediocrity, and allow for a champion to arise -- one implementation that gains mass support and recognition.
This should not be done through force or FUD; it should be done through sheer software quality and excellence. I for one am nominating PulseAudio for that place, assisted in no small part by ALSA.
Eventually, there might arise two competing solutions at the top, both of which gain mass acceptance - but this is no worse than the current GTK/QT split. They consciously interoperate with one another anyway, which is a hallmark of their excellence.
Basically, I think we are experiencing somewhat of a repeat of the early X11 days: way back then, it wasn't such an unreasonable thing for people to hack their own basic window management, widget toolkit, etc. for their applications. A vast majority agreed upon the fact that the X server was a viable foundation, but no one agreed upon where to go from there. Some people (the GTK, CDE, Motif and Qt authors) wanted to create a framework that others could build upon, discouraging application writers to directly use X11 calls; other people wanted to go their own way and directly access X11 through their own abstractions, or use one of the numerous other abstractions which were sitting around in mediocrity.
But then a tipping point came, and a vast majority of developers saw the real value of unifying on a common widget toolkit. After that, we basically had Motif, GTK, and Qt (in no particular order) arise as better-than-mediocre champions, and by 2000 it was more or less a strange thing when an application writer wanted to directly use libX11 for all their drawing operations.
I see the PulseAudio camp saying the same thing to ALSA as the GTK camp was saying to XFree86 (and now X.Org) a couple of years ago:
(Audio) I hope ALSA will continue to thrive at the kernel level, but eventually either reinvent itself or disappear out of developers' minds at the user-space level.
(Graphics) I hope X11 will continue to thrive on the server-side, but eventually either reinvent itself or disappear out of developers' minds on the client-side.
What is planned for the future?
So from my not-so-unbias perspective, I'd say that Linux audio is at the base of the final peak that we must climb to get Linux audio ready for prime time. Most of the climb will involve application developers recognizing the one or two most successful audio projects, and adopting these instead of continuing the fragmentation. The more we can get applications to use the best APIs, the better the user experience will be.
Of course, there will always be multiple champions at the top; this is necessary in the FOSS world. But when 95% of the applications use only two or three different well-supported paths, the success rate of audio interoperability (software mixing, effects, etc.) will be much higher than it currently is.
