Re: the State of Linux Audio
On Thu, Jul 16, 2015 at 06:15:42AM -0500, Martin G. McCormick wrote:
> Joel Roth writes:
> > Hi Martin,
>
> > Pulse audio requires D-Bus, and D-Bus is the underlying RPC
> > mechanism of a large and controversial software stack
> > developed to support desktop applications.
>
> Thank you for this good and quick explanation.
>
> >
> > Apparently pulseaudio is unable to get D-Bus services,
> > due to a dependency of the latter on X.
> >
> > So you need either to satisfy/finesse this dependency of
> > D-Bus, or disable/remove pulseaudio. I've read but not
> > tested apulse, a library that purports to presents a pulse
> > audio API to applications such as skype that require them,
> > relaying the audio to ALSA.
>
> It looks like pulseaudio has been the ghost in the
> basement on my system for about ten years.
>
> For now, I took it off completely and killed the
> process. Before the CS4236 went away, there was always a sound
> device for Card 0 and, if I had a second sound card, there was
> another sound device for the second card. It was typical to see
> /dev/dsp linked to Card 0, an actual /dev/dsp0 device and
> /dev/dsp1 for Card 1 and so on. Just for fun, I think I stuck in a
> USB sound card in addition to cards 0 and 1 and predictably got
> /dev/dsp2 plus a lot of strange audio that sounded like a bad
> tape transport due to all the sound cards trying to write to
> their little segments of memory at the same time on a 600-MHZ
> Pentium.
>
> Back to the present, I still had PA (pulseaudio)
> running, no official Card 0 and a USB-based Soundblaster Digi
> acting as Card 1. At least that's what aplay -l said.
>
> I could run mplayer and the playback was excellent but
> amixer for Card 0 only showed one control for left and right
> front volume.
>
> After I killed PA, mplayer said it couldn't find any
> cards as there was no /dev/dsp any more. I did however find
> /dev/dsp1 for the SBdigi so I manually forced a link with ln -s
> to link /dev/dsp to /dev/dsp1. Presto! mplayer was happy again
> and played music and other audio files but the story isn't quite
> over yet.
The snd_pcm_oss kernel module, when loaded, provides
/dev/dsp for compatibility with OSS applications.
Calling mplayer with --ao=alsa should play via the default
ALSA device, and does not require /dev/dsp.
One the main benefits of pulse audio is the convenience
of having a separate volume control for each audio application.
If you don't need that, maybe you don't need PA.
I can't comment on whether PA provides any other audio
processing, however there are many other ways to
get a low-pass filter if you need one.
I refer you to the Linux Audio Users mailing list,
the authoritative source for linux audio questions.
The ArchLinux forums and wikis are also a great
resource.
Cheers,
Joel
>
> I'm thinking that pulseaudio does some signal
> processing, also. Some of the sound files I listen to are 8-bit
> PCM voice recordings made at 8000 samples per second. They're
> just fine for recording two-way radio chatter. Mr. Nyquist is
> happy because 3 to 4 KHZ is the highest frequency you will
> usually hear over such communication so it doesn't sound much
> different than it did when first heard over the air.
>
> Before I killed PA, the audio of those raw PCM
> recordings sounded fairly normal. After I killed PA, you can now
> still hear the audio but you can also hear the 8-KHZ sampling
> which sounds like a cheap toy as most of those don't bother with
> a low-pass filter but let your ears and brain do that.
>
> It is possible that pulseaudio is using DSP techniques
> to shape the wave forms properly and then is up-converting the
> samples that the sound card sees. You can't add any fidelity
> that is not already there, but this would act as a very good
> low-pass filter.
>
> I also got in to /etc/modprobe.d and added a line to
> alsa-base.conf to make the SBDigi be card 0 until I can resurrect the CS4236 and
> this seems to have made everything work automatically again.
> amixer now reports a full-featured sound card with all the
> controls one needs to do good playback and normal recording. The
> playback is actually better than the CS4236 was so now we have
> some progress. After things settle down, I may put PA back just
> for the signal processing but for now, it's best to keep things
> as simple as possible.
>
> Again, thanks for the explanation of some of
> pulseaudio's purpose in life. It is presently used in those
> screen reader modules that allow the kernel to generate
> synthesized speech so it isn't all bad but it sure helps to know
> what it does.
>
> Martin McCormick
>
>
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>
--
Joel Roth
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