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- To: proaudio@xxxxxxxxxxxxxxxxxxx
- Subject: Re: [proaudio] Real-time for audio
- From: Mark Knecht <markknecht@xxxxxxxxx>
- Date: Fri, 7 May 2010 12:46:31 -0700
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On Fri, May 7, 2010 at 10:26 AM, Grant <emailgrant@xxxxxxxxx> wrote:
>> With a rt kernel, you can setup both the hardware priorities and the software
>> priorities. That mean that tasks accessing some hardware, the sound card, will
>> have the priority over the other tasks, and that tasks executed by a given user
>> or group [the audio group] will have priority over the other. rtirq will dot
>> that for you. Just emerge it and add it into the default run level.
>
> Is jack necessary for me then? What I want to do is use my USB DAC
> and mpd (music player app) in real time. Maybe jack is necessary if I
> want to know *how* real time it is?
>
>> Be aware that if you don't really need a rt kernel, it is best to not use it,
>> because such a kernel can cause compilation failures with gcc.
>> http://bugs.gentoo.org/show_bug.cgi?id=190462
>> http://bugs.gentoo.org/show_bug.cgi?id=20600
>> For that raeson, if you want to use a rt kernel, it is good practice to have
>> another kernel of the same version, vanilla or gentoo sources, and reboot on
>> this non rt kernel when using emerge.
>
> I noticed this when trying to emerge nvidia-drivers.
>
> Thanks,
> Grant
Hi Grant,
I thought we had this discussion maybe 6 months ago? Maybe I'm
wrong about that.
For pure audio playback the real-time kernel and Jack provide
almost NO value. The real-time kernel is able to give more responsive
attention to certain drivers or apps. Jack is one of them and if your
mpd app - whatever that is - is a Jack app then it inherits the
attention. However this only matters when you need to do something in
'real time'. There is nothing, in my opinion, 'real time' about
reading a stream of pre-recorded data off a disk, buffering it up and
then sending it to a sound card. Why does the sound card need to
receive data in 5mS vs 25mS? The only difference that the buffer time
causes is to delay the start of playback. Once playback starts as long
as there is no time the buffer runs dry there is no difference in the
sound you hear.
I think that for your application using the real-time kernel and
Jack is adding nothing but complication and that's likely to cause
more problems than it's going to solve.
Cheers,
Mark