Subject: Re: [linux-audio-dev] Another Annoying "How Do I Get Started" Question
From: Greg Berchin (76145.2455_AT_compuserve.com)
Date: Wed Jul 11 2001 - 16:22:36 EEST
Benno Senoner wrote:
>I do agree that these days it pays off to use a general purpose PC to
>perform dsp stuff rather trying to build expensive custom tailored hw
>solutions.
>Especially now that almost all CPUs do have SIMD instructions.
Even without SIMD, GP processors are getting so fast that DSPs are no
longer the only game in town.
>The only problem with PCs is that you have to be careful when wanting to
>get low latencies.
For my purposes it is not a problem. As long as the latency is low enough
to avoid lip sync problems when processing audio associated with video
(which means below approximately 30 mSec), I am happy.
>If you are careful to shut down all other stuff, not using virtual
>memory , letting you application run SCHED_FIFO (realtime scheduling)
>and using busywait memory mapping audio I/O ... then I believe you can
>get pretty close to the lower limit
Ultimately I would like to know enough to hack the kernel, removing
everything that is not needed for audio support. I chose Linux simply
because it is available and has good support; I do not consider it to be
sacred in any way. As I said, my system will be standalone,
single-application, audio-only. I believe that the kernel can be seriously
trimmed and still give me what I need. I just don't know how to do it,
yet.
>When you chose to stay in userspace (sacrificing single sample latency)
>you things get very confortable for you:
>you write a simple LADSPA host once (eg one that simply loads one/more
>plugins and then sits in a loop reading from the soundcard, calling the
>dsp stuff contained in plugins and then output it to the soundcard).
THAT is exactly what I am looking for, then. My first goal is creation or
adoption of this basic framework with a simple dummy DSP routine in the
middle. Once the I/O framework is in place, then it is a simple matter of
plugging-in the appropriate subroutine(s) in place of the dummy routine.
>Greg, your help for DSP stuff will be very welcome, especially when
>trying to squeeze out the maximum performance of applications.
I will be glad to (try to) answer any questions that come my way. If I
don't know the answer, I probably know someone who does. The audio DSP
world is rather small, and most of us know each other.
>BTW: at LinuxTag we made some experiments with Muse driving the
>disksampler and on Frank's 1Ghz machine ... 70voices ... causes a CPU
>load of about 30-35% which is quite acceptable IMHO.
That is encouraging. I do know from experience, however, that the Ideal
Gas Law applies (the application expands to fill the available processing
power). There is always some new feature to add ...
Thanks again, to you and all who have responded,
Greg Berchin
76145.2455_AT_compuserve.com (my private email, for if anyone wants to ask
signal processing questions -- please don't let it fall into the hands of
the spammongers)
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